EP1800071A2

EP1800071A2 - Cooling systems

Info

Publication number: EP1800071A2
Application number: EP05794299A
Authority: EP
Inventors: Michal K. Grabon
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2004-09-29
Filing date: 2005-09-07
Publication date: 2007-06-27
Also published as: WO2006039042A3; CN101031763A; WO2006039042A2; US20060064997A1

Abstract

An apparatus has a compressor having suction and discharge ports. One or more conduits form a main flowpath from the discharge port through a condenser, a heat exchanger first leg, a first expansion device, and an evaporator to return to the suction port. The conduits also form a bypass flowpath bypassing the heat exchanger first leg, the first expansion device, and the evaporator but passing through a second leg of the heat exchanger in heat exchange relation with the first leg.

Description

COOLING SYSTEMS

BACKGROUND OF THE INVENTION

[0001] The invention relates to cooling systems. More particularly, the invention relates to the control of refrigerant phase in evaporators of air conditioning and refrigeration systems.

[0002] Many engineering considerations attend the design and operation of closed air conditioning and refrigeration systems. Among these are a variety of efficiency and other considerations attendant to evaporator operation. Proper evaporator operation is important for obtaining efficient and reliable system operation. Considerations include the handling of refrigerant and the management of heat transfer. Among problems that must be managed is excessive icing as this may interfere with heat transfer. Accordingly, much effort has gone into evaporator design and engineering.

[0003] A particular area of emphasis has been the engineering of distributor systems. A distributor receives two-phase refrigerant from the expansion device and provides balanced delivery of liquid and gas refrigerant phases among the various coils of an evaporator so as to prevent uneven performance. Various types of distributors have been developed. These include capillary-type distributors and impingement/turbulence distributors. Exemplary distributors are shown in US 2,148,414, 2,461,876, 3,795,259, 4,543,802, 5,832,744, and 5,842351, EP 0160542, and JP 5-322378 and 10-185363.

[0004] Nevertheless there remains room for further improvement in the art.

SUMMARY OF THE INVENTION

[0005] One aspect of the invention involves an apparatus including a compressor having suction and discharge ports, a condenser, first and second expansion devices, an evaporator, and a heat exchanger having first and second portions in heat exchange relation with each other. One or more conduits form a main flowpath and a bypass flowpath. The main flowpath runs from the discharge port through the condenser, the heat exchanger first portion, the first expansion device, and the evaporator, and returns to the suction port. The bypass flowpath bypasses the neat excnanger first portion, the first expansion device, and the evaporator, but passes through the second expansion device and the heat exchanger second portion.

[0006] In various implementations, the evaporator may lack a distributor. The second expansion device may be a TXV having a bulb essentially in heat exchange relation with a suction port condition. The second expansion device may be an EXV. A controller may be coupled to the EXV and programmed to control the EXV responsive to indicated superheat. The heat exchanger first portion may be downstream of the condenser and upstream of the evaporator along the main fiowpath. The heat exchanger second portion may be downstream of the condenser along the bypass fiowpath. The heat exchanger first portion may be upstream of the first expansion device along the main fiowpath. The evaporator may be a refrigerant-to-air heat exchanger. In at least a bypass mode, a bypass flow along the bypass fiowpath may enter the heat exchanger second portion in a two-phase gas/liquid condition and exit the heat exchanger second portion in a single-phase superheated gas condition. In the bypass mode, a main flow along the main flowpath may remain essentially a single-phase liquid in said heat exchanger second portion. The compressor may be selected from the group consisting of screw compressors and scroll compressors.

[0007] Another aspect of the invention involves a method for operating such an apparatus. At least one operational parameter is detected. Responsive to the detection, at least the second expansion device is operated so as to maintain essentially single-phase liquid refrigerant entering the evaporator along the main flowpath. The at least one operational parameter may include at least one of saturated suction temperature and actual suction temperature.

[0008] Another aspect of the invention involves a method for operating a cooling system. A main flow of refrigerant is caused to pass through an evaporator. The main flow is precooled upstream of the evaporator so as to maintain the main flow essentially as a liquid entering the evaporator. The precooling may comprise controlling a bypass flow in heat exchange relation with the main flow. The method may further comprise determining whether, absent the precooling, the main flow would enter the evaporator essentially as a two-phase flow. [0009] Another aspect of the invention involves a system comprising a compressor, a condenser, an expansion device, and an evaporator, a discharge line couples the compressor to the condenser to carry at least a main flow of refrigerant from the compressor to the condenser. A suction line couples the evaporator to the compressor to carry refrigerant from the condenser to the compressor. The system includes means for precooling refrigerant entering the expansion device so as to maintain the main flow essentially as a liquid while flowing along a flowpath length at least from the expansion device to the evaporator.

[0010] In various implementations, the evaporator may lack a distributor. Within the evaporator, the main flow may transition to a two-phase liquid/gas flow and then to a one-phase superheated gas flow. The bypass flow may represent 10%~35%, by weight, of a total refrigerant flow through the compressor.

[0011] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic representation of a refrigeration or air conditioning system employing the present invention.

[0013] FIG. 2 is a phase diagram for a prior art system.

[0014] FIG. 3 is a phase diagram for the system of FIG. 1.

[0015] Like reference numbers and designations in the various drawings indicate like elements.

B¹ETAILED DESCRIPTION

[0016] FIG. 1, shows an exemplary closed refrigeration or air conditioning system 10. The system 10 has a hermetic compressor 12, from which a compressor discharge conduit or line 14 extends downstream to a condenser 16. An intermediate line 18 extends downstream from the condenser 16 to an expansion device 20 and an evaporator 22. A suction line 24 extends downstream from the evaporator 22 to the compressor 12 to complete the main circuit/flowpath 26.

[0017] To form a bypass circuit/flowpath 28, a bypass line 30 branches off from the intermediate line 18 and contains an auxiliary expansion device 32 and connects with the suction line 24. A heat exchanger 34 is located such that the bypass line 30, downstream of the expansion device 32, and the line 18, upstream of the main expansion device 20, are in heat exchange relationship.

[0018] The exemplary evaporator 22 is a cross-flow refrigerant-to-air heat exchanger having a number of parallel refrigerant coils 36 extending from inlet ends at a liquid collector or manifold 38 to outlet ends at a suction collector or manifold 40. A fan 42 drives an airflow 44 across the coils 36 so that the refrigerant passing through the coils may draw heat from the airflow.

[0019] Exemplary expansion devices 20 and 32 are electronic expansion valves (EEVs) and are illustrated as coupled to a monitoring/control system 44 (e.g., a microprocessor-based controller) for receiving control inputs via control lines 45 and 46, respectively. The exemplary control system 44 may receive inputs such as zone inputs from one or more sensors 47, system condition inputs from one or more sensors (e.g., suction temperature sensor 50 and suction pressure sensor 52), and external control inputs from one or more input devices (e.g., thermostats 60).

[0020] Alternatively to the EEVs, any of a variety of expansion devices may be used (e.g., a thermal expansion valve (TXV) 32 having a remote bulb 70, a fixed orifice device, or a capillary tube device).

[0021] A basic prior art system would lack the bypass flowpath 28 and heat exchanger 34. FIG. 2 shows pressure 100 and enthalpy 102 for the refrigerant flow in such a basic system (or'tne preseriFsystem with no bypass flow). A boundary 104 separates a two-phase gas/liquid mixture domain 106 from a single phase sub-cooled liquid domain 108 and a single phase superheated gas domain 110. Suction conditions are shown as point or condition 120 at enthalpy 122 and pressure 124. These conditions are essentially present in the flowpath downstream from the suction manifold 40 to the compressor suction port. The refrigerant is compressed (plot compression segment 125) in the compressor 12 to a compressed point 126 with increased enthalpy 128 and increased pressure 130. During the compression 125, the refrigerant may typically remain in the superheated gas domain 110 or may transition thereto from the two-phase domain 106. The refrigerant is condensed (condensing segment 131) in the condenser 16 to a condensed point 132 with reduced enthalpy 134 but at the same pressure as the compressed/discharge condition. During the condensing 131, the refrigerant state may transition from the superheated gas domain 110 to the two-phase domain 106 and even into the sub-cooled liquid domain 108. The refrigerant is expanded (expansion segment 135) in the expansion device 34 to an expanded point 136 with decreased pressure 138. During the expansion 135, enthalpy may remain essentially constant at 134. The refrigerant may reenter or remain in the two-phase domain 106 during the expansion 135. This expanded two-phase refrigerant must enter the evaporator. The refrigerant is evaporated (evaporation segment 139) in the evaporator to return to the suction point 120 with substantially increased enthalpy and slightly decreased pressure relative to the expanded point 136.

[0022] The presence of the expanded point 136 in the two-phase domain 106 presents substantial problems. With two-phase refrigerant entering the evaporator, it becomes difficult to balance the refrigerant across the evaporator coils. Namely, otherwise similar coils may see different total amounts of refrigerant and/or different ratios of the two phases. This can produce substantially different coil conditions amongst the various coils. The coils with higher amounts of refrigerant and higher relative amounts of liquid may overcool so as to produce excessive frost buildup. Overall efficiency may be reduced. Accordingly, it is known to use complicated distributor arrangements in place of the liquid manifold 38 to balance the ratios among the different coils. Distributors tend to be expensive. Advantageously, under ambient conditions that would otherwise cause the point 136 to be in the two-phase domain 106, it would be desirable to shift the point 136 into the single phase liquid domain 108 eliminating the need for a distributor. In such a situation, the single-phase liquid inlet flow to the evaporator could readily be separated into similar flows for each coil. The coils could be designed/configured for operating with such an input flow. [0023] FIG. 3 shows how the bypass flow of the present invention may be utilized to achieve advantageous refrigerant conditions entering the evaporator 22. The suction condition/point 220 may be essentially the same as point 120 of FIG. 2. After compression 225, the compressed/discharge/point 226 may be similar to the point 126 of FIG. 2. The condensing 231 brings the combined main flow and bypass flow to a condensed/point 232 which may be similar to point 132.

[0024] From the condensed condition, the bypass flow splits from the main flow. The bypass flow refrigerant is expanded (segment 233) to reach a point 234 which may be essentially at the suction pressure 124 and the enthalpy 134. Heat exchange (235 for the bypass flow and 236 for the main flow) from the main flow to the bypass flow in the heat exchanger 34 then returns the bypass flow conditions to point 220 and cools the main flow to a precooled/point 238 with further reduced main flow enthalpy 240. The main flow of refrigerant is expanded (segment 241) in the expansion device 20 to a point 242 with decreased pressure 244 (which may be essentially the same as 138). The main flow of refrigerant is evaporated (segment 245) in the evaporator 22 to return the main flow to the initial suction point 220. The heat exchange from the bypass flow to the main flow tends to shift the point 242 to a lower enthalpy condition. The required amount of heat exchange may depend upon ambient conditions.

[0025] A basic operation of the expansion device 32 may be responsive to sensed superheat of the refrigerant exiting the evaporator 22. The degree of superheat (actual temperature minus saturated temperature) may be determined based upon the output of the temperature sensor 50 for the actual temperature and the pressure sensor 52 for the saturated temperature (e.g., in view of known refrigerant properties which may be programmed into the control system 44). The expansion device 32 may be opened either in a binary fashion or a progressive fashion in response to the presence or degree of superheat or superheat parameter (e.g., superheat above a threshold). With a TXV as the device 32, control could be achieved by placing its bulb 70 in heat exchange relation with the refrigerant at suction conditions. Much more complex arrangements are also possible.

[0026] The expansion device 32 and/or other components of the bypass flowpath may be dimensioned in view of main flowpath components to permit an appropriate balance between bypass and non-bypass flows. In an exemplary binary configuration (i.e., the binary flow has only off and on conditions) an exemplary balance involves having the bypass flow be approximately 30% of the total flow through the compressor (i.e., 3/7 of the non-bypass flow) by weight/mass. Broader exemplary figures for binary operation are 25%-33%, and 10%-35%. Progressive or stepwise operation may permit maximums in such ranges and may, optionally, permit flows smaller than the lower ends of such ranges.

[0027] One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a modification or a reengineering of an existing system, details of the existing system may heavily influence details of the implementation. Although illustrated with regard to a basic system and with simplified conditions, the principles may be applied to more complex system configurations, whether existing or yet-developed. Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMS What is claimed is:

1. An apparatus comprising: a compressor having suction and discharge ports; a condenser; a first expansion device; a second expansion device; an evaporator; a heat exchanger having first and second portions in heat exchange relation with each other; and one or more conduits forming: a main fiowpath from the discharge port through the condenser, the heat exchanger first portion, the first expansion device, and the evaporator and returning to the suction port; and a bypass fiowpath bypassing the heat exchanger first portion, the first expansion device, and the evaporator, but passing through the second expansion device and the heat exchanger second portion.

2. The apparatus of claim 1 wherein: the evaporator lacks a distributor.

3. The apparatus of claim 1 wherein: the second expansion device is a TXV having a bulb essentially in heat exchange relation with a suction port condition.

4. The apparatus of claim 1 wherein: the second expansion device is an EXV.

5. The apparatus of claim 4 further comprising: a controller coupled to the EXV and programmed to control the EXV responsive to indicated superheat.

6. The apparatus of claim 1 wherein: the heat exchanger first portion is downstream of the condenser and upstream of the evaporator along the main flowpath; and the heat exchanger second portion is downstream of the condenser along the bypass flowpath.

7. The apparatus of claim 6 wherein: the heat exchanger first portion is upstream of the first expansion device along the main flowpath.

8. The apparatus of claim 1 wherein: the evaporator is a refrigerant-to-air heat exchanger.

9. The apparatus of claim 1 wherein: in at least a bypass mode, a bypass flow along the bypass flowpath enters the heat exchanger second portion in a two-phase gas/liquid condition and exits the heat exchanger second portion in a single-phase superheated gas condition; and in the bypass mode, a main flow along the main flowpath remains essentially a single-phase liquid in said heat exchanger second portion.

10. The apparatus of claim 1 wherein: the compressor is selected from the group consisting of screw compressors and scroll compressors.

11. A method for operating the apparatus of claim 1 comprising: detecting at least one operational parameter; and responsive to the detecting, operating at least the second expansion device so as to maintain essentially single-phase liquid refrigerant entering the evaporator along the main flowpath.

12. The method of claim 11 wherein: the at least one operational parameter includes at least one of: saturated suction temperature; and actual suction temperature.

13. A method for operating a cooling system comprising: causing a main flow of refrigerant through an evaporator; and precooling the main flow upstream of the evaporator so as to so as to maintain said main flow essentially as a liquid entering the evaporator.

14. The method of claim 13 wherein: the precooling comprises controlling a bypass flow in heat exchange relation with the main flow.

15. The method of claim 13 further comprising: determining whether, absent the precooling, the main flow would enter the evaporator essentially as a two-phase flow.

16. The method of claim 15 wherein: the determining includes determining that a superheat of refrigerant exiting the evaporator exceeds a threshold.

17. A system comprising: a compressor; a condenser; a discharge line, coupling the compressor to the condenser to carry at least a main flow of refrigerant from the compressor to the condenser; an expansion device; an evaporator; a suction line, coupling the evaporator to the compressor to carry refrigerant from the condenser to the compressor and comprising a first and second parallel segments; and means for precooling refrigerant entering the expansion device so as to maintain said main flow essentially as a liquid while flowing along a flowpath length at least from the expansion device to the evaporator.

18. The system of claim 17 wherein: the evaporator lacks a distributor.

19. The system of claim 17 wherein: within the evaporator, the main flow transitions to a two-phase liquid/gas flow and then to a one-phase superheated gas flow.

20. The system of claim 17 wherein: the bypass flow represents 10%-35%, by weight, of a total refrigerant flow through the compressor.