REFRIGERATION PLANT AND ROTARY POSITIVE DISPLACEMENT MACHINE

Abstract: A plant of refrigeration type and a rotary, positive displacement machine for said plant. The plant comprises said machine having at least one rotor provided with spiral lobes and intervening grooves, a condenser communicating with an outlet port (60) of the compressor through a high pressure channel (14), and an evaporator communicating with an inlet port (58) of the compressor through a low pressure channel (18), a vessel for an intermediate pressure communicating with intermediate port means (40) of the compressor through an intermediate pressure channel (36), said intermediate port means being spaced from as well said inlet port (58) as said outlet port (60), and pressure reduction means for decreasing the high pressure in the condenser to the intermediate pressure in the vessel and to the low pressure in the evaporator, respectively. The plant and machine according to the invention is characterized in selectively adjustable valve means (40) for forming a communication between said intermediate channel (36) and said low pressure channel (18).

Description: REFRIGERATION PLANT AND ROTARY POSITIVE DISPLACEMENT MACHINE The present invention relates to a plant of refrigera¬ tion type comprising a compressor, and a condenser and an evaporator with pressure reduction means therebetween and communicating with the compressor through a high pressure, outlet channel and a low pressure, inlet channel, respec¬ tively. The compressor is of a rotary, positive displace¬ ment type having at least one rotor provided with spiral lobes and intervening grooves. The plant is further provi¬ ded with an intermediate pressure vessel communicating with the condenser through pressure reduction means and with in¬ termediate port means in the compressor through an interme¬ diate pressure channel. The invention further relates to a rotary machine appropriate for use as a compressor in such a plant. Plants and compressors of such types are earlier known from US patent 3,568,466, Brandin et al. , and US patent 3,913,346, Moody et al. The intermediate pressure zone in such plants is used for internal cooling purposes within the plant at a temperature level above that of the evapora- tor. The main cooling purpose is to precool the liquified refrigerant before the supply thereof to the evaporator which results in a more effective use of the evaporator area so that the dimensions thereof can be minimized for a certain capacity simultaneously as the swept volume of the compressor and thus its dimensions can be reduced corre¬ spondingly. Furthermore the power required for recompres- sion of the gaseous refrigerant supplied at the intermedia¬ te pressure will be less than that if all the refrigerant were supplied at the evaporator pressure. A second cooling purpose applicable when the compressor is driven by an electrical motor, especially important in hermetic systems and heat pump applications, is to pass the intermediate pressure fluid through the motor in order to guarantee an efficient cooling thereof under all driving conditions. Eventhough the description of the compressor for a re¬ frigeration plant in this specification is restricted to the type comprising two intermeshing rotors of male and fe¬ male type provided with helical lands and intervening groo- ves the invention may also be applicable to other types of machines comprising at least one rotor having spiral lobes, for instance compressors of the so called single screw type and of the so called scroll type. All the machines under consideration relate to such ones where the intermediate pressure port means is spaced from the main inlet port and disposed at such a distance there¬ from that any communication therebetween through the work¬ ing space of the machine is continuously blocked by at least one rotor lobe. In order to vary the volumetric capacity of a screw compressor it is earlier known from US patent 3,314,597, Schibbye, to provide the compressor with a selectively ad¬ justable valve member controlling a bleed port in the wall of the working space so that a certain amount of the work- ing fluid supplied to the compressor may be returned to the inlet channel of the compressor. This type of volumetric capacity control has been used also for screw compressors provided with intermediate port means. This bleed port is disposed within the same phase of the compression cycle as the intermediate port means. When the bleed port is opened the pressure level inside the compressor working space de¬ creases to such an extent that the back pressure within the area of the intermediate port means will be practically the same as that in the low pressure channel. The bleed port must in order to avoid throttling losses be provided with a large area corresponding not only for the recirculation of the surplus fluid supplied through the inlet port but also for draining the fluid supplied through the intermediate port means. The size of the valve member will thus be too large for location in the end wall with regard to as well its area as the limited space available outside the rotor bearings. For this reason the valve has to be located in the barrel wall of the working space. Such a val-ve will consequently be complicated in shape and expensive to manu- facture as it not only has to sealingly cooperate with its seat in the housing but also has to sealingly cooperate with the confronting rotor or rotors in order to avoid in¬ ternal leakage in the compressor, especially when running under maximum capacity conditions. The main object of the present invention is to achieve a more effective capacity control of the machine per se as well as of a complete plant by means of simpler and less expensive valve arrangement than those used in the prior art. This object of the invention is met by providing a se¬ lectively adjustable over-flow valve between the interme¬ diate pressure channel and the low pressure channel. In this way the need for a separate bleed port is eliminated as the intermediate pressure port means will act as such a port during low volumetric capacity conditions when only the surplus supply working fluid has to be drained from the working space. Furthermore, the valve body will be consi¬ derably simpler and cheaper as it only has to seal against its seat, whereas there are no requirements whatsoever about any sealing cooperation between the valve body and the rotors. Other objects of the invention and how those are met will be evident from the following detailed description of a preferred embodiment of the invention shown in the accom- panying drawings. * Fig. 1 diagrammatically illustrates an embodiment of a refrigeration plant according to the invention, Fig. 2 shows a vertical section through a compressor taken on line 2-2 in Fig. 3, and Fig. 2 shows a horizontal section through the compressor of Fig. 2 taken on line 3-3 in Fig. 2. A refrigeration plant as shown in Fig. 1 comprises a compressor 10 communicating with a condenser 12 through a high pressure channel 14 and with an evaporator 16 through a low pressure channel 18. The condenser 12 and the evapo- 5 rator 16 are interconnected by a channel 20 in which two sets of pressure reduction means 22, 24 are disposed, each shaped as a throttling valve. An intermediate pressure ves¬ sel 26 in the shape of a flash chamber is disposed between the two throttling valves 22, 24. The flash gas side of the 10 intermediate pressure vessel 26 communicates through a channel 28 with a housing 30 enclosing an electrical motor 32 drivingly connected with the compressor 10. From the housing 30 the flash gas passes through a pressure preser¬ vation valve 34 for keeping a minimum pressure in the in- 15 termediate pressure section 26, 28, 30 of the plant and an intermediate channel 36 to intermediate port means 38 in the compressor 10. The intermediate channel 36 may further communicate with the low pressure channel 18 through a se¬ lectively adjustable valve 40. The plant is further provi- 20 ded with a channel 42 for transferring liquified refrige¬ rant from the condenser 12 through a heat exchanger 44 for cooling of the liquid by the intermediate pressure fluid, and through a valve 46 for control of the liquid flow in dependence of the temperature in the high pressure channel 25 14, to a liquid injection opening 48 in the compressor 10. The compressor 10, shown in Figs. 2 and 3, is of the in- termeshing screw rotor type comprising a male rotor 50 and a female rotor 52 and a casing 54 providing a working space 30 56 enclosing the rotors and communicating with the low pressure channel through an inlet port 58 and with the high pressure channel 14 through an outlet port 60. The compressor casing 54 is rigidly connected with a mo¬ tor housing 30 enclosing an electrical motor 32 coaxial 35 with and directly joined to the male rotor 50. The motor housing 30 is provided with an inlet opening 62 communicat¬ ing with the channel 28 and with an outlet opening .64 for intermediate pressure fluid passing through the motor 32 for cooling thereof by heat exchanging between the motor and the intermediate pressure fluid. The outlet opening 64 communicates with an adjustable valve 34 provided to keep a certain minimum pressure inside the motor housing 30. The fluid from the valve 34 passes through an intermediate channel 36 to port means shaped as an opening 38 in the high pressure end wall of the working space 56. The opening 38 is disposed at such an angular position that any commu¬ nication through the working space 56 between- said opening 38 and the inlet port 58 is continuously blocked by at least one rotor lobe on each rotor 50, 52. A selectively adjustable valve 40 is provided between the intermediate channel 36 and the low pressure channel 18 to achieve a communication therebetween. The valve 40 and the port open¬ ing 38 are so dimensioned in relation to each other that the flow area of the valve is about double that of the port opening. The compressor 10 is further provided with an axially selectively adjustable valve member 66, generally of the type shown in US patent 3,088,659, Fig. 1, in the shape of an axially extending body forming a portion of the barrel wall of the working space 56 from the low pressure end wall thereof to the outlet port 60. The end of the valve body 66 facing the outlet port 60 is provided with an edge 68 de¬ fining the angular position of the rotors in which the com¬ munication with the high pressure channel 14 through the outlet port 60 is initiated. The value body 66 is provided with an internal channel 70 communicating at one end there¬ of with the liquid refrigerant channel 42 and forming at its other end the liquid injection opening 48. This opening 48 is disposed such that when the valve member 66 is in its position for maximum size of the outlet port 60 SUBSTIT .AU/TESHEET • -i'i-ϊ _3- * r- any communication through the working space 56 between said injection opening 48 and the intermediate port opening 38 is continuously blocked by at least one rotor lobe on each rotor 50, 52. The compressor is further provided with two independent and selectively adjustable bleed valves 72, 74 for return of practically uncompressed working fluid from the working space through each of said bleed valves 72, 74 and a rela¬ ted over-flow channel 76 and 78, respectively, to the low pressure channel 18. The valves 40, 72, and 74 are all shaped as lift valves selectively operable by pressure fluid available inside the compressor system. The valves 72, 74 are further provided with an end surface curved as the adjacent barrel wall of the working space 56 and adapted to lie in flush therewith when the valve is in closed position. A plant according to the invention operates in the fol¬ lowing way. Compressed gaseous working fluid is delivered from the compressor 10 to the condenser 12 where it is li- quified by external cooling means. From the condenser 12 the main mass of the liquified working fluid passes through the first throttling valve 22, whereby the pressure is re¬ duced, to the intermediate pressure vessel 26 where the working fluid is partly evaporated as flash gas and the re- maining liquified working fluid is cooled down to the eva¬ porating temperature corresponding to the pressure in the vessel 26. This cooled liquified working fluid passes through the second throttlling valve 24 whereby the pres¬ sure is further reduced, to the evaporator 16 where the working fluid is evaporated by external heating means. The low pressure gaseous working fluid is then returned from the evaporator 16 to the compressor 10 inlet 18, recompres- sed and recirculated to the condenser 12.The flash gas pro¬ duced in the intermediate pressure vessel 26 is passed through the motor housing 30, where it cools the electrical motor 32. The cooling e fect may be f rther improved by additional supply of some liquified working fluid to the motor housing 30. From this housing the flash gas is then passed on to an intermediate channel 36 disposed within the compressor casing 54 and communicating with port means 38 in the wall of the working space 56 of the compressor 10. Preferably a pressure preservation valve 34 is disposed between the motor housing 32 and the intermediate channel 36 in order to maintain a certain minimum pressure inside the motor housing 32. The port means 38 is shaped as an opening in the high pressure end wall of the working space 56 disposed in such an angular position that it communica¬ tes with a rotor groove which by means of a trailing rotor land is always brought out of communication with the inlet port 58. At full capacity conditions of the plant the compressor 10 is filled to its maximum capacity by low pressure work¬ ing fluid from the evaporator 16 through the inlet port 58 simultaneously as the intermediate pressure gas used for precooling the liquified working fluid to the evaporator 16 and for cooling the motor 32 is supplied through the in¬ termediate port means 38 to a compression chamber where the pressure has already been increased from the inlet port conditions. In this way the power for recompression of the gas supplied through the intermediate port means is reduced as the compression thereof starts at a higher pressure le¬ vel than the inlet pressure of the compressor. Simulta¬ neously the full capacity of the compressor can be used for the gas from the evaporator which means that for a certain capacity of the plant the dimensios of the compressor can be reduced. In order to achieve part load conditions the valve 40 between the intermediate channel 36 and the inlet channel 18 is opened. In this way the intermediate pressure fluid instead of entering through the intermediate port means 38 is by-passed the compressor 10 to the inlet channel 18 and thus replaces some of the gas otherwise sucked in from the evaporator 16. The intermediate port means 38 will further instead of acting as an additional inlet port acts as a bleed port for negligibly compressed gas returning through the intermediate channel 36 and the valve 40 to the inlet channel 18, whereby the capacity of the compressor 10 is further reduced, resulting in still less working fluid to pass through the evaporator 16 so that the capacity of the plant is considerably reduced. By the pressure preservation valve 34 the pressure in the motor housing 32 and thus in the intermediate pressure vessel 26 is kept on such a level that the evaporator 16 is continuously supplied with an amount of working fluid equal to that sucked in therefrom by the compressor 10. When running under such part load conditions the pressure level inside the compressor is re¬ duced such that the pressure in a compression chamber just cut off from the intermediate port 38 will be equal to that in the inlet channel 18 instead of equal to the intermedia- te pressure vessel 26 when running at full load, whereas the pressure in the condenser 12 will be practically con¬ stant as it depends upon the pressure correspoding to the condensation temperature. In order to obtain a good effi¬ ciency the outlet port 60 has to be reduced so that the built-in volume ratio has to be changed such that the built-in pressure ratio corresponds to the ratio between the condensation and the evaporation pressures. The size of the outlet port 60 is changed by adjustment of adjustable valve 66. In order to improve the sealing and especially the cool¬ ing of the gas during compression within the compressor 10 liquified working fluid from the condenser 12 is injected into the compressor 10 through the injection opening 48 disposed such that the liquid is injected into a rotor groove after that said groove is cut off from the inter- mediate port 38 so that no liquid can pass directly from the injection opening 48 to the intermediate port 38. The amount of liquid to be injected is adjusted by the valve 46 in order to keep the temperature in the high pressure chan- nel 14 at an almost constant temperature being only some¬ what higher than the temperature in the condenser 12. Further reduction of the capacity of the compressor 10 and of the plant can be obtained in steps by means of the two bleed valves 70, 74 disposed in different angular posi- tions in relation to the rotor grooves.

Claims: Claims 1. Plant of refrigeration type comprising a rotary po¬ sitive displacement compressor having at least one rotor provided with spiral lobes and intervening grooves, a condenser communicating with an outlet port of the comp- ressor through a high pressure channel, an evaporator communicating with an inlet port of the compressor through a low pressure channel, a vessel for an interme¬ diate pressure communicating with intermediate port means of the compressor through an intermediate pressure channel, said intermediate port means being spaced from as well said inlet port as said outlet port, and pres¬ sure reduction means for decreasing the high pressure in the condenser to the intermediate pressure in the vessel and to the low pressure in the evaporator, respectively, characterized in selectively adjustable valve means for forming a communication between said intermediate chan¬ nel and said low pressure channel. 2. Plant as defined in claim 1 , in which the flow area of said adjustable valve means in its maximum opening position is larger than the area of said intermediate port means. 3. Plant as defined in claim 1 or 2, in which said in¬ termediate port means are disposed in the high pressure end wall of the compressor. 4. Plant as defined in any of claims 1 to 3, in which the compressor is provided with additional, selectively adjustable valve means cooperating with at least one bleed port communicating with the inlet channel and dis¬ posed in the wall of the working space such that the vo- lumetric capacity of the compressor may be further redu¬ ced. 5. Plant as defined in any of claims 1 to 4, in which the compressor is provided with at least one injection opening for liquified refrigerant, said opening being spaced from said intermediate port means and disposed such that any communication between said opening and said intermediate port means through the working space is continuously blocked by at least one rotor lobe. 6. Plant as defined in claim 5, in which the liquified refrigerant to be injected is precooled by said inter e- diate pressure fluid before the injection thereof. 7. Plant as defined in any of claims 1 to 6, in which the compressor is provided with an adjustable valve mem¬ ber for variation of the outlet port in dependence of the adjustment of said selectively adjustable valve means and/or the actual temperature in the condenser and the evaporator. 8. Plant as defined in claim 7, in which said a justab¬ le valve member is slidable in axial diection and provi- ded with an edge determing the angular position of the cooperating rotor in which a communication is formed between a compression chamber and the high pressure channel. 9. Plant as defined in claim 8, comprising means for adjustment of said axially slidable valve member between three different positions. 10. Plant as defined in claim 8, comprising means for continuous adjustment of said axially slidable valve member between two extreme positions. 11. Plant as defined in any of the preceeding claims, in which a heat exchanger for cooling an electrical motor drivingly connected with the compressor is disposed with¬ in the intermediate pressure section of the plant. 12. Plant as defined in any of the preceeding claims, in which said intermediate pressure and said evaporator are provided in series with pressure reduction means there¬ between, whereby said vessel acts as a flash chamber pro¬ ducing flash gas of said intermediate pressure. 13. Rotary, positive displacement machine having at least one rotor with spiral lobes and intervening grooves for an elastic working fluid provided with an inlet port communicating with an inlet channel, an outlet port com¬ municating with an outlet channel, and intermediate port means communicating with an intermediate pressure chan¬ nel, said intermediate port means being spaced from as well said inlet port as said outlet port, characterized in selectively adjustable valve means for forming a com¬ munication between said intermediate pressure channel and said inlet channel. 14. Machine as defined in claim 13, especially adapted for use as a compressor in a plant of refrigeration type, which plant further comprises a condenser communicating with said outlet channel, an evaporator communicating with said inlet channel, a vessel for an intermediate pressure communicating with said intermediate pressure channel, and pressure reduction means for decreasing the high pressure in the condenser to the intermediate pres¬ sure in the vessel and to the low pressure in the evapo- rator, respectively, in which the compressor is provided with an adjustable valve member for variation of the out¬ let port in dependence of the adjustment of said selecti¬ vely adjustable valve means and/or the actual temperatu¬ res in the condenser and the evaporator. 15. Machine as defined in claim 14, in which said adjus¬ table valve member is slidable in axial direction and provided with an edge determining the angular position of the cooperating rotor in which a communication is formed between a compression chamber and the high pressure chan¬ nel. 16. Machine as defined in claim 15, comprising means for adjustment of said axially slidable valve member between two extreme positions. 17. Machine as defined in any of claims 14 to 16, in which the compressor is provided with additional, selec¬ tively adjustable valve means cooperating with at least one bleed port communicating with the inlet channel and disposed in the wall of the working space such that the volumetric capacity of the compressor may be further re¬ duced. 18. Machine as defined in- any of claims 14 to 17, in which the compressor is provided with at least one injec¬ tion opening for liquified refrigerant, said opening be- ing spaced from said intermediate port means and disposed such that any communication between said opening and said intermediate port means through the working space is con¬ tinuously blocked by at least one rotor lobe. 19. Machine as defined in any of claims 14 to 18, in which the flow area of said adjustable valve means bet¬ ween said intermediate pressure channel and said inlet channel in its maximum opening position is larger than the area of said intermediate port means. 20. Machine as defined in any of claims 14 to 19, in which said intermediate port means are disposed in the high pressure end wall of the compressor.

Inventors: SHAW DAVID N, SHAW, DAVID, N.
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1986
Main classification: F04C18
IPC codes: F04C18, F04C28, F04C29, F25B1