Transport refrigeration system and method of operation

Abstract

A refrigerant vapor compression system using CO 2 as the working fluid and operating in a transcritical cycle, compression of the refrigerant is accomplished by use of a digital scroll compressor adapted to a variable duty cycle in a pulse width modulation manner. The compression ratio is maintained at an acceptable level by selective control of various components within the system, depending on the mode of operation.

1 . A vapor compression system of the type having CO 2  as the working fluid so as to require operation in a transcritical cycle, comprising in serial working fluid flow relationship:
 a gas cooler for receiving CO 2  vapor at a higher temperature and discharging it a lower temperature;   a charge storage vessel for receiving CO 2  refrigerant from said gas cooler and discharging CO 2  in liquid form;   an expansion valve between said gas cooler and said charge storage vessel;   an expansion valve for receiving liquid CO 2  from said charge storage vessel and discharging a mixture of CO 2  vapor and liquid;   an evaporator for receiving a mixture of CO 2  vapor and liquid and discharging a CO 2  vapor; and   a digital scroll compressor responsive to a variable duty cycle signal to vary its capacity by modulating the respective loaded stated and unloaded state times in a pulse width modulation manner, said compressor receiving low pressure CO 2  vapor from said evaporator and discharging high pressure CO 2  vapor to said gas cooler.   
     
     
         2 . A vapor compression system as set forth in  claim 1  wherein said charge storage vessel comprises a flash tank adapted to have both CO 2  vapor and a CO 2  liquid therein. 
     
     
         3 . A vapor compression system as set forth in  claim 1  and including a gas cooler fan and a multi-speed motor capable of driving said fan at either a high or a low speed. 
     
     
         4 . A vapor compression system as set forth in  claim 3  and including a control for determining whether the vapor compression system is operating under full load or part load operating conditions and for operating said fan motor at either a high speed or at a low speed as a function thereof. 
     
     
         5 . A vapor compression system as set forth in  claim 4  wherein, if the system is operating under part load conditions, said control operates said fan at a low speed. 
     
     
         6 . A vapor compression system as set forth in  claim 5  wherein said control is then operable to reduce the loaded state time of operation. 
     
     
         7 . A vapor compression system as set forth in  claim 4  wherein if the system is operating under full load operating conditions said control is adapted to operate said fan at a high speed. 
     
     
         8 . A vapor compression system as set forth in  claim 7  wherein said control is then operable to reduce the loaded state time of operation as necessary to prevent overloading of the compressor. 
     
     
         9 . A vapor compression system as set forth in  claim 8  and including a sensor for obtaining an indication of the pressure ratio under which said compressor is operating and sending a representative signal to said control. 
     
     
         10 . A method of controlling a refrigerant vapor compression system of the type having a gas cooler with a multi-speed fan and being designed to operate in a transcritical cycle, comprising the steps of:
 providing a digital scroll compressor having the capability to vary its capacity by modulating the respective loaded state and unloaded state times in a pulse width modulation manner; and   during periods of part load operating conditions, operating the fan at a low speed and reducing the loaded state time of operation for the digital scroll compressor.   
     
     
         11 . A method as set forth in  claim 10  and including the steps of:
 providing in serial flow relationship downstream of said gas cooler an expansion valve and a charge storage vessel; and 
 sensing a condition indicative of compressor pressure ratio and varying said expansion valve to control said pressure ratio. 
 
     
     
         12 . A method of controlling a refrigerant vapor compression system of the type having a gas cooler with a multi-speed fan and being designed to operate in a transcritical cycle, comprising the steps of:
 providing a digital scroll compressor having the capability to vary its capacity by modulating the respective loaded state and unloaded state times in a pulse width modulation manner;   during periods of full load operation conditions, operating the fan at a high speed; and   reducing the loaded state time of operation as necessary in order to prevent overloading of the compressor.   
     
     
         13 . A method as set forth in  claim 12  and including the steps of:
 providing in serial flow relationship downstream of said gas cooler an expansion valve and a charge storage vessel; and 
 sensing a condition indicative of pressure ratio of the compressor and varying the expansion valve to control said pressure ratio. 
 
     
     
         14 . A vapor compression system as set forth in  claim 1  wherein said vapor compression system comprises a transport refrigeration system for supplying refrigerated air to a perishable cargo storage zone associated with a transport refrigeration unit.