US2904245A - Pressure exchangers - Google Patents

Description

Sept. 15, 1959 R. D. PEARSON 2,904,245

PRESSURE EXCHANGERS Filed June 28, 1956 PRESSURE EXCHANGERS' Ronald'l). Pearson, Chesterfield, England Application June 28, 1956, Serial No. 594,464-- 7 Claims. (Cl. 230-69) This invention relates to machines, hereinafter referred to as pressure exchangers, in which each of a. plurality of cells serves cyclically to receive gas from a. source of lower pressure and discharge it to a pressure-increasing means, and to receive gas from said pressure-increasing means and discharge it to a region of lower pressure. The cells are arranged around the periphery of a rotor mounted to pass over appropriate permanently-open ports in a stator. (Of course, the terms rotor and stator are used relatively, the one to the other, so that it might be thatthe rotor is stationary in space and that the stator rotates about the rotor). The admission and discharge of the gas to and from the cell in the lower and in the higher pressure stages is hereinafter refer-red to as scavenging; being defined as a condition in which both ports of the cell being open together for a suiticient duration of time, there occurs a displacement of a substantial part of the former contents fromthe cell, and their replacement by fresh gas.

The pressure-increasing means is conveniently a combustion chamber wherein the received gas is made to burn with a fuel to increase both its volume and temperature.

Conveniently, too, but not necessarily, the motion of the gas into and out of the cell in both of the scavenging stages is unidirectional, so that it is possible to speak of an inlet to and an outlet from the cell, the inlet being on one flank of the rotor and the outlet on: the other flank.

When the machine is arranged as an engine, it serves to convert some of the pressure energy from said-pressure increasing means into kinetic energy.

Itis desirable that immediatelybefore a cellreaches a-scavenging stage, the gas in the cell should be accelerated towards that port, usually the cell outlet, from which the gas is to be discharged in said stage. Hereinafter this acceleration is referred to as prescavenging, and its main function is to prevent the formation ofunwanted compression or rarefaction pulses which would adversely affect the functioning of the subsequent scavenging stage.

A somewhat fuller exposition of the working of such machines may be found in copending applications Serial Nos. 594,461 and 594,462, both filedJune 28, 1956.

It will be understood that at the instant when one of the ports of the cell is opened, either to a region of higher pressure than that obtaining in the cell or to aregion of lower pressure than that obtaining within, then a wave will travel through the cell from that newly-opened port towards the other port, at a velocity comparable with that of the velocity of sound, being in the first case a compression wave and in the second case a rarefaction wave. When the wave reaches the far end, it will be reflected; if the far end is closed, the reflected wave will be of the same sense as the incident wave, compressioncompression or rarefaction-rarefaction: while if the far end'is open, the reflected wave will be of opposite sense.

In the same way waves are generated at the instant of closing of a port which had been open.

The present invention is concerned with improving a pressure exchanger in respect of certainvwave occurrences within the cells, especially over a wide speed range.

The following description relates to the accompanying.

drawing wherein is shown, by way of example only, one embodiment of the invention. In the drawing,

Figure l is adeveloped view of a pressure exchanger In Figure 1 a rotor R comprises a plurality of cells Cv moving from right to left as indicated by the arrow between the two flanking parts of a stator S. The generalflow of gas through the cellsis in the upward-direction, so that the cell ports may be identified as inlet ci and outlet C0.

duct Li, the high pressure scavenging delivery duct Hi, and the low pressure pre-scavenging delivery ductPLi. Similarly the cell outlet C0 passes over the low pressure scavenging receiving duct L0, the compression deliveryduct CH1, the high-pressure prescavenging receiving duct PHo, the high-pressure scavenging receiving duct H0, and the low-pressure prescavenging receiving ductPLo. Various compression and rarefaction waves are generated and reflected, some being desirable, to be utilised, and others being undesirable, to be suppressed if possible. Further details of such a pressure exchanger are to be found in the specifications of the above-mentioned copending applications.

A possible disadvantage-of the arrangement shown in- Figure 1 concerns the rarefaction wave Hand its reflection as rarefaction wave 13. In the high-pressure'scavenging stage, Whenthere is a steady flow through'a cell from Hi to H0, as the cell outlet moves out of communication with Ho an'd into communication with duct PLo at a lower pressure thanthe duct H0, a rarefaction wave 1.1 will travelback through the cell, accelerating the outflow. If thiswave. be reflected from the far end, now closed, as a rarefactio-n' wavezlj, then on reaching the outlet that reflected rarefaction wave will be reflected as a compression wave 16, and will-decelerate the outflow. That deceleration, superposed upon the deceleration following the rarefaction wave 12 created by closing the inlet Hi, may cause flow reversal.

To prevent this flow reversal, to make the pressure exchanger suitable for higher pressure ratio Working and to prevent pulse 16, 17as in Figure 1 it is preferred in accordance with the present invention to add at least one more receiving duct to give two or more such ducts as indicated at PL0 and PLo wherein especially at lowspeed PL0 has a pressure below that existing in PLo At low speed'in Figure'3 flow is substantially arrested when wave 13 is reflected in PLo as 21. In Figure 1 duct PLo is made narrower than one which would receivewave 13 so that this wave is not received over a substantial speed range.

Another disadvantage of the expansion stage of Figure 1 is that at speeds below the design speed the saidrarefaction wave 13 and a compression wave 14 generated on opening of the cell inlet Ci to the prescavenging duct PLz', no longer coincide and cancel but become increasingly separated, thereby creating a rarefaction'pulse which is repeatedly reflected from the ends of the cells and thus Patented Sept. 15, 1959 In their motion the cell ports sweep over the mouths of certain ducts in the stator. Thus the cell-- inlet Ci passes over the low pressure scavenging delivery-- interferes with the fiow in the low pressure scavenging stage L. If no dividing wall exists in PL and the reflected rarefaction wave 13 is, at the design speed, received at the outlet of the cell on a duct wall dividing the low pressure pre-scavenging discharge duct PL0 from the low pressure scavenging discharge duct L0; but at lower speeds, it falls within the duct PL0 and flow reversal may occur. To prevent flow reversal at all speeds the areas of either the cell outlet C0 or of the discharge ducts PLo must be reduced, as in the one case by bending back the trailing edges of the cells and in the other case by reducing the annular height of the duct mouths in the stator, the result in either case being that the pressure difference between duct and cells is reduced substantially to zero on wave arrival.

Again, if the wave 12 generated by closing of the cell inlet Ci to the high pressure scavenging delivery duct Hi is substantial, care must be taken that at the design speed the dividing wall when fitted between the two ducts PLo and PLa be positioned at the point of reception of said wave 12.

In Figure 3 is shown the operation of the two-stage system of Figure 2 at a speed below the design speed, each wave being here shown by a single line only. The rarefaction waves 10, 12, 13 and 15 all fall well within the ducts, instead of coinciding with the walls; and all except 15, which is substantially neutralised, are reflected as compression waves.

If the wave pattern is balanced in that waves 10, 11, 12, 13 and 18 are all equal then the amplitude as measured by change of particle velocity at each wave is only one half of the velocity in the cells towards the rear ends. On reflection of these rarefaction waves as com pression waves of equal amplitude at the open outlet ends of the cells the flow is reduced to zero, but no flow reversal occurs even though the outlet ends of the cells are unrestricted. Flow reversal begins to occur when speeds are reduced below half the design speed. i

If the wave system is unbalanced then a degree of restriction of the outlet ends of the cells is required in order to give complete freedom from flow reversal over a 2:1 speed range, the amount required being increased with increase of unbalance. It is not essential in this stage to prevent flow reversal completely and losses caused are small providing that transition from forward to reverse fiow is made gradual since under such conditions the gases sucked back into the cells are those just discharged at low relative velocity and so are still moving substantially with the rotor blades. This condition is obtained by the use of waves having non-steep fronts and by receiving ducts having no obstructions such as groups of splitters at the side nearest to the low pressure scavenging stage.

At these reduced speeds of operation pressure pulses formed by pairs of compression and expansion waves 17-11, 16-18, 2021, and 21-22, traverse the cells, but all are finally substantially neutralised when they encounter the inlet nozzles so that a substantially pulsefree flow is produced within the low pressure scavenging stage at all speeds. In producing wave neutralisation the gas velocity leaving the nozzles is caused to vary and this etfect though undesirable is not generally serious.

The two stage expansion system in which a double set of expansion waves is employed at the design speed has the advantage of enabling good performance to be obtained under off design conditions without need for excessive outlet velocities from the cells at the design speed. The method could be extended to multiple stage expansion systems in which either the same wave pattern is continued and a corresponding extra number of ducts added or the number of waves produced and number of ducts required within a given space may be increased.

A similar multiple pattern could be employed in the compression process.

The duct dividing wall separating PL0 from PL0 is best made of a width less than that of the cells or of subsidiary cell channels in order to prevent formation of a compression pulse at design speed.

The total width, circumferentially, of the two successive receiving ducts PLo and PLo is approximately equal to that of a single duct which would be capable of receiving within itself a rarefaction wave 11 caused by opening of the cell to said single duct and reflected as wave 13 at the cell inlet.

Splitters Sp and Sp are placed at the upstream side of the ducts Lo, PL0 and PLo so as to reduce opening loss. Each splitter is formed of a number of thin partition strips arranged in spaced parallel relation with the duct wall at the mouth of the duct.

It is not essential that the receiving ducts PL0 and PL0 feed the delivery ducts PLi and PLi only and in fact PL0 will usually transfer gases elsewhere for instance to effect compression whilst the duct PL0 will usually supply gases for use elsewhere in addition to feeding the delivery duct PLi or PLi As shown in Figs. 2 and 3, the low-pressure prescavenging delivery ducts PLi and PLi have their inlets inclined to the cells in the rotor in order to neutralize waves therein.

What I claim is:

1. In a pressure exchanger, a rotor provided with a plurality of cells formed about its periphery, two stator parts between which the rotor is mounted to rotate, said stator parts each provided with a duct communicating with said cells and cooperating to form a low-pressure scavenging stage, first and second low-pressure prescavenging delivery ducts formed in one stator part in advance of said scavenging stage, and first and second pre-scavenging receiving ducts formed in the other stator part in advance of said scavenging stage and in advance of said pre-scavenging delivery ducts.

2. A pressure exchanger according to claim 1 wherein the total width circumferentially of two successive prescavenging receiving ducts is approximately equal to that of a single duct capable of receiving within itself a rarefaction wave caused by opening the cell to said single duct and reflected at the cell inlet.

3. A pressure exchanger according to claim 1 in which the low-pressure pre-scavenging delivery duct inlets to the cells are highly inclined to the cells in the rotor whereby waves in the cells are neutralised.

4. In a pressure exchanger, a rotor provided with a plurality of cells formed about its periphery, two stator parts between which the rotor is mounted to rotate, said stator parts each provided with ducts communicating with said cells and cooperating to form a low-pressure scavenging stage followed by a high-pressure scavenging stage, a plurality of low-pressure pre-scavenging delivery ducts formed in one stator part between said scavenging stages, and a plurality of receiving ducts formed in the other stator part between said scavenging stages but in advance of said pre-scavenging delivery ducts.

5. A pressure exchanger according to claim 4 in which the dividing wall between said receiving ducts between the high and low pressure scavenging stages is made narrower than the width of a cell.

6. A pressure exchanger according to claim 4 and including a wave splitter located at the upstream side of each pro-scavenging receiving duct located between said scavenging stages, each splitter comprising a plurality of thin partition strips arranged in spaced parallel relation with the upstream wall of the duct at the mouth thereof.

7. A pressure exchanger according to claim 4 in which the pre-scavenging receiving ducts located between the two scavenging stages are operated at successively lower pressures.

References Cited in the file of this patent UNITED STATES PATENTS

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