In This Gas Pump Water Acts as a Piston
A British Development Using Gas Explosions to Force Water From Closed Cylinder—Usual Parts Eliminated, Water Acting in Their Place
THE development following article in gas describes pumps for an interesting water works recent in which Britthe usual working parts are eliminated entirely:
Immediately prior to the outbreak of war considerable attention was paid by water works engineers and others in England to the method introduced by A. H. Humphrey for utilizing the pressure created by exploding a mixture of gas and air in a closed cylinder to force water contained therein out of it.
A means was thuS obtained for pumping water. In action the system is precisely similar to that of the explosion of gas in the gas engine cylinder, but in place of there being a moving piston, the water itself constitutes the piston.
How the Pump Works
The Humphrey gas pump is a self contained internal combustion pump. G as or oil fuel is burned in the combustion chamber and the resulting energy is used to pump water or other liquid. In a gas oil engine driven pump the combustion acts successively upon connecting rod, crankshaft, bearings, flywheel, belt, pulleys and then either on the impeller of a centrifugal pump or the piston of a reciprocating one. Only then does the power reach the water.
The parts enuir lerated above do not exist in the gas pump, The combustion acts directly on the water lifting it at one operation. In it also the expansion is carried down to a point equal to that of atmospheric pressure—there is thus no energy carried away in the exhaust.
Comparison with the Gas Engine
To those reade rs who are familiar with the indicator diagram as portrayi ng the action of the internal combustion engine Fig. 1 will show the usual form of such diagram, the area a b c d a rep iresenting the work done per cycle,
Compared with this Fig. 2 shows the work done in the gas pump cycle, the additional area ade representing the extra expansion referred to.
This extra expansion is one of the reasons for the very high efficiency of the system. An indicator, driven by clockwork at a uniform rate will give a diagram of the form shown in Fig la. Here from a to b the charge is compressed being fired at b which causes the pressure to rise to c. From c to d the gasses expand doing useful work. The exhaust valve opens at d and from d to f the pressure of the pump chamber remains atmospheric. At f the exhaust valve closes and compression of the cushion takes place, and it will be seen that the maximum pressure attained on this return stroke is in this case higher than the explosion pressure. From g to h the cushion expands and at h the air valve opens, closing again at j on the return of the water column to compress the new charge, j being the starting point of the new cycle of operations.
The actual pumps shown in the photos act entirely on this cycle.
Compared with gas driven pumps, the fuel consumption of the gas pump is remarkably low, about 1 lb. of anthracite being used per pump horse power per hour.
While this figure may be equalled by high duty steam pumps of the reciprocating type and using a cheaper coal such pumps are not well suited to dealing with large volumes of water at low lifts, which are the particular conditions suited to gas pumps, the practical range of working lift being from 8 to 15O feet.
The System Explained
A diagrammatic arrangement of the system is shown in Fig. 3. a is the pump, b the “play” pipe and c the water column. The important factors in any installation are the suction A, the delivery B and the height above sea level of the apparatus C. The working of the pump is best explained by reference to Fig. 4, the four stroke cycle being assumed as the basis of operations.
It consists merely of a bent pipe terminating at one end in the delivery tank and at the other in a combustion chamber in which the charge of air and gas is exploded over the surface of the water. Such a charge is assumed to have been just fired. The pressure above the water surface sets in motion the whole column of water, the hot gasses expanding above it. Once the water column is set in motion its inertia carries it forward so that the pressure of the gasses in the combustion chamber ultimately falls below that of the atmosphere and as a result the exhaust valve g opens downward under its own weight.The forward motion of the water continuing, the level in c finally falls below that of the water in supply tank a and fresh water then enters through the valves v. Finally the momentum being exhausted the water begins to flow back along the play pipe 6, rising in the chamber c closing the valve g by impact and entrapping above this level in space h a cushion of the spent gas mixed with a large proportion of air.
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In This Pump Water Acts as Piston
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This air was drawn in through a scavenger valve during the forward motion of the water. This cushion of air is compressed into the head of the chamber by the continued return of the water column, the latter being thus finally brought to rest, in which condition the pressure of the cushion is considerably above that due to the static head. As a consequence the water column begins a second forward oscillation in the course of which the pressure in the cushion space falls again below that of the atmosphere. By means of an interlocking gear the exhaust valve g and the scavenger valve (not shown) are now prevented from opening. At the same time the valve f is unlocked and a charge of gas and air enters the chamber. Another surge of the water takes place which compresses this charge which is fired electrically and the operations go on as before.
The whole cycle takes 4.92 seconds. Thus it will be seen that there is a long outward power stroke terminating in scavenging, a return exhaust stroke terminating in cushioning, a short out stroke for suction of the gas charge and a final stroke for compressing it, constituting in reality an eight stroke cycle as compared to the four stroke gas engine.
The two stroke pump, which is an alternative method of working is best followed from reference to Fig. 5. The combustion chamber to is specially shaped so that the incoming charge, which may toe preceded by pure air, displaces the burnt products and mixes as little as possible with them. Thus in the figure a is the admission valve at the top of the chamber b in which the full charge volume extends down to the level c c. A number of exhaust valves d d lead to a common exhaust outlet e which may toe fitted with a nonreturn valve, or each exhaust valve may carry a light non-return valve on its own spindle. The level at which expansion reaches atmospheric pressure is say f f, but this level having been reached by the water its further movement draws in simultaneously water and also fresh combustible mixture till it occupies the space down to c c and the liquid has fallen to g g.
The column of liquid now returns and drives the exhaust products through the valves d, which open by their own weight and are finally closed toy the water. The column has now acquired kinetic energy which is spent in compressing the fresh charge which is bred and a new cycle commences. Thus each outstroke is a working stroke and no locking gear is required on the halves.
Obviously the two stroke system is the simplest, but the four stroke is preferred on account of its greater efficiency.
The compression ratio of the pump obviously depends upon the head under which the pump works. Fig. 6 shows the pump cycle. An explosion initiated at 1 drives the water out of the combustion chamber and up the delivery pipe. From 2 to 3 the kinetic energy of the water increases until the pressure is equal to that of the delivery head. After reaching point 3 the water by virtue of its kinetic energy continues moving up the delivery pipe until volume v — v 4 2is completely discharged. When the kinetic energy of the water is entirely expended and fresh water has filled the volume v — v the water column returns from 0 to 5 and closes the exhaust valve toy impact. The remaining motion of the water is one of retardation up to 6 owing to the cushioning of the trapped exhaust gasses. The water column now returns and about point 5 the inlet valve opens and the mixture of air and gas is drawn in which on the next forward surge is compressed from 0 to 1.
From curves which give the relationship (see Proc. Inst. Mech. Eng., England, Dec., 1920), between delivery head and compression ratio, it is apparent that the compression ratio increases much more rapidly for a given increase of head at high heads than at low ones. This indicates the necessity of throttling or toy-passing some of the delivery head during the compression stroke in order to prevent excessive compression pressures under high heads.
It should be noted that although the pressures and volumes derived will be relatively the same in any installation where the delivery head is fixed provided the same gaseous mixture is used the velocity of the water column will vary approximately inversely as the length of the play pipe so that high theoretical velocities could be diminished in practice by a longer play pipe.
The two fundamental quantities, that is number of cycles per minute N and the horse power HP can be obtained from the following formulas.
Vc x Ve x 60 x 0.637 3Ve (V0—V1) + Vc (V4—V1)
where Vc=the velocity during compression and Ve the velocity during expansion, the values of which depend on the compression ratio r as given in the tables below. H=delivery head.
62.4 x(V4—V1) (H—34) N 33,000
from which the following values are tabulated:
A Large Installation
The most notable installation of gas pumps so far erected is that at the Chingford Reservoirs of the Metropolitan Water Board, London.
A general view of the plant is given in Fig. 6 while Fig. 7 shows two of the pumps discharging. There are 5 pumps altogether, their total capacity being 180 million gallons per day, the maximum lift being 30 feet.
The diameter of each of the pumps is 7 feet and the maximum HP 300.
The genera! arrangement of these pumps is similar to that shown in Fig. 3, that portion marked a being the suction box of ribbed cast iron and the part immediately above it constituting the explosion head which is of cast steel.
The water towers c are of riveted steel plate, the remainder of the construction being mainly of cast iron. The delivery pipes are 48-in. in diameter and terminate as shown in Fig. 7, the water towers being 47 feet high over the bend and 15 feet in diameter at the top. The gas is supplied by a pressure producer of the Dowson pattern, a governor gear being fitted to keep the gas production constant with the rate of consumption.