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.