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.