Injection cooling of screw compressors

Abstract

A liquid injection expansion valve permits liquid refrigerant at high pressure, bled from the condenser, to be injected into the working chamber between the screws and intermediate of the suction and discharge sides of a helical rotary screw compressor for cooling the refrigerant working fluid and the captured oil. A solenoid valve limits bleeding of liquid refrigerant from the condenser at high compressor loads. A thermostat sensing the temperature of the screw compressor discharge, modulates the liquid injection expansion valve downstream of the liquid injection solenoid valve. A compressor unloader slide valve may port oil and the liquid refrigerant into the working chamber. The condenser may be positioned at a height considerably above that of the screw compressor to increase the head of the bled liquid refrigerant to a pressure higher than the screw compressor discharge pressure. system oil pressure may be supplied to a fluid pressure operated, direct acting, on-off control valve upstream of the liquid injection expansion valve and within the bleed line, under control of a solenoid valve which is responsive to the temperature of the oil leaving the oil pump, where such temperature is proportional to compressor loading.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to helical, rotary screw compressors, and more particularly, to a simplified system of liquid injection to control and limit the discharge temperature of the refrigerant working fluid and the lubricating oil carried thereby.

2. DESCRIPTION OF THE PRIOR ART

In general, compressors are pumps that are used to raise gases or refrigerant from one pressure level to a higher pressure level. In the process, the vapor or gas is superheated by the work of compression. Through thermodynamic relationships, operating temperatures can be predicted by applying isotropic or polytropic compression processes. With all types of compressors, the higher the compression ratio

[compression ratio=(discharge pressure/section pressure)]

the higher the discharge temperature that will be reached.

It is desirable to control and limit discharge temperatures so that dangerous levels are not reached that may injure components and lubricants and shorten their useful life. In the past, many methods have been employed to inject fluids into gas streams for the purpose of cooling or limiting compressor discharge temperature. In air compressors, water mist has been sprayed into the compression area, which vaporizes during compression to thereby limit temperatures. In other compressors, oil injection has been used to accomplish lower discharge temperature.

Attempts have also been made to inject liquid refrigerant into the refrigerant vapor or working fluid as it is being compressed. This has been accomplished by injecting liquid refigerant or refrigerant rich oil into the suction side of the compressor where the refrigerant evaporates and reduces the net inlet suction volume of the compressor, decreasing the capacity of the compressor. Attempts have further been made to add liquid to the gas discharge from the compressor.

In the conventional systems employing axial screw compressors, the need for oil cooling limits the discharge temperatures that the system can tolerate. In systems with water cooled oil coolers, the range of operation is usually established by water temperatures available. In the case of minimizing the use of water or in air cooled systems due to the ambient temperature of the air, there is a problem in maintaining tolerable discharge temperatures. One way of maintaining the discharge temperatures is through the use of liquid injection along with some oil injection. In such a case, location of the port for the liquid injection is critical in that, if the liquid injection port is on the inlet or suction side of the machine, the effect of the liquid being injected greatly affects the volumetric efficiency of the compressor due to the fact that the liquid will expand, flash off as it hits the low pressure environment. There is also an effect on the horsepower requirements of the machine, because the expanded liquid then is in gas form and goes through a pressure range change and exits at the machine discharge pressure.

If the point of liquid injection occurs as the discharge side of the machine or after the gas has actually left the compressor discharge area, not only is the pressure condition at the system highest and thus there is an inherent requirement for an external pump to pressurize the liquid to be injected above the maximum compressor pressure of the system, but the added unit constitutes an extra component which adds to the cost of the unit, and affects the reliability of the system. Most importantly, where compressive systems are designed hermetically, there is a rather confined distance from the point of compressor gas discharge from the screw compressor itself to where the same gas contacts and envelops the motor winding of the hermetic electric motor and the space between the two does not provide sufficient time or room for the liquid injected at this point to properly expand and cool the discharge gas prior to entering the motor compartment.

Conventionally, lubricating oil has been injected into the working chamber, that is, the space occupied by the intermeshed screws in a helical rotary screw compressor, for the dual purposes of lubricating the intermeshed screws and to provide the necessary seals between the rotating screws and the stationary housing. Further, since the load on the compressor varies at times between relatively large limits, the capacity of helical rotary screw compressors has been modified by incorporating a capacity control slide valve within the rotor housing and slidable parallel to the axis of the screw. Axial movement of the valve is programmed by a solid state, temperature initiated hydraulic actuated control arrangement. The slide valve shifts longitudinally between limits with the slide valve in closed position and against a valve stop when the compressor is fully loaded, in which case all the gas flows through the rotor housing from the intake to the discharge side of the screw compressor. Unloading is achieved by moving the valve away from the valve stop to create an opening within the rotor housing through which the suction gas can return to the inlet port area before compression of the same. Thus, in principle, enlarging the opening in the rotor housing effectively reduces compressor displacement. One mode of insuring that lubricating oil is injected into the working chamber and between the intermeshed screws has been to provide an axial passage in the mechanism connecting the slide valve to a reciprocating fluid motor and creating a closed chamber at the discharge side of the slide valve with one or more radial ports opening up into the working chamber downtream of the contact area between the end of the valve and the stationary valve stop. In this case, as the slide valve opens to reduce the capacity, oil injection occurs within the working chamber closer to the discharge side of the compressor.

It is, therefore, an object of the present invention to eliminate the necessity for a separate pump in liquid injection cooling of a screw compressor and to effect liquid injection cooling of a screw compressor without materially affecting the volumetric capacity of the compressor or increasing the horsepower required.

SUMMARY OF THE INVENTION

In general, the objects of the present invention are met in conjunction with a screw compressor operating as a component within a refrigeration system wherein compressed gas is condensed to high pressure liquid within a condenser and expanded in an evaporator or chiller for cooling a refrigeration load, and then returned as a low pressure gas to the inlet side of the screw compressor. The invention involves bleeding high pressure refrigerant liquid from the condenser and directing it through a liquid injection expansion valve and into the screw compressor working chamber intermediate of the suction and discharge sides of the same. A thermostat sensitive to the compressor discharge temperature or to compressor load either variably controls the volume of bled liquid refrigerant which is injected into the screw compressor, or cycles the bleed line on and off. A solenoid valve may be interposed in the bleed line intermediate of the condenser and the liquid injection expansion valve, with the solenoid valve being under control of a thermostat bulb sensitive to the temperature of the oil, preferably at the discharge side of the oil pump and downstream of the screw compressor. In another embodiment, the condenser is located at a height considerably above that of the screw compressor so that the gravity head acts in conjunction with the normal high pressure of the liquid refrigerant within the condenser to provide liquid refrigerant within the bleed line at a pressure considerably above the discharge gas pressure of the screw compressor so that the refrigerant may be injected into the screw compressor at the discharge side of the same or slightly upstream of the discharge side.

A helical rotary screw compressor employs a capacity control slide valve within the rotor housing movable away from a fixed valve stop for reducing the capacity of the compressor. A chamber carried by the valve may be supplied the bled liquid refrigerant which is ported into the working chamber, downstream of the end of the slide valve which contacts the stop when the compressor is fully loaded. The liquid refrigerant chamber may be sealed from a second chamber of the capacity control slide valve which ports lubricating oil into the compressor working chamber; the refrigerant chamber preferably being downstream of the oil chamber. Coaxial oil and liquid refrigerant lines may be coupled to the respective sealed chambers of the capacity control slide valve from opposite ends, or by means of concentric by expansion valve 52 and only the amount of liquid refrigerant is injected into the compressor which is sufficient to maintain the gas discharge temperature within a predetermined range. Thus, as load increases, the discharge temperature of the working fluid increases and bulb 56 senses the demand for more liquid refrigerant to be injected into the screw compressor just upstream from the discharge side of the screws. Thus, as discharge gas temperature increases, the expansion valve 52 opens wider to deliver more liquid refrigerant through bleed line 46 to the compressor 26, and conversely as the load within the evaporator or chiller falls off, the compressor discharge temperature decreases and the expansion valve will throttle back to reduce the supply of liquid refrigerant delivered to the compressor. In the illustrated embodiment, it is not necessary to employ an external separate oil cooler. Since oil seeps into and forms a small part of the working fluid passing through the compressor, it must be separated therefrom prior to passing the refrigerant through the condenser and chiller or evaporator, since the presence of oil interferes with the heat transfer function of the refrigerant. After the working fluid passes through the separator which removes the oil from the refrigerant, it passes through conduit 14 to the intake side of condenser 12. Oil, in turn, accumulates within sump 28, then is driven by a pump back to the compressor assembly 10. The precentage of oil in the refrigerant which is a liquid and at relatively high pressure in the condenser, is very small, from zero to three percent or less and does not materially interfere with the liquid refrigerant which is injected through bleed line 46 just upstream from the discharge side of the screws. Some minor flashing or vaporization may occur intermediate of the liquid expansion valve and the port (not shown) within the screw compressor 26, but most of the expansion and vaporization takes place within the space defined between the lobes of the screws which space captures the working fluid gas which is compressed as it moves from the intake side to the discharge side of the intermeshed screws. The purpose of shutoff valves 48 and 54 is to isolate the bleed line components from the main components of the refrigeration system for maintenance purposes.

The thermostat 60 which is responsive to the temperature of the oil at the oil pump discharge and which controls the on-off solenoid valve 50, is positioned to be responsive to the temperature of the oil, since the outside surface of the oil line heats up much quicker than the outside surface of the refrigerant discharge conduit 14 and therefore the change in the oil temperature conduit surface anticipates the subsequent increase in the temperature of refrigerant working fluid conduit exterior surface and effects liquid refrigerant injection into the screw compressor at the time that injection is needed rather than at some point subsequent thereto.

From the above, it is seen that the present invention truly limits the discharge temperatures at the heart of the compressor and by selecting the point of injection of the liquid refrigerant, cooling is achieved without materially affecting the volumetric capacity of the compressor. Injection occurs when the suction stroke has been completed and the compression stroke is well in process, and may in fact occur just at the point of full compression prior to discharge from the compressor. Further, the injection of liquid refrigerant during the compression stroke permits the heat required to vaporize the liquid refrigerant to remove a significant part of the heat generated in the compression process and thereby materially reducing the discharge temperature of the working fluid and, of course, the maximum temperature to which the oil is subjected. Further, since the pressure level of the injected refrigerant is only slightly less than the discharge pressure of the working fluid, the horsepower requirement to raise the vaporized refrigerant to the full discharge pressure is minimum and the very small horsepower increase necessary to achieve this end does not seriously affect the overall efficiency of the compressor.

Turning to FIG. 2, there is illustrated schematically a refrigeration system similar to that of FIG. 1 which incorporates liquid refrigerant injection for oil cooling purposes and when the liquid injection system components are modified to some extent. In this case, the screw compressor unit or assembly 110 comprises a screw compressor motor 124 for driving the screw compressor 126 which receives the gaseous or vaporous refrigerant working fluid through intake line 122 and compresses the same for discharge over the screw compressor motor 124, the high pressure vapors exiting from the screw compressor through discharge conduit 114. The oil separator 125 is illustrated schematically, in this case, as being downstream of the screw compressor motor and upstream of a conventional water cooled condenser and positioned within line 114 intermediate of a screw compressor assembly 110 and condenser 112. Again, high pressure liquid refrigerant leaves condenser 112 via conduit 118 and passes to the chiller or evaporator 116 via filter drier 120. Schematically, a liquid refrigerant bleed line 146 permits some of the high pressure liquid refrigerant to be bled to the screw compressor 126 for injection through an injection port 170 defined by the headed end of the arrow which lines intermediate of the suction and discharge sides of the screw compressor 126. Refrigerant vapor returns from the chiller or evaporator 116 to the suction side of the screw compressor through line 122.

The simplified liquid refrigerant injection control system in this embodiment, consists essentially of a thermostat in the form of a temperature sensitive bulb 156 which carries along with capillary 157, a temperature responsive material which upon expansion modulates the valve 152 to thereby variably increase the delivery of liquid refrigerant to the injection port 170 of screw compressor 126. In like manner to the embodiment of FIG. 1 equalizing line 153 provides a second input to valve 152 responsive to condensing pressure. While the system contains a temperature sensistive expansion valve 152 and while the valve modulates the flow of refrigerant in direct proportion to the temperature at the discharge side of the screw compressor unit 110 and while this variation simplifies the external components of the system, this variation in the control scheme may be less desirable, since it is necessary that the valve 152 maintain a good tight shut off condition, that is, one in which at no load or when the machine is in compressor off portion of the cycle, liquid refrigerant will not be injected into the working chamber through injection port 170. Alternatively, the bulb 156 could be operatively positioned at oil separator 125 and responsive to oil temperature at the separator or within the oil sump.

Turning to FIG. 3, a further embodiment of the present invention is shown for a refrigeration system which is identical in most respects to the refrigeration system of FIG. 2. In this case, similar components are given similar numerical designations. The only difference in this embodiment, is the fact that the bleed line 146 carries a solenoid operated on-off valve 150 similar to liquid injection solenoid valve 50 of the first embodiment and, in which case, the electric control line 157 provides electrical current to the solenoid under control of thermostat operated switch contacts at a control panel 171 intermediate a thermostat bulb 172 operatively associated with the discharge line 114 at the discharge end of the screw compressor assembly 110. Bulb 172 could alternatively be positioned so as to sense directly the temperature of the separated oil at or downstream of oil separator 125. The thermostat bulb 172 carries a temperature responsive material such as a liquid or a gas along with capillary tube 173 which is coupled to the thermostat (not shown) within panel 171. Switch contacts close the current through line 157 which includes the solenoid associated with valve 150 and which is powered from a source (not shown). The solenoid valve 150 merely cycles on and off and the function of the thermostat is to either allow injection of liquid refrigerant via port 170 to the screw compressor 126 or prevent such injection of liquid refrigerant thereon. In this case, there is no modulation or variation in the rate of flow of refrigerant to the injection port 170 and, at high temperatures, liquid refrigerant is injected into the screw compressor while at low working fluid discharge temperatures, there is an absence of liquid injection. The thermostat 172 bulb merely functions to signal the valve 150 to open or close and thus continually cycles the valve during operation of the system. Again, while the system is simplified, a possible detrimental effect is the fact that the cycling of the components may reduce the life of the same well below the design life of the major components of the machine, such as the screw compressor, etc.

Reference to FIG. 4 illustrates a fourth embodiment of the invention, again as applied to a refrigeration system essentially identical to that of FIGS. 2 and 3 and, in which case, the like components are given like numerical designations. In the embodiment of FIG. 4, however, it is desirable to increase the pressure of the liquid refrigerant in bleed line 174 prior to injection of liquid refrigerant into the compressor at injection port 176 which is very close to the discharge side of the screw compressor as compared to discharge port 170 of embodiments illustrated in FIGS. 2 and 3. In this case, the condenser 112 rather than being positioned close to and at the same height as the other components of the refrigeration system, is positioned at a much greater height. For instance, the refrigeration system may be one incorporated within a relatively tall building as, for instance, where the condenser 112 is positioned on the roof, perhaps some 10 or 15 stories above the screw compressor assembly 110, and the other components of the system which may well be in the basement of the same building. In this case, within the line 118 leading from the condenser to the drier filter 120 and at the condenser itself, the bleed line 174 creates a static pressure head relative to the injection port 176 which adds materially to the pressure of the liquid refrigerant emanating from condenser 112. In fact, the combined pressure head of the liquid refrigerant within belled line 174 may be in excess of the discharge pressure of the screw compressor discharge working fluid. Again, the system employs a temperature sensitive bulb 156 carrying a temperature expansible material as does capillary 157 to modulate operation of valve 152 proportional to the temperature of the discharge gas, further modulated by the condensing pressure for the working fluid via equalizing line 153 in similar fashion to the embodiment of FIGS. 1 and 2, the bulb 156 being operatively associated with the conduit 114 at the discharge side of the screw compressor assembly 110. Thus, the valve 152 variably controls the flow of liquid refrigerant at high pressure which enters the screw compressor through the injection port 176, in this case very close to the discharge side of the same. In each of the embodiments of FIGS. 1, 2 and 3, the condenser may be temperature condensing pressure at the discharge side of the compressor and the valve will variably inject liquid refrigerant into the compressor working chamber. The thermostat bulb 356 modulates the volume of liquid passed by the liquid injection expansion valve 352 and only the amount of liquid refrigerant is injected into the compressor which is sufficient to maintain the gas discharge temperature within a predetermined range. As load increases the discharge temperature the working fluid increases and bulb 356 senses the demand for more liquid refrigerant to be injected into the screw compressor just upstream from the discharge side of the screws. Regardless of the modulation effects on the liquid passing from the main liquid refrigerant flow line 318 connecting condenser 312 to evaporator 316 through dryer 320, valve 364 remains open as long as the load on the compressor is above the minimum value.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

1. In a closed loop refrigeration system including, in order: a screw compressor, a condenser, and an evaporator, and having a refrigerant circulating therebetween, the improvement comprising:

means for bleeding high pressure refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas, and

means responsive to compressor load for controlling said bleeding and injection means.

2. The refrigeration system as claimed in claim 1, wherein said means for bleeding and injecting refrigerant comprises a bleed line coupled at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator and wherein a valve responsive to compressor load is positioned within said line for controlling the flow of liquid refrigerant therethrough.

3. The refrigeration system as claimed in claim 2, wherein said valve comprises an on-off valve and wherein a thermostat responsive to the compressor discharge temperature is operatively coupled to said on-off valve for controlling operation of the same.

4. The refrigeration system as claimed in claim 3, further comprising a variable flow valve positioned within said bleed line and means further responsive to compressor discharge temperature operatively coupled to said variable flow valve to modulate flow of liquid refrigerant through said bleed line.

5. The refrigeration system as claimed in claim 2, further comprising an oil separator at the discharge end of said screw compressor to separate oil from the refrigerant working fluid, and wherein; said means responsive to compressor load for controlling the flow of liquid refrigerant through said bleed line comprises a thermostat for sensing the temperature of said separated oil and means operatively coupling said thermostat to said valve for controlling operation of the same.

6. The refrigeration system as claimed in claim 5 further comprising: a variable flow valve positioned within said bleed line, and a thermostat responsive to compressor discharge temperature operatively coupled to said variable flow valve to modulate flow of liquid refrigerant through said bleed line.

7. The refrigeration system as claimed in claim 3, further comprising an oil separator at the discharge end of said screw compressor to separate oil from the refrigerant working fluid, and wherein; said means responsive to compressor load for controlling the flow of liquid refrigerant through said bleed line comprises a thermostat for sensing the temperature of said separated oil and means operatively coupling said thermostat to said valve for controlling operation of the same.

8. The refrigeration system as claimed in claim 2, wherein said condenser is located at a height, considerably above that of said screw compressor, and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure.

9. The refrigeration system as claimed in claim 5, wherein said condenser is located at a height, considerably above that of said screw compressor, and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure.

10. The refrigeration system as claimed in claim 7, wherein said condenser is located at a height, considerably above that of said screw compressor, and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure.

11. The refrigeration system as claimed in claim 2, further comprising a reciprocating slide valve for variably opening the working chamber to the suction side of the screw compressor, injection passage means carried by said slide valve, and wherein, said means for coupling one end of said bleed line to said compressor working chamber comprises means operatively coupling said bleed line to said slide valve

injection passage means. 12. The refrigeration system as claimed in claim 11, 34 wherein said valve comprises an on-off valve, and wherein a thermostat responsive to compressor discharge temperature is operatively coupled to said on-off valve for controlling

the operation of the same. 13. The refrigeration system as claimed in claim 11, 34 further comprising: a variable flow valve within said bleed line, and a thermostat responsive to compressor discharge temperature operatively coupled to said variable flow valve to

modulate flow of liquid refrigerant through said bleed line. 14. The refrigeration system as claimed in claim 5, 35 further comprising: a reciprocating slide valve for variably opening the working chamber to the suction side of the screw compressor, injection passage means carried by said slide valve, and wherein, said means for coupling one end of said bleed line to said compressor working chamber comprises means operatively coupling said bleed line to said slide

valve injection passage means. 15. The refrigeration system as claimed in claim 14, wherein said valve comprises an on-off valve and wherein a thermostat responsive to the compressor discharge temperature is operatively coupled to said on-off valve for controlling operation of the

same. 16. The refrigeration system as claimed in claim 14, further comprising a variable flow valve positioned within said bleed line, and a thermostat responsive to compressor discharge temperature operatively coupled to said variable flow valve for modulating the flow of liquid refrigerant through said bleed line.

17. The refrigeration system as claimed in claim 2, wherein said valve comprises a variable position valve and wherein said means responsive to compressor load comprises means responsive to the temperature of the compressor discharge for modulating said valve.

18. The refrigeration system as claimed in claim 17, further comprising means responsive to the condensing temperature of the

refrigerant working fluid for additionally modulating said valve. 19. The refrigeration system as claimed in claim 18, 41 wherein said valve means comprises a movable valve member normally biased to valve closed position, and means responsive to the temperature of the compressor discharge tending to operate operates in opposition to said bias, and further means responsive to the condensing temperature pressure of the refrigerant working fluid acts on said movable valve element tending to maintain said valve element in valve closed position.

. The refrigeration system as claimed in claim 19, wherein said valve means comprises a tubular valve housing, annular means defines a fixed valve seat, a movable valve stem is concentrically positioned within said valve seat and includes an enlarged portion closing off said passage defined by the valve seat and said valve stem, a coil spring concentrically carried by said valve stem biases said valve to closed position, a diaphragm operatively contacts said valve stem at one end and defines with said valve casing, a first closed chamber and a second closed chamber on opposite sides thereof, a bulb is positioned in heat conducting relationship to the compressor outlet and is coupled to said first chamber by capillary means, said chamber, said capillary means and said bulb carry a thermo-expansive fluid, means fluid connects said second chamber directly to said compressor at its discharge side, whereby, said diaphragm is responsive to pressure differentials between said chambers and acts directly on said valve stem to open the valve

means against the bias of said concentric coil spring. 21. In a refrigeration system including a helical rotary screw compressor, a condenser, and an evaporator, in that order, within a closed loop, with refrigerant circulating therebetween, and wherein said screw compressor includes a compressor housing defining with a pair of intermeshed screws rotatably mounted therein, a compressor working chamber and having a capacity control slide valve movable within the rotor housing and away from a fixed valve stop downstream of the compressor intake port for variably opening the compressor working chamber to said intake port, the improvement comprising: means carried by said slide valve for injecting liquid refrigerant bled from the refrigeration system downstream of the condenser into said screw compressor working chamber intermediate of the suction and discharge sides of the same and downstream of said fixed stop,

for limiting the discharge temperature of the refrigerant gas. 22. The refrigeration system as claimed in claim 21, wherein: hydraulic motor means effects movement of said slide valve and includes a slidable piston, rod means couples said piston at one end and said slide valve at the other, said rod means includes conduit means for feeding said liquid refrigerant, said slide valve includes means coupled to said conduit means defining a liquid refrigerant chamber, and at least one passage extends through a wall of said slide valve and fluid connects said liquid refrigerant chamber to said working chamber downstream of said fixed valve

stop. 23. The refrigeration system as claimed in claim 22, wherein; said rod means comprises a plurality of concentric tubes, said slide valve includes wall means defining a cavity concentrically surrounding said rod means, said cavity is separated by wall means intermediate of the axial ends of said cavity to form upstream and downstream closed chambers extending transversely across said cavity, and wherein first conduit means carried by said tubes means is fluid connected to a source of pressurized oil and to said first chamber, and has a passage opening up into the working chamber downstream of said fixed stop, said tubes further include second conduit means fluid coupled to said bled liquid refrigerant and to said second chamber and fluid passage means fluid coupled to said second chamber, and porting into said working chamber downstream of said first passage, whereby liquid refrigerant is injected into said working chamber downstream of the point of injection of said lubricating oil regardless of

the position of said capacity control slide valve. 24. The refrigeration system as claimed in claim 22, wherein; said hydraulic motor includes a piston mechanically coupled to said slider valve by a piston rod extending therebetween, said rod extends the full length of said slide valve and axially through the center of the same, partition means define first and second axially spaced fluid sealed chambers within said slide valve, fluid passage means is carried by said shaft and extends the length of the same, plug means carried by said shaft separate said passage means at said partition means, and means fluid couple one of said fluid passages to a course source of lubricating oil, and the other of said passages on the opposite side of said plug to said bled liquid refrigerant and further passage means associated with each chamber fluid couple respective portions of said shaft passage to said chambers and said chambers to said screw compressor working chamber downstream of said fixed

valve stop. 25. In a refrigeration system including, in order, a screw compressor, a condenser, and an evaporator in a closed loop with refrigeration circulating therebetween, a port opening up into said working chamber of the screw compressor intermediate of the suction and discharge sides of said compressor, a bleed line fluid coupled to said loop downstream of the condenser and to said port, a compressor discharge temperature responsive liquid injection expansion valve positioned within said bleed line for modulating the flow of liquid refrigerant through said bleed line to said port, and on-off valve means within said bleed line upstream of said expansion valve, the improvement comprising: an external source of fluid pressure, and wherein said on-off valve comprises a direct acting fluid pressure operated, valve positioned within said bleed line upstream of said liquid expansion valve, and said system further includes means responsive to compressor load for coupling said fluid pressure

operated valve to said external source. 26. The refrigeration system as claimed in claim 25, wherein said external source of said fluid pressure comprises the compressor oil system, said pressure responsive on-off valve, is fluid coupled to said system oil pressure by an oil line including a solenoid valve therein, and said means responsive to compressor load for controlling the flow of fluid pressure from said source to said valve comprises a thermostat in heat receiving position with respect to said compressor discharge and means operatively coupling

said thermostat to said solenoid valve. 27. The refrigeration system as claimed in claim 26, wherein said thermostat comprises a thermal expansion bulb, fixed to said conduit coupling the discharge side of the compressor to the intake side of the condenser, and said means operatively coupling said thermostat to said solenoid valve comprises an electrical circuit including said solenoid valve and normally open thermostat contacts, remote from said thermal expansive bulb and responsive to expansion of

temperature responsive material carried by said bulb. 28. The refrigeration system as claimed in claim 26, further comprising a by-pass line coupled to the oil pressure supply line intermediate of the solenoid valve and the fluid pressure operated valve and acting as an oil return, and fluid restriction means carried with said by-pass line to limit flow therethrough and to insure the continued operation of said on-off valve in

response to energization of said solenoid valve. 29. The refrigeration system as claimed in claim 27, further comprising a by-pass line coupled to the oil pressure supply line intermediate of the solenoid valve and the fluid pressure operated valve and acting as an oil return, and fluid restriction means carried with said by-pass line to limit flow therethrough and to insure the continued operation of said on-off valve in response to energization of said solenoid valve.

30. In a closed loop refrigeration system including, in order, a screw compressor, a condenser, and an evaporator, a refrigerant circulating therebetween, means for bleeding high pressure liquid refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas, in response to compressor load and comprising means responsive to at least compressor discharge temperature for injecting liquid refrigerant in direct proportion to compressor load, and wherein, said means for bleeding and injecting liquid refrigerant comprises a bleed line coupled at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator, and wherein a valve responsive to compressor load is positioned within said line for controlling the flow of liquid refrigerant therethrough and a thermostat is provided for sensing the temperature of said compressor discharge and means operatively couples said thermostat to said valve for controlling operation of the same, the improvement wherein:

said condenser is located at a height, considerably above that of the screw compressor and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure.

31. In a closed loop refrigeration system including, in order, a screw compressor, a condenser, and an evaporator, and having a refrigerant circulating therebetween, and further comprising an oil separator at the discharge end of the screw compressor to separate oil from the refrigerant working fluid, means for bleeding high pressure liquid refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas in response to compressor load and comprising means responsive to at least compressor discharge temperature for injecting liquid refrigerant in direct proportion to compressor load, and wherein, said means for bleeding and injecting liquid refrigerant comprises a bleed line coupled at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator, and wherein a valve responsive to compressor load is positioned within said line for controlling the flow of liquid refrigerant therethrough and a thermostat is provided for sensing the temperature of said separated oil and means operatively couples said thermostat to said valve for controlling operation of the same, the improvement wherein:

said condenser is located at a height, considerably above that of said screw compressor, and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure.

32. In a closed loop refrigeration system including, in order, a screw compressor, a condenser, and an evaporator, and having a refrigerant circulating therebetween, and further comprising an oil separator at the discharge end of the screw compressor to separate oil from the refrigerant working fluid, means for bleeding high pressure liquid refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas in response to compressor load and comprising means responsive to at least compressor discharge temperature for injecting liquid refrigerant in direct proportion to compressor load, and wherein, said means for bleeding and injecting liquid refrigerant comprises a bleed line coupled at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator, and wherein an on-off valve responsive to compressor load is positioned within said line for controlling the flow of liquid refrigerant therethrough and a thermostat is provided for sensing the temperature of said separated oil and means operatively couples said thermostat to said valve for controlling operation of the same, the improvement wherein:

said condenser is located at a height, considerably above that of said screw compressor, and said bleed line leading from said condenser to said screw compressor is ported to said compressor working chamber in the vicinity of the discharge side of the same, such that the liquid refrigerant injected into the compressor working chamber under control of said valve is at or above the compressor discharge pressure, said valve comprising an on-off valve and said thermostat being operatively coupled to said on-off valve for controlling operation of the same.

33. In a closed loop refrigeration system including, in order, a screw compressor, a condenser, and an evaporator, and having a refrigerant circulating therebetween, means for bleeding high pressure liquid refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas, means responsive to compressor load for controlling said bleeding and injection means and including means responsive to at least compressor discharge temperature for injecting liquid refrigerant in direct proportion to compressor load, said means for bleeding and injecting refrigerant comprising a bleed line coupled at least at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator, and a valve responsive to compressor load being positioned within said line for controlling the flow of liquid refrigerant therethrough, the improvement comprising:

a reciprocating slide valve for variably opening the working chamber to the suction side of the screw compressor, injection passage means carried by said slide valve, and wherein, said means for coupling one end of the bleed line to said compressor working chamber comprises means operatively coupling said bleed line to said slide valve injection passage means.

34. In a closed loop refrigeration system including, in order: a screw compressor, a condenser, and an evaporator, and having a refrigerant circulating therebetween, means for bleeding high pressure liquid refrigerant from said system and injecting said liquid refrigerant into said screw compressor working chamber intermediate of the suction and discharge sides of the compressor for limiting the discharge temperature of the refrigerant gas, including means responsive to at least compressor discharge temperature for injecting liquid refrigerant in direct proportion to compressor load, a bleed line coupled at one end to said compressor working chamber and coupled at the other end to said system intermediate of said condenser and said evaporator, and a valve responsive to compressor load positioned within said line for controlling the flow of liquid refrigerant therethrough, the improvement wherein:

an oil separator is positioned at the discharge end of the screw compressor to separate oil from the refrigerant working fluid, said means responsive to compressor load for controlling the flow of liquid refrigerant through said bleed line comprises a thermostat for sensing the temperature of the separated oil and means operatively coupling said thermostat to said valve for controlling operation of the same, and wherein a reciprocating slide valve is operatively mounted on said compressor for opening the working chamber to the suction side of the screw compressor, injection passage means is carried by the slide valve, and wherein said means for coupling one end of the bleed line to said compressor working chamber comprises means operatively coupling said bleed line to said slide valve injection passage means.

35. A method of operating a refrigeration plant comprising: a refrigerant flow circuit including a compressor of the screw rotor type, a condenser and an evaporator; means for circulating oil and for injecting said oil into the compression chambers of said compressor; an oil separator provided in said circuit between the outlet of said compressor and the inlet of said condenser; and means for introducing liquid refrigerant into a circuit portion between the inlet of the compressor and the inlet of said oil separator;

comprising controlling the introduction of the liquid refrigerant into said circuit portion such that the temperature in the oil separator is kept at a level only slightly above the liquefaction temperature of the refrigerant at the pressure prevailing in the oil separator but is prevented from falling to said liquefaction temperature.

36. The method as defined in claim 35 comprising controlling the introduction of said liquid refrigerant into the compression chambers of said compressor.

37. A refrigeration plant comprising:

a refrigerant flow circuit including a compressor of the screw rotor type, a condenser and an evaporator;

means for circulating oil and for injecting said oil into the compression chambers of said compressor;

an oil separator provided in said circuit between the outlet of said compressor and the inlet of said condenser;

means for introducing liquid refrigerant into a circuit portion between the inlet of the compressor and the inlet of said oil separator;

means responsive to at least one parameter indicative of the difference between the temperature in the oil separator and the temperature in the condenser;

adjustable means for varying the quantity of liquid refrigerant introduced into said circuit portion; and

means connecting said responsive means with said adjustable means to control said adjustable means such that said temperature difference is kept small but is prevented from dropping down to zero.

The plant according to claim 37, wherein said liquid refrigerant is introduced into the compression chambers of said compressor and said adjustable means comprises a valve means coupled in the path of said liquid refrigerant to said compressor, said valve means being opened to increase the quantity of said liquid refrigerant introduced into said compressor.