EP2347196A1

EP2347196A1 - Heat pump system and method of operating

Info

Publication number: EP2347196A1
Application number: EP08878058A
Authority: EP
Inventors: Jing Ll; Shufu Ding; Haijun Ll
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2008-11-11
Filing date: 2008-11-11
Publication date: 2011-07-27
Also published as: CN102216700A; WO2010054498A1; US20110203299A1; CN102216700B

Abstract

A heat pump system operable in a cooling mode, a heating mode and a defrost mode includes a refrigerant compressor (20), a reversing valve (30), a first heat exchanger (40) and a second heat exchanger (50) disposed in a refrigerant circuit, and a primary expansion valve (45) disposed in the refrigerant circuit between said first heat exchanger (40) and said second heat exchanger (50); said reversing valve (30) is positionable in a first position for operation of said heat pump system in the cooling mode or defrost mode and is positionable in a second position for operation of said heat pump system in the heating mode; a refrigerant bypass circuit establishes a refrigerant flow path from the refrigerant circuit at a first location upstream of said primary expansion valve (45) and downstream of said first heat exchanger (40) with respect to refrigerant flow in the defrost mode to a liquid reservoir (70) disposed in the refrigerant circuit at a second location downstream of said primary expansion valve (45) with respect to refrigerant flow in the defrost mode.

Description

HEAT PUMP SYSTEM AND METHOD OF OPERATING

Field of the Invention

[0001] This invention relates generally heat pumps and, more particularly, to a refrigeration circuit for and method of operation of an air source heat pump.

Background of the Invention

[0002] Air source heat pumps use ambient outside air as the heat source or heat sink, respectively, for heating or cooling another heat exchange medium. Conventional air source heat pumps include a compressor, a reversing valve, a first heat exchanger, an expansion device and a second heat exchanger arranged in a refrigerant vapor compression cycle refrigerant circuit in a manner well-known in the art. Air source heat pumps are commonly switched between a heating mode and a cooling mode through selective positioning of the reversing valve. In conventional systems, the first heat exchanger is disposed outdoors and acts as a refrigerant heat rejection heat exchanger, such as a condenser of refrigerant vapor, when the heat pump is operating in the cooling mode and acts as a refrigerant heat absorption heat exchanger, such as a refrigerant evaporator, when the heat pump is operating in the heating mode. Conversely, the second heat exchanger acts as a refrigerant heat absorption heat exchanger when the heat pump is operating in the cooling mode and acts as a refrigerant heat rejection heat exchanger when the heat pump is operating in the heating mode.

[0003] When the air source heat pump is operating in the heating mode, depending upon outdoor ambient conditions, frost can form and build up on the refrigerant tube coils of the first heat exchanger, which acts as a refrigerant heat absorption heat exchanger in the heating mode. It is customary practice to periodically defrost the outdoor heat exchanger coil by switching the heat pump to operation in the cooling mode for a period of operation sufficient to heat that coil and melt the frost accumulated thereon, and then switching the heat pump back to operation in the heating mode. When switching the reversing valve from the defrost mode back to the heating mode, an excess of liquid refrigerant migrates to the suction side of the compressor. Therefore, it is customary in conventional air source heat pump systems to include an accumulator in the refrigerant circuit on the suction side of the compressor upstream of the suction inlet to the compressor. The accumulator provides a reservoir to collect the liquid refrigerant so as to prevent the carryover of liquid refrigerant into the suction inlet of the compressor. It is desirable to avoid the introduction of liquid refrigerant into the compressor as the presence of liquid refrigerant in the compressor is detrimental to compressor performance. For conventional large capacity heat pumps, the suction accumulator must be sized to handle a significant amount of liquid refrigerant. Consequently, the suction accumulator typically is a high cost item in the heat pump system. Additionally, the presence of a suction accumulator in the refrigerant circuit imparts a refrigerant pressure drop to the refrigerant passing therethrough to the suction inlet of the compressor. This added pressure drop adversely impacts the coefficient of performance of the heat pump system.

[0004] U.S. Pat. No. 4,843,838 discloses an air-to-air heat pump having alternate refrigerant lines interconnecting the indoor coil and the outdoor coil. Each of these refrigerant lines includes a check valve and a float valve. Refrigerant flows through one of these lines during operation in the cooling mode and through the other line during operation in the heating mode. The usual trap type accumulator is not required and is eliminated.

Summary of the Invention

[0005] A heat pump system operable in a cooling mode, a heating mode and a defrost mode includes a refrigerant compressor, a reversing valve, a first heat exchanger and a second heat exchanger disposed in a refrigerant circuit, and a primary expansion device disposed in the refrigerant circuit intermediate the first heat exchanger and the second heat exchanger. The reversing valve may be positioned in a first position for operation of the heat pump system in the cooling mode and may be positioned in a second position for operation of the heat pump system in the heating mode.

[0006] In an aspect of the invention, the heat pump includes a refrigerant bypass circuit establishing a refrigerant flow path from the refrigerant circuit at a first location upstream of the primary expansion device and downstream of the first heat exchanger with respect to refrigerant flow in the defrost mode to a liquid reservoir disposed in the refrigerant circuit at a second location downstream with respect to refrigerant flow in the defrost mode of the primary expansion valve. [0007] In an embodiment, the second heat exchanger defines a refrigerant collection chamber comprising the liquid reservoir. The second heat exchanger may a shell and tube heat exchanger having a shell defining the refrigerant collection chamber and a tube bank heat exchanger disposed in the refrigerant collection chamber. In an embodiment, the liquid reservoir comprises a refrigerant receiver disposed in the refrigerant circuit intermediate the primary expansion device and the second heat exchanger.

[0008] The bypass circuit may comprise a bypass refrigerant line interconnecting the refrigerant circuit at the first location upstream of the primary expansion device and downstream of the first heat exchanger with respect to refrigerant flow in the defrost mode in refrigerant flow communication with the liquid reservoir and a bypass refrigerant flow control device interdisposed in the refrigerant bypass line. The bypass refrigerant flow control device may comprise a flow control valve having a first position in which the bypass refrigerant line is open to refrigerant flow and a second position in which the bypass refrigerant line is closed to refrigerant flow. In an embodiment, the bypass refrigerant flow control device comprises an open position/closed position solenoid valve. [0009] In an aspect of the invention, a method is provided for operating a heat pump system during operation in a defrost mode. The heat pump system includes a refrigerant compressor, a reversing valve, a first heat exchanger and a second heat exchanger disposed in a refrigerant circuit, and a primary expansion device disposed in the refrigerant circuit intermediate the first heat exchanger and the second heat exchanger. The reversing valve is positionable in a first position for operation of the heat pump system in the cooling or defrost mode and is positionable in a second position for operation of the heat pump system in the heating mode. The method includes the steps of: initiating switching of the reversing valve from its second position into its first position for operation in the defrost mode; prior to terminating operation in the defrost mode, passing refrigerant flow from the refrigerant circuit through a refrigerant bypass circuit to a liquid refrigerant reservoir; and initiating switching of said reversing valve out of its first position. The method may include the further steps of: providing a flow control valve in the bypass refrigerant circuit, the flow control valve having an open position in which the bypass refrigerant line is open to refrigerant flow and a closed position in which the bypass refrigerant line is closed to refrigerant flow. [0010] The step of passing refrigerant flow from the refrigerant circuit through the refrigerant bypass circuit may comprise opening the flow control valve. The step of initiating the passing of refrigerant flow from the refrigerant circuit through the refrigerant bypass circuit by opening the flow control in defrost process may comprise opening the flow control valve when a discharge pressure of the compressor exceeds a first preselected discharge pressure set point (for example 165OkPa); or frost factor decreased to 0%, or defrost time (for example 8 minutes) has elapsed. If any one of these three conditions is met, the flow control valve will begin to open.

[0011] The step of terminating the passing of refrigerant flow from the refrigerant circuit through the refrigerant bypass circuit may comprise closing the flow control valve. The step of terminating the passing of refrigerant flow from the refrigerant circuit through the refrigerant bypass circuit by closing the flow control valve may comprise the step of closing the flow control valve after a preset time period has elapsed. The length of the preset time period may range from one second to 45 seconds or more depending upon the size of the flow control valve. In an embodiment, the preset time period may be about 5 seconds.

Brief Description of the Drawings

[0012] For a further understanding of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where:

[0013] FIG. 1 is a schematic diagram illustrating an exemplary embodiment of a heat pump system in accord with the invention illustrating operation of the heat pump system in the cooling mode;

[0014] FIG. 2 is a schematic diagram illustrating the heat pump system shown in FIG. 1 illustrating operation of the heat pump system in the heating mode; [0015] Figure 3 is a schematic diagram illustrating the heat pump system shown in FIG. 1 illustrating operation of the heat pump system at transition from the defrost mode into the heating mode;

[0016] Figure 4 is a schematic diagram illustrating another exemplary embodiment of a heat pump system in accord with the invention illustrating operation of the heat pump system at transition from the defrost mode into the heating mode;

[0017] Figure 5 is a schematic block diagram illustrating an exemplary embodiment of a method for operating a heat pump system in the defrost mode; and

[0018] Figure 6 is a schematic block diagram illustrating an exemplary embodiment of a method for starting the heat pump system in the heating mode.

Detailed Description of the Invention

[0019] The invention will be described with reference to the exemplary embodiments of the refrigerant heat pump system 10 depicted in FIGs. 1-4. In each of these figures, the direction of refrigerant flow is indicated by the arrows flanking the refrigerant circuit lines. The depicted heat pump system 10 is of the type commonly referred to as an air source heat pump. However, it is to be understood that the invention is not limited in application to air source heat pumps. [0020) The heat pump system 10 includes a compressor 20, a reversing valve

30, a first heat exchanger 40 and a second heat exchanger 50 connected in refrigerant flow communication by a plurality of refrigerant lines forming a closed- loop refrigerant circuit. A primary expansion device 45 is disposed in the refrigerant circuit intermediate the first heat exchanger 40 and the second heat exchanger 50. The heat pump system 10 may also include an economizer heat exchanger 60 disposed in the refrigerant circuit intermediate the first heat exchanger 40 and the second heat exchanger 50. A receiver 70 may also be disposed in the refrigerant circuit intermediate the primary expansion device 45 and the second heat exchanger 50.

[0021] The reversing valve 30 may comprise a selectively positionable, two- position, four-port valve having a first port 30-1, a second port 30-2, a third port 30- 3 and a fourth port 30-4. The reversing valve 30 is positioned in a first position for coupling the first port 30-1 and the second port 30-2 in fluid flow communication and for simultaneously coupling the third port 30-3 and the fourth port 30-4 in fluid flow communication for operation of the heat pump system in the cooling mode and in the defrost mode. The reversing valve 30 is also positioned in a second position for coupling the first port 30- land the fourth port 30-4 in fluid flow communication and for simultaneously coupling the second port 30-2 and the third port 30-3 in fluid flow communication for operation in the heating mode. The afore-mentioned respective port-to-port couplings established in the first and second positions may be accomplished internally within the reversing valve 30. For convenience of description, the reversing valve will be deemed energized in cooling mode and not energized in heating mode. The discharge outlet 28 of the compressor 20 is connected in fluid flow communication through refrigerant line 3 to the first port 30- 1 of the reversing valve 30. An oil separator 26 may be interdisposed in refrigerant line 3 between the compressor discharge outlet 28 for removing lubricating oil from the refrigerant passing through refrigerant line 3. The suction inlet 22 of the compressor 20 is connected in fluid flow communication through refrigerant line 5 to the third port 30-3 of the reversing valve 30.

[0022] The second port 30-2 of the reversing valve 30 is coupled externally of the reversing valve 30 in refrigerant flow communication to the fourth port 30-4 of the reversing valve 30 through a series of refrigerant lines. To operate the heat pump system 10 in a cooling mode, the reversing valve 30 is selectively positioned in its first position wherein the second port 30-2 of the reversing valve 30 is coupled in refrigerant flow communication to the fourth port 30-4 of the reversing valve 30 through refrigerant lines 7, 9A, 11, 13, 15 and 17, which are connected in series flow relationship with check valves 92 and 94 open to flow and check valves 96 and 98 closed to flow. To operate the heat pump system 10 in a heating mode, the reversing valve 30 is selectively positioned in its second position wherein the fourth port 30-4 is connected in refrigerant flow communication to the second port 30-2 of the reversing valve 30 through refrigerant lines 17, 15, 19, 11, 9B and 7, which are connected in series flow relationship with check valves 96and 98open to flow and check valves 92 and 94 closed to flow. [0023] In the cooling mode, the first heat exchanger 40 functions as a refrigerant heat rejection heat exchanger. In the heating mode, the first heat exchanger 40 functions as a refrigerant heat absorption heat exchanger. The first heat exchanger 40 is located outdoors, typically on the roof of or along side a building housing a climate controlled space. One or more fans 42 are disposed in operative association with the first heat exchanger 40 for passing ambient air through the first heat exchanger in heat exchange relationship with refrigerant passing through the refrigerant circuit. In an embodiment, the first heat exchanger 40 comprises a heat exchange coil formed of an array of finned tubes through which refrigerant passes in heat exchange relationship with ambient air passing over the exterior of the tubes and over the surfaces of the fins.

[0024] In the cooling mode, the second heat exchanger 50 functions as a refrigerant heat absorption heat exchanger. In the heating mode, the second heat exchanger 50 functions as a refrigerant heat rejection heat exchanger. In the depicted embodiment, the second heat exchanger 50 is coupled through a secondary heat exchange loop to an air side unit 80. The second heat exchanger 50 may also be located exteriorly of the climate-controlled space, typically outside of the building on the roof of the building or along side the building. In traversing the second heat exchanger 50, refrigerant from the refrigerant circuit passes in heat exchange relationship with a secondary heat exchange fluid, commonly water or glycol. The secondary heat exchange fluid may traverses a secondary heat exchange loop wherein the secondary heat exchange fluid passes in heat exchange relationship with air being drawn through the air side unit 80 from the climate controlled environment for cooling or heating the air prior to return to the climate controlled environment. [0025] In the depicted embodiment, the second heat exchanger 50 comprises a shell and tube heat exchanger having a heat exchange tube bank 52 disposed within the interior 55 of the shell 54 of the second heat exchanger 50. The interior 55 of the shell 54 defines a refrigerant collection chamber. A secondary heat exchange fluid passes through the tube bank 52 in heat exchange relationship with refrigerant from the refrigerant circuit of the heat pump system 10. In operation, the interior 55 of shell 54 is flooded with refrigerant from the refrigerant circuit. The refrigerant flows over the exterior of the tubes of the tube bank 52 in heat exchange relationship with the secondary heat exchange fluid passing through the tubes of the tube bank. The heat exchange tube bank may be partially or fully immersed in the refrigerant. In the depicted embodiment, the tube bank 52 comprises a first heat transfer module of a secondary heat exchange loop through which the secondary heat exchange fluid circulates. The secondary heat exchange loop includes a second heat transfer module which comprises a heat exchanger tube coil 82 of the air side unit 80, such as for example, but not limited to, an air handling unit or a fan coil unit, wherein the secondary heat exchange fluid passes through the heat exchanger tube coil 82 in heat exchange relationship with indoor air drawn from the climate- controlled space. In passing over the heat exchanger tube coil 82, the indoor air is cooled during operation of the heat pump system 10 in the cooling mode and is heated during operation of the heat pump system 10 in the heating mode. [0026] As depicted in the embodiment of the heat pump system 10 illustrated in FIGS. 1-4, the heat pump system 10 may include an economizer heat exchanger 60. In the depicted embodiment, the economizer heat exchanger 60 may comprise a refrigerant-to-refrigerant heat exchanger having a first refrigerant circuit branch 62 and a second refrigerant circuit branch 64 disposed in heat exchange relationship. The first refrigerant circuit branch 62 comprises a portion of the refrigerant line 1 1 , The second refrigerant circuit branch 64 comprises a portion of economizer refrigerant line 21 which taps into the refrigerant line 11 at a location upstream of the first refrigerant circuit branch 62 and provides a refrigerant flow passage from that point to an intermediate pressure port 24 of the compressor 20 through which refrigerant vapor may be injected into an intermediate pressure, that is a pressure intermediate the suction pressure and the discharge pressure, chamber of the compressor 20. A secondary expansion device 65 is disposed in the economizer refrigerant line 21 at a location upstream with respect to refrigerant flow of the second refrigerant circuit branch 64.

[0027] The economizer 60 is typically in operation in both the cooling mode and the heating mode, but generally not operated in the defrost mode. When the economizer is in operation, a portion of the liquid refrigerant passing through refrigerant line is diverted to flow through refrigerant line 21 and to traverse the secondary expansion device 65. The secondary expansion device 65 functions to expand refrigerant passing therethrough from a higher pressure, higher temperature refrigerant liquid to a lower pressure, lower temperature refrigerant vapor or liquid/vapor mix. The lower pressure, lower temperature refrigerant vapor or vapor/liquid mix passes through the second refrigerant circuit branch 64 in heat exchange relationship with the higher pressure, higher temperature refrigerant liquid passing through the first refrigerant circuit branch 62 whereby the refrigerant liquid is further cooled prior to traversing the primary expansion device 45 and the refrigerant vapor passing through the second refrigerant circuit branch 64 is heated prior to injection into an intermediate pressure stage of the compression process. The primary expansion valve 45 is disposed in refrigerant line 1 1 downstream with respect to refrigerant flow of the first refrigerant circuit branch 62. [0028] Referring now to FIG. 1 in particular, in operation of the heat pump system 10 in the cooling mode, hot, high pressure refrigerant vapor discharging from the compressor 20 through refrigerant line 3 passes through the reversing valve 30 from the first port 30-1 to the second port 30-2, thence through refrigerant line 7, thence through the first heat exchanger 40, thence through refrigerant line 9A, thence through refrigerant line 11 traversing the economizer heat exchanger 60 and the primary expansion valve 45, thence through refrigerant line 13, thence through refrigerant line 15, traversing the receiver 70 and the second heat exchanger 50, thence through refrigerant line 17 to the reversing valve 30. When the heat pump system 10 is operating in the cooling mode, the secondary heat transfer medium is passed through the second heat exchanger 50 in heat exchange relationship with the refrigerant within the second heat exchanger 50, whereby refrigerant is evaporated and the secondary heat transfer medium is cooled. In the cooling mode of operation, the refrigerant leaving the second heat exchanger 50 and passing through refrigerant line 17 consists of refrigerant vapor with little or no liquid refrigerant carryover. The refrigerant vapor passes from refrigerant line 17 into the fourth port 30-4 of the reversing valve 30 and out the third port 30-3 of the reversing valve 30 into and thence through the refrigerant line 5 to return to the compressor 20 through the suction inlet 22 to the compressor 20.

[0029] Referring now to FIG. 2 in particular, in operation of the heat pump system 10 in the heating mode, hot, high pressure refrigerant vapor discharging from the compressor 20 through refrigerant line 3 passes through the reversing valve 30 from the first port 30-1 to the fourth port 30-4, thence through refrigerant line 17, thence through the second heat exchanger 50, thence through refrigerant lines 15 and 19, traversing the receiver 70, thence through refrigerant line 1 1 traversing the economizer heat exchanger 60 and the primary expansion valve 45, thence through refrigerant line 9B, thence through the first heat exchanger 40 in heat exchange relationship within ambient outdoor air, thence through refrigerant line 7 to the reversing valve 30. When the heat pump is operating in the heating mode, the first heat exchanger 40 functions as a refrigerant evaporator, whereby the refrigerant leaving the first heat exchanger 40 and passing through refrigerant line 7 consists of refrigerant vapor with little or no liquid refrigerant carryover. The refrigerant vapor passes from refrigerant line 7 into the second port 30-2 of the reversing valve 30 and out the third port 30-3 of the reversing valve 30 into and thence through the refrigerant line 5 to return to the compressor 20 through the suction inlet 22 to the compressor 20.

[0030] When the heat pump system 10 is operating in the heating mode, the second heat exchanger 50 receives hot, high pressure refrigerant vapor discharged from the compressor. As the secondary heat transfer medium is passed through the second heat exchanger 50 in heat exchange relationship with the refrigerant vapor within the second heat exchanger 50, the refrigerant vapor is condensed and the secondary heat transfer medium is heated. Thus, in the heating mode of operation, the second heat exchanger 50 is operating as a refrigerant heat rejection heat exchanger, that is, a refrigerant condenser. In the heating mode of operation, the refrigerant leaving the second heat exchanger 50 and passing through refrigerant line 15 consists of liquid phase refrigerant.

[0031] In heat pump applications in temperate climates, depending upon ambient conditions, such as outdoor air temperature and humidity, during operation of the heat pump system in the heating mode, frost may form on the refrigerant conveying heat exchange coils of the first heat exchanger 40. Therefore, when operating the heat pump system in the heating mode, it is necessary to periodically interrupt operation in the heating mode and operate the heat pump system in a cooling mode for a limited period in order to defrost the heat exchange coils of the first heat exchanger 40. Switching into the defrost mode may be done automatically after a preset time period of operation in the heating mode or it may be done in response to a frost sensor, such as for example, but not limited to, a coil temperature sensor 41 operatively associated with the heat exchange coils of the first heat exchanger 40, or in response to an operating parameter. Termination of the defrost mode and switch back into the heating mode may also be done automatically after a preset time period of operation in the defrost mode or it may be done in response to a frost sensor operatively associated with the heat exchange coils of the first heat exchanger 40 or in response to an operating parameter.

[0032] Referring now to FIGs. 3 and 4 in particular, the heat pump system

10 is equipped with a refrigerant bypass line 23 which taps into refrigerant line 1 1 at a location downstream with respect to refrigerant flow of the economizer 60 and upstream with respect to refrigerant flow of the primary expansion valve 45. In the exemplary embodiment of the heat pump system 10 depicted in FIG. 3, the refrigerant bypass line 23 opens into the interior chamber of the receiver 70. In the exemplary embodiment of the heat pump system 10 depicted in FIG. 4, the refrigerant bypass line 23 opens into the refrigerant collection chamber 55 defined by the interior of the shell 54 of the shell and tube heat exchanger 50. A bypass flow control device 75 is disposed in the refrigerant bypass line 23. The bypass flow control device 75 may be selectively positioned in at least a first open position and a second closed position. With the bypass flow control device 75 positioned in its open position, refrigerant flows through the refrigerant bypass line 23 to either the receiver 70 or the shell and tube heat exchanger 50, bypassing the primary expansion device 45. With the bypass flow control device 75 positioned in its closed position, refrigerant flow through the refrigerant bypass line 23 is blocked and the refrigerant flowing through refrigerant line 1 1 continues on to pass through the primary expansion device 45. In an embodiment, the bypass flow control device 75 may comprise, for example but not limited to, a two-position on/off solenoid valve.

[0033] During steady state operation of the heat pump system 10 in either the heating mode or the cooling/defrost mode, the bypass flow control device 75 is positioned in a closed position. Upon termination of operation in the defrost mode, the reversing valve 30 is repositioned from the cooling/defrost mode position into the heating mode position. Prior to transition of the heat pump system 10 from the defrost mode into the heating mode by repositioning the reversing valve 30, the bypass flow control device 75 is positioned in an open position to divert liquid refrigerant flowing through refrigerant line 1 1 through the bypass refrigerant line 23 directly into either the receiver 70 or the refrigerant collection chamber 55 of the shell and tube heat exchanger 50 without traversing the primary expansion valve 45. Because the refrigerant flowing through the bypass refrigerant line 23 does not pass through the primary expansion valve 45, this refrigerant remains in the liquid phase. By diverting the refrigerant flow through bypass refrigerant line 23 to collect in either of the receiver 70 or the chamber 55 of the shell and tube heat exchanger 50 during the transition out of defrost, liquid refrigerant is permitted to pass from the first heat exchanger 40 directly into the receiver 70 or the chamber 55 of the shell and tube heat exchanger 50. In this manner, liquid refrigerant drains from the first heat exchanger 40 so that upon entering back in the heating mode of operation at the end of the transition from the defrost mode, little or no liquid refrigerant will be resident in the first heat exchanger 40, thereby minimizing or eliminating the potential for liquid phase refrigerant to be introduced into the compressor 20 through the suction inlet 22 when operation upon commencement of operation in the heating mode.

[0034] Typically, the reversing valve 30 remains positioned in the heating mode position after a heating mode cycle is completed and the compressor 20 is powered off, for example if heating demand has been satisfied or in the event of an emergency electrical power loss. With the reversing valve 30 remaining in the heating position, when restarting the compressor 20, liquid refrigerant in the heat exchange coil of the first heat exchanger 40 would flush into the compressor 20 because the heat exchange coil of the heat exchanger 40 is connected in fluid communication with the suction port 22 of the compressor 20 when the reserving valve 30 is positioned in the heating mode. Since the introduction of a significant amount of liquid refrigerant into the compressor 20 with the suction inlet 22 would be detrimental to the compressor 20, in the heat pump system 10 a flow control valve 43 and a flow check valve 47, disposed in a parallel arrangement, are interdisposed in refrigerant line 9 between the heat exchange coil of the first heat exchanger 40 and the intersection of refrigerant branch lines 9A and 9B. Thus, in reference to the heating mode, the flow control valve 43 and the flow check valve 47 are disposed upstream with respect to refrigerant flow through the circuit of the first heat exchanger 40 and downstream with respect to refrigerant flow of the primary expansion device 45. The flow check valve 47 provides a bypass circuit for refrigerant flow to bypass the flow control valve 43 when the flow control valve 43 is closed and the refrigerant is flowing from the first heat exchanger 40 through refrigerant line 9 as in the cooling or defrost mode. When refrigerant is flowing into the first heat exchanger 40 from refrigerant line 9 and through the flow control valve 43 as in the heating mode, the flow check valve 47 is inherently closed to flow. 10035] When the compressor 20 restarts in the heating mode, for example after completion of a defrost mode, the flow control valve 43, which may be a solenoid valve, such as a solenoid valve having an open position and a closed position, is positioned closed so that no liquid refrigerant will flow into the heat exchange coil of the first heat exchanger 40. Thus the refrigerant pressure, and therefore the saturated evaporation temperature, SET, of the refrigerant within the heat exchange coil of the heat exchanger 40 will decrease. Consequently, the liquid refrigerant within the heat exchange coil will start to evaporate into refrigerant vapor.

[0036] When the refrigerant liquid is almost completely evaporated, the refrigerant pressure and, correspondingly, the saturate evaporation temperature will further decrease. Once the temperature difference between ambient temperature and the saturated evaporation temperature reaches a preset temperature differential set point, such as for example 12 degrees Kelvin (21.6 degrees Fahrenheit), the flow control valve 43 is opened to allow the refrigerant from refrigerant line 9 to pass into and through the heat exchange coil of the heat exchanger 40. When the flow control valve 43 is initially opened, there will generally be liquid refrigerant carried mixed in the refrigerant vapor passing from the heat exchange coil of the first heat exchanger 40 into the compressor 20 through the suction inlet 22. Because the compressor 20 is operating at a low capacity at the beginning of compressor startup, the relatively small amount of liquid refrigerant carryover into the compressor 20 for a short period after opening the flow control valve 43 will have no detrimental effect on the compressor 20 and is relatively safe. After compressor restarting, the reversing valve starts to reposition to the heating mode from the defrost mode only when the refrigerant pressure differential between the discharge pressure, PD, and the suction pressure, PS, exceeds a preset pressure differential, such as for example 35OkPa (kilopascals), substantially reduces the risk of liquid carryover upon restarting. If the compressor 20 is tripped in response to an alarm condition while operating in the defrost mode, the compressor 20 will be restarted in the defrost mode after reset.

[0037] Referring now to FIG. 5, there is depicted in block diagram format the steps of a method for operating the heat pump system 10 in a defrost mode of operation. In the embodiment of the method as depicted in FIG. 5, the period of operation of the heat pump system 10 in the defrost mode is initiated either in response to a frost accumulation sensor, such as for example, but not limited to, the coil temperature sensor 41, for example a thermistor, opera tively associated with the heat exchanger coil of the outdoor heat exchanger 40 or in response to a low refrigerant suction pressure override being initialized. At 102-1, the system controller (not shown) monitors the output of the coil temperature sensor 41 and determines the magnitude of a coil frost factor based upon the output received from the coil temperature sensor. If the frost factor reaches 100%, the system controller, at 104, will first energize the solenoid valve 75 and then initiate the repositioning of the reversing valve 30 from a heating mode position to a defrost mode position. At 102-2, the system controller monitors the temperature of the refrigerant entering the compressor 20 through the suction inlet 22 and determines whether the suction saturation temperature is less than a lower limit set point suction temperature, such as for example minus 26.4 degrees Celsius. If the suction saturation temperature is less than the lower limit set point suction temperature for a preset period of time, such as for example fifty- five seconds, the system controller will, at 104, will first energize the refrigerant bypass flow control valve 75 and then initiate the repositioning of the reversing valve 30 from a heating mode position to a defrost mode position. [0038] With the reversing valve 30 positioned in the defrost mode, refrigerant will flow through the refrigerant circuit of the heat pump system 10 as described with respect to the cooling mode and illustrated in FIG. 1. During the defrost mode, the system controller continuously monitors: the coil temperature via temperature sensor 41, the elapsed time in defrost, for example by means of a software program, and the refrigerant pressure at the compressor via a pressure sensor, such as for example a pressure transducer, for example by means of a pressure sensing transducer (not shown) operatively associated with the compressor refrigerant discharge line for sensing the refrigerant pressure at the compressor discharge. At 106-1, the system controller compares the sensed compressor refrigerant discharge pressure with an upper limit set point refrigerant discharge pressure, such as for example, but not limited to, 1650 KPa (kilopascals). At 106-2, the system controller compares the sensed coil temperature to a set point coil temperature, such as for example 14 degrees Celsius, or to a temperature value calculated by the controller based upon a preprogrammed function and indicative of a zero frost factor. At 106-3, the system controller compares the elapsed time in the defrost mode with a preprogrammed maximum time period permitted in defrost. [0039] If any one of the three conditions is met, that is if the sensed compressor discharge temperature has reaches the upper limit set point, or the sensed coil temperature has reached the coil temperature set point, or the elapsed time in defrost has reached the maximum time limit for the defrost mode, the system controller, at 108, opens the solenoid opens the refrigerant bypass flow control valve 75. With the refrigerant bypass flow control valve 75 now open, refrigerant flows through the bypass refrigerant line 23 directly to a liquid reservoir, namely either to the receiver 70 or to the refrigerant collection chamber 55 of the second heat exchanger 50, bypassing the primary expansion valve 45. When the bypass flow control valve 75 is open, the primary expansion valve 45 may be maintained at an initial fixed opening set upon entry into the defrost mode or reset upon opening of the bypass flow control valve 75 and maintained at a new fixed opening. At 110, the system controller will monitor the time elapsed since the bypass flow control valve 75 was opened and determine when the bypass flow control valve has been open for a preprogrammed time period, such as for example 5 seconds, at which point the system controller will at 1 12 closed the refrigerant bypass flow control valve 75 and at 114 initiate repositioning of the reversing valve 30 out of the defrost mode. [0040J When compressor 20 is in standby mode or in delay status, the reversing valve 30 will maintain the original status in which the compressor 20 was most recently operating, unless an accidental or emergency electrical power loss has occurred when the compressor was in idle. Thus, under normal circumstances, if the compressor 20 stopped in the heating mode, the reversing valve 30 will remain in the heating mode position, and if the compressor 20 stopped in the cooling or defrost mode, the reversing valve 30 will keep remain in the cooling/defrost mode position. If the compressor 20 restarts in the cooling mode, refrigerant liquid carryover into the compressor 20 is not a concern because sufficient capacity exists within the interior 55 defined by the shell 54 of the second heat exchanger to collect the liquid refrigerant and prevent the liquid refrigerant from proceeding directly into compressor directly.

[0041] Referring now to FIG. 6, if the system controller, at 301, anticipates that the compressor 20 restarts in the heating mode, to prevent all the liquid refrigerant in the heat exchange coil of the first heat exchanger 40 from flushing through refrigerant line 7 and refrigerant line 5 to the suction inlet 22 and directly into the compressor 20, the system controller will first, at 302, close the solenoid valve 43 so that no refrigerant fluid can enter the heat exchange coil of the first heat exchanger 40. The system controller will then start the compressor 20. With the compressor 20 now running at 303 and the solenoid valve 43 closed, the pressure and saturated temperature of refrigerant within the heat exchange coil of the first heat exchanger 40 will drop down gradually. Thus, the liquid refrigerant inside of heat exchange coil will evaporate into vapor by absorbing heat from the ambient air passing over the heat exchange coil. As the refrigerant pressure and saturated temperature continue to drop lower and lower, the system controller will monitor the saturated suction temperature and, at 304, compare the saturated suction temperature to the ambient air temperature. Once a temperature differential defined as the ambient air temperature minus the saturated suction temperature exceeds a set point temperature difference, such as for example 12 degrees Celsius, the system controller, at 305, will open the solenoid valve 43 so that the expanded refrigerant from expansion device 45 may now pass through the valve 43 into the heat exchange coil of the first heat

[0042] Operation of the heat pump system 10 in accordance with the afore- described method for operation during the defrost mode ensures that all or substantially all of the liquid refrigerant resident in the coil of first (outdoor) heat exchanger 40 and the various refrigerant circuit lines at termination of the defrost mode drains into the either the receiver 70 or the refrigerant collection chamber 55 of the second heat exchanger 50. Therefore, little or no liquid refrigerant remains in the coil of the first heat exchanger 40 or the refrigerant circuit lines that could possibly be carried over into the suction inlet 22 of the compressor 20 upon switching from the defrost mode into the heating mode. As a result, the accumulator 74 may be substantially reduced in size in comparison to the typical size of an accumulator of a conventional prior art heat pump system of comparable capacity. In some applications, with the heat pump system of the present invention, the accumulator 74 may even be eliminated from the refrigerant circuit such as depicted in the exemplary embodiment of the heat pump system 10 depicted in FIG. 4. [0043] The method of operation in the defrost mode has been described hereinbefore with respect to a heat pump system equipped with a frost sensor, such as, for example, the coil temperature sensor 41 operatively associated with the first (outdoor) heat exchanger 40. It is to be understood, however, that the heat pump system 10 of the invention may be also operated in a defrost mode entered into at periodic time intervals of operation in the heating mode and extending for a limited period of time of operation in the defrost mode before returning to the heating mode, without departing from the spirit of the method of operation of the present invention. [0044] The terminology used herein is for the purpose of description, not limitation. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as basis for teaching one skilled in the art to employ the present invention.

[0045] While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. Those skilled in the art will also recognize the equivalents that may be substituted for elements described with reference to the exemplary embodiments disclosed herein without departing from the scope of the present invention. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims.

Claims

We Claim:

1. A heat pump system operable in a cooling mode, a heating mode and a defrost mode and including a refrigerant compressor, a reversing valve, a first heat exchanger and a second heat exchanger disposed in a refrigerant circuit, and a primary expansion device disposed in the refrigerant circuit intermediate said first heat exchanger and said second heat exchanger, said reversing valve being positionable in a first position for operation of said heat pump system in the cooling mode or defrost mode and being positionable in a second position for operation of said heat pump system in the heating mode, said heat pump characterized by: a refrigerant bypass circuit establishing a refrigerant flow path from the refrigerant circuit at a first location upstream of said primary expansion device and downstream of said first heat exchanger with respect to refrigerant flow in the defrost mode to a liquid reservoir disposed in the refrigerant circuit at a second location downstream with respect to refrigerant flow in the defrost mode of said primary expansion valve.

2. The heat pump system of claim 1 further characterized in that said second heat exchanger defines a refrigerant collection chamber comprising said liquid reservoir.

3. The heat pump system of claim 2 further characterized in that said second heat exchanger comprises a shell and tube heat exchanger having a shell defining the refrigerant collection chamber and a tube bank heat exchanger disposed in the refrigerant collection chamber.

4. The heat pump system of claim 1 further characterized in that said liquid reservoir comprises a refrigerant receiver disposed in the refrigerant circuit intermediate said primary expansion device and said second heat exchanger.

5. The heat pump system of claim 1 further characterized in that said bypass circuit comprises: a bypass refrigerant line interconnecting the refrigerant circuit at the first location upstream of said primary expansion device and downstream of said first heat exchanger with respect to refrigerant flow in the defrost mode in refrigerant flow communication with said liquid reservoir; and a bypass refrigerant flow control device interdisposed in said refrigerant bypass line.

6. The heat pump system of claim 5 further characterized in that said bypass refrigerant flow control device comprises a flow control valve having a first position in which said bypass refrigerant line is open to refrigerant flow and a second position in which said bypass refrigerant line is closed to refrigerant flow.

7. The heat pump system of claim 5 further characterized in that said bypass refrigerant flow control device comprises an open position/closed position solenoid valve.

8. The heat pump system of claim 1 further characterized by: a flow control valve disposed in the refrigerant circuit upstream of the said first heat exchanger and downstream of primary expansion device with respect to refrigerant flow through the circuit in a heating mode; and a flow check valve disposed in the refrigerant circuit in parallel relationship with said flow control.

9. A method of operating the heat pump system during a defrost mode, the heat pump including a refrigerant compressor, a reversing valve, a first heat exchanger and a second heat exchanger disposed in a refrigerant circuit, and a primary expansion device disposed in the refrigerant circuit intermediate said first heat exchanger and said second heat exchanger, said reversing valve being positionable in a first position for operation of said heat pump system in the cooling or defrost mode and being positionable in a second position for operation of said heat pump system in the heating mode, said method characterized by the steps of: initiating switching of said reversing valve from its second position into its first position for operation in the defrost mode; prior to terminating operation in the defrost mode, passing refrigerant flow from the refrigerant circuit through a refrigerant bypass circuit to a liquid reservoir; and initiating switching of said reversing valve out of its first position.

10. The method of claim 9 further characterized by: providing a flow control valve in said bypass refrigerant circuit, said flow control valve having an open position in which said bypass refrigerant line is open to refrigerant flow and a closed position in which said bypass refrigerant line is closed to refrigerant flow; and in that the step of passing refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit comprises opening said flow control valve.

11. The method of claim 10 further characterized in that the step of passing refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit by opening said flow control comprises opening said flow control valve in defrost mode when a discharge pressure of said compressor exceeds a first preselected discharge pressure set point.

12. The method of claim 10 further characterized in that the step of passing refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit by opening said flow control comprises opening said flow control valve in defrost mode when a frost factor for the first heat exchanger drops to 0%.

13. The method of claim 10 further characterized in that the step of passing refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit by opening said flow control comprises opening said flow control valve in defrost mode when the time elapsed in a defrost mode reaches eight minutes.

14. The method of claim 10 further characterized by the step of terminating the passing of refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit by closing said flow control valve.

15. The method of claim 14 further characterized in that the step of terminating the passing of refrigerant flow from the refrigerant circuit through said refrigerant bypass circuit comprises closing said flow control valve upon expiration of said preselected period of time.

16. The method as recited in claim 15 further characterized in that said preselected period of time ranges from one second to forty-five seconds.

17. The method as recited in claim 16 further characterized in that said preselected period of time is about five seconds