Abstract:
A transport refrigeration system, which uses the heat of compression to selectively provide heat to a cargo space by way of the evaporator coil, is provided with enhanced heating capacity during lower ambient conditions by causing the heat from an engine radiator to flow over the condenser coil to thereby increase the condensing pressure and temperature and thereby increase the heat of compression and the heat being provided to the space. Provision is also made to cause the hot air from the engine itself to flow over the engine radiator and the condenser coil to further enhance the heating capacity. Various damper and shutter arrangements are also provided as alternative embodiments.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to refrigeration systems and, more particularly to transport refrigeration systems operating in low temperature ambient conditions. 
         [0002]    For the transportation of goods that are required to be kept cold or frozen, vehicles such as trucks, trailers, rail cars, or refrigerated containers are provided with a refrigeration system which interfaces with the cargo space to cool the cargo down to a predetermined temperature. During periods in which the vehicle is located in an area of relatively low ambient temperature conditions, the temperature within the cargo space may fall to undesirable low temperatures such that the cargo could be damaged. Accordingly, it is necessary to provide heat to the internal cargo space so as to prevent the temperatures from falling to such levels. 
         [0003]    One method that has been used to provide heat to a cargo container is that of using the refrigerant heat of compression. However, in extremely cold ambients there is a minimal heat of compression that can be generated because much of the heat is lost to the surrounding atmosphere in the condenser and interconnecting piping. If the heat of compression is insufficient to overcome the lower ambient temperature conditions damage to the cargo may result. 
         [0004]    A transport refrigeration unit normally includes a diesel engine for driving the compressor of the system. 
         [0005]    The diesel engine normally has a liquid coolant system that includes a radiator for cooling the liquid by way of a liquid-to-air heat exchanger or radiator. In this way, the heat from the engine is passed to ambient by way of the radiator. It is common to place the radiator adjacent to the condenser with a single fan to draw cooling air first through the condenser and then through the radiator after which it passes to ambient. 
       SUMMARY OF THE INVENTION 
       [0006]    Briefly, in accordance with one aspect of the invention, during periods in which the refrigeration system is operating in very low ambient temperature conditions, the normal heating system is supplemented by a heating system in which waste heat from the engine radiator is used to increase the condensing pressure and temperature so as to thereby increase the heat of compression and the amount of heat that is available to maintain the temperature of the cargo. 
         [0007]    By another aspect of the invention, a fan, which normally operates to draw cooling air first through a condenser coil and then through the radiator coil, is operated in reverse during the heating cycle to cause air to pass over the radiator coil and then over the condenser coil to thereby increase the heat of compression in the system. 
         [0008]    By yet another aspect of the invention, in addition to the heat from the radiator coil, the heat from the engine is caused to flow over the condenser coil to thereby further increase the heat of compression on the system. 
         [0009]    In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic illustration of a transport refrigeration system operating in the cooling mode in accordance with the prior art. 
           [0011]      FIG. 2  is a schematic illustration of a transport refrigeration system operating in the heating mode in accordance with the prior art. 
           [0012]      FIG. 3  is a schematic illustration of a side view showing the airflow through the system during a cooling mode in accordance with the present invention. 
           [0013]      FIG. 4  is a schematic illustration of a side view showing the airflow through the system during the heating cycle in accordance with the present invention. 
           [0014]      FIG. 5  is a side view of an alternative embodiment thereof. 
           [0015]      FIG. 6  is a schematic side view of the airflow during a cooling mode in accordance with an alternative embodiment. 
           [0016]      FIG. 7  is a schematic side view of the heating mode in accordance with such an alternative approach. 
           [0017]      FIG. 8  is a schematic illustration of another alternative embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring now to  FIG. 1 , there is shown a conventional transport refrigeration system that includes the primary components of a compressor  11 , a condenser  12 , an expansion valve  13  and an evaporator  14 , all connected in serial flow relationship to operate as a vapor compression refrigeration system in a normal manner. 
         [0019]    The compressor  14  raises the pressure and the temperature of the refrigerant and forces it through the discharge check valve  16  and into the condenser tubes. The condenser fan circulates surrounding air over the outside of the condenser tubes. The tubes have fins designed to improve the transfer of heat from the refrigerant gas to the air. This removal of heat causes the refrigerant to liquefy. Liquid refrigerant leaves the condenser  12  and flows through the solenoid valve  17  (normally open) and to the receiver  18 . 
         [0020]    The receiver  18  stores the additional charge necessary for low ambient operation and for the heating and defrost modes of operation. 
         [0021]    The refrigerant leaves the receiver  18  and flows through the manual liquid line service valve  19  to the subcooler  21 . The subcooler  21  occupies a portion of the main condensing coil surface and gives off farther heat to the passing air. 
         [0022]    The refrigerant then flows through a filter-drier  22  where an absorbent keeps the refrigerant clean and dry, and then to the electrically controlled liquid line solenoid valve  23 , which, when open, allows for the flow of liquid refrigerant to the “liquid/suction” heat exchanger  24  where the liquid is further reduced in temperature by giving off some of its heat to the suction gas. The liquid then flows to the expansion valve  13  which is preferably an externally equalized thermostatic expansion valve which reduces the pressure of the liquid and meters the flow of liquid refrigerant to the evaporator  14  to obtain maximum use of the evaporator  14  heat transfer surface. 
         [0023]    The refrigerant pressure drop caused by the expansion valve is accompanied by a drop in temperature such that the low pressure, low temperature fluid that flows into the evaporator tubes is colder than the air that is circulated over the evaporator tubes by the evaporator fan. The evaporator tubes have aluminum fins to increase heat transfer; therefore heat is removed from the air circulated over the evaporator. This cold air is circulated throughout the box to maintain the cargo at the desired temperature. 
         [0024]    The transfer of heat from the air to the low temperature liquid refrigerant causes the liquid to vaporize. This low temperature, low pressure vapor passes through the “suction line/liquid line” heat exchanger  24  where it absorbs more heat from the high pressure/high temperature liquid and then returns to the compressor  11  through the suction modulation valve  26 . The suction modulation valve  26  controls the compressor suction pressure, thereby matching the compressor capacity to the load. 
         [0025]    While the primary concern with a transport refrigeration system is with the cooling mode of operation, it should be recognized that in certain seasons and localities the ambient temperatures are lower than the desired temperature for the internal confines of the box. Accordingly, it is necessary to provide heat to the box during these periods in order to prevent the cargo from being exposed to temperatures below the desired temperatures. Further, there are times when operating in the cooling mode that the evaporator coil has a frost buildup thereon that needs to be removed in order to continue to operate efficiently. This is accomplished by a defrosting process. Both the heating and defrosting is commonly accomplished by use of the “heat of compression” of the system. That is, when vapor is compressed to a high pressure and temperature in the compressor  11 , the mechanical energy necessary to operate the compressor  11  is transferred to the gas as it is being compressed. This energy is referred to as the “heat of compression” and is used as a source of heat during the heating cycle. 
         [0026]    Referring to  FIG. 2 , when the unit controller calls for heating, the hot gas solenoid valve  27  opens and the condenser pressure control solenoid valve  17  closes. The condenser coil  12  then fills with refrigerant, and hot gas from the compressor  11  enters the evaporator  17 . Also the liquid line solenoid valve  23  will remain energized (valve open) until the compressor discharge pressure increases to a pre-determined setting in the microprocessor. The microprocessor de-energizes the liquid line solenoid valve  23  and the valve closes to stop the flow of refrigerant to the expansion valve  13 . When additional heating capacity is required the microprocessor opens the liquid line solenoid valve  23  to allow additional refrigerant to be metered into the hot gas cycle through the expansion valve  13 . 
         [0027]    The function of the hot gas bypass line  28  is to raise the receiver pressure when the ambient temperature is low (below −17.8° C./0° F.) so that refrigerant flows from the receiver  18  to the evaporator  14  when needed. 
         [0028]    The applicants have recognized that in cold ambients, there is a minimal heat of compression that can be generated and this heat of compression may not be sufficient to provide the necessary heat to maintain the desired temperature in the box. It is therefore desirable to provide additional heat during these periods. 
         [0029]    The compressor  14  is traditionally driven by an internal combustion engine and preferably a diesel engine. Such an engine requires some method of cooling so as to prevent excessive temperatures therein. This is normally accomplished by way of a radiator with liquid coolant passing through the engine and through the radiator where it is exposed to the flow of air therethrough for the cooling of the coolant. 
         [0030]    Referring now to  FIG. 3 , the relative placement of the engine  29  and its fluidly connected radiator  31  is shown in relation to the condenser coil  12  and the evaporator coil  14 . As will be seen, the radiator coil  13  is located directly behind the condenser coil  12  such that when the condenser fan  32  is driven by the motor  33 , the cooling air is caused to pass first through the condenser coil  12  and then through the radiator  31 . A portion of the air then passes over the engine  29  as shown, and a portion passes out the opening  34  to ambient. A damper  36  may be provided to be used in a manner to be described hereinafter. 
         [0031]    In order to boost the heat of compression during low ambient conditions, it is the intent of the present invention to use the heat that is rejected by the engine radiator  31  to provide an additional heat source for the purpose. This is  12  such that the warmer air being re-circulated into the condenser inlet air stream is used to boost the condensing pressure and temperature. Higher pressure leads to the compressor  11  producing more heat of compression, and therefore more heat can be generated to maintain cargo temperature. 
         [0032]    In addition to the waste heat from the radiator, the relative position of the components as shown in  FIG. 4  also allows heat from the engine  29  to be drawn-in by the fan  32  and passed to the radiator  31  and the condenser coil  12  to thereby further boost heat performance of the system. 
         [0033]    Although the damper  36  as shown in  FIGS. 3 and 4  is in the open position, it may be moved to a closed position for the purpose of directing warmer air into the radiator and condenser that has circulated past the warm engine to further raise condensing temperature and pressure, as opposed to just pulling colder air from the outside ambient. 
         [0034]    An alternative embodiment is shown in  FIG. 5  wherein, because of packaging constraints, there is a minimum depth available for the unit. Accordingly, the evaporator section  37  has a dedicated fan  38  and drive motor  39  to circulate air through the evaporator coil  41 . The condenser fan rather than being centrally located in the space  42 , is located at the lower end thereof such that the motor  43  is located in the space  42  and the fan  44  is located between the space  42  and the space occupied by the engine compartment which includes the engine, generator and compressor shown at  30 . 
         [0035]    In operation during the heating process, the fan is operated in a direction such that the hot air from the engine compartment flows into the space  42  and through the radiator  31  and the condenser  12  so as to raise the condensing pressure in the manner as described hereinabove. In the cooling mode, the fan  44  is operated in the opposite direction such that the air flows first through the condenser  12  the radiator  31 , the space  42  and then through the engine compartment. 
         [0036]    Referring now to  FIGS. 6 and 7 , an alternative embodiment is shown to include a plurality of shutters  46  and a damper  47  as shown. During operation in the cooling mode, the fan motor  33  is driving the fan  32  in a direction such that the air is pulled through the condenser  12  and the radiator  31 , and the shutters  46  are open such that the air passes through them, through the condenser  12  and through the radiator  31 . The damper  47  is in the closed position as shown. 
         [0037]    During operation in the heating mode, the shutters  46  are closed and the damper  47  is open as shown. The fan motor  33  rotates the fan  32  in a blow through direction such that the air then passes first through the radiator  31 , then through the condenser  12  and out the opening of the open damper  47  as shown. These additional dampers serve to block air from entering the condenser and radiator from the undesired direction that opposes the airflow path described above when the refrigeration unit is being transported. 
         [0038]    A further alternative approach is shown in  FIG. 8  wherein the fan  32  is driven by a belt  48  and is unidirectional. It is thus necessary to provide other means of reversing the direction of flow when changing from the cooling to the heating mode. For that purpose, an air recirculation passageway  49  is provided at one end of the unit as shown. Also provided is a gate  51  which is open (as shown in solid line) during the heating mode and closed (as shown in dashed line) during the cooling mode. Thus, during the cooling mode of operation, the air flows from the fan into the air circulation passageway  49  and then, with the shutters  46  in the closed position, the air passes through the condenser coil  12  and through the radiator  31 . 
         [0039]    During the cooling mode of operation, the gate  51  is in the closed position and the shutters  46  are in the open position such that the air passes first through the condenser coil  12  and then through the radiator  31 , and out through the open shutters  51 . In heating mode the fan direction can&#39;t be reversed with a belt drive approach, so air is directed into passageway  49  and recirculated to the condenser.