CA2090390A1

CA2090390A1 - Fail safe mechanical oil shutoff arrangement for screw compressor

Info

Publication number: CA2090390A1
Application number: CA002090390A
Authority: CA
Inventors: Jerome C. Roach
Original assignee: Wabco Standard Trane Inc
Current assignee: Individual
Priority date: 1992-08-07
Filing date: 1993-02-25
Publication date: 1994-02-08
Also published as: DE4325200A1; GB9304508D0; ITRM930539A1; GB2269424A; ITRM930539A0; JPH06108982A; IT1262389B; FR2694602A1

Abstract

Title FAIL SAFE MECHANICAL OIL SHUTOFF ARRANGEMENT
FOR SCREW COMPRESSOR

Inventor JEROME C. ROACH

Abstract Apparatus as disposed internal of a refrigeration screw compressor which, upon compressor shutdown, shuts off the flow of oil injected into the compressor's working chamber and the oil which is directed to the compressor rotor bearings and which at compressor startup opens oil flow to those locations by the use of ambient internal compressor conditions that inherently exist at those respective times. The operation of the apparatus is therefore "fail safe" and the need for external oil flow cutoff valving and the need to monitor and/or prove oil flow within the compressor is eliminated.

Description

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D E S C R I P T I O N

Title .
FAIL SAFE MECHANICAL OIL SHUTOFF ARRANGEMENT
FOR SCREW COMPRESSOR

Back round of the Invention .~ .
The present invention relates generally to the art of compressing a gas in an oil-injected rotary screw compressor. More specifically, the present invention relates to apparatus for isolating rotor bearing lubricant passages and the oil in~ection port, which opens into the working chamber of an oil inJected screw compressor, from their oil supply upon compressor shut down, Screw compressors employed in refrigeration systems , are comprised of complementary male and female sc~ew rotors i' disposed within a working chamber defined by a rotor housing.
The working chamber can be characterized as a volume generally shaped as a pair of parallel intersecting cylindrical bores and is closely toleranced to the outside length and diameter dimensions of the intermeshed screw rotor set~ The rotor housing has low and high pressure ends which define unvalved suction and discharge ports in open-Elow communication with the working chamber.
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In operatlon, refrigerant gas at suction pressure enters the working chamber via the suction port and is enveloped in a chevron shaped pocket i`ormed between che counter-rotating screw rotors. The pocket closes, its volume :
decreases and it is displaced toward the high pressure end of the compressor as the rotors meshingly rotate within the working chamber. The gas within such a pocket is compressed by virtue of the decreasing volume in which it is contained until the pocket opens to the discharge port at the high pressure end of the working chamber where it is expelled through the discharge port.
Due to the extremely close tolerances between the rotor set and the walls of the working chamber, the bearing arrangement in which the rotor set is mounted is critical to compressor operation and }ife. This is particularly true because the bearings and rotors in a screw compressor are ~ub;ect to high and variable axial and radial loads.
Protection and lubrication of rotor bearings is therefore o paramount concern in the design and operation of rotary screw compressors.
In addition to being delivered to the rotor bearings, oil is in many instances injected into the working chamber of a screw compressor through an injection port to perform several functions. First, the oil in~ec~ed into the working chamber acts as a sealant between the rotors and the surfaces of the working chamber in which the rotors are dlspo~ed, .:

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The oil also acts as a lubricant becween the driving and driven screw rotor. In that regard, one of the two screw rotors is driven by an external source, such as an electric ~otor, while the other rotor is driven by virtue of its meshing relationship with the motor-driv~n rotor. Oil -injected into the working chamber of the compressor therefore acts to prevent excessive wear between the driving and driven rotors.
Finally, injected oil is used to cool the refrigerant undergoing compression within the working chamber which in turn reduces the thermal expansion of the rotors that :~
would otherwise occur as a result of the heat generated by the compression process. Such injection cooling therefor permits tighter rotor to housing clearances from the outset.
At compressor shut down, when the drive motor is de-energized, the backflow of discharge pressure gas from the high (downstream) side of the refrigeration system in which a screw compressor is employed back through the compressor discharge port, if allowed to occur, causes the high speed reverse direction rotation of the no longer driven screw rotors within the working chamber and causes the compressor to act as an expander with respect to gas downstream of the discharge port. Such reverse direction freewheeling of che rotors can occur at speeds greater than the maximum desLgn RPM of the ;`
rotor set for normal operation. `
Additionally, to the extent gas bAckPlow is cutoPf at shutdown, such as by a chock valve arrangcment, the initial rush of downstream discharge pressure gas back through the compressor toward the low pressure side of the refrigeration ..
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system may still be sufficient to cause the pressure at the suction end of the compressor to exceed that which exists immediately downstream of the discharge port. This situation can occur when the compressor, acting as an expander in its reverse direction rotation, pumps against the closed discharge : check valve, and can result in the developmerlt of large axial forces on the screw rotor set and rotor bearings in a direction opposite that which is normally enc~untered and compensated for :~
during compressor operation.
; 10 Also, many screw compressor bearing lubrication i: schemes are predicated on the development and maintenance of relatively high pressure downstream of the compressor which is used to drive lubricating oil from a sump or reservoir to the rotor bearings and/or in;ection port. The high speed reverse rotation of the rotor set at compressor shutdown and momentary ; development of relatively higher pressure at the upstream or low side end of the working chamber, i allowed to occur, ~ could, under some circumstances, cause oil to be sucked from :~ the bearings or not to be delivered to tha be~rings Ln :~ 20 sufficient quantity with potentially catascrophic results.
Finally, unless the oil injection port opening into the working chamber of a screw compressor is isolated from its ~ typically pressurized oil supply upon compressor shutdown, oil will continue to flow through the injection port into the working chamber after shutdown, until the system pressures equalize, by virtue of the pressure differential which exists between the oil supply and the working chamber at compressor shutdown. Absent means for reliably isolating the oil in~ection port ~rom Its oil supply under such circumstances, : ' . ':
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the worklng chamber can become flooded with oil. As a result, the compressor lubrication system can become starved for oil due to the dislocation of the oil supply from the oil sump to the working chamber and insufficient oil may be av~ilable for delivery to the necessary locations within the compressor when the compressor next starts with potentially catastrophic results.
; The need, therefore, continues ~o e~ist for a fail safe arrangement i'or preventing the continued flow of oil to the bearings and through the injection port into the working `
chamber of a refrigeration screw compressor upon compressor shut down and for permitting such oil flow at compressor startup.

Summar~ of the Invention It is an object of the present invention to isolate the bearing lubrication passages and the oil injection port which opens into the working chamber of a screw compressor from their oil supply upon compressor shutdown in a manner which is : ~
actuated by the existence of discharge pressure gas immedlately :.
~, ~ downstream of the compressor's working chamber when the `
I compressor is in operation.
: A urther ob~ect of the present invention is to provide an arrangement which, by the act of compressing gas and ;, discharging it i'rom the compressor's working chamber upon "~' compressor s~art up, immcdiately and mechanically places ~he 1. `
,I bearing lubricati.on passAges and oil in~ection port into 10w'I communiFation wlth their oil supply. ~
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It i5 also an object of the present invention to provide mechanical apparatus for closing the bearing lubrication passages and oil injection port of a screw compressor immediately upon compressor shutdown and for opening them i~mediately upon startup in a manner which, by its use of , ambient conditions which are inherent within the compressor at ; those respective times, is "fail safe" and eliminates the need for external check valves, solenoid valves or sensors to : "prove" oil flow within the compressor.
These and other objects of the present invention, which will become apparent when the Drawing Figures and the Description of the Preferred Embodiment hereof are considered, are accomplished by apparatus disposed within a screw compressor which shuts off the flow of injection and beflring ` 15 lubrication oil in the compressor ac compressor shutdown and which permits flow to occur at compressor startup by the use of the internal pressure differentials and gas flow which are inherent in the compressor and its operation at those respective times.
Discharge pressure, which exists immediacely ,~ downstream of the compressor's discharge port when the compressor is in operation, is used to position a spool valve against internal compressor suction pressure to a position ' which permits the flow of lubricating oil from an oil supply to ~:
bearing locations and to the oil injection port opening into the compressor's working chamber. At compressor shutdown the backflow of discharge preasure gas to the compressor's working chamber closes an internal discharge check valve causing an immediate pressure differential to develop across the spool .

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valve. The pressure dLfferential operates to posi~ion the spool valve to isolate the oil supply from the bearings and in;ection port. Upon compressor startup discharge pressure develops downstream of the compressor's working chamber and :-acts on the spool valve causing it to be positioned to permit oil flow within the compressor so that oil is immediately directed to the bearings and oil injection port.
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Brief Description of the Drawin~ FiQures ~
, . ~: , - Figure 1 is a cross sectional view of the compressor of the present invention and its schematic disposition in a refrigeration system.
Figure 2 is an enlarged partial view of the oil ~;
; 15 shutoff valve installation in the co~pressor of Flgure 1.

DescriPtion of the Preferred Embodiment , , ':
Referring concurrently to Drawing Figures 1 and 2, : .
refrigeration system 10 is comprised of a compressor housing assembly 12, condenser 14, expansion valve 16 and evaporator 18 all of which are serially connected to form a hermetic closed loop refrigeration system. Rotor housing 20 of compressor ;~
assembly 12 houses a pair of screw rotors one of which, rotor 22, is illustrated. The rotor set is disposed ln working chamber 24 of the rotor housing which further defines a suction port 26 and discharge port 28 which are, respectively, the cntry and exit lo~atlons for refrigerant gas passing through the working chamber during compressor operation.

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Rotor 22, in the embodiment of Figure 1, is the driven one of ~he pair of screw rotors and is mounted for rotation within the rotor housing in bearings 30 and 32. Rotor 22 has a shaft 34 extending from one of its ends which is driven by motor 36. Bearing housing 38 of the compressor assembly i5 attached to the discharge end of ro~or housing 20 and serves to house bearing 32 and to close the discharge end of the working chamber.
Bearing housing 38 defines a discharge passage 40 in flow communication with discharge port 28 which channPls ~ dischar~e gas out of the compressor assembly. Discharge ; passage 40 is also in flow communication with oil separator 42 in which lubricant, which has been carried out of compressor housing assembly 12 in the discharge gas stream, is separated from the discharge gas prior to the use of that gas in the refrigeration system.
It is to be noted that a relatively large amount of oil is typically carried out of the compressor~s working chamber in the discharge gas stream in an oil-injected screw compressor and that as much of that entrained oil as possible must be removed from the refrigerant gas ~o as not to degrade downstream refrigeration system performance and to ensure that sufficient lubricant continues to be available ~o the compressor.
Disposed in discharge passage 40 is a discharge ; check valve member 41. While check valve member 41 in Figure l is illustrated as being a spherical member trapped in volume 43 against open spider 45, it will be appreciated chat a very large number and variety of discharge check valve arrangements are coneemplated within the scope of the present invent_on.

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The discharge check valve assembly may be disposed in the bearLng housing or in the discharge piping which connects che compressor assembly to the oil separator. It must, however, serve to isolate the compressor's working chamber from the oil ~1 sump 44 upon compresscr shutdown. ~ .:
;~ Compressor assembly 10 defines a plurality of oil ;~
passages including lubrication passages 46 and 48 which communicate with the bearings that support the screw ~otors -;
: within the compressor assembly and with an oil injection passage 50 which opens into the compressor's working chamber.
In the smbodiment illustrated in Figures 1 and 2, all three passages flow into common oil supply passage 52.
Oil supply passage 52 is in flow communication with sump 44 oi oil separator ~2. It is to be noted that oil separator 42 and swnp 44 may be integral to the campressor :`~
` assembly and that sump 44 might communicate with supply passage 50 via passages which are entirely internal of the compressor assembly in such instances. Also, oil sump 44 may be physically removed and in a vessel separate from oil separator ' 20 42. Once again, however, some means for preventing ~as ; backflow to the working chamber at compressor shutdown must bedisposed between the working chamber and oil separator/sump wherever the separatorjsump may be located.
Interposed in oil supply passa~e 52 in rotor : 25 housing 20 is a volume 54 in which a valve member 56 is disposed. Volume 54, ln addition to being in flow eommunication with oil supply passage 52 and ther~Eore, internal eompressor oil passages 46, 48 and 50, i9 in tlow '.':1 ~;
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communlcation with an area in ~he compressor assembly which is at a pressure less than discharge pressure and an area within the compressor assembly which, when the compressor is in operaeion, is at high side or discharge pressure.
S In that regard, volume 54 communicates through a passage 58 to area 60 which is a volume within rotor housing 20 that is at suction pressure during compressor operation. Area 60 is in flow communication with suction port 26 within the compressor assembly and is, in effect, upstream thereof within the refrigeration system.
As was indicated above, area 60, rather than being an area of the compressor which is at suction pressure, can be an area within the compressor which is at an intermediate pressure. Area 60 will, however, always be an area which is at less than discharge pressure when the compressor is in operation. Volume 54 also opens into area 62 within which ls an area immediately downstream of discharge port 28 that is at dlscharge pressure. Area 62 is therefore on the high side of the refrigeration system when the compressor is in operation.
It will be appreciated that valve 56 is slideably .
disposed for axial movement within volume 54 between a first position, illustrated in Figure 1, in which oil is permitted to flow through passage 52 and chamber 54 to oil passages 46, 48 and 50 around relieved portion 64 of valve member 56 and a second position, illustrated in Figure 2, in which an unrelieved portion of valve 56 blocks the flow oi` oil through chamber 54. Valve 56 is posicioned to the posi~Lon Lllustrated ln Figure 1 by the exposure of its high side end face 66 to the discharge pressure which exists in discharge pressure area 62 whenever the compressor is in operation.

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Low side end face 68 of valve 56, on the other hand, is exposed, as earlier mentioned, to an area of the compressor which is at low side or suction pressure through passage 58. The high to low side pressure differentiaI across valve 56 which exists whenever the compressor is in operation .
ensures that valve 56 is positioned to permit oil flow through chamber 54, as is illustrated in Figure 1, at all times during compressor operation. This assures, in a fail safe manner which relies on an operating condition which is inherent in the ~ `
compressor when it is in operation,- that oil is permitted to ; ;~
flow fro~ sump 44 to the oil injection port and to the ~ compressor bearings whenever the compressor is operating.
; ~pon de-energization of motor 36 the compressor is shut down and previously compressed discharge pressure gas will immediately flow back to the working chamber of the de- ~ ;
energlzed compressor from downstream thereof. The immediate effect of the backflow of the discharge pressure gas is to carry check valve member 41 to the position in which it is illustrated in phantom in Figure 1. ~
As soon as check valve member 41 seats in the ;`
phantom position illustrated in Figure l, the backflow of previously compressed gas from downstream of the compressor to the working chamber will stop. The immediate initial backflow of gas to the working chamber prior to the discharge check valve having seated will, however, have caused the rotors to begin to rotate in a direction opposite the direction they are caused to rotate in operation by motor 36.

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This reverse rotation of the ro~ors has the effect of evacuating 8as from discharge area 62 as soon as valve member 41 seats and of lowering the pressure in that area to a pressure ~hich is less than system low side pressure. This is because the rotors, which function as a gas expander by virtue of their reverse direction rota~ion, act to pump gas from the discharge area against the closed discharge check valve 41 under this condition when it is in its backflow preventing position.
As che pressure in discharge pressure area 62 drops under these circumstances the pressure on high pressure end 66 of valve 56 quickly drops to a pressure which is less than the low side pressure in suction area 60. Under tha~ circumstance the pressur~ on low side end face 68 of the spool valve will b~ ~
greater than the pressure on the high end side face 66 oE the .
valve and the pressure differential across the valve will act to move the valve into the position illustrated in ~igure 2.
Once again, it will be appreciated that an ambien~ condition ~ inherent i~ the compressor at a particular point in ics .`
;, 20 operation is used to cause oil flow passage 52 to be closed to `i flow at an appropriate time.
It will be noted fro~ Figure 2 that valve 56 may be biased by spring 70 toward the Figure 2 position in which an unrelieved portion of valve member 56 occludes oil supply passage 52. It will also be noted that a retainer ring 72 is disposed in volume 54 and protrudes thereinto per~itting valve 56 to travel no Eurther within volume 54 than to the position illustrat~d in Figure 2. While spring 70 is not ~andatory, it `i will preierably be used since in addition to assisting the ~:
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movemen~ of valve 56 to the position in which oil ~low is ~
prevented upon compressor shutdown it assists ln maintaining .~ ~;
the valve in that position as conditions in discharge area 62, which are somewhat transient by nature at compressor shutdown, assume a steady state condition.
. When the compressor next starts up subsequent to ;~ having been shutdown, the compression of gas between the screw . :~
; rotors will immediately commence and discharge pressure will quickly build in discharge pressure area 62 causing valve 56 to be urged in~o the position illustrated in Figure 1 in which oil ; supply passage 52 is open to flow. Pressure will concurrently build up in oil separator 42 which will cause oil to flow from sump 44 through oil supply passage 52 to the compressor .
bearings and oil in~ection port.
~ 15 It will be appreciated that since the oil shutofP
:' arrangement of the present invention Ls mechanical and fail safe, relying on inherent internal compressor operating conditions for aceuation at appropriate times, the need for monitoring the position oi' the shutoff valve and/or the need to l 20 "prove" oil ~low to the compressor bearings and oil injection :~
.' port at compressor startup is avoided. The arrangement of the present i~vention likewLse eliminates the need for electrical or electronic sensing and/or monitoring with respect to oil ,~
., flow during compressor operation and, wieh respect to some systems, the need to employ a relatively expensive solenoid operated valve, which is sub~ect to electrical failure, in the ;~, compressQr oil supply line, I What is claimed is:
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Claims

1. A rotary screw refrigerant gas compressor comprising:
a housing defining a working chamber, said housing further defining a suction port, a discharge port and an oil supply passage, all in flow communication with said working chamber, said housing further defining an oil flow cutoff passage in flow communication with said oil supply passage and with an area in said compressor downstream of said discharge port which is at compressor discharge pressure when said compressor is in operation; and a pair of screw rotors meshingly disposed for rotation in said working chamber; and valve means disposed within said compressor and positionable to (i) occlude said oil supply passage to prevent the flow of oil therethrough and to (ii) open said oil passage to permit the flow of oil therethrough in direct response to ambient conditions in said compressor downstream of said discharge port which inherently exist at compressor shutdown and startup respectively.

2. The screw compressor according to claim 1 wherein said valve means is comprised of means for stopping the backflow of said refrigerant gas to said working chamber from downstream thereof immediately subsequent to compressor shutdown; and, a discrete valve member disposed in said oil flow cutoff passage.

3. The screw compressor according to claim 1 wherein said oil flow cutoff passage is also in flow communication with an area in said compressor which is at a pressure less than compressor discharge pressure when said compressor is in operation.

4. The screw compressor according to claim 3 wherein said valve member has an upstream surface exposed to the pressure which exists in said area which is at less than discharge pressure when said compressor is in operation and a downstream surface exposed to pressure in said area downstream of said discharge port, the position of said valve means being dependent upon the pressure differential across said valve member.

5. The screw compressor according to claim 4 wherein the stoppage of gas backflow subsequent to compressor shutdown causes the pressure in said area downstream of said discharge port to become less than pressure on said upstream surface of said valve member so that said valve member is urged by the pressure on said upstream surface into said position in which said oil supply passage is occluded and wherein, upon compressor startup, pressure in said area downstream of said discharge port is caused to increase due to the compression of said gas in said working chamber to an extent such that the pressure in said area downstream of said discharge port and on said downstream valve member surface surpasses the pressure on said area upstream valve member surface thereby urging said valve member into a position in which oil flow through said oil supply passage is permitted.

6. The screw compressor according to claim 5 wherein said valve member is a spool valve defining a relieved portion between said upstream surface and said downstream surface, the flow of oil through said oil supply passage being permitted by the registry of said relieved portion of said spool valve with said oil supply passage internal of said compressor and the flow of oil through said oil supply passage being prevented by the movement of said spool valve to a position in said oil cutoff passage in which said relieved portion is out of registry with said oil supply passage.

7. The screw compressor according to claim 6 further comprising means for mechanically biasing said spool valve to a position within said oil cutoff passage in which the flow of oil through said oil supply passage is prevented.

8. The screw compressor according co claim 6 wherein said area which is less than discharge pressure when said compressor is in operation is at suction pressure when said compressor is in operation.

9. A screw compressor comprising:
a housing defining a working chamber, an oil supply passage and an oil cutoff passage, said housing further defining a suction port and a discharge port in flow communication with said working chamber, said oil cutoff passage being in flow communication with said oil supply passage and communicating between an area in said compressor downstream of said discharge port and an area in said compressor which is at a pressure less than compressor discharge pressure when said compressor is in operation, said downstream area being in flow communication with said discharge port;
a pair of screw rotors meshingly disposed in said working chamber;
means for driving one of said screw rotors;
means for stopping the backflow of gas to said working chamber from downstream thereof immediately subsequent to the stoppage of said means for driving one of said screw rotors motor; and valve means disposed in said first passage, said valve means having an upstream surface exposed to pressure in said area which is at less than discharge pressure when said compressor is in operation and a downstream surface exposed to pressure in said area downstream of said discharge port, said valve means being positionable to occlude the flow of oil through said oil supply passage by the pressure differential which develops across said valve means subsequent to the operation of said means for stopping backflow to stop gas backflow.

10. The compressor according to claim 9 wherein said screw rotors are mounted in bearings for rotation in said working chamber and wherein said oil supply passage is in flow communication with said bearings and an injection port opening into said working chamber.

11. The compressor according to claim 10 wherein said valve means is a spool valve mechanically biased to occlude said oil supply passage, said mechanical biasing being overcome, so that said spool valve is positioned to permit flow through said oil supply passage, by the development of pressure in said area downstream of said discharge port which is sufficient to overcome the mechanical bias and the effect of the pressure on the side of said spool valve which is exposed to said area which is at less than discharge pressure when said compressor is in operation.

12. What is claimed is a screw compressor-based refrigeration system comprising:
an oil supply;
a condenser;
an expansion valve;
an evaporator; and a screw compressor, said compressor, condenser, expansion valve and evaporator being serially connected to form a hermetically closed refrigeration system having a high pressure side, downstream of said compressor and upstream of said expansion valve, and a low pressure side, downstream of said expansion valve and upstream of said compressor, said compressor having locations which are at a pressure intermediate compressor discharge pressure and compressor suction pressure when said compressor is in operation said compressor (i) defining a working chamber in which a pair of screw rotors are disposed, an oil supply passage in flow communication with said oil supply, a suction port in open flow communication with said working chamber and with said low pressure side of said system, a discharge port in open flow communication with said working chamber and said high pressure side of said system, an oil cutoff passage in flow communication with the high pressure side of said system and said oil supply passage; and (ii) having valve means disposed in said cutoff passage, said valve means being positioned to prevent flow through said oil supply passage by the ambient conditions which inherently exist internal of said compressor immediately subsequent to compressor shutdown and being positioned to permit the flow of oil through said oil supply passage by the ambient conditions which inherently exist internal of said compressor immediately subsequent compressor startup.

13. The screw compressor-based refrigeration system of claim 12 wherein said valve means is comprised of means for stopping the backflow of refrigerant gas to said working chamber from downstream thereof immediately subsequent to compressor shutdown; and, a discrete valve member disposed in said oil cutoff passage.

14. The screw compressor-based refrigeration system according to claim 13 wherein said oil flow cutoff passage is in flow communication with one of said intermediate pressure locations in said compressor; and, wherein said valve member has an upstream surface exposed to pressure in said one of said intermediate pressure locations in said compressor and a downstream surface exposed to pressure in said high pressure side of said refrigeration system, the position of said valve member being dependent upon the pressure differential across said valve member.

15. The screw compressor-based refrigeration system according to claim 14 wherein the stoppage of gas backflow subsequent to compressor shutdown causes the pressure to which said downstream surface of said valve member is exposed to become less than the pressure to which said upstream valve member surface is exposed so that said valve member is urged by the pressure on said upstream surface into a position which prevents the flow of oil through said oil supply passage.

16. The screw compressor-based refrigeration system according to claim 15 wherein said valve member is a spool valve defining a relieved portion between said upstream surface and said downstream surface, the flow of oil through said oil supply passage being permitted by the registry of said relieved portion of said spool valve with said oil supply passage and the flow of oil through said oil supply passage being prevented by the movement of said spool valve to a position in said oil cutoff passage in which said relieved portion is moved out of registry with said oil supply passage.

17. The screw compressor-based refrigeration system according to claim 16 further comprising means for mechanically biasing said spool valve to a position within said oil cutoff passage in which the flow of oil through said oil supply passage is prevented.

18. The screw compressor-based refrigeration system according to claim 13 wherein said oil flow cutoff passage is in flow communication with said low pressure side of said refrigeration system; and, wherein said valve member has an upstream surface exposed to pressure in said low pressure side of said refrigeration system and a downstream surface exposed to pressure in said high pressure side of said refrigeration system, the position of said valve member being dependent upon the pressure differential across said valve member.

19. The screw compressor-based refrigeration system according to claim 18 wherein said valve member is a spool valve which defines a relieved portion between said upstream surface and said downstream surface, the flow of oil through said oil supply passage being permitted by the registry of said relieved portion of said spool valve with said oil supply passage and the flow of oil through said oil supply passage being prevented by the movement of said spool valve to a position in said oil cutoff passage in which said relieved portion is moved out of registry with said oil supply passage.

20. The screw compressor-based refrigeration system according to claim 19 further comprising means for mechanically biasing said spool valve to a position with said oil cutoff passage in which the flow of oil through said oil supply passage is prevented.