HK1151088A - Capacity modulation of refrigerant vapor compression system - Google Patents

Description

Capacity modulation for refrigerant vapor compression system

Technical Field

The present invention relates generally to refrigerant vapor compression systems, and more particularly to efficient capacity modulation of refrigerant vapor compression systems, including efficient capacity modulation of transport refrigeration refrigerant vapor compression systems using carbon dioxide refrigerant and operating in a transcritical cycle.

Background

Refrigerant vapor compression systems are well known in the art and are commonly used in transport refrigeration systems to refrigerate the air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable items. Refrigerant vapor compression systems are also commonly used in commercial refrigeration systems associated with supermarkets, convenience stores, restaurants and other commercial establishments to refrigerate the air supplied to a cold room or refrigerated display case where perishable food items are stored. Refrigerant vapor compression systems are also commonly used to condition air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Typically, such refrigerant vapor compression systems include a compressor, an air-cooled condenser, an evaporator, and an expansion device, typically a thermal expansion valve or an electronic expansion valve, disposed upstream of the evaporator and downstream of the condenser with respect to refrigerant flow. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit in accordance with a known refrigerant vapor compression cycle arrangement.

Traditionally, these refrigerant vapor compression systems have been operated mostly at subcritical refrigerant pressures. Refrigerant vapor compression systems operating in the subcritical range are typically charged with conventional fluorocarbon refrigerants such as, but not limited to, Hydrochlorofluorocarbons (HCFCs), such as R22, and more typically Hydrofluorocarbons (HFCs), such as R134a, R410A and R407C. In the current market, there is increasing interest in "natural" refrigerants, such as carbon dioxide, for air conditioning applications, commercial refrigeration applications, and transport refrigeration applications in place of HFC refrigerants. However, since carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed to operate in a transcritical pressure range. For example, a transport refrigerant vapor compression system having an air-cooled refrigerant heat rejection heat exchanger, operating in an environment with an ambient air temperature that exceeds the critical temperature point of carbon dioxide by 31.1 ℃ (88F), such transport refrigerant vapor compression system must also operate at a compressor discharge pressure that exceeds the critical point pressure of carbon dioxide by 7.38MPa (1070psia) and thus will operate in a transcritical cycle. In a refrigerant vapor compression system operating in a transcritical cycle, the refrigerant heat rejection heat exchanger operates as a gas cooler rather than a condenser and operates at refrigerant temperatures and pressures above the refrigerant critical point, while the evaporator operates at refrigerant temperatures and pressures in the subcritical range.

In transport refrigeration applications, refrigerant vapor compression systems must be capable of operating in a pulldown mode and in a set-point control mode. When the cargo space is loaded with perishable items having temperatures significantly exceeding the storage temperature required for transporting the perishable items, the refrigerant vapor compression system operates in a pull-down mode. For example, fruits and vegetables are typically loaded directly from a plucked field into the cargo storage space of a truck, trailer, or intermodal container. There is therefore a need to be able to rapidly reduce the temperature of the product within the cargo storage space from the ambient temperature at which the product is loaded to the storage temperature required for shipping, typically between about 1 c to about 5 c (about 34F to about 40F) for refrigerated food items and below 0 c (32F) for frozen food items. Therefore, the refrigerant vapor compression system must be designed to have sufficient capacity to adequately cool the air circulating from the cargo storage space through the evaporator of the refrigerant vapor compression system at full load operation in order to rapidly pull down the product temperature within the cargo storage space.

However, once the cargo storage space has been cooled to a storage temperature required for transportation of the particular cargo being transported, the refrigerant vapor compression system is operated in a set point control mode. In this mode of operation, the refrigerant vapor compression system must maintain the temperature within the cargo storage space within a relatively narrow range of plus/minus equal to the set point temperature of the desired transport temperature for the particular product stored within the cargo storage space. To avoid excessive cooling of the product, the refrigerant vapor compression system must operate at a reduced refrigeration capacity that is significantly less than the full load refrigeration capacity of the system to avoid excessive cooling of the air circulating from the cargo storage space.

Disclosure of Invention

In one aspect of the invention, a refrigerant vapor compression system comprises: a refrigerant compression device, a refrigerant heat rejection heat exchanger, an expansion device, and a refrigerant heat absorption heat exchanger disposed in series refrigerant flow communication in the primary refrigerant circuit; and an unloading circuit operatively associated with said compression means. A first flow control device is disposed in the primary refrigerant circuit downstream of a discharge outlet of the compression device with respect to refrigerant flow and upstream of the refrigerant heat rejection heat exchanger with respect to refrigerant flow. The unload circuit includes an unload refrigerant line having an inlet in refrigerant flow communication with the main refrigerant circuit at a first location downstream with respect to refrigerant flow of the discharge outlet of the compression device and upstream with respect to refrigerant flow of the first flow control device, and an outlet in refrigerant flow communication with the main refrigerant circuit at a second location downstream with respect to refrigerant flow of said refrigerant heat absorption heat exchanger and upstream with respect to refrigerant flow of the suction inlet of the compression device, and an unload circuit flow control device disposed in the unload refrigerant line. The refrigerant vapor compression system also includes a controller operatively associated with the first flow control device and the unload circuit flow control device. The controller switches the refrigerant vapor compression system by operating between a first mode of operation in which the compression device operates in a duty cycle and a second mode of operation in which the compression device operates in an unload cycle.

A controller positions the unload circuit flow control device in a closed position and the first flow control device in an open position in a first mode of operation to operate the refrigerant vapor compression system in a duty cycle. In a second mode of operation, the controller positions the unload circuit flow control device in an open position and positions the first flow control device in a closed position to operate the refrigerant vapor compression system in an unload cycle. In the first mode of operation, the controller may also adjust the expansion device. In the second mode of operation, the controller may also position the expansion device in a closed position.

In one embodiment, the refrigerant vapor compression system further includes a second flow control device disposed in the primary refrigerant circuit downstream, with respect to refrigerant flow, of the refrigerant heat absorption heat exchanger and upstream, with respect to refrigerant flow, of the suction inlet of the compression device. In this embodiment, the outlet of the unload refrigerant line is in refrigerant flow communication with the main refrigerant circuit at a second location downstream with respect to refrigerant flow of the second flow control device and upstream with respect to refrigerant flow of the suction inlet of the compression device. In this embodiment, in the first mode of operation, the controller positions the unload circuit flow control device in a closed position and each of the first and second flow control devices in an open position to operate the refrigerant vapor compression system in a duty cycle. In a second mode of operation, the controller positions the unload circuit flow control device in an open position and each of the first and second flow control devices in a closed position to operate the refrigerant vapor compression system in an unload cycle.

In one aspect of the invention, a method is provided for adjusting capacity of a refrigerant vapor compression system including a refrigerant compression device, a refrigerant heat rejection heat exchanger, an expansion device, and a refrigerant heat absorption heat exchanger arranged in series flow arrangement in a primary refrigerant circuit, the method comprising the steps of: operating the compression device in a duty cycle for a first period of time; operating the compression device in an unload cycle for a second period of time; and repeatedly alternating operation of the compression device between the load cycle operation and the unload cycle operation.

Drawings

For a further understanding of these and other objects 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 drawings, wherein:

FIG. 1 is a schematic diagram showing a first exemplary embodiment of a refrigerant vapor compression system according to the present invention;

FIG. 2 is a schematic diagram illustrating a second exemplary embodiment of a refrigerant vapor compression system according to the present invention; and

fig. 3 is a schematic diagram illustrating a third exemplary embodiment of a refrigerant vapor compression system according to the present invention.

Detailed Description

Referring now to fig. 1-3, the refrigerant vapor compression system 10 will be described herein in connection with the refrigeration of a temperature controlled cargo space 200 of a refrigerated container, trailer or truck for transporting perishable items. It should be understood that the refrigerant vapor compression system described herein may also be used to cool air supplied to a refrigerated display case (merchandis) or cold room associated with a supermarket, convenience store, restaurant or other commercial establishment or to condition air supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. The refrigerant vapor compression system 10 includes a compression device 20 (driven by an electric motor 30 operatively associated therewith) connected in a series refrigerant flow arrangement in a closed loop refrigerant circuit by various refrigerant lines 2, 4 and 6, a refrigerant heat rejection heat exchanger 40, a refrigerant heat absorption heat exchanger 50. Further, an expansion device 55 operatively associated with the evaporator 50 is disposed in the refrigerant line 4 downstream of the refrigerant heat rejection heat exchanger 40 with respect to refrigerant flow and upstream of the refrigerant heat absorption heat exchanger 50 with respect to refrigerant flow. In the embodiment of the refrigerant vapor compression system 10 depicted in fig. 1 and 3, the expansion device 55 comprises an electronic expansion valve. However, in the embodiment depicted in fig. 2, expansion device 55 comprises a thermostatic expansion valve 57 or a fixed orifice device (such as a capillary tube) disposed in series refrigerant flow with an open/close flow control device 53 (such as a two-position solenoid valve).

The refrigerant heat rejection heat exchanger 40 operates in the subcritical pressure range and functions as a refrigerant vapor condenser when the refrigerant vapor compression system 10 is operating in a subcritical cycle, such as when charged with a conventional hydrochlorofluorocarbon refrigerant (HCFC), such as R22, or a hydrofluorocarbon refrigerant (HFC), such as R134a, R410A, and R407C, or with carbon dioxide refrigerant when operating at a compressor discharge pressure that is 7.38MPa (1070psia) below the critical point pressure of carbon dioxide. However, when the refrigerant vapor compression system 10 is operating in a transcritical cycle, such as when charged with carbon dioxide refrigerant and operating at a compressor discharge pressure exceeding the critical pressure point of carbon dioxide, the refrigerant heat rejection heat exchanger 40 operates at a supercritical pressure and acts as a refrigerant vapor cooler, rather than by operating to condense carbon dioxide refrigerant vapor. The tube bundle 42 of the heat rejection heat exchanger 40 may comprise, for example, a finned tube heat exchanger, such as a plate fin and round tube heat exchanger coil, or a fin and multichannel tube heat exchanger, such as a fin and mini-channel or microchannel flat tube heat exchanger. In passing through the refrigerant heat rejection heat exchanger 40, the refrigerant passes through the heat exchanger tubes of the tube bundle 42 in heat exchange relationship with a secondary fluid (typically ambient air, typically outdoor air) drawn through the tube bundle 42 by a blower 45, such as one or more fans, operatively associated with the tube bundle 42 of the heat rejection heat exchanger 40.

Whether the refrigerant vapor compression system 10 is operating in a subcritical cycle or a transcritical cycle, the refrigerant heat absorption heat exchanger 50, located downstream of the expansion device 55 with respect to refrigerant flow in the refrigerant circuit, is always operating at a subcritical pressure and acts as a refrigerant liquid evaporator. In passing through the heat absorption heat exchanger 50, the refrigerant passes through the heat exchanger tubes of the tube bundle 52 in heat exchange relationship with the air to be conditioned, typically air drawn from and returned to a climate controlled environment, which is drawn through the tube bundle 52 by a blower 55 (such as one or more fans) operatively associated with the tube bundle 52 of the heat absorption heat exchanger 50 to cool the air and heat and evaporate the refrigerant. The tube bundle 52 of the refrigerant heat absorption heat exchanger 50 may comprise, for example, a finned tube heat exchanger, such as a plate fin and round tube heat exchanger coil, or a fin and multi-tube heat exchanger, such as a fin and mini-channel or micro-channel flat tube heat exchanger.

The compression device 20 is used to compress and circulate a refrigerant through a refrigerant circuit, as will be discussed in further detail below. The compression device 20 may be a single stage compressor, such as a scroll compressor, reciprocating compressor, rotary compressor, screw compressor, centrifugal compressor, as depicted in fig. 1 and 2. It should be understood that the compression device 20 could also be a multi-stage compression device having at least a lower pressure compression stage and a higher pressure compression stage with refrigerant flow passing from the lower pressure compression stage to the higher pressure compression stage, such as depicted in fig. 3. In such embodiments, the multi-stage compression device may comprise a single multi-stage compressor, such as a scroll compressor, or a screw compressor having a staged compression chamber or a reciprocating compressor having at least a first bank of cylinders and a second bank of cylinders, or a pair of single-stage compressors connected in series refrigerant flow relationship with the discharge outlet of an upstream compressor connected in series refrigerant flow communication with the suction inlet of a downstream compressor.

The drive motor 30 operatively associated with the compression mechanism of the compression device 20 may be a fixed speed motor that operates on power from a fixed frequency power source. The compression device 20 receives refrigerant vapor at suction pressure from the evaporator 50 through refrigerant line 2 in refrigerant flow communication with a suction inlet of the compression device 20, and discharges refrigerant vapor at discharge pressure to the refrigerant heat rejection heat exchanger 40 through refrigerant line 2 in refrigerant flow communication with a discharge outlet of the compression device 20. The flow control device 65 is interposed in the refrigerant 6 at a position downstream of the evaporator 50 with respect to refrigerant flow and upstream of the suction port of the compression device 20 with respect to refrigerant flow. Further, the flow control device 75 is interposed in the refrigerant 2 at a location downstream of the discharge outlet of the compression device 20 with respect to refrigerant flow and upstream of the refrigerant heat rejection heat exchanger 40 with respect to refrigerant flow. Each of the flow control devices 65, 75 is selectively positionable in at least a fully open position in which refrigerant may flow through the flow control device and a fully closed position in which refrigerant may not flow through the flow control device. In one embodiment, each of the flow control devices 65, 75 comprises a two-position solenoid valve having an open position and a closed position.

Referring now to fig. 3, in the exemplary embodiment of the refrigerant vapor compression system 10 depicted herein, the primary refrigerant circuit includes an economizer circuit operatively associated therewith. The economizer circuit includes an economizer refrigerant line 14, an economizer expansion device 73, an economizer heat exchanger 70 and an economizer flow control valve 95. An economizer refrigerant line 14 establishes refrigerant flow communication between refrigerant line 4 of the main refrigerant circuit and an intermediate pressure stage of the compression process. The economizer heat exchanger 70 may comprise a refrigerant-to-refrigerant heat exchanger having a first refrigerant flow pass 72 and a second refrigerant flow pass 74. A first refrigerant flow pass 72 is interposed in refrigerant line 4 of the primary refrigerant circuit downstream with respect to refrigerant flow of the refrigerant outlet of the refrigerant heat rejection heat exchanger 40 and upstream with respect to refrigerant flow of the expansion device 55. The second refrigerant flow path 74 is interposed in the economizer refrigerant line 14. The economizer expansion device 73 can be an electronic expansion valve, a thermostatic expansion valve, or a fixed orifice flow metering device disposed in the economizer refrigerant line 14 upstream of the second pass 74 of the refrigerant-to-refrigerant heat exchanger 70 with respect to refrigerant flow through the second pass 74. The economizer flow control device 95 can be a two-position, open/close solenoid valve interposed in the economizer refrigerant line 14 downstream of the second pass 74 of the economizer heat exchanger 70. In operation of the main refrigerant circuit, the controller 100 may selectively open or close the economizer flow control device 95 to bring the economizer circuit on-line or off-line, as in conventional practice, to switch between the economized and non-economized refrigeration cycles. If the expansion device 73 is an electronic expansion valve, the economizer flow control valve 95 may be omitted and the controller 100 may open and close the electronic expansion valve to bring the economizer circuit online or offline.

The refrigerant vapor compression system 10 also includes a compressor unload circuit including an unload refrigerant line 8 interconnecting refrigerant line 2 of the refrigerant circuit with refrigerant line 6 of the refrigerant circuit and an unload valve 85 disposed in the unload refrigerant line 8 operable to control the flow of refrigerant through the unload refrigerant line 8 of the compressor unload circuit. In one embodiment, the unloader valve 85 comprises a two-position solenoid valve having an open position and a closed position. The unload refrigerant line 8 taps into refrigerant line 2 at a location between the compression device 20 and the flow control valve 75 (i.e., downstream of the discharge outlet of the compression device 20 with respect to refrigerant flow and upstream of the flow control valve 75 with respect to refrigerant flow) and to refrigerant line 6 at a location between the flow control valve 65 and the compression device 20 (i.e., downstream of the flow control valve 65 with respect to refrigerant flow and upstream of the suction inlet of the compression device 20 with respect to refrigerant flow). Thus, when the unload control valve 85 is positioned in its open position, refrigerant vapor may flow from the discharge outlet of the compression device 20 directly back to the suction inlet of the compression device 20 through the unload refrigerant line 8.

The refrigerant vapor compression system 10 further includes a controller 100, the controller 100 being operatively associated with each of the respective flow control devices 65, 75 and 85 interposed in the refrigerant lines 6, 2 and 8, respectively, to selectively position each of the respective flow control devices in either an open position or a closed position. The controller 100 also monitors the temperature of the ambient air passing into the refrigerant heat rejection heat exchanger 40 as a cooling medium via temperature sensor 101, the temperature of the air and/or product within the temperature controlled cargo storage space 200 via temperature sensor 201, and various system operating parameters with various sensors operatively associated with the controller 100 and disposed at selected locations throughout the system. For example, in the exemplary embodiment depicted in fig. 1-3, a temperature sensor 103 and a pressure sensor 104 may be provided to sense refrigerant suction temperature and pressure, respectively, and a temperature sensor 105 and a pressure sensor 106 may be provided to sense refrigerant discharge temperature and pressure, respectively. The pressure sensor may be a conventional pressure sensor, such as a pressure transmitter, and the temperature sensor may be a conventional temperature sensor, such as a thermocouple or thermistor.

The controller 100 controls operation of the refrigerant vapor compression system 10 and selective positioning of the flow control devices 65, 75 and 85 and the economizer flow control valve 95 (if present). The controller 100 also controls the operation of the compressor drive motor 30 that drives the compression mechanism of the compression device 20 and controls the operation of the fans 44 and 54 by controlling respective fan motors (not shown) operatively associated with the fans. Controller 100 determines the desired mode of operation based on a comparison of the sensed temperature of the air and/or product within cargo storage space 200 to a set point temperature representing a desired storage temperature during transport of the product stored within cargo storage space 200. If the product temperature within the cargo storage space 200 exceeds the set point temperature by more than a few degrees, such as in the event that the refrigerant vapor compression system 10 is initially started after loading product into the space 200, the controller 100 operates the refrigerant vapor compression system 10 at a high capacity in a pulldown mode. However, if the product temperature within the cargo storage space 200 is within a preselected range of the set point temperature, the controller 100 operates the refrigerant vapor compression system 10 in the set point control mode at a reduced capacity.

To operate the refrigerant vapor compression system 10 in the pulldown mode, the controller 100 closes the unloader valve 85 and opens the suction flow control valve 65 and the discharge flow control valve 75 such that refrigerant circulates through refrigerant lines 2, 4, and 6 of the main refrigerant circuit but not through refrigerant line 8 of the unloader circuit. The controller 100 selectively opens the unload circuit control device 75 in response to a sensed temperature of air entering the evaporator from the temperature controlled space 200 (which is indicative of the temperature of the air or product within the temperature controlled space), the unload circuit control device 75 including a fixed flow area valve, such as a fixed orifice solenoid valve. However, it should be understood that other system parameters may be used by the controller 100 to determine when to open the unload circuit control valve 85.

In the embodiment depicted in fig. 1 and 3, in the pull-down mode, the controller 100 also adjusts the refrigerant flow to the evaporator 50 by varying the flow area of the flow path in response to the refrigerant suction temperature or pressure sensed on the suction side of the compression device 20 by sensors 103 and 104, respectively. If the suction pressure drops too slowly, this indicates that the system is providing too much capacity. In the embodiment depicted in fig. 3, in the pull-down mode, the controller 100 may also selectively switch between non-economized refrigeration cycle operation and economized refrigerant cycle operation by selectively opening or closing the economizer flow control valve 95.

The controller 100 may unload the compression device 20 as necessary to control the capacity of the refrigerant vapor compression system 10. For example, when the sensed temperature of air and/or product within cargo storage space 200 has dropped to within a few degrees of a set point temperature (which represents a temperature required for transporting the product stored in the cargo storage space), controller 100 switches operation from the pull-down mode to the set point control mode. In so doing, the controller 100 opens the flow control device 85 interposed in the unload refrigerant line 8 and simultaneously closes the flow control valves 65 and 75 and the economizer flow control valve 95 (if present) interposed in the main refrigerant circuit. With the unloader flow control device 85 open, refrigerant vapor discharged from the compression device 20 flows through the unloader circuit refrigerant line 8 to return directly to the suction side of the compression device 20, thereby unloading the compression device 20. This unloading of the compression device 20 through the unloading circuit may also be performed in response to the high compressor discharge refrigerant temperature or pressure.

To operate the refrigerant vapor compression system in a setpoint control mode, the controller 100 regulates the capacity of the refrigerant vapor compression system 10 by selectively loading and unloading the compression device 20. The controller 10 does so by alternately opening and closing the unloader flow control devices for short periods of time while closing and opening the flow control devices 65 and 75. Thus, the controller 100 operates the system 10 in a duty cycle by closing the unloader flow control device 85 and opening the flow control devices 65 and 75 in synchronization therewith for a first period of time, and then operates the system 10 in a unload cycle by opening the unloader flow control device 85 and closing the flow control devices 65 and 75 in synchronization therewith for a second period of time.

In an alternate embodiment of the refrigerant vapor compression system 10, the flow control device 65 may be eliminated. In this case, if the expansion device 55 is an expansion valve 55 as depicted in fig. 1, the controller 100 closes the expansion valve 55 when the unloader flow control device 85 is open, i.e., when the system is operating in an unload cycle, but when the system 10 is operating in a load mode with the unloader flow control device closed and the flow control device 75 open, the controller 100 adjusts the degree of opening of the expansion device 55 to control the refrigerant flow to the evaporator 50. However, if the expansion device 55 comprises a thermostatic expansion valve 57, or a fixed orifice device, in conjunction with the flow control valve 53 depicted in fig. 2, then the controller closes the flow control valve 53 when the unloader flow control device 85 is open, i.e., when the system is operating in an unload cycle. When the system is operating in the charging mode, thermostatic expansion valve 57 controls the flow of refrigerant to evaporator 50 in a conventional manner responsive to the temperature of the refrigerant vapor leaving evaporator 50 as sensed by a bulb 59, bulb 59 typically being mounted on refrigerant line 6 downstream of the outlet of evaporator 50.

The compression device 20 remains in operation during the load and unload cycles, but during the unload cycle, the compression device 20 does not experience a pressure rise from suction to discharge because the refrigerant vapor discharged from the compression device 20 passes back through the unload circuit to the suction port of the compression device 20 with a minimal pressure drop. In the event that the flow control device 75 is closed during the unload cycle, backflow of refrigerant in the high pressure side of the main refrigerant circuit is prevented. Instead, the refrigerant continues to slowly flow from the refrigerant heat rejection heat exchanger 40 through the expansion device 55 into the refrigerant heat absorption heat exchanger 50. To prevent backflow of refrigerant during operation of the system 10 unload cycle, the controller 100 closes not only the flow control device 75, but also the flow control device 65, or (if the flow control device 65 is not present) the expansion valve 55 or the flow control valve 53, and also the economizer flow control valve 95 (if present). Preventing backflow of refrigerant during operation facilitates a quick and efficient switch from an unloaded cycle operation to a loaded cycle operation, since refrigerant mass in the main refrigerant circuit does not have to be redistributed, which would occur if the pressure within the refrigerant circuit between the high pressure side (i.e., upstream of the expansion valve 55 with respect to refrigerant flow) and the low pressure side (i.e., downstream of the expansion valve 55 with respect to refrigerant flow) became equal due to the backflow.

The controller 100 also keeps the evaporator fan 54 operating during the unloading cycle so that air continues to circulate from the cargo storage space 200 over the heat exchanger coil 52 of the evaporator 50. Thus, the refrigeration of this air continues even during operation of the refrigerant vapor compression system 10 in the unloaded cycle, albeit at a much lower refrigeration capacity than when the system is operating at full load. By repeatedly alternating the operation of the refrigerant vapor compression system 10 between a first period of the duty cycle and a second period of the unload cycle, the total cooling capacity of the system during the set point control mode (averaged over time) is a relatively small fraction of the cooling capacity of the system 10 when operating at full load during the pull-down mode of operation. Also, the power consumed by the compression device 20 during the unloading cycle is a relatively small fraction of the power consumed by the compression device 20 when operating at full load during the pull-down mode of operation. Keeping the compression device 20 running during the unloading cycle, as opposed to shutting down the compression device 20 during the unloading cycle, reduces the number of compressor starts, which extends the life expectancy of the compression device by reducing the risk of premature failure and also reduces energy consumption due to motor inefficiencies during start-up.

Accordingly, the refrigerant vapor compression system 10 includes a main refrigerant circuit and an unload circuit arranged in parallel with respect to refrigerant flow. When the system is operating in a duty cycle, refrigerant flows through the primary circuit from the compression device 20 through the refrigerant heat rejection heat exchanger 40, then the expansion device 55, and the refrigerant heat absorption heat exchanger 50, and then back to the compression device 20. During the load cycle, the unload circuit is closed to refrigerant flow. When the system is operating in an unload cycle, the main refrigerant circuit is closed to refrigerant flow and refrigerant flow returns through the unload circuit from the discharge outlet of the compression device 20 to the suction inlet of the compression device, while bypassing the refrigerant heat rejection heat exchanger 40, the expansion device 55, and the refrigerant heat absorption heat exchanger 50.

In full load operation, the controller 100 opens the flow control devices 65 and 75 and simultaneously closes the unloader flow control device 85, such that the primary refrigerant circuit is open to refrigerant flow and the unloader circuit is closed to refrigerant flow. In part load operation, the controller 100 adjusts the capacity of the refrigerant vapor compression system 10 by: first operating the compression device 20 in a duty cycle for a first period of time, and then operating the compression device 20 in an unload cycle for a second period of time; and thereafter repeating the operation of alternating the compression device 20 between the load cycle operation and the unload cycle operation.

The application of the embodiment depicted in fig. 1-3 is intended to be exemplary, and not limiting, of a compression device unloader circuit in parallel with a main refrigerant circuit of a refrigerant vapor compression system. It should be understood that the main refrigerant circuit may include other conventional components and associated refrigerant circuits. For example, the main refrigerant circuit may also include an interstage cooling circuit operatively associated with the compression device for passing refrigerant from one compression stage through the refrigerant cooler to another compression stage.

The controller 100 may be an electronic controller such as a microprocessor controller or any other controller of the type conventionally used to control the operation of refrigerant vapor compression systems. For example, in transport refrigeration applications, the controller 100 may be a MicroLink^TMA series of microprocessor controllers such as the ML2 model, the ML2i model, or the ML3 model available from Carrier Corporation, Syracuse, n.y., USA. It should be understood that any controller capable of performing the functions discussed above with respect to the control 100 can be used in the refrigerant vapor compression system of the present invention and for performing the method of the present invention.

The foregoing description is only exemplary of the teachings of the present invention. It will be recognized by those skilled in the art that various modifications and changes may be made to the invention specifically described herein and its equivalents without departing from the spirit and scope of the invention as defined by the following claims.

Claims (11)

1. A refrigerant vapor compression system comprising:

the following devices disposed in series refrigerant flow communication in the main refrigerant circuit: a refrigerant compression device having a refrigerant discharge outlet and a refrigerant suction inlet; a refrigerant heat rejection heat exchanger for passing refrigerant at high pressure received from the compression device in heat exchange relationship with a cooling medium; and a refrigerant heat absorption heat exchanger for passing a refrigerant at low pressure, the refrigerant being in heat exchange relationship with a heating medium;

an expansion device disposed in the primary refrigerant circuit downstream of the refrigerant heat rejection heat exchanger and upstream of the refrigerant heating heat exchanger;

a first flow control device disposed in the primary refrigerant circuit downstream of a discharge outlet of the compression device with respect to refrigerant flow and upstream of the refrigerant heat rejection heat exchanger with respect to refrigerant flow;

an unload circuit operatively associated with said compression device and comprising an unload refrigerant line and an unload circuit flow control device disposed in said unload refrigerant line, said unload refrigerant line having an inlet in refrigerant flow communication with said main refrigerant circuit at a first location downstream with respect to refrigerant flow of a discharge outlet of said compression device and upstream with respect to refrigerant flow of said first flow control device, and in refrigerant flow communication with said main refrigerant circuit at a second location downstream with respect to refrigerant flow of said refrigerant heat absorption heat exchanger and upstream with respect to refrigerant flow of a suction inlet of said compression device; and

a controller operatively associated with the first flow control device and the unload circuit flow control device, the controller switching the refrigerant vapor compression system by operating between a first mode of operation in which the compression device operates in a duty cycle and a second mode of operation in which the compression device operates in an unload cycle.

2. A refrigerant vapor compression system as recited in claim 1 wherein said controller positions said unload circuit flow control device in a closed position and said first flow control device in an open position to operate said refrigerant vapor compression system in said first mode of operation.

3. A refrigerant vapor compression system as recited in claim 2 wherein said control regulates said expansion device in said first mode of operation.

4. A refrigerant vapor compression system as recited in claim 1 wherein said controller positions said unload circuit flow control device in an open position and said first flow control device in a closed position to operate said refrigerant vapor compression system in said second mode of operation.

5. A refrigerant vapor compression system as recited in claim 4 wherein in said second mode of operation said control positions said expansion device in a closed position.

6. A refrigerant vapor compression system as recited in claim 1 further comprising:

a second flow control device disposed in the main refrigerant circuit downstream of the refrigerant heat absorption heat exchanger with respect to refrigerant flow and upstream of the suction inlet of the compression device with respect to refrigerant flow.

7. A refrigerant vapor compression system as recited in claim 6 wherein said controller positions said unload circuit flow control device in a closed position and positions each of said first and second flow control devices in an open position to operate said refrigerant vapor compression system in said first mode of operation.

8. A refrigerant vapor compression system as recited in claim 7 wherein said control regulates said expansion device in said first mode of operation.

9. The refrigerant vapor compression system as recited in claim 6 wherein the controller positions the unload circuit flow control device in an open position and each of the first and second flow control devices in a closed position to operate the refrigerant vapor compression system in the second mode of operation.

10. A method for adjusting capacity of a refrigerant vapor compression system including a refrigerant compression device, a refrigerant heat rejection heat exchanger, an expansion device, and a refrigerant heat absorption heat exchanger disposed in a series flow arrangement in a primary refrigerant circuit, the method comprising the steps of:

operating the compression device in a duty cycle for a first period of time;

operating the compression device in an unload cycle for a second period of time; and the number of the first and second groups,

repeatedly alternating operation of the compression device between a load cycle operation and an unload cycle operation.

11. A method for adjusting capacity of a refrigerant vapor compression system as recited in claim 10 comprising the steps of: a compression device unloader circuit is provided in parallel refrigerant flow relationship with the main refrigerant circuit, the unloader circuit connecting a refrigerant discharge outlet of the compression device in direct refrigerant flow communication with a refrigerant suction inlet of the compression device.