US3385508A

US3385508A - Compressor capacity control

Info

Publication number: US3385508A
Application number: US354714A
Authority: US
Inventors: David N Shaw
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 1964-03-25
Filing date: 1964-03-25
Publication date: 1968-05-28
Anticipated expiration: 1985-05-28

Description

y 23, 1968 D. N. SHAW 3,385,508

COMPRESSOR CAPACITY CONTROL Filed March 25, 1964 2 Sheets-Sheet 1 34 f as \J) 82 31 9 .8 62 57 as K M1 FIG 5 1 5g 60M 45 m FIG. I

INVENTOR. 45 I2 DAVID M. sum.

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114M442? aw ATTORNEY.

May 28, 1968 D. N. SHAW COMPRESSOR CAPACITY CONTROL 2 Sheets-Sheet 2 Filed March 25, 1964 FIG. 2

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INVENTOR.

DAVID N. SHAW.

United States Patent ce 3,385,508 7 COMPRESSOR CAPACITY CONTROL David N. Shaw, Liverpool, N.Y., assignor to Carrier Corporation, Syracuse, N.Y., a corporation of Delaware Filed Mar. 25, 1964, Ser. No. 354,714 3 Claims. (Cl. 23022) This invention relates to control arrangements, and more particularly to control arrangements for regulating the capacity of fluid compressors.

In copending application, 'Ser. No. 198,854, filed on May 31, 1962, now Patent No. 3,184,151 there is disclosed a control apparatus for regulatin the capacity of a reciprocating compressor by effecting loading and unloading of at least one cylinder of a multi-cylinder compressor. The control apparatus therein disclosed regulates the position of a piston type valve disposed between the discharge and suction sides of a compressor cylinder which in response to load conditions imposed on the compressor interrupts communication between the cylinder discharge and suction sides to load the cylinder in response to increased compressor load and which permits communication between the cylinder discharge side and the suction side to unload the cylinder in response to decreased compressor load.

The control apparatus disclosed in copending application, Ser. No. 198,854, is adjustable to accommodate different applications. However, the individual control requirements of the different applications with which that control apparatus may be used exceed the adjustment capabilities of a single control apparatus of that type necessitating that a number of control apparatuses having different ranges of adjustment be stocked.

It is a principal object of the present invention to provide a new and more versatile control arrangement for fluid compressors.

It is a further object of the present invention to provide an improved control arrangement for regulating operation of fluid compressors having an extended range of adjustment.

It is an object of the present invention to provide a control arrangement for fluid compressors having unique adjusting means whereby the operational response of the compressor may be more closely delimited.

It is an additional object of the present invention to provide a control arrangement of the pressure type for regulating the capacity of fluid compressors, incorporating a unique pressure differential adjusting mechanism. Other objects of the invention will be readily perceived from the ensuing description.

This invention relates to a control arrangement for a fluid compressor having suction and discharge sides with partitioning means therebetween having an opening therethrough for communicating the compressor discharge side with the suction side, and pressure actuated piston means for interrupting the partition means opening, the combination of a bleed passage serving to impose compressor discharge pressure against the piston means to move the piston means relative to the partitioning means opening, and means for controlling the passage of discharge gas through the bleed passage to control the piston means, the controlling means including rst and second passages for communicating the bleed passage with the compressor suction side, and valve means for regulating flow of gas through the first passage in response to pressure conditions between the bleed passage and the' piston means.

The attached drawings illustrate :a preferred embodiment of the invention, in which FIGURE 1 is a cross-sectional view of a four cylinder reciprocating type compressor embodying the control arrangement of the present invention;

3,385,508 Patented May 28, 1968 FIGURE 2 is a sectional view of a portion of a compressor embodying the control arrangement of the present invention, the components of the control being shown in the compressor unloaded condition;

FIGURE 3 is a sectional view of a portion of the compressor including the control arrangement of the present invention, the components of the control being shown in the compressor loaded condition;

FIGURE 4 is an enlarged sectional view of a modified control arrangement, the components of the control being shown in the compressor unloaded condition; and

FIGURE 5 is an enlarged sectional view of the modified control arrangement, the components of the control being shown in the compressor loaded condition.

Referring to FIGURE 1 of the drawings, a fluid compressor 10 embodying the control arrangement of the present invention is shown. While the control arrangement of the present invention is shown and described as applied to a compressor of the reciprocating type, it is understood that the control arrangement is applicable to other types of compressors.

The compressor 10 is a four-cylinder compressor having cylinder banks 3 and 4. Each of the cylinder banks 3 and 4 include a pair of cylinders 11 having pistons 14 disposed therein. Pistons 14 are connected to the crank shaft 5 by connecting rods 6.

A suction manifold 22 is provided for introducing low pressure gas into the respective cylinders. A discharge manifold 23 is provided for receiving the pressurized gas discharged from the respective cylinders. Discharge manifold 23 is provided with an outlet 7.

In the compressor 10, cylinder bank 3 is provided with a discharge manifold check valve 24, more fully described hereinafter. Cylinder bank 3 is also provided with control arrangement 40 for regulating the capacity of compressor 10.

While compressor 10 is shown as a four-cylinder compressor having two banks of paired cylinders 3 and 4, it is understood that additional cylinder banks may be provided, some of all of which may be provided with capacity controls.

Referring to FIGURES 2 and 3, the cylinder 11 i cludes a cylinder head 12 secured to the compressor 10 by bolts 13. The piston 14 moves within the cylinder 11. A valve plate 15 is placed between the cylinder head 12 and the cylinder 11. Cylinder head gasket 16 and valve plate gasket 17 are disposed on opposite sides of valve plate 15. The cylinder communicates with the cylinder head through the discharge port 18 and suction port 19 in the valve plate 15.

Afiixed to the valve plate are a discharge valve 20 and a suction valve 21. On the suction stroke of the piston, gases are drawn into the cylinder through the suction port 19 and chamber 32 from the suction manifold 22 through chamber 32. On the discharge stroke the compressed gas is forced into the discharge chamber 29 through the discharge port 18. When the cylinder is in loaded condition, the cylinder discharge pressure will force open the check valve 24 to permit passage of refrigerant gas into the discharge manifold 23. Check valve 24 is urged toward closed position by spring 26 disposed between valve 24 and support 25.

Within the cylinder head is a partition 27 which divides the head into a discharge chamber 29 and a suction chamber 30. The

chambers

29 and 30 can be considered to be merely extensions of the discharge manifold and the suction manifold, respectively. The partition 27 includes an opening 33 to place the

chambers

29 and 30 in communication with one another and a wall 31 defining the chamber 32. Chamber 32 is placed in communication with suction manifold 22 by internal passages (not shown) in the compressor.

A piston type valve 34 serves to close the opening 33 in response to a predetermined pressure. The piston is urged by spring 35 to the open position. Spring 35 is positioned in piston chamber 39 on spring guide 36. Spring guide 36 is connected to the control apparatus body 45. One end of spring 35 abuts retaining ring 37 secured to piston 34 and the other end engages retaining lip 38 of the guide 36.

The control arrangement 40 includes body secured to the cylinder head 12 by bolts 41. Gasket 42 is provided between the cylinder head 12 and the control body 45 to prevent the leakage of the gases from the compressor. A pair of opposed chambers 46, 47 are formed in control body 45. Opening 43 in control body 45 is disposed between chambers 46, 47. Guide 36 is secured to control body 45 opposite chamber 46. Control chamber 46 includes valve seat 57.

Valve stem 56 having an outer dimension less than the dimension of opening 43 is positioned in opening 43 with opposite ends extending into chambers 46, 47. Stem 56 is reduced at 44 to define a stop 66. Control valve 55 is slidably disposed on reduced end 44 of stem 56. Stop collar 61 on reduced end 44 of valve stem 56 limits movement of control valve 55 in a valve opening direction relative to stem 56. Stop 60 limits movement of valve 55 relative to stem 56 in a valve closing direction. Control spring 62 urges valve 55 into abutment with stop collar 61.

Retainer 63 is secured to the opposite end of valve stem 56. Spring 64, disposed between retainer 63 and control body 45, urges valve stem 56 in a valve closing direction. Spring 64 is capable of overriding valve control spring 62.

A bleed passage 51 communicates control valve chamber 46 with discharge passage 52. Passage 48 communicates control valve chamber 46 with piston chamber 39. Cylinder head passage 53 communicates discharge passage 52 with compressor discharge manifold 23. Interconnecting suction manifold passages 49 and in control body 45 and cylinder head 12 respectively communicate bleed passage 51 with the suction manifold 30 through control valve chamber 46. Control valve regulates communication between bleed passage 51 and suction manifold 30.

A pressure differential control means 75 for regulating pressures in control valve chamber 46 is provided. Pressure differential control means 75 includes passage 71 between suction passage 50 and control valve chamber 46, and closure means, for example, ball valve 72,

therefor. Spring 73 urges valve 72 in a closing direction.

Cooperating screw and nut type adjusting means 74 regulate tension of spring 73.

A diaphragm assembly 65 is attached to the outer end of the control body 45 by suitable means as by threaded portion 76. Diaphragm assembly 65 includes diaphragm 66 adapted to abut valve stem 56. A spring 67, acting through diaphragm 66, urges valve stem 56 and valve 55 positioned thereon in a valve open direction, that is, cylinder unloaded position. Spring 67 is capable of overriding valve stem spring 64. Threaded cap 68 is provided for the purpose of adjusting the bias of spring 67. Diaphragm chamber 69 in control body 45 communicates with suction manifold passage 50 by means of the clearance between valve stem 56 and the wall of opening 43 in control body 45, chamber 47 and passage 59.

During operation of the compressor 10, gas is drawn, on the suction stroke of the piston into the cylinder 11 from the suction manifold 22, through chamber 32, suction port 19 and suction valve 21. On the discharge stroke of the piston the cylinder suction valve 21 is closed and the gas is forced through discharge port 18 and discharge valve 20 into the discharge chamber 29. From the chamber 29, the gas flows by the check valve 24 into the discharge manifold 23 for discharge through discharge outlet 7.

Inasmuch as at least one of the other compressor cylinders is loaded at all times, a suitable pressure head is maintained in the discharge manifold 23. When the cylinder is loaded, that is, opening 33 is closed by control piston 34, the discharge pressure of the loaded cylinder forces check valve 24 open to discharge pressurized gas into the manifold 23. At all other times, check valve 24 is closed. The purpose of check valve 24 is to prevent pressurized gas in discharge manifold 23 from flowing into the suction manifold 30 while permitting the pressurized gas obtained from the cylinder during the loaded condition thereof to discharge into the discharge manifold 23.

When control valve 55 communicates discharge bleed passage 51 with suction passage 49 through control valve chamber 46, the reduced pressure in piston chamber 39 is insufficient to overcome spring 35. Accordingly, piston 34 is retracted. The compressor cylinder is therefore unloaded due to the communication of discharge chamber 29 with suction chamber 30 by means of opening 33. Valve 72 of pressure different control means 75, under the influence of spring 73, interrupts passage 71.

The reduced pressure in the piston chamber 39 is effected by bleeding the gas from the compressor discharge manifold 23 into the suction manifold 30 through bleed passage 51, control valve chamber 46, and

passages

49, 50. The pressure of the compressor discharge gas is reduced because the rate of bleed through the

passages

49, 50 into suction manifold 30 is more than the rate of supply through the bleed passage 51. This reduced pressure is insufiicient to overcome the spring 35 which maintains pistons 34 in retracted position. Discharge pressure in the discharge manifold 23 (from the working cylinders) will close discharge check valve 24 isolating the compressor discharge manifold from the individual unloaded cylinder manifolds.

An increase in load on the compressor is reflected by an increase in pressure in the suction manifold 30. Diaphragm chamber 69 communicating with suction manifold 30 via

passages

49, 50 and opening 43 in control body 45 reflects a similar pressure increase. The increased pressure in chamber 69 moves diaphragm 66 against spring 67 whereby valve stem 56, under the influence of spring 64, moves control valve 55 toward closed position. It is understood that spring 64 is capable of overriding valve spring 62 holding control valve 55 against stop collar 61. The increase in suction manifold pressure required to permit diaphragm 66 to overcome spring 67 may be varied by adjustment of cap 68.

As control valve 55 moves toward valve seat 57 and closed position, the rate of bleed of discharge gas from bleed passage 51 into the suction manifold 30 through valve chamber 46 decreases with a resultant increase in pressure in control valve chamber 46. As noted valve 72 of pressure differential control means is closed. Accordingly, valve 55 alone regulates the rate at which discharge gas is bled into suction manifold 30.

As control valve 55 nears closed position, the build up of pressure in valve chamber 46 overcomes the force of valve control spring 62. Valve 55 is rapidly moved in a closing direction relative to valve stem 56 and snaps shut to interrupt the bleed of discharge gas through valve chamber 46 into suction manifold 30. The increased pressure in piston chamber 39 moves piston 34 against the urging of spring 35 to close bypass 33 thereby loading the cylinder.

At a predetermined valve chamber pressure, the force of spring 73 holding'valve 72 of pressure differential control means 75 closed is overcome to uncover passage 71 and bleed gas from the control valve chamber 46 into the suction manifold 30. By this arrangement a predetermined piston closing pressure may be maintained in valve chamber 46 and piston chamber 39. It is understood that the force of spring 73 and thus the control pressure in valve chamber 46 may be regulated by adjustment of screw means 74.

A decrease in compressor load is evidenced by a decrease in pressure in suction manifold 30. Diaphragm chamber 69, in communication with the suction manifold through

pasages

49, 50, opening 43 in control body 45, chamber 47 and passage 59, reflects a similar decrease. The diaphragm 66, in response to a decrease in suction manifold pressure, moves valve stem 56 in a valve opening direction against the force of spring 64. Control valve 55, under the influence of pressure in control valve chamber 46 remains closed. Movement of valve stem 56 brings stop 60 thereof into engagement with control valve 55. Continued movement of valve stem 56 in a valve opening direction is opposed by pressure in control valve chamber 46.

With a further decrease in compressor suction pressure, diaphragm 66, acting through valve stem 56 and stop 60, opens control valve 55. The initial slight opening of valve 55 permits relatively high pressure gas in valve chamber 46 to seep around control valve 55 through

passages

49, 59 into suction manifold 30. The seepage or leakage of relatively high pressure gas around control valve 55 into suction manifold 30 tends to reduce pressures in control valve chamber 46. Valve 72 of pressure differential control means 75, responsive to changes in pressure in control valve chamber 46, moves toward closed position to maintain the predetermined piston closing pressure in control valve chamber 46 and piston chamber 39.

The continued movement of control valve 55 in an opening direction by diaphragm 66 increases the rate of bleed of relatively high pressure gas in control valve chamber 46 into the suction manifold 30. As pressur in control valve chamber 46 falls below the predetermined pressure setting of differential control means 75, valve 72 thereof closes to interrupt the flow of gas from control valve chamber 46 through passage 71 into the suction manifold 30.

As pressure in control valve chamber 46, urging control valve 55 in a closed direction, decreases, movement of control valve 55 in an opening direction accelerates. As pressure in valve chamber 46 and accordingly in piston chamber 39 decreases with movement of control valve 55 in an opening direction, spring retracts piston 34 to open bypass 33 and unload the compressor cylinder, and control spring 62 moves control valve 55 against stop 61. Reduced pressure is maintained in piston chamber 39 by the flow of discharge gas through bleed passage 51 and control valve chamber 46 into suction manifold 38 in the manner described heretofore.

Valve stem 56, positioned in opening 43 in control body 45 communicating diaphragm chamber 69 with suction passage 49, restricts the effective size of opening 43. During movement of control valve in an opening direction, the bleed of relatively high pressure gas in control valve chamber 46 around valve 55 into suction passage 49 may result in a surge of pressure in suction passage 49. The reduced size of opening 43 effectively prevents this pressure surge from affecting diaphragm 66.

In the modification illustrated in FIGURES 4 and 5 of the drawings, wherein like numerals refer to like parts, bleed passage 80 communicates with control valve chamber 46 through passage 71 of pressure differential control means 75.

Connecting passages

82, 83 communicate bleed passage 80 with cylinder head passage 53 and discharge manifold 23. A piston type valve 85 movably disposed in passage 71 of pressure differential control means 75 regulates the flow of discharge gas through bleed passage 80 into control valve chamber 46 in a manner to be more particularly explained hereinafter.

Piston type valve 85 has an outer dimension slightly less than the dimension of passage 71 in pressure differential control means 75. The outer periphery of piston type valve 85 is provided with an annular groove 88. Passage 89 in valve 85 communicates groove 88 with con trol valve chamber 46.

With valve 72 of pressure differential control means 75 closed to interrupt passage 71, groove 88 of piston type valve 85 is disposed opposite bleed passage 80. It is understood that pressure in control valve chamber 46 urges valve 85 in an upward direction as viewed in FIGURES 4 and 5 of the drawings, valve 72 of pressure differential control means 75 limiting upward movement of valve 85. Movement of valve 85 in the opposite direction is limited by control valve 55.

During operation of the compressor 10, with piston 34 retracted as shown in FIGURE 2 of the drawings, the compressor cylinder is unloaded. An increase in pressure in suction manifold 30 indicative of increased load on the compressor, results in the closure of control valve 55 by diaphragm 66 in the manner described heretofore in connection with the embodiment illustrated in FIGURES 1-3 of the drawings.

Closure of control valve 55 interrupts the bleed of high pressure discharge gas from bleed passage through passage 89 in valve 85, chamber 46 and

passages

49, 50 to suction manifold 30 to increase pressure in piston chamber 39 whereby piston 34 interrupts bypass opening 33 to load the compressor cylinder.

Referring particularly to FIGURE 5 of the drawings, with control valve 55 closed, pressure in control valve chamber 46 increases. At a predetermined pressure spring 73, biasing valve 72 of pressure differential control means 75 closed, is overcome. Valve 72 opens to permit relatively high pressure gas in control valve chamber 46 to bleed through passage 71 and

passages

49, 50 into suction manifold 30. Piston type valve maintained in abutting relation with valve 72 by pressure in control valve chamber 46 moves with valve 72. Movement of valve 85 offsets groove 88 of valve 85 relative to bleed passage 80 to restrict communication of bleed passage 80 with control valve chamber 46 through passage 71 of pressure differential control means 75. Valve 85 and valve 72 of pressure differential control means 75 cooperate to establish and maintain predetermined piston closing pressure in valve chamber 46. With the establishment of an equilibrium condition, a relatively small amount of discharge gas bleeds around valve 85 and valve 72 into suction manifold 30.

At a predetermined decrease in compressor load, control valve 55 is opened in the manner described heretofore in connection with the embodiment illustrated in FIGURES 1-3 of the drawings to permit relatively high pressure gas in the control valve chamber 46 to bleed around control valve 55 and through passages 49-50 into suction manifold 30. The reduction in pressure in control valve chamber 46 permits spring 35 to retract piston 34 to open bypass 33 and unload the compressor cylinder.

Referring to FIGURE 4 of the drawings, movement of control valve 55 in an opening direction reduces pressure in control valve Chamber 46. Valve 72 of pressure differential control means 75, responsive to changes in pressure in control valve chamber 46, moves toward closed position. Piston type valve 85 is similarly moved to bring groove 88 thereof opposite bleed passage St). The bleed of discharge gas from passage 80 through passage 89 in valve 85 and control valve chamber 46 into suction manifold 30 reduces pressure control valve chamber 46 whereby spring 35 retracts piston 34 to unload the cylinder.

While I have described a preferred embodiment of the invention, it will be understood that the invention is not limited thereto, since it may be otherwise embodied in the scope of the following claims.

I claim:

1. In a control arrangement for a fluid compressor having suction and discharge sides with partitioning means therebetween having an opening therethrough for communicating the compressor discharge side with the suction side, and pressure actuated piston means for interrupting the partition means opening, the combination of a bleed passage serving to impose compressor discharge pressure against said piston means to move said piston means relative to said partitioning means opening, and means for controlling the passage of discharge gas through said bleed passage to control said piston means, said controlling means including first and second passages for communicating said bleed passage with said compressor suction side, valve means for regulating flow of gas through said first passage in response to pressure conditions between said bleed passage and said piston means, and valve means for regulating flow of gas through said first passage valve means including a first valve operable at a predetermined pressure to bleed gas through said first passage into said suction side; and a second valve operable at said predetermined pressure to reduce communication between said bleed passage and said piston means.

2. In a control arrangement for a fluid compressor having suction and discharge sides with partitioning means therebetween having an opening therethrough for communicating the compressor discharge side with the suction side, and pressure actuated piston means for interrupting the partition means opening, the combination of a bleed passage serving to impose compressor discharge pressure against said piston means to move said piston means relative to said partitioning means opening, and mean for controlling the passage of discharge gas through said bleed passage to control said piston means, said controlling means including first and second passages for communicating said bleed passage with said compressor suction side, valve means for regulating flow of gas through said first passage in response to pressure conditions between said bleed passage and said piston means, and valve means for regulating flow of gas through said second passage in response to suction side conditions, said second passage means including a valve; a movable valve stem, said valve being disposed on said valve stem for limited movement relative thereto; bias means urging said valve in an opening direction; and actuator means for said valve stem operable in response to a predetermined suction side pressure to move said valve stem in a closing direction to reduce the flow of gas through said second passage to increase pressure between said bleed passage and said piston means, a determined increase in pressure between said bleed passage and said piston means overcoming said valve biasing means to rapidly move said valve in a closing direction relative to said valve stem to interrupt the flow of gas through said second passage to move said piston means.

3. In a compressor having a cylinder including a movable piston therein, a suction manifold, a cylinder head, a discharge manifold, a partition in the cylinder head including an opening for communicating the discharge and suction manifolds to unload the cylinder, and movable piston means including a pressure chamber for closing the partition opening, the combination comprising a restricted passage communicating said discharge manifold with said pressure chamber to move said piston means in a first direction to load said cylinder, means for controlling pressure in said pressure chamber including a first passage for bleeding discharge gas from said pressure chamber into said suction manifold, and first valve means for regulating the bleed of gas through said first passage in response to pressure conditions in said pressure chamber to maintain a predetermined pressure in said pressure chamber while said cylinder is loaded, and means for reducing said predetermined pressure to move said piston means in a second direction to unload said cylinder and render said first valve means inoperative including a second passage for communicating said pressure chamber with said suction manifold and second valve means for regulating the flow of gas through said second passage in response to changes in suction manifold pressures, said second valve means including a valve between said second passage and said pressure chamber, a movable operator for said valve, said valve being positioned on said operator for limited movement relative thereto, means biasing said valve in an opening direction, and actuator means for moving said operator in response to changes in suction manifold pressure, so that on a predetermined change in suction manifold pressure, said actuator means moves said operator in a valve closing direction whereby said valve reduces flow of gas through said second passage to increase pressure in said pressure chamber, a predetermined increase in pressure in said pressure chamber overcoming said valve biasing means to quickly move said valve relative to said operator to interrupt communication between said second passage and said pressure chamber.

References Cited UNITED STATES PATENTS 2,522,762 9/1950 Neeson 23031 2,673,025 3/1954 Labus et al 23031 2,715,992 8/1955 Wilson 230-31 3,021,790 2/1962 Brunson 10342 3,119,550 2/1964 West et al. 230-22 3,184,151 5/1965 Shaw 2303l DONLEY J. STOCKING, Primary Examiner.

MARK NEWMAN, MARTIN P. SCHWADRON,

Examiners.

W. I. KRAUSS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,385,508 May 28, 1968 David N. Shaw It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 40, "of" should read or Column 6,

line :62 before "control" insert in Column 7 line 10, after "said" insert second passage in response to suction side conditions, said Signed and sealed this 4th day of November 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

Claims

1. IN A CONTROL ARRANGEMENT FOR A FLUID COMPRESSOR HAVING SUCTION AND DISCHARGE SIDES WITH PARTITIONING MEANS THEREBETWEEN HAVING AN OPENING THERETHROUGH FOR COMMUNICATING THE COMPRESSOR DISCHARGE SIDE WITH THE SUCTION SIDE, AND PRESSURE ACTUATED PISTON MEANS FOR INTERRUPTING THE PARTITION MEANS OPENING, THE COMBINATION OF A BLEED PASSAGE SERVING TO IMPOSE COMPRESSOR DISCHARGE PRESSURE AGAINST SAID PISTON MEANS TO MOVE SAID PISTON MEANS RELATIVE TO SAID PARTITIONING MEANS OPENING, AND MEANS FOR CONTROLLING THE PASSAGE OF DISCHARGE GAS THROUGH SAID BLEED PASSAGE TO CONTROL SAID PISTON MEANS, SAID CONTROLLING MEANS INCLUDING FIRST AND SECOND PASSAGES FOR COMMUNICATING SAID BLEED PASSAGE WITH SAID COMPRESSOR SUCTION SIDE, VALVE MEANS FOR REGULATING FLOW OF GAS THROUGH SAID FIRST PASSAGE IN RESPONSE TO PRESSURE CONDITIONS BETWEEN SAID BLEED PASSAGE AND SAID PISTON MEANS, AND VALVE MEANS FOR REGULATING FLOW OF GAS THROUGH SAID FIRST PASSAGE VALVE MEANS INCLUDING A FIRST VALVE OPERABLE AT A PREDETERMINED PRESSURE TO BLEED GAS THROUGH SAID FIRST