Patent Publication Number: US-8522564-B2

Title: Temperature control system with refrigerant recovery arrangement

Description:
BACKGROUND 
     The present invention relates to temperature control systems. More particularly, the present invention relates to a temperature control system for a transport vehicle. 
     Generally, transport vehicles (e.g., straight trucks and tractor-trailer combinations) are used to transport temperature sensitive cargo that is maintained at predetermined conditions using a temperature control system during transportation to preserve the quality of the cargo. The cargo is transported, stored, or otherwise supported within a cargo space of the transport vehicle. 
     In some transport units, the temperature control system must be capable of cooling and heating the cargo space to maintain a desired temperature (i.e., a setpoint temperature). A controller switches the temperature control unit between heating and cooling modes based on the relative difference between a sensed temperature and the setpoint temperature to regulate the condition of the cargo space. Typically, the temperature control system is capable of operating a conventional refrigeration cycle utilizing a phase-change refrigerant to cool the cargo space. Refrigerant is compressed by a compressor, condensed, and evaporated in a heat exchanger in thermal communication with the cargo space to cool the cargo space. Heating is typically accomplished by bypassing the condenser and directing hot compressed refrigerant directly to the heat exchanger in thermal communication with the cargo space to heat the cargo space. 
     SUMMARY 
     In one embodiment, the invention provides a temperature control system including a compressor configured to compress a heat transfer fluid, a first heat exchanger in fluid communication with the compressor and configured to receive the heat transfer fluid from the compressor and to cool and condense the heat transfer fluid (e.g., up to saturated vapor state), and a second heat exchanger in fluid communication with the first heat exchanger and the compressor and configured to exchange heat with a temperature-controlled space. The system further includes a receiver in fluid communication with each of the first and second heat exchangers. The receiver is positioned between the first heat exchanger and the second heat exchanger and is configured to receive condensed heat transfer fluid from the first heat exchanger and to direct heat transfer fluid to the second heat exchanger. A valve is positioned between the receiver and the second heat exchanger. An accumulator is in fluid communication with the second heat exchanger and the compressor and is configured to receive a mixture of liquid and vapor heat transfer fluid from the second heat exchanger and direct a vapor portion of the heat transfer fluid to the compressor. An evacuation line has a first end in fluid communication with and positioned between the valve and the receiver, and a second end in fluid communication with the accumulator. The evacuation line provides for flow of the heat transfer fluid from both of the first heat exchanger and the receiver to the accumulator during an evacuation mode of operation of the temperature control system. The valve substantially prevents the flow of heat transfer fluid from the second heat exchanger into the receiver 
     In another embodiment the invention provides a method of operating a temperature control system having a compressor, a first heat exchanger downstream of the compressor, a receiver downstream of the first heat exchanger, a valve downstream of the receiver, a second heat exchanger downstream of the valve, and an accumulator downstream of the second heat exchanger. The system can be operated in a cooling mode by compressing a heat transfer fluid with the compressor, directing the heat transfer fluid from the compressor to the first heat exchanger with a valve arrangement in a first configuration, cooling and condensing the heat transfer fluid from the compressor in the first heat exchanger, exchanging heat with a temperature-controlled space with the second heat exchanger, receiving a mixture of liquid and vapor heat transfer fluid from the second heat exchanger into the accumulator, and directing a vapor portion of the heat transfer fluid in the accumulator to the compressor. The system can be operated in an evacuation mode by opening a purge valve coupled to an evacuation line. The evacuation line has a first end in fluid communication with and between the valve and the receiver, and a second end in fluid communication with the accumulator. The evacuation line provides flow of the heat transfer fluid from both of the first heat exchanger and the receiver to the accumulator without passing through the second heat exchanger. The valve substantially prevents the flow of heat transfer fluid from the second heat exchanger into the receiver. The system can be further operated in a heating mode by moving the valve arrangement from the first configuration to a second configuration, directing the heat transfer fluid from the compressor to the second heat exchanger without passing through the first heat exchanger, and maintaining the purge valve open. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a temperature control system according to one embodiment of the present invention, illustrating a cooling mode. 
         FIG. 2  is a schematic of the temperature control system of  FIG. 1 , illustrating a condenser evacuation mode. 
         FIG. 3  is a schematic of the temperature control system of  FIG. 1 , illustrating a heating/defrost mode. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
       FIGS. 1-3  illustrate a temperature control system  10  for a transport vehicle. The temperature control system  10  is capable of cooling and heating a cargo space of the transport vehicle to maintain a desired temperature (i.e., a setpoint temperature). The temperature control system  10  includes a controller  12  that switches the temperature control system  10  between heating and cooling modes based on the relative difference between a sensed temperature and the setpoint temperature to regulate the condition of the cargo space. It is to be understood that the refrigeration system  10  may be utilized for other refrigeration applications and is not limited to transport refrigeration applications. 
     The temperature control system  10  includes a compressor  14 , a first heat exchanger or condenser  16 , a receiver  18 , a filter/dryer  19 , an expansion valve  20 , a second heat exchanger or evaporator  22 , and an accumulator  24  connected in series by fluid conduits. The first heat exchanger  16  is in thermal communication with air outside of the transport vehicle. The second heat exchanger  22  is in thermal communication with air inside the cargo space of the transport vehicle. In other constructions, there may be more than one heat exchanger in thermal communication with air inside the cargo space. A distributor  40  may be employed, as is well known in the art, to distribute refrigerant to a plurality of second heat exchangers (not shown). A first portion  44  of the temperature control system  10 , including the second heat exchanger  22 , is preferably positioned within the cargo space. A second portion  46  of the temperature control system  10 , including the first heat exchanger  16 , is preferably positioned outside of the cargo space. 
     In the cooling mode or refrigeration cycle (see  FIG. 1 ), the receiver  18  receives a heat transfer fluid (e.g., refrigerant) from the first heat exchanger  16  and directs refrigerant through the filter/dryer  19  and to the second heat exchanger  22 . The expansion valve  20  reduces the pressure of the refrigerant just upstream of the second heat exchanger  22 . The illustrated expansion valve  20  is a conventional thermostatic expansion valve having a bleed port. The accumulator  24  receives a liquid and gaseous mixture of refrigerant from the second heat exchanger  22 . Liquid refrigerant accumulates at the bottom of the accumulator  24  and gaseous refrigerant is displaced to the top. The accumulator  24  includes a U-tube  42  (i.e., a U-shaped tube) for drawing gaseous refrigerant from the top of the accumulator  24  into the compressor suction line, as is well known in the art. 
     The temperature control system  10  also includes a suction line heat exchanger  26  and a throttling valve  28 . The suction line heat exchanger  26  is a shell and tube heat exchanger that transfers heat between the warm liquid refrigerant entering the second heat exchanger  22  and cold vapor refrigerant leaving the second heat exchanger  22 . The throttling valve  28  is positioned between the suction line heat exchanger  26  and the accumulator  24 . It is to be understood that other types of heat exchangers may be used to accomplish the same results. 
     First and second compressor outlet solenoid valves  30  and  32  together define a valve arrangement operable to direct high pressure refrigerant exiting the compressor  14 . The first solenoid valve  30  is sometimes referred to as a condenser block-off solenoid, and the second solenoid valve is sometimes referred to as a hot gas defrost solenoid. In a first configuration, the first compressor outlet solenoid valve  30  is opened and the second compressor outlet solenoid valve  32  is closed ( FIGS. 1 and 2 ). In this first configuration, refrigerant is directed from the compressor  14  to the first heat exchanger  16 . In a second configuration, in which the first compressor outlet solenoid valve  30  is closed and the second compressor outlet solenoid valve  32  is open ( FIG. 3 ), refrigerant is directed from the compressor  14  to the second heat exchanger  22 . In the first configuration, a refrigeration or cooling circuit is formed. In the second configuration, a heating/defrost circuit is formed. The controller  12  controls the position and energization of the solenoid valves  30  and  32  in a manner well understood in the art. In other embodiments, a conventional three-way valve could be substituted for the two solenoid valves  30 ,  32 . In yet other constructions, other types of valve arrangements or switching mechanisms may be employed to switch the system between a refrigeration circuit and a heating/defrost circuit. 
     The temperature control system  10  is operable in a cooling mode, an evacuation mode, and a heating/defrost mode. The controller  12  communicates with the first and second compressor outlet solenoid valves  30 ,  32  to achieve the first configuration during the cooling mode and the evacuation mode, and to achieve the second configuration during the heating/defrost mode. 
     During the cooling mode, illustrated in  FIG. 1 , the first and second compressor outlet solenoid valves  30 ,  32  are the first configuration to direct high pressure gas refrigerant from the compressor  14  to the first heat exchanger  16 . The high pressure gas refrigerant is condensed in the first heat exchanger  16  to a high pressure liquid refrigerant, which is directed to the receiver  18 . The receiver  18  ensures that only liquid refrigerant is directed toward the second heat exchanger  22 . The high pressure liquid refrigerant is directed through a line  34  to the filter/dryer  19  and then through a line  36  to the suction line heat exchanger  26  to be pre-cooled by low pressure refrigerant exiting the second heat exchanger  22 . A one-way check valve  38  is positioned in the line  36 , downstream of the filter/dryer  19 , to substantially prevent back flow of refrigerant from the suction line heat exchanger  26  toward the filter/dryer  19 . One of ordinary skill in the art will understand that while the check valve  38  is intended to completely prevent back flow, there may or will be, in actuality, a small amount of leakage past the check valve  38  in normal operation during the evacuation mode and the heating mode. 
     After passing through the suction line heat exchanger  26 , the high pressure liquid refrigerant is passed through the expansion valve  20  to lower the pressure of the refrigerant. At least a portion of the refrigerant evaporates in the second heat exchanger  22  to create a mixture of low pressure liquid and low pressure gas. The mixture passes through the second heat exchanger  22  and absorbs heat from air being directed into the cargo space to thereby cool the cargo space. Vapor refrigerant exits the heat exchanger  22  and then passes through the suction line heat exchanger  26  and exchanges heat with the high pressure liquid refrigerant approaching the second heat exchanger  22 . Then, the vapor refrigerant goes to the compressor suction line via the accumulator  24 . 
     The illustrated compressor  14  is a scroll compressor that is cooled during the cooling mode using a portion of the high pressure liquid refrigerant. A liquid injection cooling line  50  is fluidly connected between the compressor  14  and the line  34  such that a portion of the high pressure liquid refrigerant can flow from the line  34  (at a point downstream of the receiver  18 ) back to the compressor  14  to help cool the internal components of the compressor  14 . A solenoid valve  54  controls flow of the refrigerant through the liquid injection cooling line  50 . Specifically, the valve  54  is opened during the cooling cycle to permit the flow of high pressure liquid refrigerant back to the compressor  14 , and is closed during the evacuation mode and the heating/defrost modes. Other embodiments of the invention could use other types of compressors (e.g., a Swash plate compressor/reciprocating compressor), and may or may not incorporate the liquid injection cooling line  50  and the associated solenoid valve  54 . 
     The temperature control system  10  remains in the cooling mode until a heating or defrost operation is needed. When a heating or defrost operation is needed, the controller  12  switches to the evacuation mode. 
     The evacuation mode, illustrated in  FIG. 2 , is operated between the cooling and heating/defrost modes to move refrigerant from the high pressure side to the low pressure side for use during the heating/defrost mode. During the evacuation mode, the first and second compressor outlet solenoid valves  30 ,  32  are in the first configuration. The system  10  includes an evacuation line  58  that fluidly connects the first heat exchanger  16 , the receiver  18 , the filter/dryer  19 , and the liquid injection cooling line  50  (if present) to the accumulator  24 , bypassing the suction line heat exchanger  26 , the expansion valve  20  and the second heat exchanger  22 . A purge valve  62  controls the flow of liquid refrigerant through the evacuation line  58 , and in the illustrated embodiment is a solenoid valve that can be opened to allow the flow of refrigerant through the evacuation line  58  or closed to prevent the flow of refrigerant through the evacuation line  58 . During the cooling mode, the controller  12  ensures that the purge valve  62  is closed to prevent the movement of high pressure liquid refrigerant into the accumulator  24 . 
     Upon entering the evacuation mode, the controller  12  opens the purge valve  62  to allow passage through the evacuation line  58  of high pressure liquid refrigerant from the receiver  18  and the first heat exchanger  16  into the accumulator  24 . The controller  12  may switch OFF evaporator fans  68  (or may close the damper door if the evaporator fans are directly-driven, mechanical fans). Also, the controller  12  may switch OFF condenser fans  70  to build the head pressure for better evacuation in low ambient temperatures. High pressure liquid refrigerant also flows through the evacuation line  58  and into the accumulator  24  from the filter/dryer  19  and possibly from the liquid injection cooling line  50 . The check valve  38 , which is in the line  36  downstream of the filter/dryer  19  and upstream of the suction line heat exchanger  26 , substantially prevents refrigerant in the second heat exchanger  22  and in the suction line heat exchanger  26  from flowing back to the filter/dryer  19 , the line  34 , the evacuation line  58 , and/or the receiver  18  where it may tend to condense, especially in cold ambient temperature conditions. The check valve  38  thereby substantially prevents the evaporator-side refrigerant from flowing back through the evacuation line  58  during the evacuation cycle. In some embodiments, in which the expansion valve does not include a bleed port, the check valve  38  can be eliminated since the expansion valve, which is closed due to elevated evaporator pressure during the heating mode, would operate to substantially prevent the flow of heat transfer fluid from the second heat exchanger  22  into the filter/dryer  19 , the evacuation line  58 , and the receiver  18 . In contrast to the bleed port design, which intends to allow flow through the bleed port when the expansion valve  20  is closed, this alternative expansion valve without a bleed port is intended to completely prevent back flow when closed, although, in actuality, a small amount of leakage may occur past this expansion valve design. 
     Refrigerant introduced to the accumulator  24  through the evacuation line  58  becomes available for use during the heating/defrost cycle. The amount of refrigerant moved into the accumulator  24  is large enough to enhance the capacity for heating, and in the illustrated embodiment is all of the refrigerant from the first heat exchanger  16 , the receiver  18 , and the filter/dryer  19 . 
     When the controller determines that enough time has passed to evacuate the refrigerant from the first heat exchanger  16 , the receiver  18 , and the filter/dryer  19  to the accumulator  24 , it switches the first and second compressor outlet solenoid valves  30 ,  32  to the second configuration and initiates the heating/defrost mode. The purge valve  62  remains in the open position during the heating/defrost cycle so that any refrigerant that may leak past the check valve  38  or the compressor outlet solenoid valve  30  can be evacuated to the accumulator  24  via the evacuation line  58  for use during the heating/defrost cycle. A check valve  72  downstream of the purge valve  62  prevents the back flow of refrigerant if the pressure in the accumulator  24  is higher than the pressure in evacuation line  58 . This may happen in low ambient temperatures. 
     During the heating/defrost mode, illustrated in  FIG. 3 , refrigerant bypasses the first heat exchanger  16  and hot gas refrigerant is directed from the compressor  14  to the second heat exchanger  22  to heat the cargo space or to defrost the second heat exchanger coil. A pressure regulating device  66  (e.g., a differential pressure regulating (DPR) valve) is positioned between the compressor  14  and the second heat exchanger  22 . Refrigerant is directed from the compressor  14  to the pressure regulating device  66  to the second heat exchanger  22 , bypassing the expansion valve  20 . The refrigerant goes through the second heat exchanger coil  22  where it is cooled and/or partially condensed. As the refrigerant passes through the second heat exchanger  22 , the refrigerant releases heat either to ice formed on the external surfaces of the second heat exchanger  22  to thereby defrost the second heat exchanger  22 , to air being directed into the cargo space to thereby heat the cargo space, or to both. 
     The refrigerant enters the accumulator  24 , where condensed liquid refrigerant is separated from vapor refrigerant. The vapor refrigerant returns to the compressor by way of U-tube  42  and is compressed to a high pressure and high temperature gas, and the cycle repeats. 
     If cooling is demanded, the controller  12  switches to the refrigeration mode by switching the first and second compressor outlet solenoid valves  30 ,  32  to the first configuration and closing the purge valve  62 . 
     Various features and advantages of the invention are set forth in the following claims.