Compressor refrigeration system with demand cooling

Abstract: A refrigeration system is disclosed which incorporates apparatus for preventing overheating of the compressor (10, 10') by selectively feeding liquid refrigerant from the outlet of the condenser (16, 16') to the compressor (10, 10'). In one embodiment the refrigerant fluid from the condenser (16) is injected into the suction manifold (52) of the compressor (10). In another embodiment this fluid is injected directly into the compression chamber or chambers (88, 90). Control means are provided which include a temperature sensor (32, 70, 112) located within the compressor discharge chamber (58, 114) and valve means (38, 100, 102) responsive thereto to control the flow of liquid refrigerant to the suction manifold (52) or compression chamber (88, 90).

Description: The present invention relates generally to refrigeration systems and more particularly to refrigeration systems incorporating means to prevent overheating of the compressor by selectively injecting liquid refrigerant into the suction manifold. In response to recent concerns over depletion of the ozone layer due to release of various types of refrigerants such as R12, the government has imposed increasingly stricter limitations on the use of these refrigerants. These limitations will require refrigeration systems of the future to utilize substitute refrigerants. Presently, the available substitutes for commonly used refrigerants such as R-12 and R-502 are not well suited for low temperature applications because they result in high discharge temperatures which may damage or shorten the life expectancy of the compressor particularly under high load situations and high compression ratios. Liquid injection systems have long been used in refrigeration systems in an effort to limit or control excessive discharge gas temperatures which cause overheating of the compressor and may result in breakdown of the compressor lubricant. Typically, these prior systems utilized capillary tubes or thermal expansion valves to control the fluid injection. However, such systems have been very inefficient and the capillary tubes and thermal expansion valves were prone to leaking during periods when such injection cooling was not needed. This leakage could result in flooding of the compressor. Additionally, when the compressor was shut down, the high pressure liquid could migrate from the receiver to the low pressure suction side through these capillary tubes or expansion valves, thereby resulting in slugging of the compressor upon startup. Also, the thermal sensors utilised by these prior systems were typically located in the discharge line between the compressor and the condenser. This positioning of the sensor often resulted in inadequate cooling as the sensed temperature could vary greatly from the actual temperature of the discharge gas exiting the compression chamber due to a variety of factors such as the ambient temperature around the discharge line and the mass flow rate of discharge gas. Thus, overheating of the compressor could occur due to an erroneous sensed temperature of the discharge gas. Typical of such a prior system is the one illustrated in US-A-4 049 410 which provides the basis for the prior art portion of claim 1. The present invention, as defined in claim 1, overcomes these problems by providing a liquid injection system which utilises a temperature sensor positioned within the discharge chamber of the compressor in close proximity to and in direct contact with the compressed gas exiting the compression chamber. Thus, a more accurate indication of the compressor heating is achieved which is not subject to error due to external variables. Further, the present invention employs in a presently preferred embodiment a positive acting solenoid actuated on/off valve coupled with a preselected orifice which prevents leakage of high pressure liquid during periods when cooling is not required. Additionally, the orifice is sized for a maximum flow rate such that it will be able to accommodate the cooling requirements while still avoiding flooding of the compressor. The term "liquid injection" is used herein to denote that it is liquid refrigerant which is taken from the condenser in such systems but in reality a portion of this liquid will be vaporised as it passed through the capillary tube, expansion valve or other orifice, thus providing a two-phase (liquid and vapour) fluid which is injected into the compressor. The present invention also injects the fluid (i.e. two-phase fluid) directly into the suction chamber at a location selected to ensure even flow of the injected fluid to each compression chamber so as to thereby maximise compressor efficiency as well as to ensure a maximum and even cooling effect. While GB-A-1 327 055 discloses the provision of a temperature sensor in the discharge outlet chamber of a refrigerant compressor, this is in a system where a plurality of sensors are provided, one for each compressor chamber, and are arranged to prevent operation of the compressor, should anyone of the chambers overheat. It contains no suggestion of actively cooling the compressor or of the specific valve and orifice arrangement required by the present invention. In another embodiment of the present invention, the refrigerant fluid is injected directly into the compression chamber, preferably immediately after the suction ports or valve has been closed off, thus acting to cool both the compression chamber and suction gas contained therein. While this arrangement offers greater efficiency in operation, it tends to be more costly as additional controls and other hardware are required for its implementation. Additional advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings. Figure 1 is a schematic diagram of a refrigeration system incorporating a demand cooling liquid injection system in accordance with the present invention; Figure 2 is a side view of a refrigeration compressor having the injection system of the present invention installed thereon all in accordance with the present invention; Figure 3 is a fragmentary section view of the refrigeration compressor of Figure 1, the section being taken along lines 3-3 of Figures 2 and 4; Figure 4 is a top view of the refrigeration compressor of Figure 2 with the head removed therefrom; Figure 5 shows an exemplary plot of discharge temperature as a function of time for a compressor employing the injection cooling system of the present invention; Figure 6 is a section view similar to that of Figure 4 but showing another refrigeration compressor having the demand cooling liquid injection system of the present invention installed thereon; and Figure 7 is a schematic view of a refrigeration system similar to Figure 1 but showing an alternative embodiment of the present invention incorporated therein. Referring now to the drawings and more particularly to Figure 1, there is shown a typical refrigeration circuit including a compressor 10 having a suction line 12 and discharge line 14 connected thereto. Discharge line 14 extends to a condenser 16 the output of which is supplied to an evaporator 18 via lines 20, receiver 22 and line 24. The output of evaporator 18 is thence fed to an accumulator 26 via line 28 the output of which is connected to suction line 12. As thus described, this refrigeration circuit is typical of such systems employed in both building air conditioning or other refrigerating systems. The present invention, however, provides a unique demand cooling fluid injection system indicated generally at 30 which operates to prevent potential overheating of the compressor. Fluid injection system incorporates a temperature sensor 32 positioned within the compressor 10 which operates to provide a signal to an electronic controller 34 which signal is indicative of the temperature of the compressed gas being discharged from the compressor 10. A fluid line 36 is also provided having one end connected to line 20 at or near the output of condenser 16. The other end of fluid line 36 is connected to a solenoid actuated valve 38 which is operatively controlled by controller 34. The output from solenoid valve 38 is fed through a restricted orifice 40 to an injection port provided on compressor 10 via line 42. As best seen with reference to Figures 2 through 4, compressor 10 is of the semi-hermetic reciprocating piston type and includes a housing 44 having a pair of compression cylinders 46, 48 disposed in longitudinally aligned side-by-side relationship. Housing 44 has a suction inlet 50 disposed at one end thereof through which suction gas is admitted. Suction gas then flows through a motor chamber provided in the housing and upwardly to a suction manifold 52 (indicated by the dotted lines in Figure 4) which extends forwardly and in generally surrounding relationship to cylinders 46, 48. A plurality of passages 54 serve to conduct the suction gas upwardly through a valve plate assembly 56 whereupon it is drawn into the respective cylinders 46, 48 for compression. Once the suction gas has been compressed within cylinders 46, 48, it is discharged through valve plate assembly 56 into a discharge chamber 58 defined by overlying head 60. As best seen with reference to Figures 3 and 4, line 42 is connected to an injection port 62 provided in the sidewall of housing 44 and opening into suction manifold 52 at a location substantially centered between cylinders 46, 48 and directly below passage 54. The location of this injection port was determined experimentally to optimize efficiency and to insure even cooling of each of the two cylinders. Preferably this location will be selected for a given compressor model such that the compressed gas exiting from each of the respective compression chambers will be within a predetermined range relative to each other (i.e. from hottest to coolest) and more preferably these temperatures will be approximately equal. It should be noted that it is desirable to inject the liquid as close to the cylinders as possible to optimize operational efficiency. Also as best seen with reference to Figures 2 and 3, temperature sensor 32 is fitted within an opening 64 provided in head 60 and extends into discharge chamber 58 so as to be in direct contact with the discharge gas entering from respective cylinders 46, 48. Preferably sensor 32 will be positioned at a location approximately centered between the two cylinders 46, 48 and as close to the discharge valve means 66 as possible so as to insure an accurate temperature is sensed for each of the respective cylinders. It is believed that this location will place the temperature sensor closest to the hottest compressed gas exiting from the compression chambers. Solenoid actuated valve 38 will preferably be an on/off type valve having a capability for a very high number of duty cycles while also assuring a leak resistant off position so as to avoid the possibility of compressor flooding or slugging. Alternatively, solenoid valve could be replaced by a valve having the capability to modulate the flow of liquid into suction manifold 52 in response to the sensed temperature of the discharge gas. For example, a stepping motor driven valve could be utilized which would open progressively greater amounts in response to increasing discharge temperature. Another alternative would be to employ a pulse width modulated valve which would allow modulation of the injection fluid flow by controlling the pulse duration or frequency in response to the discharge temperature. In order to limit the maximum flow of fluid into suction manifold 52 via injection port 62 as well as to reduce the pressure of the fluid to approximately that of the suction gas flowing from the evaporator, an orifice 40 is provided downstream of valve 38. Preferably orifice 40 will be sized to provide a maximum fluid flow therethrough at a pressure differential of about 20 bar (300 psi) which corresponds to an evaporator temperature of about -40°C (-40°F) and a condenser temperature of about 54°C (130°F) so as to assure adequate cooling liquid is provided to compressor 10 to prevent overheating thereof. Evaporator temperature refers to the saturation temperature of the refrigerant as it enters the evaporator and has passed through the expansion valve. Condenser temperature refers to the saturation temperature of the refrigerant as it leaves the condenser. This represents a worst case design criteria. The maximum flow will vary between different compressors and will be sufficient to prevent the discharge temperature of the compressor from becoming excessively high yet not so high as to cause flooding or slugging of the compressor. It should be noted that it is important that orifice 40 be sized to create a pressure drop thereacross which is substantially equal to the pressure drop occurring between the condenser outlet and the compressor suction inlet across the evaporator so as to prevent subjecting the evaporator to a back pressure which may result in excessive system efficiency loss. In operation, upon initial startup from a "cold" condition, valve 38 will be in a closed condition as the temperature of compressor 10 as sensed by sensor 32 will be low enough not to require any additional cooling. Thus, the refrigeration circuit will function in the normal manner with refrigerant being circulated through condenser 16, receiver 22, evaporator 18, accumulator 26 and compressor 10. However, as the load upon the refrigeration system increases, the temperature of the discharge gas will increase. When the temperature of the discharge gas exiting the compression chambers of compressor 10 as sensed by sensor 32 reaches a first predetermined temperature as shown by the spikes in the graph of Figure 5, controller 34 will actuate valve 38 to an open position thereby allowing high pressure liquid refrigerant exiting condenser 16 to flow through line 36, valve 38, orifice 40, line 42 and be injected into the suction manifold 52 of compressor 10 via port 62. It should be noted that the liquid refrigerant will normally be partially vaporized as it passes through orifice 40 and hence the fluid entering through port 62 will typically be two phase (part gas, part liquid). This cool liquid refrigerant will mix with the relatively warn suction gas flowing through manifold 52 and be drawn into the respective cylinders 46, 48. The vaporization of this liquid refrigerant will cool both the suction gas and the compressor itself thereby resulting in a lowering of the temperature of the discharge gas as sensed by sensor 32 and as shown in the graph of Figure 5. Once the discharge temperature sensed by sensor 32 drops below a second predetermined temperature, controller 34 will operate to close valve 38 thereby shutting off the flow of liquid refrigerant until such time as the temperature of the discharge gas sensed by sensor 32 again reaches the first predetermined temperature. Preferably, the first predetermined temperature at which valve 38 will be opened will be below the temperature at which any degradation of the compressor operation or life expectancy will occur and in particular below the temperature at which any degradation of the lubricant utilized within compressor 10 occurs. The second predetermined temperature will preferably be set sufficiently below the first predetermined temperature so as to avoid excessive rapid cycling of valve 38 yet high enough to insure against possible flooding of the compressor. In a preferred embodiment of the present invention the first predetermined temperature was set at about 143°C (290°F) and the second predetermined temperature was set at about 138°C (280°F). The graph of Figure 5 shows the resulting discharge temperature variation as a function of time for these predetermined temperatures at -32°C (-25°F) evaporating temperature, 43°C (110°F) condensing temperature and 18°C (65°F) return temperatures. Return temperature refers to the temperature of the refrigerant returning from the evaporator as it enters the compressor. As noted above, positioning of the sensor 32 and the injection port 62 is very important for insuring proper even cooling of the compressor and for maximizing operating efficiency of the system. Figure 6 shows the position of injection port 68 and discharge gas sensor 70 in a semi-hermetic compressor 72 having three compression cylinders 74, 76, 78. Port 68 opens into suction manifold 80 (outlined by dotted lines and extending along both sides of the two rearmost cylinders) provided within the compressor housing and is preferably centered on the middle cylinder 76. Similarly, sensor 70 extends inwardly through the head (not shown) and is positioned in closely overlying relationship to the center cylinder 76 so as to be exposed to direct contact with the compressed discharge gas exiting from each of the three cylinders. Again, it is believed that this location will place the sensor closest to the hottest compressed gas exiting from the respective compression chambers as is believed preferable. The operation of this embodiment will be substantially identical to that described above. Referring now to Figure 7, there is shown a refrigeration system similar to that shown in Figure 1 incorporating the same components indicated by like reference numbers primed. However, this refrigeration system incorporates an alternative embodiment of the present invention wherein the refrigerant fluid is injected directly into each of the respective cylinders as soon as the piston has completed its suction stroke (i.e. just as the piston passes its bottom dead center position). This embodiment offers even greater improvements in system operating efficiency in that the fluid being injected does not displace any of the suction gas being drawn into the compressor but rather adds to the fluid being compressed thus resulting in greater mass flow for each stroke of the piston. As shown in Figure 7, compressor 10' has a crankshaft 82 operative to reciprocate pistons 84, 86 within respective cylinders 88, 90. A plurality of indicia 92 equal in number to the number of cylinders provided within compressor 10' are provided on a rotating member 94 associated with crankshaft 82 which are designed to be moved past and sensed by sensor 96 as crankshaft 82 rotates. Indicia 92 will be positioned relative to sensor 96 such that sensor 96 will produce a signal indicating that a corresponding piston is moving past bottom dead center. These signals generated by sensor 96 will be supplied to controller 98. In order to supply refrigerant fluid to each of the respective cylinders 88, 90, a pair of suitable valves 100, 102 are provided each of which has an input side connected to fluid line 36' and is designed to be actuated between on/off positions by controller 98 as described in greater detail below. An orifice 104, 106 is associated with each of the respective valves 100, 102. Orifice 104, 106 perform substantially the same functions as orifice 40 described above except that they will be designed to maintain the fluid to be injected into the cylinders somewhat above the pressure of the suction gas within the cylinders at the time the fluid is to be injected which pressure may be above that of the suction gas returning from the evaporator. The outputs of respective valves 100, 102 and orifices 104, 106 will be supplied to respective cylinders 88, 90 via fluid lines 108, 110 respectively which may communicate with cylinders 88, 90 through any suitable porting arrangement such as openings provided in the sidewall of the respective cylinders or through a valve plate associated therewith. Additionally, suitable check valves may be provided to prevent any backflow of refrigerant during the compression stroke if desired. A sensor 112 is also provided being disposed within a discharge chamber 114 defined by head 116 and operative to send a signal indicative of the temperature of the compressed gas exiting cylinders 88, 90 to controller 98. Sensor 112 is substantially identical to sensors 32 and 70 described above and will be positioned within discharge chamber 114 in a substantially identical manner to and will function in the same manner as described with reference to sensors 32 and 70. In operation, when sensor 112 indicates to controller 98 that the temperature of the compressed gas exiting cylinders 88, 90 exceeds a predetermined temperature, controller 98 will begin looking for actuating signals from sensor 96. As indicia 92 carried by crankshaft 82 passes sensor 96, a signal indicating that one of pistons 84 and 86 is passing bottom dead center is provided to controller 98 which in turn will then actuate the corresponding one of valves 100 and 102 to an open position for a brief predetermined period of time whereby refrigerant fluid will be allowed to flow into the corresponding cylinder thus mixing with and cooling the suction gas previously drawn into the cylinder for compression. This cycle will be repeated for the other of cylinders 88, 90 as the next indicia 92 moves past sensor 96 carried by crankshaft 82 thereby providing a supply of cooling refrigerant fluid to that cylinder. The actual time periods for which valves 100 and 102 are maintained in an open position will be selected so as to provide a sufficient cooling to avoid excessive overheating of compressor 10' while avoiding the possibility of causing a flooding or slugging of the respective cylinders. In some applications it may be desirable to vary the length of time the respective valves are maintained in an open condition in response to the magnitude by which the temperature of the discharge gas as sensed by sensor 112 exceeds a predetermined temperature. In any event, once the temperature of the compressed gas sensed by sensor 112 drops below a second predetermined temperature, controller 98 will cease actuation of respective valves 100 and 102 and the refrigerant system will operate in a conventional manner without any fluid injection. It should be noted that while the present invention has been described in connection with reciprocating piston type compressors, it is also equally applicable to other types of compressors such as rotary, screw, scroll or any other type thereof. Because the present invention employs a sensor exposed directly to the discharge gas as it exits the compression chamber or chambers, the possibility of erroneous readings due to external factors is substantially eliminated. Further, the use of a positive control valve insures that cool liquid will only be supplied at those times that it is necessary to effect cooling of the compressor. Also, the provision of a properly sized orifice limits maximum liquid flow so as to insure that flooding of the compressor will not occur.

Claims: A refrigeration system including a compressor (10) having a suction manifold (52) and a discharge chamber (58), a condenser (16), an evaporator (18) connected to said compressor (10) in a serial closed loop system, and means (30) for preventing overheating of said compressor (10) comprising a sensor (32) for sensing the temperature of compressed gas, a fluid line (36,42) connected between the outlet (20) of said condenser (16) and said compressor (10) and control means (34,38) operative selectively to control fluid flow from said condenser outlet to said compressor (10) in response to the sensed temperature of said compressed gas, characterised in that the sensor (32) is within the discharge chamber (58) of said compressor (10) and in the direct flowpath of said compressed gas, the control means comprises a positively acting control valve (38) capable of shutting off the flow of fluid through said fluid line (36,42) and in that an orifice (40) is provided between the valve (38) and the compressor (10) sized to provide a pressure drop thereacross when the valve (38) is open to limit fluid flow through the fluid line thereby to inhibit flooding of the compressor (10). A refrigeration system as claimed in claim 1, wherein said valve (38) is actuable between open and closed positions thereby selectively to control said fluid flow. A refrigeration system as claimed in claim 1, wherein said valve (38) is operable to modulate said fluid flow. A refrigeration system as claimed in claim 3, wherein said valve (38) is a pulse width modulated valve. A refrigeration system as claimed in claim 2, wherein said control means (34) is operable to actuate said valve (38) to an open position at a first predetermined temperature and to actuate said valve (38) to a closed position at a second predetermined temperature. A refrigeration system as claimed in any preceding claim, wherein said compressor includes a plurality of compression chambers (46,48), each of said chambers receiving suction gas from a suction manifold (52) and discharging compressed gas into said discharge chamber (58), said fluid line (36,42) opening into said suction manifold (52). A refrigeration system as claimed in any one of claims 1 to 5, wherein said compressor includes a plurality of compression chambers (84,86), an injection fluid line (108,110) opening into each of said chambers, control valves (100,102) being provided in each of said injection fluid lines, said fluid line (36') being connected to each of said valves and a controller (98) being provided operable to actuate selective ones of said valves thereby to control fluid flow from said condenser outlet to selective ones of said compressor chambers. A refrigeration system as claimed in any preceding claim, wherein the compressor has a plurality of compression chambers (74,76,78), each having a discharge port associated therewith, said sensor (112) being positioned within said discharge chamber substantially centrally of said discharge ports so as to be in direct contact with said compressed gas entering said discharge chamber therefrom. A refrigeration system as claimed in any one of claims 1 to 7, wherein the compressor includes a plurality of compression chambers (74,76,78), each of the chambers discharging the compressed gas into the discharge chamber via respective discharge ports, said sensor (70) being located within the discharge chamber closest to the discharge port through which the compressed gas having the highest temperature enters the discharge chamber. A refrigeration system as claimed in any preceding claim, further comprising timing means (92,94,96) for providing a signal to said control means (34) indicating that filling of the or a compression chamber of the compressor with suction gas has been completed. A refrigeration system as claimed in claim 10, when appendant to claim 7, wherein the valves (100,102) are actuable to open position at, or subsequent to, completion of filling of the respective compressor chamber (84,86). A refrigeration system as claimed in claim 10 or 11, wherein said compressor is a reciprocating piston compressor and said timing means is operative to provide a signal to said controller indicating that said piston is at bottom dead centre.

Inventors: DIAB TARIQ ABDEL RAHIM, DIAB, TARIQ ABDEL RAHIM
Applicants: COPELAND CORP, COPELAND CORPORATION
Application year: 1990
Main classification: F25B31
IPC codes: F25B49, F25B1, F04B39, F25B31, F04C18