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Title: Pneumatic conveying
Author: E. G. Phillips
Release date: December 2, 2023 [eBook #72290]
Language: English
Original publication: London: Sir Isaac Pitman & Sons, 1921
Credits: Bob Taylor, deaurider and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK PNEUMATIC CONVEYING ***
Transcriber’s Note
Italic text displayed as: _italic_
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PITMAN’S TECHNICAL PRIMER SERIES
_Edited by R. E. NEALE, B.Sc., Hons. (Lond.)
A.C.G.I., A.M.I.E.E._
PNEUMATIC CONVEYING
PITMAN’S TECHNICAL PRIMERS
Edited by R. E. NEALE, B.Sc. (Hons.), A.C.G.I., A.M.I.E.E.
In each book of the series the fundamental principles of some
sub-division of engineering technology are treated in a practical
manner, providing the student with a handy survey of the particular
branch of technology with which he is concerned. Each 2s. 6d. net.
=THE STEAM LOCOMOTIVE.= By E. L. AHRONS, M.I.Mech.E.
=BELTS FOR POWER TRANSMISSION.= By W. G. DUNKLEY, B.Sc.
=WATER-POWER ENGINEERING.= By F. F. FERGUSSON, A.M.I.C.E.
=PHOTOGRAPHIC TECHNIQUE.= By L. J. HIBBERT, F.R.P.S.
=HYDRO-ELECTRIC DEVELOPMENT.= By J. W. MEARES, M.Inst.C.E.
=THE ELECTRIFICATION OF RAILWAYS.= By H. F. TREWMAN, M.A.
=CONTINUOUS-CURRENT ARMATURE WINDING.= By F. M. DENTON.
=MUNICIPAL ENGINEERING.= By H. PERCY BOULNOIS, M.Inst.C.E.
=FOUNDRYWORK.= By BEN SHAW and JAMES EDGAR.
=PATTERNMAKING.= By BEN SHAW and JAMES EDGAR.
=THE ELECTRIC FURNACE.= By FRANK J. MOFFETT, B.A., M.I.E.E.
=SMALL SINGLE-PHASE TRANSFORMERS.= By E. T. PAINTON, B.Sc.
=PNEUMATIC CONVEYING.= By E. G. PHILLIPS, M.I.E.E., A.M.I.Mech.E.
=BOILER INSPECTION AND MAINTENANCE.= By R. CLAYTON.
=ELECTRICITY IN STEEL WORKS.= By W. MACFARLANE, B.Sc.
=MODERN CENTRAL STATIONS.= By C. W. MARSHALL, B.Sc.
=STEAM LOCOMOTIVE CONSTRUCTION AND MAINTENANCE.= By E. L. AHRONS,
M.I.Mech.E., M.I.Loco.E.
=HIGH TENSION SWITCHGEAR.= By H. E. POOLE, B.Sc., A.C.G.I.
=HIGH TENSION SWITCHBOARDS.= By the Same Author.
=POWER FACTOR CORRECTION.= By A. E. CLAYTON, B.Sc., A.K.C.
=TOOL AND MACHINE SETTING.= By P. GATES.
=TIDAL POWER.= By A. STRUBEN, O.B.E., A.M.I.C.E.
=SEWERS AND SEWERAGE.= By H. G. WHYATT, M.Inst.C.E. M.R.S.I.
=ELEMENTS OF ILLUMINATING ENGINEERING.= By A. P. TROTTER, M.Inst.C.E.
=COAL-CUTTING MACHINERY.= By G. E. F. EAGAR, M.Inst.Min.E.
=GRINDING MACHINES AND THEIR USE.= By T. R. SHAW, M.I.Mech.E.
=ELECTRO-DEPOSITION OF COPPER.= By C. W. DENNY, A.M.I.E.E.
=DIRECTIVE WIRELESS TELEGRAPHY.= By L. H. WALTER, M.A.
=TESTING OF CONTINUOUS-CURRENT MACHINES.= By C. F. SMITH, D.Sc.,
M.I.E.E.
=ELECTRICAL TRANSMISSION OF ENERGY.= By W. M. THORNTON, D.Sc.
=STEAM ENGINE VALVES AND VALVE GEAR.= By E. L. AHRONS, M.I.Mech.E.
=MECHANICAL HANDLING OF GOODS.= By C. H. WOODFIELD.
=INDUSTRIAL AND POWER ALCOHOL.= By R. C. FARMER, D.Sc.
=POSITION AND DIRECTION FINDING.= By L. H. WALTER, M.A., A.M.I.E.E.
=HIGH TENSION TRANSFORMERS.= By W. T. TAYLOR, M.Inst.C.E.
LONDON: SIR ISAAC PITMAN & SONS, LTD.
[Illustration:
_Frontispiece_
INTAKE END OF PNEUMATIC CONVEYOR.
Adjustable to follow rise and fall of vessel on tidal waters.]
PNEUMATIC
CONVEYING
A CONCISE TREATMENT OF THE
PRINCIPLES, METHODS AND APPLICATIONS
OF PNEUMATIC CONVEYANCE OF MATERIALS
WITH SPECIAL REFERENCE TO THE CONVEYING
AND ELEVATING OF HEAVY SOLID MATERIALS
FOR ENGINEERS, WORKS MANAGERS,
AND STUDENTS
BY
E. G. PHILLIPS
M.I.E.E., A.M.I.MECH.E.
[Illustration: PITMAN’S
TECHNICAL
PRIMERS]
LONDON
SIR ISAAC PITMAN & SONS, LTD.
PARKER STREET, KINGSWAY, W.C.2
BATH, MELBOURNE, TORONTO, NEW YORK
1921
PRINTED BY
SIR ISAAC PITMAN & SONS, LTD.
BATH, ENGLAND
PREFACE
Pneumatic Conveying has recently attracted great and widespread
interest amongst engineers and others interested in the economical
handling of materials. The information hitherto available on the
subject has, however, been so meagre and so indefinite that the author
offers this concise treatment of principles, methods and applications
in order that those who are anxious to avail themselves of this
flexible means of transportation may know just how, and why, it is
worthy of their attention.
The high cost of labour, coupled with the desire to improve the
condition of the worker who has had to work in a dust-laden atmosphere
of an objectionable, or even poisonous nature, will create such a
demand for pneumatic conveying plant that present day methods of
transportation will be revolutionized in many branches of industry.
The author has had three years’ experience with one of the first
pneumatic plants erected in this country, for use with coal, ashes,
and flue dust, and he has conducted numerous experiments on other
materials, such as sand, oxide, potatoes, various chemicals, etc., and
has endeavoured to add his quota to the information available for the
benefit of other engineers and works managers.
Experiments proved that with certain materials the discharger was the
weak link in the chain, as wet sticky materials entering at a high
velocity “packed” tight and would not discharge freely. This led to
experiments which, with the valuable assistance of Mr. H. B. Clarke,
A.M.I.E.E., resulted in overcoming this difficulty by means which are
described fully in Chapter VI.
No attempt has been made to go fully into the questions of domestic
vacuum cleaners, removing dust from manufacturing processes, etc. The
chief aim has been rather to bring into prominence the flexibility and
other advantages of moving air as a means of conveying and elevating
heavy solid materials which hitherto it has been thought could not be
handled in this manner.
The author gladly acknowledges the interest and information given by
the following firms and individuals Mr. H. B. Clarke, A.M.I.E.E.;
Messrs. Ashwell & Nesbit; Boby, Ltd.; H. J. H. King & Co., Ltd.; The
Lamson Store Service Co.; and the Sturtevant Engineering Co., Ltd.
E. G. PHILLIPS.
NOTTINGHAM.
CONTENTS
CHAP. PAGE
PREFACE vii
I. SYSTEMS AND APPLICATIONS OF PNEUMATIC CONVEYING 1
II. DETAILS OF PLANT—PUMPS, EXHAUSTERS AND AIR FILTERS 10
III. DETAILS OF PLANT (CONTD.)—DISCHARGERS, PIPE LINES AND SUCTION
NOZZLES 28
IV. TYPICAL INSTALLATIONS FOR GRAIN 45
V. PNEUMATIC COAL HANDLING PLANTS 54
VI. THE INDUCTION CONVEYOR 62
VII. STEAM JET CONVEYORS 72
VIII. MISCELLANEOUS APPLICATIONS OF PNEUMATIC CONVEYING 80
BIBLIOGRAPHY 105
INDEX 107
ILLUSTRATIONS
FIG. PAGE
Intake end of Pneumatic Conveyor, with rise and fall
adjustment _Frontispiece_
1. King’s pneumatic system. Steam-driven air pump 12
2. Sturtevant rotary blower 17
3. Nash “Hydro-Turbo” exhauster 19
4. Sturtevant “Cyclone” dust separator 21
5. Automatic bag filter 23
6. Mollers’ air filter 24
7. Sturtevant wet filter 26
8. Fixed discharger with glass receiver 30
9. Sturtevant patent junction 33
10. Course taken by material round bend 35
11. Segment-back bend 35
12. Lobster bend 35
13. King full-way junction valve 37
14. Suction nozzles for high pressure systems 40
15. Sturtevant equipment removing wood refuse from double tenoning
machine 41
16. Sturtevant equipment removing dust from sand papering machines 42
17, 18. Ancient and Modern 46
19. Typical grain-handling plant 47
20. Floating pneumatic transport plant 49
21. Portable pneumatic plant on railway truck 50
22. Portable railway plant in operation 52
23. Pneumatic unloading of coal 55
24. Discharger for coal conveying plant 57
25. Brady steam jet ash conveyor 75
26. Typical Lamson inter-communication carrier 82
27. Tube central in wholesale drug house 87
28. Lamson distributing station 88
29. Stationary turbo-exhauster with dust separator 91
30. Portable turbo-exhauster 91
31. Suction cleaning for railway carriage cushions 93
32. Sturtevant equipment for office cleaning 93
33, 34. Air-lift pumping 98
PNEUMATIC CONVEYING
CHAPTER I
SYSTEMS AND APPLICATIONS OF PNEUMATIC CONVEYING
Conveying by mechanical means has existed for many centuries, and is
one of the earliest forms of man’s ingenuity towards labour saving
as we know it to-day. The pneumatic conveyance of materials from one
position to another, either horizontally or vertically, is the most
recent form of automatic handling of solid substances.
=Genesis and Applications of Pneumatic Conveying.= The need of water
for human consumption and for irrigation purposes caused the ancient
inventor to carry out nearly all his experiments with that substance,
and the first instance we have of anything approaching pneumatic
conveying is the well known injector in which steam is passed through
one pipe, placed at right angles to a second pipe, at such a velocity
as to reduce the pressure in the second pipe to below the atmospheric
pressure. The excess of atmospheric pressure over the reduced pressure
in the second pipe then drives up the latter the material (in this case
water) in which the end of the pipe is submerged. This invention is
about a century old, if it emanated from the Marquis Mammonry d’Eclet,
in 1818, as is usually believed.
The first practical application of this invention to other than liquid
materials is supposed to have been in connection with the conveying of
cotton in a loose form, as an improvement upon the manual shifting of
large bales. This development was made about 1867, and after this date
great progress was made, principally in connection with the handling of
grain, wheat, malt, etc., and largely owing to the work of an American
named A. K. Williams.
At the present day it is hardly possible to enumerate all the
successful schemes for the pneumatic handling of materials. In addition
to installations for the conveyance of materials such as those
mentioned above is the pneumatic tube, for conveying papers, messages
and cash in offices and between shop counter and cash desk, etc.
Also, there is the suction cleaner, ranging in application from the
handling of refuse and dust from saw mills and woodworking machinery
to the removal of fine abrasive and poisonous particles in certain
manufacturing processes. Small suction cleaners are, of course, now
quite familiar domestic appliances. The sand-blasting machine is really
another example of a self contained pneumatic conveyor on a small
scale.
Ashes, coal, oranges, sugar, chemicals, spent oxide, iron ore, spent
tanning bark, and many other materials are now actually transported,
elevated or conveyed pneumatically, and it must here be acknowledged
that the first really successful plant in this country was due entirely
to the initiative and inventive genius of Mr. Frederic Eliot Duckham,
late Chief Engineer to the Millwall Dock Co., who in 1888 commenced
experiments in grain-handling by suction. In 1892 he produced a very
successful floating plant for unloading ships into land silos, and this
installation was the prototype of many similar plants which were placed
in commission all over the world. Many improvements have been carried
out and numerous patents issued for pneumatic handling apparatus, but
the original scheme as designed by Mr. Duckham has never been departed
from seriously.
=Fundamental Principles and Components.= The pneumatic conveyance of
materials along pipes is most easily understood when the equipment
is considered as a pump producing a high velocity stream of air in
which the material to be transported is floating, and with which it is
carried through the pipe system. It is necessary fully to understand
this, as it is otherwise difficult to realize how it can be possible to
lift solids at the rate of 100 tons or more per hour, several hundred
feet up a pipe in which the vacuum does not exceed about 7 inches
mercury column (say, 11 lb. per sq. in. absolute).
A modern pneumatic conveying plant of the _suction type_ comprises:
(1) An _exhauster_, or an air pump, of either the reciprocating or
the rotary type. (2) A _suction nozzle_. (3) A _discharger_, whereby
the material is extracted from the pipe line at the desired position,
without “breaking” or losing the vacuum. (4) One or more appliances
for _filtering_ the air and extracting any foreign material which may
have been carried over from the discharger and which would damage the
cylinder walls of the exhauster if allowed to enter the plant.
It is necessary here to mention that all pneumatic conveying is not
done by exhausting, but frequently by the use of _pressure_, that is,
that the air is not sucked along the transport line but is actually
blown in under pressure by fans, air compressors, or rotary blowers,
according to the circumstances.
The _suction system_ is preferable when it is required to convey
materials from several outlying points to one central storage bunker
or area. On the other hand, the _pressure system_ is less expensive in
first cost, when it is required to transport from one central point to
numerous outlying plants in the area to be served. A combination of the
suction and pressure systems is now being developed for the handling of
materials in cases where neither the “suction” nor the “blowing” scheme
alone can be said to be successful. This combined system is known as
the _induction system_ and is discussed in Chapter VI.
=Advantages over Mechanical Conveyors.= The reasons why pneumatic
methods for elevating and conveying are now receiving such attention
are to be found in the advantages of pneumatic over mechanical
conveying. These may be summarized as follows: Economy in labour;
flexibility of plant both in design and operation; elimination of dust
and its harmful effects on the employees; and in many instances the
recovery of dust which is valuable and which would otherwise represent
a serious financial loss. Wear and tear on moving parts is reduced to
a minimum. All obstacles such as buildings, roads, rivers, railways,
etc., can be overcome easily since the conveyor is “only a pipe.”
Should circumstances not permit of a straight single run for such
appliances as bucket elevators, it is usually necessary to bag and
cart the material from one position to another, but this is avoided by
pneumatic elevating and conveying, because the pipe can be carried up
or down, round corners, over or under roads, etc.
Hundreds of instances still exist in which loose material in barges is
shovelled into bags, which are lifted by an ordinary friction hoist
into a building over the quay-side. The sacks are wheeled by manual
labour into the building, and emptied into hoppers or silos, the empty
bags being lowered again for a further cycle of operations. This costly
multiple handling may be obviated by the erection of a small pipe
line which will automatically feed itself at one end, and discharge
evenly and continuously into the receptacle provided. The whole of the
“moving parts” may be situated in one position, which enables them to
be carefully inspected, oiled and kept in repair, etc.
Cranes, hoists, telphers, and belt conveyors all have their special
spheres of usefulness, but no other plant can claim all the advantages
of the pneumatic system as outlined above.
=Pneumatic Conveying Systems.= As mentioned above there are three main
systems of pneumatic conveying, viz.—
(1) Conveying by air _above_ atmospheric pressure.
(2) Conveying by air _below_ atmospheric pressure.
(3) Conveying by air _above and below_ atmospheric pressure, by a
combination of (1) and (2), known as the _induction_ system.
Although these are the main headings under which the subject may be
considered, they must be sub-divided further as follows—
=(1) Conveying above Atmospheric Pressure.= (_a_) _Low pressure
systems_ using single-stage centrifugal fans, and suitable for
conveying such materials as wool, cotton, bark, chopped straw, paper
clippings, sawdust, shavings, jute, and fibrous materials of many kinds.
(_b_) _High pressure systems_ using multi-stage fans and blowers of the
rotary type or positive design, and suitable for conveying materials
of a denser nature such as dry sand, sugar, etc., also for pneumatic
despatch tubes.
(=2=) =Air Suction Systems.= (_a_) _Large pipe systems_ using ordinary
steel plate centrifugal fans, and applied to handling the waste
products and injurious dust in many industries, such materials being
sawdust and shavings from woodworking machinery; emery dust from
grinding wheels; dust and lint from polishing mops; leather dust from
heel and sole scouring plants; starch dust in confectionery works;
colour dust in works manufacturing pigments; and bronze dust in
printing works. Many other examples will no doubt occur to the reader.
(_b_) _Small pipe systems_ using multi-stage centrifugal fans or rotary
blowers. Under this section the most important plants are undoubtedly
those for domestic vacuum cleaning. Large stationary plants for this
purpose are installed in many hotels, offices, buildings, workshops,
theatres, large private houses, etc.
(_c_) _Heavy commercial systems_ in which the fan is replaced by
a reciprocating exhauster, producing currents of air suitable for
conveying such materials as coal, ashes, sugar, leather tanning bark,
ores, granite chippings, wood blocks, oranges, etc., which it would be
impossible, or undesirable, to pass through a fan.
(=3=) =Induction System.= A compressed air injector is used to produce
a partial vacuum on the suction or inlet end of the pipe system, and to
produce a pressure above atmospheric at the delivery end, thus avoiding
the necessity for a discharger. This system, which is suitable for use
with heavy sticky materials—such as hot sugar, saturated sand, finely
ground heavy ores, spent oxide as used in gas works, etc.—is now being
developed so as to make possible pneumatic conveyance of all manner of
materials which are unsuitable for passing through the discharger, or
air lock, of a suction system.
=Factors influencing Design.= In the strict sense of the term,
“pneumatic conveying” really implies the conveyance of a quantity of
material from one point to another, using air as the conveying agent.
As will be seen from the above, the subject also embraces the removal
of comparatively light dust produced in many industrial processes.
Although the same name is implied in both cases, the methods to be
adopted vary, and each case must be considered on its own merits.
With regard to a conveying plant proper, the points for consideration
in the initial stages are as follows—
(1) The nature of the material to be handled and the quantity required
in a given time.
(2) The size of the largest and smallest pieces of materials and the
density of the material.
(3) The distance over which the material is to be conveyed.
(4) The difference in level between the point at which the material is
fed into the system, and the point where it is delivered.
(5) The method and regularity of feeding the material into the system.
(6) The means to be adopted for separating the material from the
conveying air at the desired point of delivery.
There is a very important distinction between plants in which the
material passes _through_ the fan or blower and those in which the
conveying is carried out entirely under suction.
The latter system has many advantages, but it carries with it the
necessity of providing automatic delivery of the material without
seriously impairing the suction. In order to accomplish this, some
form of “air lock” is required, and a necessary feature of this is a
device with close-fitting surfaces or more or less air-tight valves.
The simpler plan is undoubtedly to pass the material through the fan,
as there is then no question of breaking the suction in order to get
the material out of the system. It is common practice to convey for
long distances such materials as wood chippings and sawdust, cotton,
jute, wool, esparto grass, paper chippings and many other materials.
In all cases, however, the designer is confronted with the problem of
separating the material from the air, and in many instances to do this
satisfactorily is more difficult than the actual conveying; especially
is this so with certain sticky materials or materials which will
readily “pack” or build up when entering any mechanical discharger at
the high velocity necessary in the suction system.
CHAPTER II
DETAILS OF PLANT PUMPS, EXHAUSTERS AND AIR FILTERS
A pneumatic conveying plant of practically any type comprises the
following five main components: (1) A _pump_ or _exhauster_ to create
a partial vacuum in the pipe line and so induce a high velocity air
current in which the material will be conveyed. (2) An _air filter_
in which any light dust carried over beyond the receiver is trapped,
to prevent its entering the exhauster where it would quickly damage
the piston rings and cylinder walls of reciprocating pumps. (3) A main
_receiver_ or _discharger_, and occasionally one or more subsidiary
receivers, in which the conveyed material is extracted from the system
and discharged into the receptacle or on to a dump as required. (4) The
_pipe line_, _junctions_, etc., to connect the point of supply with the
desired point of delivery. (5) The _suction nozzle_ through which the
material enters the system, together with the “free air” which is to
act as the conveying medium.
=Pumps and Exhausters.= The type of apparatus used for creating the
flow of air varies according to the ideas of the numerous makers of
the plants. It must be remembered that for a pump working under the
conditions required for pneumatic plants it is not a high vacuum that
is required; the most important function is to handle very large
quantities of air at a comparatively low vacuum.
The most efficient type of plant for dealing with large quantities of
air is certainly the reciprocating pump, although several makers are
now devoting a lot of attention to the multi-stage turbine type of
blower, or exhauster. The probability is that this type may shortly be
as efficient as the reciprocating pump, and if so it is almost sure to
be used extensively, as it has distinct advantages in other directions
compared with the cylinder and piston type. This remark has special
reference to the high pressure system for conveying sand, coal, sugar,
etc., and does not apply to the “small pipe system” detailed later.
In dealing with ashes, flue dust, crushed iron ore, and similar
abrasive materials, it is essential that the air passing through
the pump should be filtered thoroughly so that no dust should enter
the cylinders and cut the walls and piston rings. The turbo-blower,
having no rubbing surfaces, would suffer little or no damage from a
small percentage of dust, hence the filtering equipment might be less
elaborate and less costly. No lubrication would be necessary in the
rotary pump (except, of course, at the bearings), hence no difficulties
would arise owing to oil acting as a dust trap, as is sometimes the
case in reciprocating pumps with defective air filters.
[Illustration: FIG. 1.—KING’S PNEUMATIC SYSTEM. STEAM-DRIVEN AIR PUMP.]
KEY TO FIG. 1.
_A_ = cylinder of double-acting steam engine driving the whole machine.
_B_ = semi-rotary valve controlling admission of steam to _A_.
_C_ = steam gland for piston rod connecting _L_ and _M_.
_D_ = Corliss-type air-inlet valves with semi-rotary motion. These
valves are alternately open to the pipe system and the air cylinder
_E_ when the piston is travelling in one direction, and are closed
(both to cylinder and air pipe) directly the piston reverses its
travel.
_E_ = cylinder of double-acting air pump.
_F_ = valve gear for operating the Corliss valves, _D_ this gear is
driven by a pin out of centre on the flywheel.
_G_ = cross-head and slide.
_H_ = big-end bearing and crank.
_I_ = flywheels on the main shaft.
_J_ = eccentric and strap operating the valve, _B_, on the steam
cylinder.
_K_ = leather air-outlet valves held on two sides by metal strips. The
leathers are opened by air pressure on the exhaust stroke, and are
drawn on to their gridiron seatings during the inlet stroke.
_L_ = steam piston.
_M_ = air piston.
_Note._—If preferred, the steam cylinder may be omitted and the air
pump driven by electric motor direct-coupled or geared to the main
shaft. Belt drive on to the flywheels or special pulley is possible
but not advisable.
=Typical Reciprocating Pump.= Fig. 1 shows the general construction
of a vertical steam-driven air pump such as is generally used in
present day plants. The pump shown has a stroke of 14 ins., with an
air-cylinder diameter of 28 ins. The machine is a dry air pump, and is
fitted with Corliss inlet valves and special leather exhaust valves:
it is fitted with ring-oiling bearings and automatic continuous
lubrication, and under test it shows a mechanical efficiency of 78 per
cent. from power input to air horse-power delivered. Such a pump is
suitable for a plant handling 20 to 25 tons per hour and, if preferred,
the pump may be driven through gearing from an electric motor on the
same bed-plate.
=Rotary Blowers and Exhausters.= The turbine blower and exhauster
depend entirely upon centrifugal force for their power to compress or
exhaust air or gas, etc. The use of centrifugal force makes this type
of machine resemble the centrifugal water-pump, but radical differences
in design have to be introduced, seeing that water is not compressible,
whereas air is capable of compression, and alterations are also
necessary, due to the great difference in specific gravity of the two
substances.
Owing to the very low specific gravity of air, the machine must run
at a much higher speed than would be required with water to develop a
given pressure. The high speed of the steam turbine has given impetus
to the design of large exhausters on the turbine principle.
It will be recognized that, when air is made to flow steadily along
a conduit or pipe of gradually diminishing cross-sectional area, the
velocity of the air must increase correspondingly, in order that
the constant quantity may flow through the smaller section of pipe.
Increasing the velocity will diminish the pressure due to the greater
kinetic energy to increase the velocity. The converse is the result of
passing air at constant pressure through a channel gradually increasing
from a smaller to a larger area, the velocity being then reduced and
the pressure increased.
Now if a number of impellers be mounted turbine fashion on a high
speed shaft, and the casing be so designed that the area between
stages gradually increases, the air will enter the first stage and
will be caught up by the impeller and accelerated until it leaves
No. 1 impeller at a higher pressure and velocity. Leaving the casing
through a diffuser which gradually increases in area the velocity is
transformed into pressure in the diffuser. The air therefore travels
to the second stage with its initial pressure plus the pressure due to
the conversion of velocity in the first diffuser. This process may be
repeated stage by stage until almost any desired pressure is obtained.
As there are no rubbing surfaces in this type of machine it is
particularly suitable for the work under consideration, and when
developed for efficient running in small sizes it will be very
effective in “booming” pneumatic conveying.
=Sturtevant Blower or Exhauster.= The Sturtevant Engineering Co.,
Ltd., has developed a special rotary blower or exhauster suitable for
use with pneumatic conveying installations and, although this machine
has not a water-seal for surfaces under pressure (as in the Nash
Hydro-Turbo described below), it has a number of distinctive points,
and the discharge of the air, or the intake of air, as the case may be,
occurs at a more nearly constant pressure and with smaller pulsations
than with any other rotary blower known to the writer.
The sectional diagrams (Fig. 2) show the four successive stages in the
movement of the rotors. In position (1), chamber _D_ has been filled
with air, while chamber _E_ is discharging air against pressure in the
delivery pipe. In position (2), chamber _D_ is cut off from the inlet,
and the air in it is being carried round. Blade _C_ has entered pocket
_Z_, which is filled with air under pressure; this air, however, in
turn is released into pocket _Y_ through leakage passage _O_. Continued
rotation carries the rotors _A_, _B_ and _C_ to position (3), and the
remaining pressure in _Z_ is now being transferred to _X_ by leakage
passage _N_.
As the fourth position is reached, chamber _F_ is filling and pocket
_Y_ is discharging its air. When further rotation brings impeller blade
_B_ into the discharge passage, the air in space _D_ will be delivered
from the blower. After leaving position (4), the rotors again reach a
position similar to that shown at (1), and the cycle is then repeated.
[Illustration: FIG. 2.—STURTEVANT ROTARY BLOWER.]
A study of the above will convince the reader that this blower is
ingenious and very suitable for the class of work under investigation.
For certain conditions the Roots blower and the multifarious types of
drum pumps or blowers can be used, seeing that it is quantity rather
than pressure that is required, but it is essential that the most
reliable and efficient exhauster should be installed in accordance
with the conditions for each installation, bearing in mind always the
questions of dust, speed, lubrication, etc.
=Nash Hydro-Turbine.= An entirely new type of rotary exhauster was
recently illustrated in the _Chemical Age_, and a study of Fig. 3
will show the principle of this pump. This pump was built just before
the war by Messrs. Siemens-Schuckert, and similar pumps are now being
introduced by the Nash Engineering Co. (U.S.A.), and are known as the
Nash Hydro-Turbine.
This exhauster has a cylindroid external casing, inside which is
a shaft carried on the two end brackets and having mounted upon
it a crude type of water-wheel. It should be noted here that this
arrangement has the effect of bringing the edges of the wheel into a
position of eccentricity in relation to the inside of the external
casing.
The wheel shaft is connected directly to a high speed electric motor,
and when the pump is running the water which is fed into the casing is
thrown by centrifugal force to the periphery of the casing and thus
forms certain air pockets into which the air of the system is drawn.
The air is now locked between the hub of the wheel and the sheet of
water surrounding the outside of the wheel. As the wheel revolves,
the air in each pocket in turn is compressed into a smaller capacity
and eventually, when it arrives opposite the outlet point, it at once
escapes into the atmosphere due to its additional pressure.
[Illustration: FIG. 3.—NASH “HYDRO-TURBO” EXHAUSTER.]
Tracing the wheel right round we then find that another pocket is
formed into which the air is again sucked, and so the cycle of
operations continues, two compressions and extractions occurring per
revolution of the spindle. For many materials such a pump has the great
advantage that the water acts as a wet filter and traps all the dust
in suspension which can be washed out in the form of sludge. Where the
dust is valuable this characteristic would be a disadvantage, but it is
very useful in plants dealing with corrosive and abrasive materials.
=Lubrication.= The lubrication of cylinders in the reciprocating type
of pump has been mentioned in connection with the necessity for taking
special care in filtering _all_ dust from the air pulled over from
the discharger. Ordinary oil makes a sticky surface to which any dust
adheres readily, and the two combined make an abrasive mixture which
will quickly score and damage the walls of the cylinders. One firm, at
least, has gone a long way towards removing this trouble by inserting
pieces of solid graphite into the piston: this provides dry lubrication
and produces a very smooth surface on which dust finds great difficulty
in obtaining a “footing.”
The use of “Aquadag” is also fairly successful: this lubricant consists
of deflocculated graphite in such very fine particles that they will
remain in suspension in water even without mechanical agitation.
“Aquadag” is fed into the cylinders in the same way as oil and deposits
its graphite in the pores of the cylinder walls, whilst the water is
atomized and blown out with the air.
The turbine exhauster solves the problem by eliminating the necessity
for internal lubrication, and the perfect filtering of the air is then
not so important.
=Air Filters.= It is impossible to deal with practically any granular
material without carrying over more or less dust beyond the discharger,
and to obviate the damage and inconvenience which would result from
allowing this to enter the exhauster, an air filter is fitted between
the discharger and the pump. Many types of air filters have been
introduced, and representative examples are described below.
[Illustration: FIG. 4.—STURTEVANT “CYCLONE” DUST SEPARATOR.]
=“Cyclone” Separators.= For such materials as grain, malt, etc.,
the ordinary type of cyclone separator is frequently mounted inside
the receiver. This separator consists of an inverted cone: the dust
laden air enters at the top and the heavy material circulates inside,
and gradually falls to the bottom from which it is discharged into
suitable containers (see Fig. 4).
The cyclone is an excellent separator, and has the advantage that it is
self-cleaning, and offers little or no resistance to the flow of air,
but so large a cone would be required to separate very fine dusts that
it frequently becomes impossible to use this type of plant.
=Bag Filters.= An alternative type of air filter is the ordinary bag
filter, which consists of a number of closely woven fabric “stockings”
in a cast iron container. The air is led into the casing so that it
passes down through the inside of the tubes, through the fabric, where
it deposits the greater proportion of the dust, and then out to the
atmosphere.
Naturally, after working a certain period, the fabric becomes choked
with deposited dust, and it is then necessary to dislodge the dust by
shaking the “stocking” somewhat violently. This is usually carried out
by a mechanically operated vibrating apparatus, but cleaning is done
more effectively and time is saved if the air is shut off from the
filter while the dust is dislodged. This is done most conveniently by
having two similar filters installed, working in parallel; then, when
cleaning becomes necessary, all the air is passed through each filter
in turn, whilst the other is cleaned.
The bags or tubes hang vertically in the casing, and as the air is
brought in at the top and discharged in a downward vertical direction,
it naturally discharges the heavier material straight into the bottom
of the casing, owing to the inertia of the heavy particles and the high
velocity of the incoming air.
[Illustration: FIG. 5.—AUTOMATIC BAG FILTER.
Showing filter bags through the open access doors]
Fig. 5 illustrates a Sturtevant automatic bag filter. In this case the
air is brought in at the bottom. The cleaning of the bags is effected
automatically. At frequent intervals and in rotation, each section
of bags is cut off automatically from the supply of dust-laden air
by closing the outlet valve of that section. At the same time, that
section is opened to the atmosphere at the top, causing a reverse
current of air downwards. The bags are then agitated automatically and
the dust adhering to the fabric drops, and is blown into the discharge
hopper below. The opening and closing of the valves is accomplished
by a simple mechanism driven by the pulley shown at the top of the
illustration. Where the amount of dust to be handled is large and
continuous it can be extracted from the hopper by a worm or screw
conveyor, or, as in some other patterns, by a rotary valve placed in
the bottom of the hopper.
[Illustration: FIG. 6.—MOLLERS’ AIR FILTER.]
=Mollers’ Air Filter.= This apparatus consists of separate fabric
pockets mounted on a frame. The pockets are rectangular and taper
towards the top. Referring to Fig. 6, _A_ is the frame on which the
fabric is stretched, and _B_ is the fabric pocket. As many of these
units as necessary are mounted side by side in an adjustable frame,
each unit having fixing and tension bolts. This arrangement permits a
filter of any desired capacity to be built up quickly, and facilitates
repairs and cleaning of filter bags.
=Wet Filters.= The type of filtering apparatus used for dealing
with poisonous material, emery dust from grinding wheels, sand from
sand-blasting apparatus, etc., is usually of the wet type, of which
Fig. 7 is a good example. This apparatus consists of a tank having a
wire shelf on which a layer of coke is supported. The tank is partially
filled with water, the level of which is regulated by an overflow.
The dust-laden air impinges on the surface of the water, and the major
portion of the dust is trapped by being driven actually into the water.
The dust particles too light to be brought into contact with the water
are compelled to pass through the coke screen and are there arrested.
If necessary, the scheme can be made to deal with finer dusts by having
the coke constantly sprayed with water. The dust is reclaimed from the
tank in the form of sludge or mud through the door or valve provided
for that purpose.
[Illustration: FIG. 7.—STURTEVANT WET FILTER.]
Another type of air filter which might be developed in connection with
the pneumatic handling of material consists of a very slowly revolving
drum or cylinder, which is fitted with a continuous corrugated tape
running spirally from the centre to the extreme edge of the casing.
The space between the corrugated sheets is very small, and as a stream
of water is continually running over and around the divisions, the
air passing through the very tortuous path provided is bound to come
into contact with these wet surfaces and give up its dust or other
contamination, which is washed off when it arrives at the bottom of
the cylinder. Naturally, this or any other wet filter would not be
used where the recovery of dust in dry form was desired.
Another form of wet filter consists of a chamber of suitable
proportions (according to the amount of air to be cleansed), fitted
with racks in which are placed strips of glass at an angle of 45° to
the flow of air, and at 90° to one another. The glass strips have a
serrated or prismatic face, and the air carries the dust forward into
the angles of the glass. A very fine water spray keeps the glass moist
and eventually trickles down to the drain channels, washing the glass
in its course.
The development of apparatus for _air-washing_ has received
considerable attention during recent years, owing to the necessity of
having pure, dust-free air for ventilating turbo-generators, etc., and
no difficulty should present itself in obtaining satisfactory results
for pneumatic conveying plants, except in cases when the collection and
retention of dust is required. In these cases the bag or fabric sheet
filter is the only type available.
CHAPTER III
DETAILS OF PLANT—(_Contd._)
DISCHARGERS, PIPE LINES AND SUCTION NOZZLES
=Dischargers.= The advocates of “blowing” material, instead of
“sucking” it through the pipe, often lay great stress on the alleged
difficulties of extracting the material from the system without
allowing air leakage. This, however, has been overcome successfully
by several designers and is not the serious difficulty so frequently
suggested, providing that the material is suitable for this means
of treatment. The only exception to this is, when the high velocity
at which the material enters the discharger—say, 45 to 50 ft. per
sec.—causes it to pack or bind so that it will not fall by gravity into
the rotary valves and then out into the storage bins.
The function of the discharger is to cause the incoming material to
lose its velocity and to fall into a compartment which can eventually
be discharged after it has been moved from the low pressure to which
the chamber itself is subjected.
This is best accomplished by the use of a rotary valve, somewhat
similar to a paddle-wheel, which is revolved slowly but continuously.
This wheel can be placed either vertically or horizontally at the
bottom of the receiving chamber; the material enters the large chamber
above, loses its velocity, drops by gravity, is caught in the box
formed by the revolving paddle-wheel, and gradually is carried forward
out of the chamber, eventually passing over an aperture through which
it again gravitates to the bunker, silo, or other container. Probably
the horizontal type of rotary valve is preferable because, owing to the
increased surface exposed to the vacuum, the “suction” effect assists
in holding the valve up to its seat.
When dealing with such materials as malt and grain, it is an advantage
to be able to inspect the material entering the receiver, and at least
one maker secures this advantage by constructing the chamber of a glass
cylinder to which are bolted cast iron top and bottom pieces carrying
the necessary pipe connections and discharge valves. Fig. 8 illustrates
this construction and also shows fairly clearly the method of driving
or revolving the discharge valves. The top flange of the valve has
a worm wheel tooth cut around its periphery and the actuating worm
engages in this wheel, thereby obtaining a large reduction in speed.
In other words, the worm can be driven by a light, high speed belt and
pulley, and still revolve the valve at a very low speed; such gearing
is smooth, silent running, and altogether admirable for such a purpose
as the one under consideration.
[Illustration: FIG. 8.—FIXED DISCHARGER WITH GLASS RECEIVER.]
A very common form of separator, which is used almost invariably on
plants dealing with wood shavings, sawdust and similar materials,
is known as the cyclone or centrifugal separator. This is usually
constructed with a sheet steel body with the inlet for the dust-laden
air at the top, and so arranged that the air enters tangentially.
Inside there is a smaller cylinder of sheet steel forming an air
outlet, and the laden air sweeps round the annular space between the
body and the inner cylinder. This results in a whirling action and the
material entrained in the air is projected by centrifugal force against
the side of the separator body. In some instances an internal ledge,
or plate of “corkscrew” form, leads the material downwards towards the
outlet at the bottom of the separator.
In the case of some of the denser materials which can be conveyed by
air, it is sufficient to connect the discharge pipe to an open bin or
chamber, the material in such cases being heavy enough to separate
itself by the action of gravity.
Mr. Gordon S. Layton, describing dischargers in his paper, before the
Engineering Group of the Society of Chemical Industry (Birmingham,
April, 1920), stated—
“There are two types of dischargers in use: The first consists of a
large steel box divided into two compartments. This box is arranged
to oscillate about a horizontal axis, so that each compartment
alternately is brought under the lower opening of the receiver vessel.
“The other type of discharger consists of a bucket wheel rotating
continuously inside a closely fitting casing. The material which is
being conveyed falls into the pockets of the bucket wheel when these
are on the top of their revolution, and is passed out through an
opening in the lower side of the casing.
“It will be obvious that the working of both types is liable to be
interrupted by the jamming of a foreign body (such as a piece of wood
or a bolt) in the working parts; in each case, special mechanism for
driving the discharger has been devised, to avoid the interruption
resulting from such blocking, and to enable the discharger to keep on
working continuously.”
The rotary type of discharger is preferable to the tipping box type,
for the following reasons: because the rotary discharger is more easily
kept air-tight, works without vibration, and gives a practically
continuous stream, whereas the discharge from the tipping box occurs as
large isolated masses of material.
It is impossible to give specific details concerning the discharger
because, in all cases, the conditions under which the plant has to
work affect the whole design. For instance, where the working is
only intermittent, e.g. the removal of ashes from a boiler-house,
the discharger can be eliminated, provided that the ash bunker is
large enough to hold the quantity of ashes to be dealt with at each
operation. In such a case, the ash container would be capable of being
exhausted, and the material entering as before would simply drop by
gravity into the container and remain there until the pump was shut
down; it would then be allowed to gravitate into the truck or other
conveyance for disposing of the ashes.
Hand-holes for cleaning, and easy access to the interior are essential
in the design of a discharger, especially if the material to be handled
is liable to “pack” when entering at a high velocity.
=Pipe Lines.= One of the most important points in the designing of a
pneumatic conveying system is the correct lay-out of the pipe line.
[Illustration: FIG. 9.—STURTEVANT PATENT JUNCTION.]
A fatal mistake often made in low pressure and exhausting systems is
that the numerous branch pipes are added to or altered after the makers
have left the original installation. Almost invariably, branches thus
added are made to approach the main trunk at too great an angle, with
the result that eddies and whirlpools are created within the pipe,
seriously reducing the output of the main trunk. So essential is it
that this junction should be correct and that the two streams should
run as nearly parallel as possible, that the Sturtevant Engineering
Co. has patented a special junction (Fig. 9) to bring together the two
streams of air in the main and the branch pipe practically parallel, as
shown.
=Bends.= In connection with high pressure systems, the following points
are of great importance. All vertical and horizontal straight lengths
may be of a light section constructed in steel. Bends should be avoided
whenever possible, and those which are inevitable should be made in
hard cast iron, with every possible provision for easy replacement of
wearing parts.
The wear takes place at the point of actual contact which, in elbows,
is practically confined to one place only. The material rushes to
the end, strikes the bend, and—suddenly changing its direction of
travel—whirls off down the next straight length. The impact and the
resulting wear on the pipe, as well as the breaking of the material
conveyed, are naturally much greater in elbows than in easy bends, but
if the breaking of the material is not detrimental, elbows should be
employed, as they are less costly and can be replaced more quickly and
easily.
Certain raw materials—such as salt, soda, lime and various
chemicals—which have to be ground before use, may be prepared to
a considerable extent for this operation by the use of elbows. On
the other hand, easy bends should be employed for material which it
is desired to convey without damage, e.g. malt, coal, and granular
substances, which are finally required in granular form and not as
powder.
The wear in bends is only on the external radius of the bend, and then
is inclined to be localized at certain points rather than distributed
over the full sweep of the bend (see Fig. 10); this being so, it is
often desirable so to construct the bends that the back is in segments
which can be renewed easily (see Fig. 11). Alternatively, the bend may
be constructed on the “lobster” principle (Fig. 12), only the worn
sections being replaced when overhauling. It is not necessary always to
take a bend at an angle of 90°, and if the small short angle sections
are interchangeable, then almost any angle can be constructed by
building up with the necessary number of sections.
[Illustration: FIG. 10.—COURSE TAKEN BY MATERIAL ROUND BEND.]
[Illustration: FIG. 11.—SEGMENT-BACK BEND.]
[Illustration: FIG. 12.—LOBSTER BEND.]
With regard to the straight lengths of pipe, it is necessary to ensure
a smooth internal bore, especially at the joints. It is therefore
desirable that the joints should be self-aligning as, if this is not
the case, eddies will be formed which will cause the material to
deviate from the centre of the pipe, striking the side at one or more
definite points where holes will eventually be worn through the pipe.
In vertical pipes the evidence of wear is negligible, in fact the
conveyed material presumably does not touch the pipe at all, but
travels up the centre of it as a core.
=Capacity of Pipes.= The velocity of the air passing up the pipe should
be from 40 to 50 ft. per sec., equivalent to about 35 miles per hour.
The conveying capacity of an efficient pipe line is approximately
15 per cent. of the total capacity: in other words, if the total
cross-sectional area of the pipe be taken as 100, the effective
cross-sectional area as regards conveying is 15.
=Flexible Connections.= The flexible connections attached to the
permanent pipes may be ordinary tubing, as made by the Flexible
Metallic Co. Phosphor bronze and other metals have been tried, but the
extra cost is not justified. A loose screw collar connection makes
possible easy fixing, and permits the flexible connection to be removed
easily to prevent damage when not in use. Rubber tubing, reinforced
with steel wires, would be best where it is important that the material
conveyed should not be damaged, but the wear on such tubing is so
rapid that the extra cost is not recovered by the saving in damage to
ordinary materials. The length of the flexible pipe should be such that
the movement over the greatest area to be covered does not put too
great a strain on the bending properties of the tubing. On the other
hand, unnecessary increase in the length of the flexible pipe merely
increases the cost of one of the shortest lived portions of the plant,
the “scrap value” of which is almost negligible.
[Illustration: Showing Three Positions of the Valve. FIG. 13.—KING
FULL-WAY JUNCTION VALVE.]
=Valves, etc.= For use where branches are inserted in the main pipe
line for convenience in either lifting over, or discharging over, a
large area, special appliances have been designed and these should
be used, as they do not create eddies or increase the pipe friction
appreciably, or reduce the carrying capacity of the pipe line. The King
patent full-way junction valve is an excellent example, and is shown
in Fig. 13, from which it will be seen that a full bore circuit can be
completed in any of three directions. This valve has no corners where
the material can collect, hence the pipes are sucked perfectly clean
the moment the feed is shut off.
Another convenient fitting of this description is the Boby patent pipe
switch. This device is similar to a switch as used on a railway track,
and by its use three separate side positions may be connected with
one part on the main transport line. When the switch is thrown over
so as to connect to any one particular branch, all other branches are
disconnected.
=Telescopic Pipes.= When the unloading of ships is carried out by
“suction” it is necessary to make provision for lengthening or
shortening the vertical suction pipe, or pipes (see Frontispiece),
because the ship will rise as relieved of its cargo, and the suction
nozzle will simultaneously move towards the bottom of the hold as the
cargo is discharged.
A still greater difficulty is encountered in tidal rivers, where the
rise and fall may be many feet and must be allowed for continuously.
This is best done by the introduction of telescopic pipes in the
vertical downright pipes. These must be so constructed that while it
is fairly easy to increase or decrease their length, the pipes must
remain air-tight at the joints and connections.
Where the rise and fall is small the difference in level may be
compensated by a ball and socket joint, and a counter balance on the
jib arm, but this method has its limitations.
=Pipes for High Pressure Systems.= Coming now to the “small pipe,” high
pressure systems as used for vacuum cleaning plants, the pipe lines
must be designed and installed carefully and on a liberal basis. It
is mistaken policy to attempt to economize by using a smaller main on
branch pipes. Small diameter pipes cause excessive losses by friction,
and naturally are less efficient as regards power consumption.
The frictional losses in a system of this description vary directly
as the length of the pipe, and inversely as the fifth power of the
diameter. Large pipes are therefore very desirable, not only because
of their greater carrying capacity—which is very desirable—but also
because such things as matches, hairpins, etc., are picked up every day
by an installation as fitted in hotels, restaurants and theatres. Such
material quickly clogs small pipes and causes endless trouble and delay.
The flexible hose should be as short as is consistent with ease of
working, because the frictional losses in this class of tubing are very
great. It would be preferable to increase the number of wall plugs,
rather than have to use very long lengths of flexible hose.
[Illustration: FIG. 14.—SUCTION NOZZLES FOR HIGH PRESSURE SYSTEMS.]
=Suction Nozzles.= Probably more patents have been taken out on new
suction nozzles than on any other portion of a pneumatic suction
plant. The chief desiderata for a nozzle on a _high pressure system_
for wheat, coal, ashes, etc., are that it be of light construction
to allow of easy manipulation by the operator, and that it have some
means of allowing a “free air” inlet, making it impossible to choke
the nozzle by burying the end. It is an advantage to be able to
regulate the free air inlet according to the conditions existing with
different materials. The same nozzle that will act best while dealing
with a large bulk of material, may be quite unsuitable when it becomes
necessary to “clean up” in the corners of the hold or waggon. Fig. 14
shows different types of nozzles for high pressure plants, but as the
efficiency and capacity of the plant can be affected seriously by
the design of this portion of the apparatus, it is highly advisable
to allow the designer to have a free hand and to make use of the
experience already gained.
[Illustration: FIG. 15.—STURTEVANT EQUIPMENT REMOVING WOOD REFUSE FROM
DOUBLE TENONING MACHINE.]
Nozzles designed for _low pressure systems_, dealing with dust,
shavings, etc., have to be built to suit the machine to which they are
attached, and they therefore vary indefinitely in details of design and
construction. The same remarks apply to the nozzles for use on suction
cleaning plants. Figs. 15 and 16 show how the suction nozzles are
adapted to the machines.
[Illustration: FIG. 16.—STURTEVANT EQUIPMENT REMOVING DUST FROM
SAND-PAPERING MACHINES.]
It must be remembered that in low pressure systems handling shavings,
dust, etc., the problem is quite different from that in high pressure
systems handling wheat, etc. In the case of removing dust or shavings
from a machine, the material is already in motion, and only requires
drawing forward and into the pipe system, but in the case of conveyors
for wheat, coal, etc., and in the case of suction cleaners, the
material to be moved is heavy and stationary and has to be lifted and
started in motion before it can be carried away. This necessitates a
much higher air velocity through the collecting nozzles.
=Idle Nozzles should be Closed.= It is perhaps advisable to draw
attention at this point to the disadvantages of using more than one
suction nozzle on one receiver at one and the same time. The reader is
asked to recall the fact that the material is not lifted by vacuum, but
that the production of a partial vacuum causes a stream of air to pass
up the pipe at high velocity. The material to be conveyed is entrained
with the air, and due to the frictional contact between the particles
of air and the particles of material, the latter is lifted and carried
forward.
If the conveying plant is to be efficient and of reasonable capacity,
the pipes must be relatively large, and in order that the desired
partial vacuum may be maintained in them (establishing a vigorous air
current) without the use of an unduly large pump, it is important that
air be admitted only through those nozzles which are actually in use.
Also, when more than one nozzle is in use at the same time, it is
necessary to keep each nozzle covered with material to such an extent
that the same amount of air passes into each pipe. Unless this is done
a large quantity of air will pass up one pipe, and a small quantity
up the other, and the amount of material taken in at each nozzle will
vary as the quantity of air varies. To consider an extreme case,
suppose that the man operating at one of the pipes allows his nozzle
to become exposed. Air will rush in at this nozzle to the full capacity
of the pump, with the result that little or no air will pass up the
second pipe, and consequently no material either. Thus, if one man is
sufficiently neglectful to leave his nozzle idle and open, he renders
practically useless the other nozzle or nozzles on the same main.
Even with care this is bound to occur to a certain extent, as is shown
by the figures given by makers for the estimated power consumption,
viz., about 1 h.p. per ton on single-nozzle plants, and 1½ h.p. on
double-nozzle plants.
Under these conditions it should be considered whether it is more
advisable to install one large plant with two nozzles, or two small
plants, each with only one nozzle. The decision depends upon the extra
cost of power for the double-nozzle plant compared with the higher
capital charges on the two single-nozzle plants.
CHAPTER IV
TYPICAL INSTALLATIONS FOR GRAIN
The pneumatic principle has been applied to the handling of grain in
bulk to a much greater extent than to any other material. As previously
stated, the original successful plant invented and designed by Mr.
Frederic Duckham was for the handling of wheat for the Millwall Docks
Co., and the success of this plant was such that it was imitated by
engineers in this country and abroad with equally satisfactory results.
Grain lends itself admirably to pneumatic transport because it is
easy of flow, regular in size, and practically self-feeding. Also,
the removal of dust, which is incidental to pneumatic conveying, is
a special advantage where grain is concerned. Figs. 17 and 18 show
clearly the advantages of pneumatic conveying in point of simplicity
and labour saving.
=Typical Quayside Plant.= Fig. 19 shows a typical lay-out for a
comparatively small plant handling 50 tons per hour. The diagram is
almost self explanatory.
_A_ represents the special suction nozzle through which the grain
enters the system, together with the “free air” which acts as the
conveying medium.
[Illustration: FIG. 17.—ANCIENT.]
[Illustration: FIG. 18.—MODERN.]
[Illustration: FIG. 19.—TYPICAL GRAIN-HANDLING PLANT; 50 TONS PER
HOUR.]
_B_ is a length of flexible pipe to enable the nozzle to sweep the
width of the barge, the length being dealt with by moving the boat to
within the radius of the swivel arm _E_.
_C_ is a permanent pipe carried on the jib arm which is mounted on
a swivel joint; _B_ is another short flexible pipe to permit of the
swivelling already mentioned.
_F_ indicates the receiver into which the grain is deposited, and _G_
the special rotary valves, for releasing the material from the system
without interfering with the vacuum of the conveying line.
_H_ illustrates the twin air filters which are provided with valves,
so that each of the filters can be isolated in turn from the rest of
the plant for cleaning of the fabric tubes, without interrupting the
continuous working of the plant.
_J_ indicates the connecting pipes between the receiver, air filters,
and pump, and it should be noted they are considerably larger than
those on the suction end _C_, this being necessary to allow for the
expansion of the air under vacuum.
_K_ is the reciprocating air pump, driven electrically by the motor
_M_ through large helical gear wheels, which act as flywheels and are
helpful in equalizing the torque required by the pump.
_L_ is the outlet pipe for the air as it is exhausted from and by the
pump.
[Illustration: FIG. 20.—FLOATING PNEUMATIC TRANSPORT PLANT; 200-250
TONS PER HOUR.]
[Illustration: FIG. 21.—PORTABLE PNEUMATIC PLANT ON RAILWAY TRUCK.]
This plant is simple, but every possible modification can be added
that circumstances make desirable; such as discharging on to a belt
conveyor, for feeding the silos or other storage bins.
Where grain is bought by bulk it would be necessary to check the
weight, and in this case an automatic weigher would be installed, into
which the material would be discharged from the receiver. With a bag
filter the collected dust can also be weighed, the total weight being
then obtained by addition.
=Typical Floating Plant.= The most popular development of the above
type of plant is the floating plant, designed for erection on a hulk,
or barge, and intended to suck grain from an ocean-going steamer
and discharge it into barges or lighters on the other side of the
discharging plant, as indicated in Fig. 20. The plant shown is one
of many in actual operation and its construction may be followed
by reference to the description of the plant shown in Fig. 19, the
same index letters being used in each case. One great advantage of
floating plants is that the difference in height due to tides is of no
consequence.
=Portable Plants.= Still another modification consists of a complete
quay-side plant mounted like a travelling crane, so that it can be
moved longitudinally along steel rails on the quay-side. This plant is
particularly useful where large cargo ships have to be emptied and then
allowed to remain at their berth until reloaded with another cargo.
[Illustration: FIG. 22.—PORTABLE RAILWAY PLANT IN OPERATION.]
Complete plants have also been mounted on railway trucks, the engine
and pump in this case being on a second waggon (see Figs. 21, 22). In
such a case the plant has to be mounted very low, and it is necessary
to lift from the under-side of the truck to, say, a railway waggon, by
means of an adjustable belt or bucket elevator (shown at the extreme
left of Fig. 21).
Many other applications will suggest themselves to the reader, and
sufficient has been said to prove that for the handling of wheat the
pneumatic system is distinctly flexible and convenient; also, it
effects a great saving in labour, which is an important consideration
nowadays. “Bushelling,” conveying, and weighing by hand used to cost
well over a shilling per ton, which figure was reduced to just over
1½d. per ton by pneumatic conveying; these are pre-war costs in both
instances, but the relative saving effected by pneumatic conveying is
certainly not less under present conditions.
All the previous remarks also apply to linseed and cotton seed in
bulk, maize, oats, and in fact all cereals. Such materials have to be
accepted as and when the ships arrive irrespective of convenience, and
it is an important advantage of pneumatic conveying that the material
can be lifted and discharged in the most convenient position; also,
when the barge or ship has departed the same apparatus can be utilized
to lift the material from its position in store to the cleaning or
grinding plant.
CHAPTER V
PNEUMATIC COAL-HANDLING PLANTS
The writer was directly interested in the erection and installing of
one of the first plants installed in this country for the elevation and
conveying of coal, and a description of the various details may give a
good idea of a complete plant, handling coal on a commercial scale.
The conditions to be complied with are as follows: 20 tons of “slack”
per hour, to be raised 90 ft. above canal level or 80 ft. above road
level.
The coal is brought alongside the power-house by canal barges of 25
tons capacity, or by tipping steam waggons from the railway sidings,
a distance of one mile away. In both cases the coal is required to be
elevated into overhead bunkers of 600 tons capacity placed vertically
over the boilers.
[Illustration: FIG. 23.—PNEUMATIC UNLOADING OF COAL AT MESSRS. BOOTS,
LTD. (NOTTINGHAM).]
In the first case, immediately the barge is alongside, the flexible
suction pipe is lowered into the barge (Fig. 23), and coal immediately
begins to rise in the pipe and is discharged as required. Little or
no handling of the coal is required after the suction nozzle has once
reached the bottom of the barge; all that it is necessary to do is to
bring the barge gradually up to the nozzle, the coal then “avalanching”
down to the nozzle. Fig. 24 shows the discharger placed on girders
over the bunkers into which it discharges continuously. The coal enters
at _A_ and the major portion of the fuel is discharged through the
rotary valves _B_.
The _coal discharger_ itself consists of a cast iron vessel with two
King’s patent rotating valves. These are designed in the form of a
slightly conical taper divided into four sections, one portion of
the circular valve being under vacuum, and the other under ordinary
atmospheric pressure. The outlet of the valves is larger than the inlet
to allow the coal which is in the valve to drop out easily. Over each
valve is also provided a four-armed sweeper to prevent any damp coal
from forming a cone inside. The discharger is provided with two inlets
with full-way bored valves, so that the coal can be drawn either from
the water side or from the land side at will. After the coal has been
deposited in the main discharger, there is provided a supplementary
discharger consisting of a vessel 6 ft. high by 30 ins. diameter, with
two inlet pipes of 8 ins. diameter, to provide a contra-flow, so that
any particles of coal dust in the air will meet one another in the 30
ins. box at equal velocity and be deposited. The small particles are
delivered by a supplementary rotary discharge valve which is set to run
very much slower than the main discharger valves.
[Illustration: FIG. 24.—DISCHARGER FOR COAL CONVEYING PLANT.]
The main discharger valves are driven by worm gearing, the latter
having one right-hand thread and one left-hand thread, so that the end
thrust on the worms is neutralized. Ball bearings are provided and
the small motor which drives all three valves is coupled up with an
electrical device designed by the author. This device ensures that if
anything happens to the top discharge valves—so that the 3 h.p. motor
driving them cuts out, owing to an overload or other cause—then the
main motor also is cut out by the opening of its circuit breaker. This
prevents any “flooding” of the pipes and dischargers.
It may be mentioned that the valves are so designed that a portion of
the weight of each valve is carried by the vacuum, so that the vertical
wearing lift on the valves when at work is very slight.
The _intake pipes_ for the coal are 5 ins. diameter, and they are
provided with heavy cast iron bends, having extra thick metal on the
outside radius to allow for the wearing effect of coal passing at the
rate of 20 tons per hour.
The pipe into the barge is provided with a flexible steel pipe at the
suction nozzle end, for convenience of handling. India-rubber piping
has been tried, but the extra cost does not justify its continued use.
The nozzle is made as light as possible for convenience of handling,
and is fitted with a special “free air” inlet for the regulation of the
amount of air necessary to blend with the coal.
=Ash Handling.= In addition to unloading coal, the above plant is
capable of dealing with hot ashes which are first crushed in a
portable clinker breaker, electrically driven, which runs under all
the ash hoppers of the boilers. The ash when crushed gravitates into
funnel-topped tee-pieces, inserted in the main ash-conveying pipe,
whence it is immediately sucked up into an overhead ash hopper to await
the convenience of the waggons which dispose of it on the “tips.”
=Flue Cleaning.= A 3 in. suction pipe has been run round the
boiler-house in such positions that flexible hose can be attached
for flue cleaning purposes. In this case the cleaners simply use an
enlarged nozzle such as is supplied with a domestic equipment and the
dust is removed from the flues, economizer soot chambers, etc., into
the ash hopper without trouble or dust.
The success of this plant is best indicated by the fact that, at
the moment of writing, a duplicate plant is being erected. Owing to
the growth of the business, and its demand for power and steam, the
original plant has to be worked continuously on coal, so that the ash
and flue dust problem has become acute again.
=Portable Floating Plant.= A third plant ordered by the same firm is
of considerable interest. This is intended to be mounted in a barge
so as to be portable. Owing to lack of space in close proximity to
the power-house, considerable difficulty is found in keeping adequate
stocks of coal on the site except the 600 tons in the overhead bunkers.
In order to secure continuity of working, it is essential that as much
fuel as possible be stored, and for this purpose a coal pile has been
made about half a mile away from the works, adjoining the canal. Ashes
can be disposed of on certain fields a few miles outside the city in
swamps and pools caused by subsidences, due to colliery workings.
The portable plant is therefore arranged to operate as follows: the
barge is self-propelled by a 30 h.p. paraffin engine which can be
coupled by clutches to either the propeller or a Roots blower, the
latter being the exhauster for the portable suction plant.
The barge is loaded with ashes for disposal, and then proceeds under
its own power to the site where they are to be dumped. The clutch is
operated disconnecting the propeller and operating the blower. The
suction side of the blower is coupled up with the pipe line in the
boat and the barge feeds the plant by means of the flexible hose: the
discharge pipe is raised over the towing path so as not to interfere
with passing traffic, and the ashes are blown out into the swamps
previously mentioned. It will readily be recognized how simple this
unloading becomes compared with trying to dig out the ashes with either
a spade or a fork.
The empty barge then returns to the coal pile and takes up a load of
coal in a similar manner, then proceeding to the power-house under
its own power and being unloaded by the original fixed pneumatic
installation in the ordinary way.
The coal arriving by road is tipped into a concrete hopper excavated
below the ground level, and so designed with sloping sides that it is
self feeding into a suction pipe connected to the bottom of the hopper.
The same procedure occurs except that in this case the coal enters the
main discharger at the top (_E_, Fig. 24).
It is interesting to note that the very fine dust collected from the
air filter is eagerly sought after by the foundry trade, and what would
at first appear to be a waste product impossible to burn, is actually a
valuable by-product of the plant.
CHAPTER VI
THE INDUCTION CONVEYOR
Numerous means have been devised to cause the necessary current of air
to flow along the conveyor pipe, but the ideal method is probably yet
to seek. Probably the most satisfactory and economical system, until
recently, was the positive pump exhausting a vacuum chamber; the latter
receiving the material, and discharging it into the receptacle provided
for that purpose.
The difficulties arising in practice, however, incited the inventive
genius of engineers responsible for the operation of these plants, and
a number of attempts were made to induce an air current by other means.
=Ejector Systems.= Steam ejectors were fitted to the closed tank
provided for the reception of the material, thus converting the tank
into a vacuum chamber, and eliminating the discharger. In other cases
injectors, also operated by steam, were placed at intervals along the
conveyor pipe, usually at such convenient points as 90° bends, and the
slight vacuum created by the condensation of the steam and also by the
velocity of the jet, induced an air current which swept the material
along with it into the receiver chamber.
Although both these methods are in practical use, their applications
are strictly limited to materials which do not suffer by contact with
heat and moisture; the methods are therefore used principally for
conveying ashes and soot from boiler furnaces and flues. Ashes formed
by the combustion of coal contain large amounts of abrasive matter, and
it is very important that all this matter should be extracted from the
air, before entering the exhauster of the suction system. The steam jet
cuts out the exhauster entirely, but absorbs an excessive amount of
power in the form of steam. It has the advantage, however, of quenching
the ashes on their way to the settling tank. For flue dust, however,
the steam jet is unsuitable, as the condensed vapour causes the
material to cake in the pipes, and the latter rapidly become choked,
involving considerable delay and trouble in cleaning out. The ejector
system is used for this material, the tank being of the closed type,
and the necessary vacuum being created by a steam ejector fixed in a
branch at the side near the top, the dust striking a baffle and falling
by gravity to the bottom of the tank. A special air-tight gate or valve
is opened to empty the tank.
=Air Induction.= The cardinal feature of the induction system is the
ease with which materials may be handled which cannot be conveyed by
the suction method.
Sand, sugar, salt, soda ash, and many other substances of a granular
nature, which are very troublesome when conveyed by the suction
method, may be dealt with economically by the induction system and,
although the latter is only in its experimental stages at the moment of
writing, it is possible that it may displace all other systems in the
near future.
The induction system differs from the suction system in that the air
flow along the pipe is induced by a jet of air, at very high velocity,
fixed at any convenient distance from the intake nozzle of the conveyor
pipe, and the material conveyed is discharged either from an open
end into an open container, or by some form of cyclone. The closed
discharger or container, with its baffles and rotary valves or air
locks, is eliminated, and the substance to be handled has a free and
unrestricted flow throughout the length of the pipe.
=Advantages of the Induction System.= The advantages of the induction
conveyor may be summarized as follows: (1) Low first cost, the power
unit being the only expensive item. (2) Low maintenance cost, there
being no moving parts and little wear. (3) Low labour cost, practically
no attendance being required. (4) Flexibility and ease of handling. (5)
High efficiency of power unit and reliability of system. (6) Ability to
handle materials which are easily damaged.
In the case of a suction plant handling grain or coal, the intake end
of the conveyor is fairly flexible, and the nozzle may be operated
over a fairly large radius, say, all over the floor of a vessel’s
hold. The discharge end, however, is fixed, unless a cumbersome and
expensive gantry is provided to permit of the discharge apparatus being
moved about. Even should the discharger be mounted on rails, the area
over which it can operate is limited by the rails on which it runs.
With a large plant, this would mean that a number of dischargers would
be required to lift from a ship into a warehouse or store, from the
latter into bunkers or silos, or perhaps into trucks or waggons. Each
discharger would require a separate exhauster and a separate intake,
and valuable space would be occupied by the plant and expense incurred
for machinery which would not be in use for a considerable part of the
time.
The induction system, however, is flexible at both intake and discharge
ends of the pipe. It is only necessary to lower the nozzle into the
material to be removed, and to place the delivery pipe over the
receptacle for the material, and to turn on the air jet. The delivery
may be handled easily while working, and the material distributed where
required; or suitable valves and branches may be fixed, and a number
of discharge pipes used in turn to deliver into different bins or into
various floors.
The source of power for the operation of the induction conveyor is
the air compressor. As every operating engineer is well aware, all
machinery is kept in better condition and runs more economically, when
it is housed in proper environment and receives skilled attention. The
compressor, in this case, need not be erected near the work, but may
be placed some distance away, preferably in the power station, as the
pipe line connecting the conveyor with the machine will have a very
small bore, compared with the air pipe to the exhauster on a suction
plant, and will also be inexpensive to erect and maintain. In the
instance quoted above, where a number of suction and delivery points
are required, only these small pipes need be run from a common main,
and turned on and off as needed, the compressor running continuously at
or near its most economical load.
=Construction of Induction Plant.= The induction conveyor may be said
to be a compromise between the suction and blowing methods. The air jet
is fixed in the conveyor pipe at a suitable angle, some distance above
the intake nozzle, and a stream of air at high velocity is passed along
the pipe in the direction of the discharge. This air jet is designed
carefully for the duty it has to perform, and its discharge entrains
the free air in the pipe, causing it to move in the required direction.
A partial vacuum is created in the conveyor pipe, behind the jet, and
free air rushes in at the intake, carrying the material along with it.
In order to effect the greatest economy in the operation of this plant,
it is important that the power unit should be carefully chosen, and
that the pipe system should be designed to give the full pressure at
the jet. With modern multiple-stage compressors of the rotary or
reciprocating type, working at about full load, very high efficiencies
can be obtained, and the pressure pipe line should be arranged to
avoid loss by friction as far as possible. The receiver should be of
sufficient capacity to absorb any pulsations, and to throw down oil and
moisture before the air enters the pipes. A separator of good design
should also be incorporated.
In designing the conveyor pipe line, bends should be avoided when
possible, by erecting the lifting pipes at an angle with the
horizontal. It is not sufficiently well appreciated that bends and
angles rapidly increase the frictional resistance to the flow of the
conveying medium, and mean loss of power; in fact there is no doubt
that the difference between success and failure in pneumatic conveying
is largely a matter of design. Many substances which are otherwise
quite suitable for handling in this manner are very fragile, and any
friction on pipe walls or contact with metal baffles at high velocity,
so reduces or pulverizes them that their value is reduced considerably.
In the case of ashes from boiler furnaces, this effect is advantageous
rather than otherwise, but when dealing with coal it is necessary to
arrange the system so that the minimum amount of damage is done to the
material. Some coals, such as Derbyshire bituminous, is not easily
broken or abraded, and can be lifted very satisfactorily by the usual
suction method. Welsh coal, on the other hand, is very friable, and if
conveyed into the usual discharger, will emerge in a finely divided
state, even though it may be fed to the intake in large pieces. For
handling such materials, the induction method is most suitable, as the
discharge end may be arranged so that the material is not delivered at
high velocity, and does not strike any obstacle which would reduce it
or break it up. It is possible to elevate potatoes and even oranges by
the induction process, and it is quite within the bounds of probability
that eggs may be delivered in this manner, without more than the usual
percentage of breakages.
In conveying many materials, which are conveyed whilst hot, it is
better if they can be kept at practically the same temperature at the
delivery as when they enter the pipe. This is provided for by heating
the air to a suitable temperature just before it enters the jet.
This is also an additional source of economy in operation. As is
well known, air, like all other gases, increases in volume with the
temperature, and if the heat lost by the air cooling after compression
be replaced at the jet, considerably more power is obtained. If the
compressor is situated in such a position that most of the heat of
compression is delivered at the jet, there is little to be gained by
reheating. In most cases, however, the air has returned to normal
temperature by the time it reaches the point where it is to be used,
and, if a suitable air heater is installed at this point, the volume
may be increased greatly by a comparatively small expenditure.
A heater consisting of tubes through which the air passes, these
tubes surrounded by water under high steam pressures, offers the most
convenient and satisfactory method of heating the air. The air pipes
between the heater and conveyor pipe should be lagged in order to
retain the heat.
The pressure of the air may be increased by 50 per cent. by heating to
the temperature of steam at 200 lbs. per sq. in. gauge pressure, while
the cost will be comparatively small. Theoretically, a gain of about
40 per cent. in economy should be obtained, and the practical results
should be reasonably close to this figure.
=Air Receivers.= It is a decided advantage in practice to install an
efficient separator between the ordinary receiver of the compressor and
the pipe line, as large quantities of moisture will travel over with
the air, and will be condensed directly they meet some cooler surface.
The ordinary receiver is supposed to fulfil this function, but it does
not do so because it is, in effect, an enlargement of the pipe line,
and, being filled with hot air under pressure, has no tendency to
condense the moisture. The latter does not begin to cool to any extent
until it reaches the small diameter pipes, with the consequence that
these pipes contain quantities of oil and water which eventually reach
the jet, and are blown into the material handled.
Where compressors of the rotary or turbine type are installed, there
will be only water in expansion, but it is good practice to remove
this, even though the air be re-heated, because the moisture will
recondense in the conveyor pipe, and tend to choke the latter when
small grained substances are being conveyed.
=Types of Compressors.= Reference has already been made to the power
unit, and it is hardly within the scope of this work to describe in
detail the various machines available. As, however, the economy of
air conveying depends in a large measure on the cost of power, it is
evident that the compressor should be of the most suitable type for the
duty to be performed.
For small installations, single-stage reciprocating machines, driven
directly by steam engines or by electric motors are, no doubt, the most
suitable. In the case of large plants, using the air continuously in
a number of air jets, where the load factor is high, it is certainly
more important to install a two or three-stage compressor, owing to
the greater economy of working. The larger capital expenditure will
be compensated by the considerable saving of energy. As compared with
single-stage compression to 100 lbs. gauge pressure, a saving of 20
per cent. can be effected by three-stage working, and with a constant
load of from 75 per cent. to 100 per cent. of full load, a turbine or
electrically driven rotary multiple-stage compressor is decidedly the
best type to adopt.
In plants where exhaust steam can be used to advantage, as in large
generating stations, a steam turbo-compressor, multiple-stage,
exhausting to a feed water heater will show great economy, and the
operating costs of a large plant of this type are very low compared
with any other form of conveyor. This will be obvious when it is
pointed out that maintenance costs on the conveyor are confined to
renewals of bends and junctions in the pipe lines, and of flexible
hose. There are no discharge valves or air locks to be kept vacuum
tight, no filter strainers or sleeves to renew, and the power unit is
not subjected to undue wear through extraneous matter entering the
cylinders and scoring the walls or wearing the valves.
Compared with other forms of mechanical conveying, the pneumatic
induction system is very low in maintenance costs, while the serious
charges incurred in employing human labour are reduced to a minimum.
CHAPTER VII
STEAM JET CONVEYORS
A method of removing ashes from boiler furnaces which has been
developed extensively in America is essentially a pneumatic system,
although steam is the conveying medium instead of moving air. Steam
is used because the apparatus is always in use on boiler plants, from
which steam can be taken as conveying medium. No air compressor or
other special plant is required. On the other hand, the simple use of a
connection from the steam main is a matter of very little importance,
and no check is ever made as to the amount of steam so used, hence 99
per cent. of the users consider that the steam jet costs practically
nothing for “power” compared with a compressor which would have a
certain sized motor connected, and could not escape attention as an
additional power consumer.
=Steam Consumption.= Investigation into the actual consumption of steam
jets would often give very startling results, especially after the
plant had been in operation for some time and the nozzles had begun
to cut and wear. As proof of the waste of steam possible in such a
plant, it is interesting to note that Mr. David Brownlie, in a paper
on Automatic Stokers,[1] gave results of actual tests made on steam
jets as used in certain classes of stokers in which steam jets are
allowed to blow down the hollow furnace bars. These tests showed that,
whereas the makers estimated the steam consumption of the jets to be
about 2 per cent. of the boiler output, the tests on 80 plants showed a
consumption varying from 0·5 per cent. up to as much as 21·4 per cent.
of the total output of boiler.
As further evidence of the waste of steam that can occur due to neglect
of the cutting effect on the nozzles, one American firm has designed
an ingenious warning or “tell-tale.” A small hole is drilled almost,
but not quite, through the nozzle. While the nozzle retains its initial
shape and size the apparatus acts normally, but as soon as the small
amount of metal covering the end of the hole has worn away, the hole
is exposed, and a certain amount of steam passes through it to a
steam whistle which blows continuously until a new nozzle has been
inserted in place of the one which is now worn so much as to make it
uneconomical in steam consumption.
Provided that means are taken to prevent waste of steam due to worn
nozzles, the steam jet conveyor is very serviceable and, being flexible
and convenient, it is very useful for the purpose for which it has been
developed.
The following estimated steam consumptions are given for what they
are worth; they are of comparative value in relation with the power
consumption on the “suction” scheme: One firm claims, in an actual
proposal for a plant to be erected in this country, a consumption of
30 lbs. of steam per min. to deal with 150 lbs. of ashes per min., or
4 tons per hr. This is approximately equivalent to 72 electrical h.p.
for dealing with 4 tons of ashes per hr. A second firm states that a
steam jet plant dealing with 12 tons per hr. will require 3,466 lbs. of
steam per hr. at 130 lbs. pressure: this, if passed into a modern steam
driven generator, would produce over 130 h.p. hours. These figures
indicate how variable are the estimates of power required. _Note._—The
“suction” schemes for wheat actually work out at slightly more than 1
h.p. per ton per hr. in single-nozzle plants, and 1½ h.p. per ton per
hr. in twin nozzle plants.
=Lay-out of Plant.= The plant is usually designed on the following
lines: Immediately under the ash hoppers are funnel-shaped tee-pieces
fitted to a cast iron pipe laid on the floor, or preferably in a small
trench just below the ground level. These funnel inlets are usually
covered with a cap when not in use, a tight joint being established by
the “suction” in the pipe line. When used on Lancashire boilers having
no ash basement the ashes are raked from under the furnaces on to the
floor, and swept into the inlets mentioned (see Fig. 25). Large pieces
of clinker are broken by hand until they enter the intake pipe, when
they are immediately conveyed through the rest of the system.
[Illustration: FIG. 25.—BRADY STEAM JET ASH CONVEYOR.]
In all large boiler houses with a proper ash basement it is usual to
have a travelling clinker breaker, motor driven, which can be moved on
light rails under each ash hopper and over each intake. The breaker
receives all the ashes when released by the hopper valve, crushes them
to a suitable size and discharges them by gravity over the intake
funnels, whence they are transported to the ash tank or hopper.
The method of creating the moving air currents is by passing steam
through specially designed nozzles which are placed at the extreme end
of the intake pipe, and force the air out of the pipe, thus inducing
a stream of air to enter at the intake openings, and carry forward
the ashes which have been fed into the pipe with the air. When the
underground pipe has to rise vertically to cross roads, etc., or
to reach an overhead tank, it is usually found necessary to insert
“booster” jets to impart additional velocity to the ashes, which are
naturally retarded seriously in changing their direction at the bend or
elbow. Should circumstances necessitate many bends being employed in
the pipe line the number of “booster” jets has to be increased, and the
total cost of steam for operation is increased seriously.
The capacity of the conveyor depends upon the volume of air passed
through the pipe in a given time, and the ashes must not be slacked
before handling, but must be handled either straight from the furnaces
or allowed to cool and then conveyed to the ash hopper.
An 8 in. pipe is the largest used, and this will handle approximately 8
tons of ash per hour. Any increase over this size of pipe necessitates
a consumption of steam which makes the scheme impracticable.
The conveyor pipe may be run at any angle, elevation or level,
and therefore is not handicapped by the rigid straight line,
point-to-point, requirements of bucket elevators, skips, etc.
The abrasive action of ashes is well known, and when they are
travelling at the high speed necessary with this form of conveyor they
cause considerable wear at the bends and elbows in the pipe line.
To overcome this a special mixture of iron has been obtained, which
is extremely hard and wear-resisting. Steel is quite unsuitable and
ordinary cast iron is too soft for these conditions.
=Steam Jets.= The James Brady Foundry Co. (Chicago, U.S.A.) state,
in their Bulletin on this subject, that the special steam jet elbows
are usually placed at the top and bottom of a vertical riser. The jet
of steam from the nozzle enters the elbow directly in front of, and
parallel to, the face of a special wearing liner. This prevents or
reduces the wear on the liners, as the jet protects the liners from the
pounding action of the ash. A renewable sectional liner is provided of
specially hard metal at all points in which the material makes actual
contact with the pipe or fittings. These liners are interchangeable in
all elbows, and each individual liner can be turned end for end when
affected by wear.
In cases where the length of horizontal run exceeds 125 ft. it is
necessary to supplement the primary nozzles by “booster” steam jets to
maintain the velocity of the air current.
=Buffer Boxes.= At the discharge end of the pipe line it is necessary
to insert a baffle or buffer box to take the impact of the ashes, and
thus prevent wear on certain parts that are not designed to stand up
to the destructive effects of the impact. The function of the box is
to bring the ashes to rest, so that they may fall by gravity into the
ash tank or on to the storage pile. When delivering into a tank it is
very essential to install a buffer box, as otherwise the velocity with
which the ashes enter the tank will pack them so tightly that they will
not discharge automatically through the valve or gate. The location of
the ash hopper can be wherever most convenient for loading the vans,
railway trucks, or barges, etc., but preference should be given to a
site which makes possible a pipe run with a minimum number of bends.
One American firm of engineers, the Vacuum Ash and Soot Conveyor Co.,
New York, have done away with the numerous steam jets and the blowing
effect produced thereby, and rely entirely on suction by using a sealed
ash tank and exhausting the container and pipe system by means of a
single steam jet injector built into the roof of the ash tank, and
discharging its steam directly into the air.
By this means it is claimed that the following advantages are obtained:
(1) No sand-blast effect, such as is inevitable when blowing at high
velocity. (2) No steam enters the ash tank and consequently there
are no condensation troubles. (3) Much less dust is blown into the
atmosphere as the steam is never in contact with the dust. (4) Conveyor
pipes are cleaner since no steam enters them, as in “blowing,” and
there is therefore no condensation, caking and corrosion.
FOOTNOTES:
[1] _Inst. Mechanical Engineers Journal_, March, 1920, p. 291.
CHAPTER VIII
MISCELLANEOUS APPLICATIONS OF PNEUMATIC CONVEYING
=Pneumatic Despatch Tubes.= The ordinary pneumatic conveyor picks
up material at one point and unloads it at another and continues
this course consistently, whereas the “pneumatic despatch tube” is
a conveyor of small articles enclosed in a special cartridge which
is built to fit the tube and which travels to and fro as required,
carrying a variety of articles, or if necessary, the same articles,
backwards and forwards between the same two stations or a series of
fixed stations.
The despatch tube thus constitutes an effective “mechanical messenger.”
One or more tubes are run between the points to be connected, with a
despatch and receiving terminal at each end, or if necessary, a single
line to operate in both directions can be designed. The tubes vary from
1½ ins. to 4 ins. diameter, and they are also made of oval sections up
to 4 ins. × 7 ins.; rectangular tubes have been installed in special
installations, chiefly in telephone exchanges for convenience in
dealing with certain cards there employed.
=Tubes.= The tubes are of lead and are usually encased for protection
against mechanical damage, and the erection is carried out with great
care so as to preserve the smooth interior. Joints occur at intervals
of 28 ft. or less, and are “wiped” with an ordinary plumber’s joint
over an internal mandril which is heated previous to insertion in the
tube. Air-tight joints and smooth interiors are absolutely essential to
a successful installation.
=Carriers.= The carriers or cartridges in which the material to be
transmitted is placed are made of gutta-percha covered at the ends with
felt. One end of the container is closed and the other end is left
open, but a “skirt” of felt surrounds the open end, and, as this is the
“trailing” end and the air pressure is behind it, the air forces open
the “skirt,” making a tight fit and preventing leakage of air past the
carrier. The nose of the carrier is usually fitted with a felt “buffer”
which also assists in making an air-tight fit. A carrier for a 2½ in.
tube is 6¾ ins. long and weighs empty about 3 ozs. Fig. 26 shows a
large carrier.
=Methods of Working.= Pneumatic tubes are worked either by air above
atmospheric pressure or by reducing the pressure below atmospheric. In
the pressure system the usual pressure is about 10 lbs. per sq. in.
above atmospheric pressure, whilst in the suction system the vacuum
employed is equivalent to about 6½ lbs. per sq. in.
[Illustration: FIG. 26.—TYPICAL LAMSON INTERCOMMUNICATION CARRIER.]
Also, the method of working may be either “continuous” or
“intermittent”; in the first system the air, either above or below
atmospheric pressure, is circulating continuously and the cartridge
or carrier is inserted into a stream of air already in circulation,
whilst in the “intermittent” system the power, either pressure or
suction, is admitted to the conveyor tube only after the carrier has
been inserted, and it is again cut off when the carrier reaches the end
of its journey.
To a great extent the success of a pneumatic tube system is the speed
at which it can transmit the message sent by this means. In the
“continuous” system, working above atmospheric pressure, the speed is
not so great as in the “intermittent” scheme, because the pressure in
the tube is the same in front of and behind the carrier, which has
to displace the air in front of it. In the “intermittent” system the
pressure is turned on after the carrier is in place, and the advancing
carrier has only to move the air at atmospheric pressure. On the other
hand, if suction is employed, the “intermittent” system is slower
than the “continuous” system because the air has to be exhausted to a
certain point before the carrier begins to travel. It is true that it
will begin to move as soon as the difference in pressure amounts to a
few ounces, but there is a distinct “time lag” compared with inserting
the cartridge into a tube continuously exhausted when it starts off at
practically full pressure and speed immediately.
The difference in time is stated by Kemp to be 3 per cent. longer with
“continuous” pressure, compared with “intermittent” pressure at 6 lbs.
per sq. in.; the difference increasing to 6 per cent. when the pressure
is raised to 14 lbs. per sq. in. The average working speed of these
tubes is from 25 to 30 miles per hour.
=Power Required for Operation.= It is difficult to determine the actual
amount of power necessary to carry a cartridge through a tube. _Kemp’s
Engineer’s Year Book_ states that, working at the standard pressure
of 10 lbs. per sq. in., the power required is theoretically 3·35 h.p.
for a 2½ in. tube, 1 mile long, but actual experience suggests that at
least 50 per cent. should be added to allow for losses from various
causes, making the actual power, say, 5 h.p. per 2½ in. tube per mile.
Pressure receivers or tanks are inserted between the pump and the
travelling tube to compensate for the impulses due to the irregularity
of the pumps and also to act as reservoirs furnishing additional power
during periods of abnormal working.
The vacuum system takes less power (for a definite time of
transmission) than is required by the pressure method of working,
but local conditions always influence results considerably, and it
is inadvisable to give any definite figure as to the power required,
without actual knowledge of the system and conditions involved.
The air compressors are usually driven electrically, but they can, of
course, be operated by any other prime mover such as oil, gas, or steam
engines. It is economical to combine the pressure and suction systems
by arranging the air compressor to draw air from the vacuum receiver
into the compressor cylinders whence it is returned to the pressure
line.
Automatic valves keep the pressures in the pressure and vacuum sides
of the system within pre-determined limits. “Make up” air is admitted
by the automatic opening of an atmospheric valve when the pressure
side of the system is low and the vacuum side high, so that the pump
is deprived of sufficient air to operate the system efficiently.
Should the conditions become reversed, that is, a low vacuum and a
high pressure, then the pump is working against a high back pressure,
and this is reduced by the opening of an atmospheric relief valve
which remains open until the vacuum is restored to normal pressure.
This system is preferable to and more economical than the use of two
separate pumping and exhausting machines.
Elaborate and valuable tables of horse-power required by compressors
and of “transit times” for distances up to 4,500 yds. with 1½ in., 2¼
in., and 3 in. tubes are given in _Kemp’s Engineer’s Year Book_.
The Lamson Tube Co., Ltd., have brought what was originally invented as
a means of conveying persons to a practical business accessory, capable
of saving a great amount of time by despatching sketches, papers, small
articles, money, etc., here, there, and everywhere at the rate of 30
miles an hour.
The utility of these plants has long been recognised by banking
establishments, the General Post Office, large stores, factories,
newspaper publishing offices, etc. (see Figs. 27 and 28).
In addition to the conveyance of messages and papers, they are
frequently installed to convey money and bills from the numerous
departments of a large store to the cashiers, thus saving time and
effecting economy in labour and floor space. One cashier can attend to
from 10 to 15 stations, or in small establishments all the stations can
be centralized around the book-keeper.
The installation of a power-driven plant is not essential, providing
that the service required is not too great. A foot power pneumatic
service is available and it is in use in many business establishments.
In this system the methods of transportation are similar to those in
a power plant, but the tubes are brought to a special cabinet 15 ins.
square by 2 ft. 6 ins. high, in which is mounted a foot-operated pump
of patented design without bellows or cords. The pump is operated
as and when the service is required, and there is no loss of any
description when the apparatus is not in use.
=Pneumatic Tubes for Heavy Articles.= It is interesting to recall,
especially in view of the proposed use of pneumatically-propelled
parcel-conveying trains by the G.P.O. in London, the proposal made by
Mr. Medhurst, in 1810, when it was suggested that a carriage somewhat
similar to the modern railway carriage should be moved through a
tunnel by pneumatic means. So long ago as 1667, Denin Papin read before
the Royal Society a paper entitled “A Double Pneumatic Pump,” and
definite mention of despatch tubes was made in this paper.
[Illustration: FIG. 27.—TUBE CENTRAL IN WHOLESALE DRUG HOUSE,
DISTRIBUTING ORDERS TO ALL DEPARTMENTS.]
[Illustration: FIG. 28.—LAMSON DISTRIBUTING STATION IN WELL-KNOWN
PUBLISHING HOUSE.]
In 1840 a pneumatic railway was actually built and worked between
London and Croydon, and in view of its success was followed by others
between Dalkey and Kingstown and between Exeter and Plymouth. From this
it will be seen that transportation by pneumatic means is not modern in
its application, and was originally intended for very large tubes and
weights, but modern development has been toward small tubes and light
weights.
=The Vacuum Cleaner.= The pneumatic transporting of material in the
form of dust has been brought to a very high state of perfection during
recent years and an enormous number of plants is now in use, ranging
from the hand-propelled machine to very large stationary equipments.
Certain hand-propelled machines have been constructed in such a way
that the fan is directly operated by gearing from the running wheels,
and after a few moments a very considerable speed is attained and the
suction of the fan is used for lifting the dust from the surface over
which the apparatus is travelling.
Numerous designs of more powerful machines actuated by hand bellows
have been placed on the market and these possess the advantage that
they are independent of the use of power; but it is not altogether easy
to operate a machine by one hand and to manipulate the nozzle with the
other.
Electrically driven machines of almost numberless designs are
available. These usually employ a high speed fan of the single-stage
type, but a piston pump is embodied in some designs.
In the removal of dust the same principle applies as in the conveying
of heavier materials, i.e. it is not so imperative to obtain a high
vacuum as it is to have a large volume of air moving at high velocity,
hence the multi-stage turbine machine has distinct advantages as
regards weight of material moved and economy of power.
The multi-stage exhauster consists of turbine wheels mounted on a
single shaft, the air being drawn into the first wheel, from this
to the second wheel and so on right through the machine, each wheel
increasing the suction on the intake end according to the total number
of wheels or stages. This style of machine is procurable in either the
stationary or portable type, and in both it is made in various sizes,
the portable machines ranging from 1/12 h.p. up to ½ h.p. for domestic
purposes, and from 1½ to 3 h.p. on trucks for cleaning electrical
machinery, railway carriages, etc. Figs. 29 and 30 illustrate typical
stationary and portable plants respectively.
[Illustration: FIG. 29.—STATIONARY TURBO-EXHAUSTER WITH DUST SEPARATOR.]
[Illustration: FIG. 30.—PORTABLE TURBO-EXHAUSTER DRIVEN BY 1½ H.P. D.C.
MOTOR.]
It is not generally recognized what enormous amounts of dust and dirt
may be extracted by these machines. From one London hotel a ½ h.p.
cleaner removed 166 lbs. of dust from the carpets of the public
rooms only. On a cleaning test in a first class dining car on one of
the English railways 25 lbs. of dust was removed from 38 sq. yds. of
carpet. A rug in front of a lift in a London stores yielded 91 lbs. 1
oz. of dirt to a small machine.
The stationary plants are usually installed in the basements of large
office buildings, theatres, hotels, clubs, etc., and the whole building
is piped suitably, wall plugs or connectors being fitted to which the
staff make connection by flexible hose as and when required. The free
end of the flexible hose is fitted with one or other of a series of
special nozzles, the latter being adapted to the varying requirements
of everything in the room from floor to ceiling.
With the portable hand sets or even with the larger truck type,
the design is complete as a working unit; the equipment is used as
manufactured and there is little or no chance for the user to endanger
the working efficiency of the plant. In permanent plants, however, as
installed in hotels, etc., it is necessary that all points previously
mentioned regarding pipe lines, valves, junctions, bends, etc., should
be considered and acted upon.
The pipe lines should be too large rather than restricted in any
way, the suction flexible should be kept as short as possible, and
if necessary extra connections should be allowed rather than require
flexibles too long for use without “kinking.”
[Illustration: FIG. 31.—SUCTION CLEANING FOR RAILWAY CARRIAGE CUSHIONS.]
[Illustration: FIG. 32.—STURTEVANT EQUIPMENT FOR OFFICE CLEANING.]
Fig. 31 illustrates a stationary suction cleaning plant applied to
cleaning railway carriage cushions, and Fig. 32 shows a similar
installation in use in an office building.
=Cleaning by Air Blast.= By transferring the hose from the suction
side to the discharge, a suction cleaner may be used to blow dust from
machinery of all kinds and from places that are high up and cannot be
cleaned economically by suction. For cleaning electric generators and
motors by blast, these machines have many advantages, and on account of
the large volume of air handled they are much to be preferred to the
small-volume high pressure jet of the ordinary air compressor often
used for this purpose. With the portable turbo-blower there is no
danger of damage to the insulation through high pressure, or through
the carrying of moisture and oil into the windings with the air jet.
=Pumping by Compressed Air.= Although, generally speaking, the
raising of water by compressed air is not an economical method, it is
frequently adopted in mining and tunnelling where the use of steam or
electricity is objectionable. In these cases, cost of operation is a
minor factor, and it may be interesting to give a few particulars of
this form of pneumatic conveying.
The simplest form of compressed air pump consists of a closed chamber
or tank immersed in the water, to be raised or fixed at such a level
that the water will flow into the tank. An air pipe is connected to
the top of the chamber, and the rising main is carried inside the tank
to the bottom. On opening the air valve, pressure is exerted on the
surface of the water in the tank, and the water is expelled through the
lift pipe or rising main. On closing the air valve, water again fills
up the tank, and the process is repeated.
A decided improvement on this pump is the return air pump, which
consists of two closed chambers connected through valves with the
rising main. The compressed air pipe passes through a two-way valve,
either into one tank or the other, this valve being positively
operated. The method of working is similar to that of the single acting
pump, considering each chamber separately, but one tank is filling
while the other is being emptied.
The air expelled from the filling tank, instead of being discharged
to atmosphere, and part of its expansive power lost, is carried back
through the pipe, which would be the air intake pipe when discharging,
through a port in the two-way valve, and into the compressor intake
pipe. The air leaving the filling tank is naturally above atmospheric
pressure, and assists the piston on entering the compressor, thus
reducing the power absorbed in driving the latter.
=Air-lift Pumping.= The air-lift pump is a common means of conveying by
pneumatic means and should not be confused with the above methods of
raising water by compressed air.
In the air-lift method of pumping air under pressure is admitted at the
foot of a pipe already submerged in the well. The air does not merely
bubble through the water, as might be supposed, but passes up the pipe
as a mixture of air and water. The introduction of the air into the
rising column of water makes the latter as a whole less dense than
the water around the tube, and therefore we have a difference in head
between the internal and external columns of water which will carry the
internal column considerably higher than the external column.
As the lifting force depends upon the “head” of water outside the
rising main, it follows that the maximum height to which the water can
be raised depends upon the depth to which the air pipe and rising main
are submerged below the standing level of the water in the bore-hole.
In other words, the greater the lift, the greater the depth to which
the air pipe must be carried before releasing the air into the rising
main.
Experience shows that the water pipe should be submerged 18 ins. for
every 1 ft. lift above the water level in the bore-hole, and allowance
must be made for the “depression” of the water level in the bore-hole,
which will probably take place when pumping is in progress. This
depression will vary according to the water bearing capacity of the
strata, in which the hole has been bored, hence it is necessary to go
carefully into the conditions before boring the hole. If available,
data should be studied concerning the standing water level, and the
pumping depression in other bore-holes in the immediate neighbourhood.
Also tests should be made before the boring machinery is removed
because, although the initial depth of bore-hole may be satisfactory
on the basis of standing level calculations, it may be found that when
pumping the depression is so great that the bore-hole has to be carried
to a greater depth.
The air is supplied at a pressure suitable for the conditions, and can
be carried down a separate tube and connected to the rising main at the
correct depth (Fig. 33), or, as is often done, one pipe may be lowered
and the rising main supported centrally inside the casing tube, the
annular space between the two being used as the air pipe (see Fig. 34).
The amount of free air required is from 0·6 to 1·0 cu. ft. per gallon
of water raised per min., provided that all the details have been
studied carefully and the design of the plant worked out accordingly.
If the air pipe is too small the air will bubble slowly through the
water, while if it is too large it will blow out with great force,
spraying and losing the water: the ratio between the cross-sectional
area of the air and water pipes is about 1½ to 4.
Advantages of air-lift pumping are that a greater amount of water can
be obtained from a hole of given size than by ordinary pumping; and
that one compressing plant can deal with several wells instead of
needing a separate pump to each well.
AIR-LIFT PUMPING
[Illustration: FIG. 33.—AIR PIPE OUTSIDE RISER.]
[Illustration: FIG. 34.—AIR BETWEEN CASING AND RISER.]
The disadvantages are, that the mechanical efficiency is low; that
a considerable amount of air is entrained in the water, and aerated
water is very unsuitable for boiler feed purposes; and that means
must be provided to allow air to escape by passing the discharge from
the pump over a weir or similar contrivance. It is necessary to have
some reliable form of oil trap between the compressor and the well to
prevent contamination of the water by oil carried over by the air from
the cylinders of the compressor; this is difficult, because the oil is
not only “atomized” but is actually vaporized while in the compressor
cylinders and as a gas it is difficult to reclaim. The air must be kept
as low in temperature as possible, and it is usually passed through a
cooler before being delivered down the well.
At times, air-lifts are installed for conveying other liquids to a
height, and when these can be treated at a high temperature it is
advisable, as the efficiency is then much improved. Even under these
conditions it is advisable to cool the air to the lowest feasible
temperature, before using it as a lifting medium.
When starting up, the column of water in the rising main has to be
moved as a solid column, and consequently a higher pressure of air is
required at starting than when the column has been set in motion, as
the water and air then pass up in alternative “pellets.”
In chemical works and allied industries this pneumatic method is
frequently used for pumping acids, and other corrosive liquids from
one place to another. Compressed air is a very handy medium for this
class of work as ordinary mechanical methods are ruled out, due to
the impossibility of introducing corrosive liquids into the pumps and
syphons unless great expense is incurred by the use of acid-proof
materials.
The air-lift is also very advantageous for pumping water which contains
a large amount of sand or similar gritty material which would cut
and score the walls of an ordinary piston pump. Air-lift pumping is
frequently used, therefore, on new bore-holes until the sand, etc., has
been eliminated, after which the final pump can be installed without
fear of damage.
The question of submergence will frequently make it impossible to
use air-lift without boring many feet deeper than would otherwise
be necessary, but when the water bearing strata is low this form of
pumping is frequently very convenient.
=Miscellaneous Applications of Pneumatic Conveying.= Several other
interesting applications of pneumatic conveying may be enumerated but,
being somewhat outside the primary scope of this book, they will not be
discussed in detail. The main object of the author is to raise interest
in the handling of solid materials in a manner practically unknown to
the general reader.
The housing problem has developed the _pneumatic handling of cement_
in a liquid form, and houses are now being built of reinforced cement
in the following manner. An expanded steel frame is supported between
concrete or brick piers, and on wood sheeting where necessary, and
liquid cement is blown on to the metal in the form of a liquid spray:
the first coat dries quickly and leaves a certain amount of cement
covering the framework. Then follows another coat, and again another
and so on, until the whole of the framework has been covered to an
appreciable thickness. The result is a thin wall or slab of cement
reinforced with the steel and of great combined strength. Slightly
domed roofs constructed in this manner have proved very strong and
durable.
The _Aerograph_ is an instrument working on the same principle for the
application of paint, and it is used a great deal in the art world, in
the manufacture of Christmas cards, in panel painting, and in interior
decoration generally. Excellent “tones” and shades are obtained by the
simple method of varying the thickness of the colour or the number of
coats applied. It is usual to convey the surplus colour and fumes away
from the operator by means of a stream of air through a special hood
placed at the back of the work, thus maintaining clean pure air for the
operator.
A similar machine of more crude design is used for whitewashing
walls of stables, cattle pens, etc. All these plants comprise an air
compressor, either power or hand operated, from which the air is led
to a special injector which draws up through a second pipe a certain
amount of the material to be sprayed. The paint or other material is
then atomized and impelled with considerable force on to the surface to
be covered.
The _sand blast_ is another application of pneumatic conveying in
which the medium conveyed is sand, which has well-known cutting and
erosive effects when it impinges on a surface at high velocity. This
plant is used for decorating glassware, obscuring sheet glass, and also
for cleaning stone buildings by the actual removal of the face of the
previously discoloured stone.
The _pneumatic conveyance of energy_ is exemplified by rock drills,
riveting machines, coal-cutters and innumerable other portable tools.
Energy is expended in compressing air which is transmitted through
pipes and made to yield its stored energy by driving the air motors of
the tools or other apparatus in question.
=Conclusion.= Enough has been said to show that pneumatic conveying
has made great progress, and that the possibilities of this method of
dealing with the moving of solid materials are much greater than has
been generally recognized.
Almost anything that will enter a pipe up to about 9 ins. diameter can
be conveyed in this way, either by “blowing” or “suction” or by the
“induction” method.
Weight and size is an advantage rather than otherwise, and bricks can
be dealt with more successfully than flour. The writer’s experience,
in the results of actual working with pneumatic conveying, indicates
that no problem should be considered too difficult to be tackled by
this method, and that even the most unlikely materials can be conveyed
successfully by pneumatic means.
BIBLIOGRAPHY
Readers wishing to amplify their knowledge of pneumatic conveying may
find useful the following references—
“Pneumatic Dispatch,” by H. R. Kemp, M.I.C.E., M.I.E.E., M.R.M. Paper
before the Inst. of Post Office Engineers. October, 1909.
“Power Plant for Pneumatic Tubes in the Post Office,” by A. B. Eason,
M.A., A.M.I.E.E. Paper before the Inst. of Post Office Engineers. 18th
October, 1913.
“Portable Plant.” Editorial article in _Cassier’s Engineering
Monthly_, June, 1919.
“Pneumatic Handling Machinery.” _Engineering and Industrial
Management_, 5th June, 1919.
“History of Conveying,” by G. F. Zimmer, A.M.I.C.E. _Engineering and
Industrial Management_, July to Sept., 1920.
“Boots as Power Users,” by E. G. Phillips, M.I.E.E., A.M.I.Mech.E.,
describing the coal handling plant used by Messrs. Boots Pure Drug
Co., Ltd., Nottingham. _Power User_, March, 1920.
“Pneumatic Handling Installation for Calcium Sulphide,” by G. F.
Zimmer, A.M.I.C.E. _Chemical Age_, 10th April, 1920.
“Pneumatic Conveying of Granular Substances, including Chemicals,” by
G. S. Layton. Paper before the Society of Chemical Industry, Third
Conference (Birmingham), 23rd April, 1920.
“Pneumatic Conveying of Coal and Similar Substances,” by J. H. King,
M.I.Mech.E. Paper before the Society of Chemical Industry, Third
Conference (Birmingham), 23rd April, 1920.
Instructive catalogues on this and allied subjects are issued by
the following firms (amongst others): Messrs. Ashwell & Nesbit,
Ltd., Leicester; R. Boby, Ltd., Bury St. Edmunds; H. J. King & Co.,
Nailsworth, Gloucester; The Lamson Store Pneumatic Co., Ltd., London;
and The Sturtevant Engineering Co., Ltd., London.
INDEX
Advantages of system, 5
Aerograph, 101
Air compressors, 67, 70, 71
—— filters, 10, 21-27
—— induction, 63
—— lift, advantages, 97
—— ——, air required, 97
—— —— “depression,” 96
—— —— pumping, 95-100
—— —— submergence, 96, 100
—— receivers, 69
—— reheating, 68, 69
—— velocity, 36
“Aquadag,” 20
Ash handling, 58, 74-77
Bag filters, 22-24
Bends and elbows, 34, 35, 67
Bibliography, 105
“Blowing” system, 4
Breaking of materials, 34, 35, 67
Buffer boxes, 78
Capacity of pipe lines, 36
Cement handling plants, 101
Cleaning with air blast, 94
Coal-handling plants, 54-58, 60, 61
Comparative costs, 53
Conveying above atmospheric pressure, 6
—— below atmospheric pressure, 6
—— above and below atmospheric pressure, 6, 64-66
Cyclone separators, 21
Design, factors influencing, 8
Despatch tubes, 80, 81
Dischargers design, 10, 28-32, 56, 57
—— difficulties, 9
—— valves, 28, 31, 32, 56, 58
Exhausters, 10, 14
Factors influencing design, 8
Flexibility, 5
Flexible suction pipes, 36
Floating plants, 3, 46, 51, 59-61
Flue cleaning, 59
Foot power pumps, 86
Fundamental principles, 3
Grain-handling, 45, 47
Heavy commercial systems, 7
High pressure systems, 6, 39
Historical, 1, 2
“Induction” system, 4, 6, 62-66
“Intermittent” tube system, 81, 83
Junctions in pipe lines, 33
King’s exhauster, 10-14
—— three-way valve, 37
“Kinking” to be avoided, 92
Large pipe systems, 7
Lime washing, 101
Low pressure systems, 6
Lubrication, 20
Materials, breaking of, 34, 35, 67
Mollers’ air filter, 24
Nash hydro-turbine, 18, 19
Nozzles (suction), 10, 40-43
Oil contamination, 11
Pipe lines, 10, 33, 36, 39
—— ——, capacity of, 36
Pneumatic tube carriers, 81, 82
—— ——, “continuous,” 81, 83
—— —— foot power, 86
—— ——, “intermittent,” 81, 83
—— ——, power required, 84, 85
—— —— pressure system, 81, 85
—— —— vacuum system, 84
Portable quay-side plant, 5
—— railway plant, 49-51
—— vacuum cleaners, 92
Power required, 44
Pressure systems, 4, 6
Pumping by compressed air, 94, 95
Quayside plants, 51
Reheating compressed air, 68, 69
Rotary blowers, 14, 15
Sand-blasting, 102
Stationary plants, vacuum, 92
Steam consumption, 72-74
—— jet conveyors, 72, 74, 76
—— jets, 77
—— jets, economy of, 72, 73
Sturtevant blowers, 16, 17
“Suction” nozzles, 10, 40-43
—— systems, 4-7
Systems, advantages of, 5
Telescopic pipes, 38
Turbo-blowers, 11
Vacuum cleaners, 89
—— ——, tests, 92
—— required, 3
Valves in pipe line, 37
Velocity of air in pipes, 36
Water pumping, 95-100
Waterside plants, 45, 47, 48, 59
Wear of pipes and bends, 34, 35
Wet air filters, 25-27
_Printed by Sir Isaac Pitman & Sons, Ltd., Bath, England_
Transcriber’s Notes
pg 59 Changed: await the convenience of the wagons
to: await the convenience of the waggons
pg 69 Changed: practice to instal an efficient separator
to: practice to install an efficient separator
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