§ 206. The different classes of pumps correspond almost exactly to the
different classes of water motors, although the mechanical details of
the construction are somewhat different. They are properly reversed
water motors. Ordinary reciprocating pumps correspond to water-pressure
engines. Chain and bucket pumps are in principle similar to water wheels
in which the water acts by weight. Scoop wheels are similar to undershot
water wheels, and centrifugal pumps to turbines.
_Reciprocating Pumps_ are single or double acting, and differ from
water-pressure engines in that the valves are moved by the water instead
of by automatic machinery. They may be classed thus:--
1. _Lift Pumps._--The water drawn through a foot valve on the ascent of
the pump bucket is forced through the bucket valve when it descends, and
lifted by the bucket when it reascends. Such pumps give an intermittent
discharge.
2. _Plunger or Force Pumps_, in which the water drawn through the foot
valve is displaced by the descent of a solid plunger, and forced through
a delivery valve. They have the advantage that the friction is less
than that of lift pumps, and the packing round the plunger is easily
accessible, whilst that round a lift pump bucket is not. The flow is
intermittent.
3. _The Double-acting Force Pump_ is in principle a double plunger pump.
The discharge fluctuates from zero to a maximum and back to zero each
stroke, but is not arrested for any appreciable time.
4. _Bucket and Plunger Pumps_ consist of a lift pump bucket combined
with a plunger of half its area. The flow varies as in a double-acting
pump.
5. _Diaphragm Pumps_ have been used, in which the solid plunger is
replaced by an elastic diaphragm, alternately depressed into and raised
out of a cylinder.
As single-acting pumps give an intermittent discharge three are
generally used on cranks at 120°. But with all pumps the variation of
velocity of discharge would cause great waste of work in the delivery
pipes when they are long, and even danger from the hydraulic ramming
action of the long column of water. An air vessel is interposed between
the pump and the delivery pipes, of a volume from 5 to 100 times the
space described by the plunger per stroke. The air in this must be
replenished from time to time, or continuously, by a special air-pump.
At low speeds not exceeding 30 ft. per minute the delivery of a pump is
about 90 to 95% of the volume described by the plunger or bucket, from 5
to 10% of the discharge being lost by leakage. At high speeds the
quantity pumped occasionally exceeds the volume described by the
plunger, the momentum of the water keeping the valves open after the
turn of the stroke.
The velocity of large mining pumps is about 140 ft. per minute, the
indoor or suction stroke being sometimes made at 250 ft. per minute.
Rotative pumping engines of large size have a plunger speed of 90 ft.
per minute. Small rotative pumps are run faster, but at some loss of
efficiency. Fire-engine pumps have a speed of 180 to 220 ft. per minute.
The efficiency of reciprocating pumps varies very greatly. Small
reciprocating pumps, with metal valves on lifts of 15 ft., were found by
Morin to have an efficiency of 16 to 40%, or on the average 25%. When
used to pump water at considerable pressure, through hose pipes, the
efficiency rose to from 28 to 57%, or on the average, with 50 to 100 ft.
of lift, about 50%. A large pump with barrels 18 in. diameter, at speeds
under 60 ft. per minute, gave the following results:--
Lift in feet 14½ 34 47
Efficiency .46 .66 .70
The very large steam-pumps employed for waterworks, with 150 ft. or more
of lift, appear to reach an efficiency of 90%, not including the
friction of the discharge pipes. Reckoned on the indicated work of the
steam-engine the efficiency may be 80%.
Many small pumps are now driven electrically and are usually three-throw
single-acting pumps driven from the electric motor by gearing. It is not
convenient to vary the speed of the motor to accommodate it to the
varying rate of pumping usually required. Messrs Hayward Tyler have
introduced a mechanism for varying the stroke of the pumps (Sinclair's
patent) from full stroke to nil, without stopping the pumps.
§ 207. _Centrifugal Pump._--For large volumes of water on lifts not
exceeding about 60 ft. the most convenient pump is the centrifugal pump.
Recent improvements have made it available also for very high lifts. It
consists of a wheel or fan with curved vanes enclosed in an annular
chamber. Water flows in at the centre and is discharged at the
periphery. The fan may rotate in a vertical or horizontal plane and the
water may enter on one or both sides of the fan. In the latter case
there is no axial unbalanced pressure. The fan and its casing must be
filled with water before it can start, so that if not drowned there must
be a foot valve on the suction pipe. When no special attention needs to
be paid to efficiency the water may have a velocity of 6 to 7 ft. in the
suction and delivery pipes. The fan often has 6 to 12 vanes. For a
double-inlet fan of diameter D, the diameter of the inlets is D/2. If Q
is the discharge in cub. ft. per second D = about 0.6 [root]Q in average
cases. The peripheral speed is a little greater than the velocity due
to the lift. Ordinary centrifugal pumps will have an efficiency of 40 to
60%.
The first pump of this kind which attracted notice was one exhibited by
J. G. Appold in 1851, and the special features of his pump have been
retained in the best pumps since constructed. Appold's pump raised
continuously a volume of water equal to 1400 times its own capacity per
minute. It had no valves, and it permitted the passage of solid bodies,
such as walnuts and oranges, without obstruction to its working. Its
efficiency was also found to be good.
[Illustration: FIG. 209.]
Fig. 209 shows the ordinary form of a centrifugal pump. The pump disk
and vanes B are cast in one, usually of bronze,
and the disk is keyed on the driving shaft C. The casing A has a
spirally enlarging discharge passage into the discharge pipe K. A cover
L gives access to the pump. S is the suction pipe which opens into the
pump disk on both sides at D.
Fig. 210 shows a centrifugal pump differing from ordinary centrifugal
pumps in one feature only. The water rises through a suction pipe S,
which divides so as to enter the pump wheel W at the centre on each
side. The pump disk or wheel is very similar to a turbine wheel. It is
keyed on a shaft driven by a belt on a fast and loose pulley arrangement
at P. The water rotating in the pump disk presses outwards, and if the
speed is sufficient a continuous flow is maintained through the pump and
into the discharge pipe D. The special feature in this pump is that the
water, discharged by the pump disk with a whirling velocity of not
inconsiderable magnitude, is allowed to continue rotation in a chamber
somewhat larger than the pump. The use of this whirlpool chamber was
first suggested by Professor James Thomson. It utilizes the energy due
to the whirling velocity of the water which in most pumps is wasted in
eddies in the discharge pipe. In the pump shown guide-blades are also
added which have the direction of the stream lines in a free vortex.
They do not therefore interfere with the action of the water when
pumping the normal quantity, but only prevent irregular motion. At A is
a plug by which the pump case is filled before starting. If the pump is
above the water to be pumped, a foot valve is required to permit the
pump to be filled. Sometimes instead of the foot valve a delivery valve
is used, an air-pump or steam jet pump being employed to exhaust the air
from the pump case.
[Illustration: FIG. 210.]
§ 208. _Design and Proportions of a Centrifugal Pump._--The design of
the pump disk is very simple. Let r_i, r_o be the radii of the inlet
and outlet surfaces of the pump disk, d_i, d_o the clear axial width
at those radii. The velocity of flow through the pump may be taken
the same as for a turbine. If Q is the quantity pumped, and H the
lift,
u_i = 0.25 [root](2gH). (1)
2[pi]r_i d_i = Q/u_i.