Abstract:
A transportation refrigeration system operable in a cooling mode, a defrost mode, and a heating mode to condition air in an air-conditioned space. The system comprises a refrigeration circuit fluidly connecting a compressor, a condenser, a tank, and an evaporator. The evaporator is in thermal communication with the air-conditioned space. A heating circuit fluidly connects the compressor, the tank, the evaporator, and a heater. A defrost circuit fluidly connects the compressor, the tank, the evaporator, and the heater. The tank has a base and the first opening is spaced a first distance above the base and the third opening is spaced a second greater distance above the base.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to provisional patent application No. 60/311,637, filed on Aug. 10, 2001. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to air conditioning and refrigeration systems, and more specifically to a refrigeration system having a cooling mode, a heating mode, and a defrost mode and a method of operating the refrigeration system in the cooling, heating, and defrost modes. 
     BACKGROUND OF THE INVENTION 
     Conventional refrigeration systems are commonly used-to maintain the temperature of an air-conditioned space at or near a set point temperature and typically include a compressor, a condenser, an expansion valve, and an evaporator. Generally, refrigeration systems operate in cooling, heating, and defrost modes, depending, at least in part, upon the temperature of the air-conditioned space and the ambient temperature outside the air-conditioned space. When the temperature of the air-conditioned space is above the set point temperature, the refrigeration systems operate in the cooling mode to pull down the temperature in the air-conditioned space. During operation in the cooling mode, refrigerant is directed along a refrigerant circuit, which includes the condenser, a receiver tank, the expansion valve, the evaporator, and an accumulator tank. 
     When the temperature of the air conditioned space is below the set point temperature, the refrigeration systems operate in a heating mode. Additionally, to minimize the formation of ice and frost on the evaporator coil and to ensure that the refrigeration system is operating in the most efficient manner, refrigeration systems periodically operate in a defrost mode. When the system is switched from the cooling mode to the heating or defrost modes, hot refrigerant vapor is directed out of the compressor through a heating circuit, which includes a pan heater, the evaporator and the accumulator tank. 
     The heating and defrosting capacity of the system depends, at least in part, upon the volume of refrigerant being directed through the heating circuit. Therefore, it is desirable to ensure that a maximum amount of refrigerant is directed through the heating circuit during heating and defrost modes. Moreover, during heating and defrost modes, refrigerant which has accumulated in the condenser is unavailable for heating and defrosting. Therefore, it is desirable to ensure that refrigerant does not accumulate in condenser during heating and defrost modes. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a transportation refrigeration system operable in a cooling mode, a defrost mode, and a heating mode to condition air in an air-conditioned space comprises a refrigeration circuit fluidly connecting a compressor, a condenser, a tank, and an evaporator. The evaporator is in thermal communication with the air-conditioned space. The system further comprises a heating circuit fluidly connecting the compressor, the tank, the evaporator, and a heater. The system also comprises a defrost circuit fluidly connecting the compressor, the tank, the evaporator, and the heater. 
     In some embodiments of the transportation refrigeration system, a substantial quantity of a refrigerant is directed through the refrigeration circuit during operation in the cooling mode. The substantial quantity of the refrigerant is also directed through the heating circuit during operation in the heating mode and the defrost mode. 
     The transportation refrigeration system includes a controller operable to selectively direct a quantity of a refrigerant through the refrigeration circuit, the defrosting circuit, and the heating circuit during operation in the cooling mode, the defrost mode, and the heating mode, respectively. A plurality of adjustable valves are distributed throughout the refrigeration system and are operable to selectively alter the flow of refrigerant through the refrigeration circuit, the heating circuit, and the defrost circuit. 
     The tank includes a first opening, a second opening, and a third opening. The refrigeration circuit is coupled to the first and second openings. The heating circuit is coupled to the first and third openings, and the defrost circuit is coupled to the first and third openings. The tank includes a base and the first opening is spaced a first distance above the base and the third opening is spaced a second smaller distance above the base. A quantity of oil is periodically mixed with the refrigerant in the compressor and the tank periodically separates at least some of the oil from the refrigerant. 
     Also according to the present invention, a method of conditioning air in an air-conditioned space with a transportation refrigeration system having a refrigerant comprises directing substantially all of the refrigerant through a refrigeration circuit during operation in a cooling mode. The refrigeration circuit includes a tank and an evaporator. The evaporator is in thermal communication with the air-conditioned space. The method further comprises directing substantially all of the refrigerant through a heating circuit during operation in a heating mode. The heating circuit includes the tank, the evaporator coil, and a heater. The method also includes directing substantially all of the refrigerant through a defrost circuit during operation in a defrost mode. The defrost circuit extends through the tank, the heater, and the evaporator. 
     In some embodiments, the refrigeration circuit includes a condenser and the method includes removing at least a substantial quantity of refrigerant from the condenser before initiating the defrost mode. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described with reference to the accompanying drawings, which show preferred embodiments of the present invention. However, it should be noted that the invention as disclosed in the accompanying drawings is illustrated by way of example only. The various elements and combinations of elements described below and illustrated in the drawings can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present invention. 
     In the drawings, wherein like reference numerals indicate like parts: 
     FIG. 1 is a schematic drawing of the refrigeration system according to a first embodiment of the present invention; and 
     FIG. 2 is a schematic drawing of the refrigeration system according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a transportation refrigeration system  10  in accordance with the present invention. The refrigeration system  10  is operable to control the temperature of an air-conditioned space  14  to a predetermined set point temperature (“SP”). As shown in FIG. 1, the refrigeration system  10  is mounted on the front wall  16  of a truck or trailer. In other applications, the refrigeration system  10  can alternatively be used on other vehicles, such as a tractor-trailer combination, a container, and the like. Similarly, the refrigeration system  10  can be used to control the temperature in the passenger space of a vehicle, such as, for example, a bus or the passenger compartment of a truck. 
     As used herein and in the claims, the term “air-conditioned space” includes any space to be temperature and/or humidity controlled, including transport and stationary applications for the preservation of foods, beverages, plants, flowers, and other perishables, maintenance of a proper atmosphere for the shipment of industrial products, space conditioning for human comfort, and the like. 
     Additionally, as used herein and in the claims, the term “refrigerant” includes any conventional refrigerant including, but not limited to, chloroflourocarbons (CFCs), hydrocarbons, cryogens (e.g., CO 2  and N 2 ), and other known refrigerants. Additionally, one having ordinary skill in the art will appreciate that during operation of the present system additives and inclusions may be intentionally or inadvertently mixed with the refrigerant, such as, for example, motor oil, dirt, debris, rust, and the like. 
     In order to maintain the temperature in the air-conditioned space  14  at or near the set point temperature SP, the refrigeration system  10  can operate in a cooling mode, a heating mode, and a defrost mode. If the temperature in the air-conditioned space  14  is above the set point temperature SP, the refrigeration system  10  operates in the cooling mode. Conversely, if the temperature in the air-conditioned space  14  is below an acceptable range surrounding the set point temperature SP (e.g., in relatively cold climates), the refrigeration system  10  operates in the heating mode. Also, the refrigeration system  10  periodically operates in the defrost mode to maintain system efficiency and to prevent the formation of ice and frost. 
     With reference first to operation in the cooling mode, the refrigeration system  10  directs a refrigerant along a closed refrigeration circuit  20  (represented by arrows  20  in the figures), which extends through a compressor  22  that is driven by a prime mover, such as, for example, an internal combustion engine or an electric motor. In the illustrated embodiment, the compressor  22  is a hermetically sealed compressor and a compressor motor  24  is enclosed within the compressor housing. In other embodiments (not shown), other compressors, including open shaft, can also or alternately be used. 
     The compressor  22  includes an inlet  26  located on the low pressure or suction side of the compressor  22 , a discharge  28  located on the high pressure or discharge side of the compressor  22 , and a liquid injection port  30 . The discharge  28  is connected to a flow path  34 , which includes a first solenoid valve  38 . A branch  36  intersects the flow path  34  upstream from the first solenoid valve  38  and includes a second solenoid valve  40 . 
     During operation in the cooling mode, the second solenoid valve  40  is closed and the first solenoid valve  38  is opened. Therefore, refrigerant flowing out of the compressor  22  travels along the refrigeration circuit  20  through the flow path  34  and into a condenser  42 . The condenser  42  includes a condenser fan  46  and a condenser coil  48  having an inlet  50  connected to the flow path  34 . During operation in the cooling mode, the refrigerant continues along the refrigeration circuit  20  out of the condenser coil  48  into a liquid line  53 , which includes a first check valve  54 , a first throttling valve  55 , and a third solenoid valve  56 . The first check valve  54  is a one way check valve that enables liquid refrigerant to flow out of the condenser coil  48  and prevents liquid refrigerant from flowing back into the condenser coil  48 . During operation in the cooling mode, the third solenoid valve  56  is closed, causing refrigerant leaving the condenser coil  48  to enter a tank  57 . 
     The tank  57  includes a first opening  58 , a second opening  60 , and a third opening  62 . During operation in the cooling mode, refrigerant travels along the refrigeration circuit  20 , into the tank  57 , through the first opening  58 , and out of the tank  57  through the second opening  60 . Preferably, a quantity of liquid refrigerant accumulates at the base of the tank  57 . 
     From the second opening  60 , the refrigerant continues along the refrigeration circuit  20  through a line  64 . The line  64  has a first branch  66  and a second branch  68 . The second branch  68  includes a throttling device  69  and intersects line  53  at a junction between the first throttling valve  55  and the third solenoid valve  56 . The third solenoid valve  56  is closed during operation in the cooling mode. Additionally, the check valve  54  prevents refrigerant from flowing through the line  53  toward the condenser coil  48 . Therefore, during operation in the cooling mode, refrigerant flows along the refrigeration circuit  20  through the first branch  66 , a filter  74 , and an internal heat exchanger  76 . 
     The refrigerant traveling along the refrigeration circuit  20  continues through a liquid line  78  in the internal heat exchanger  76  to an expansion valve  80 . The outlet of the expansion valve  80  is connected to a distributor  82  with a small orifice (not shown), which distributes refrigerant to inlets on the inlet side of an evaporator coil  84 . The evaporator coil  84  is in thermal communication with the air-conditioned space  14  to control the temperature of air in the air-conditioned space  14 . Specifically, during operation in the cooling mode, relatively warm air-conditioned space air is blown across the evaporator coil  84  by a fan or blower  86  and is cooled by contact with the relatively cold evaporator coil  84 . At the same time, warmer air-conditioned space air warms the refrigerant in the evaporator coil  84 , vaporizing and superheating most (if not all) of the refrigerant. 
     The vaporized and superheated refrigerant continues along the refrigeration circuit  20  out of the evaporator coil  84  via a vapor line  88  and is directed through a vapor portion  90  of the internal heat exchanger  76 . The vaporized refrigerant passes through an adjustable throttling device  96  before re-entering the compressor  22  via the compressor inlet  26 . Additionally, a second check valve  92  prevents saturated vapor and saturated liquid from reentering the vapor portion  90  of the internal heat exchanger  76  and the evaporator coil  84 . The fourth solenoid valve  94  is closed to prevent superheated gas from the compressor discharge  28  from entering the inlet  26  via line  111 . Once the refrigerant is returned to the compressor  22 , the compressor  22  recompresses the refrigerant and recycles the refrigerant through the refrigerant circuit  20 . 
     The refrigeration system  10  also includes a controller  100 , which is preferably a microprocessor. The controller  100  receives temperature and pressure data from sensors (not shown), located throughout the refrigeration system  10 . The controller  100  determines whether heating, cooling, or defrost is required by comparing the data collected by the sensors with the set point temperature SP. 
     During operation in the cooling mode, the controller  100  controls the cooling capacity of the refrigeration system  10  by opening and closing the first, second, third, and fourth solenoid valves  38 ,  40 ,  56 ,  94 , adjusting the operating speed of the compressor  22 , and opening and closing the adjustable throttling device  96 . 
     Additionally, the controller  100  is programmed to inject liquid refrigerant into the compressor  22  to maintain the discharge temperature below a desired temperature limit. With reference to FIG. 1, a liquid injection line  108  extends from the second opening  60  of the storage tank  57  to the liquid injection port  30  on the compressor  22 . When the controller  100  determines that liquid injection is required (e.g., sensors adjacent the discharge  28  record temperature values below a desired discharge temperature), the controller  100  opens a liquid injection valve  109 , positioned along the liquid injection line  108 , directing liquid refrigerant from the bottom of the storage tank  57  into the working space of the compressor  22  through the liquid injection line  108 . 
     Also, if the compressor  22  is a hermetically sealed or semi-hermetically sealed compressor, as shown in the figures, the system preferably includes a line  111 , which is used during extreme working conditions (e.g., during startup and when the ambient temperature is relatively high). In these circumstances, the adjustable throttling device  96  is incremented, reducing the flow of refrigerant through the refrigeration circuit  20 , which reduces the discharge pressure of the compressor  22  and/or reduces the compressor supply power. In this manner, the system  10  cools the compressor motor  24  and maintains the discharge temperature below the maximum allowable limit. Additionally, if necessary, the liquid injection valve  109  can be opened simultaneously with the fifth solenoid valve  110  to maintain the desired compressor discharge temperature. Regardless of the type of compressor used, once the system  10  is no longer operating in extreme conditions, the controller  100  is programmed to maintain the compressor discharge temperature by periodically opening and closing the liquid injection valve  109 , as described above. 
     Additionally, liquid injection through the fifth solenoid valve  110  may be required when the compressor  22  is a hermetically sealed compressor and the shaft speed is at a minimum. Once the fifth solenoid valve  110  is opened, the pressure in the tank  57  forces liquid refrigerant out of the bottom of the tank  57  through the second opening  60  and the liquid line  112  into the compressor  22 . The liquid refrigerant cools the compressor motor  24  and the compressor  22 . The compressor  22  then compresses the refrigerant before forcing the refrigerant out of the discharge  28  to be recycled through the refrigeration system  10 . 
     When the fifth solenoid valve  110  is opened, less refrigerant is directed through the evaporator coil  84 , thereby reducing the cooling capacity of the refrigeration system  10 . Therefore, after the compressor temperature or the discharge pressure drop below the maximum allowable temperature and pressure values or after a predetermined time period, the controller  100  is programmed to close the fifth solenoid valve  110 , increasing the flow of refrigerant through-the evaporator  84  and increasing the rate of cooling of the air-conditioned space  14 . If either or both of the compressor temperature and discharge pressure again rise above the maximum allowable values after the fifth solenoid valve  110  is closed, the controller  100  is programmed to wait a predetermined time period (e.g., two minutes) and reopen the fifth solenoid valve  110 . This process is repeated until the compressor temperature and discharge pressure drop below the maximum allowable values. 
     In some cases, particularly when the ambient temperature is below the set point temperature SP, it may be necessary to heat the air-conditioned space  14 . In these cases, the controller  100  is programmed to operate the refrigeration system  10  in the heating mode. When the controller  100  determines that heating is required, the controller  100  closes the adjustable throttling device  96  and the first solenoid valve  38  and opens the second solenoid valve  40 . Also, the third solenoid valve  56  remains closed. Because the first solenoid valve  38  is closed, refrigerant is not allowed to enter the condenser coil  48 . However, some residual refrigerant may remain in the condenser coil  48  even after the first solenoid valve  38  is closed. It is desirable to evacuate most of the refrigerant from the condenser coil  48  at the beginning of operation in the heating mode because any refrigerant that remains in the condenser coil  48  is unavailable for heating and therefore reduces the heating capacity of the refrigeration system  10 . Therefore, at the beginning of operation in the heating mode, the controller  100  is programmed to operate the refrigeration system  10  in a first or condenser evacuation phase. In the condenser evacuation phase, the fourth solenoid valve  94  is opened, and the compressor  22  operates to create an area of reduced pressure adjacent the inlet  26 , thereby drawing residual refrigerant out of the condenser coil  48  into the compressor  22  via a line  111  and the fourth solenoid valve  94 . 
     Once the condenser coil  48  has been evacuated or after a predetermined time period, the controller  100  enters a second heating phase. Alternatively, the controller  100  can be programmed to enter the second heating phase when a pressure sensor (not shown) determines that a desired suction pressure has been achieved in the line  53 . In the second heating phase, the controller  100  closes the fourth solenoid valve  94  and opens the third solenoid valve  56 . In this manner, refrigerant flows along a heating circuit  118  (represented by arrows) from the compressor  22  through the branch  36 , the bypass  98 , and through a pan heater  120 . 
     The warm gas continues along the heating circuit  118  from the pan heater  120  through the distributor  82  and into the evaporator coil  84 . Air-conditioned space air is blown past the evaporator coil  84  by the blower  86  to facilitate heat transfer between the warm refrigerant in the evaporator coil  84  and the air-conditioned space air. The air-conditioned space air is heated by contact with the warm evaporator coil  84  and is then returned to the air-conditioned space  14  to raise the temperature of the air-conditioned space  14 . The refrigerant continues out of the evaporator coil  84  along the heating circuit  118  through the vapor line  88 , the vapor portion of the heat exchanger  90 , the second check valve  92 , the tank  57 , the first fixed throttling device  55 , and the third solenoid valve  56 , and is returned to the compressor  22  to be recycled through the refrigeration system  10 . 
     Preferably, the controller  100  monitors the temperature and pressure of the refrigerant exiting the compressor  22  during operation in the heating mode via sensors (not shown), at least some of which are located adjacent the discharge  28 . Additionally, the controller  100  preferably adjusts the temperature and pressure in the heating circuit  118  in response to data collected by the sensors by opening and closing the first fixed throttling device  55  and by selectively injecting liquid refrigerant into the compressor  22  via the liquid injection line  108 . Alternatively or in addition, if the pressure of the heating circuit  118  rises above a desired operating pressure, the controller  100  is programmed to reduce the operating speed of the compressor  22 . Also, if the pressure in the heating circuit  118  remains above the desired operating pressure after the compressor speed has been reduced, the controller  100  is programmed to open the first solenoid valve  38 . Once the first solenoid valve  38  is opened, at least some of the refrigerant is directed through the condenser  42  and is condensed to a liquid and stored there. 
     To further reduce the pressure in the heating circuit  118 , to reduce the load experienced by the compressor  22 , or to modulate the temperature of the air-conditioned space  14 , the controller  100  is programmed to close the third solenoid valve  56  and to open the adjustable throttling device  96 . If the pressure in the heating circuit  118  drops below the desired pressure range, the controller  100  is programmed to reestablish pressure in the heating circuit  118  by reinitiating the condenser coil evacuation phase, as described above. 
     During operation in the cooling mode, water vapor from the air-conditioned space  14  can condense on the evaporator coil  84  and form frost if the coil temperature is below freezing. To remove frost from the evaporator coil  84 , the controller  100  is programmed to operate the refrigeration system  10  in the defrost mode. In various applications of the present invention, the defrost mode can be initiated in a number of manners. For example, the controller  100  can initiate the defrost mode at predetermined intervals (e.g., every two hours). Alternatively or in addition, the controller  100  can be programmed to initiate the defrost mode, based on temperature and pressure data collected by sensors (not shown) distributed throughout the refrigeration system  10 . 
     As discussed above with respect to operation in the heating mode, refrigerant can occasionally accumulate in the condenser coil  48 . The accumulation of refrigerant in the condenser coil  48  reduces the amount of refrigerant available for defrosting the evaporator coil  84 . Additionally, the accumulation of refrigerant in the condenser coil  48  causes a drop in the discharge pressure and temperature of the compressor  22 , thereby further reducing the efficiency of the refrigeration system  10 . Therefore, once the defrost mode is initiated, the controller  100  determines the pressure at the compressor discharge  28  via sensors (not shown). If the condenser discharge pressure is below an acceptable range, the controller  100  is programmed to evacuate the condenser coil  48  by opening the second and fourth solenoid valves  40 ,  94  and closing the adjustable throttling device  96 , the first solenoid valve  38 , and the third solenoid valve  56 . In this manner, the compressor  22  draws most or all of the refrigerant out of the condenser coil  48 , thereby increasing the pressure in the refrigeration system  10 . 
     Once the discharge pressure reaches the acceptable pressure range, the controller  100  closes the fourth solenoid valve  94  and opens the third solenoid valve  56 . Refrigerant then travels along a defrost circuit  126  (represented by arrows) from the discharge  28  via the second branch  36  into the bypass  98 . From the bypass  98 , refrigerant continues along the defrost circuit  126  through the pan heater  120 . From the pan heater  120 , the refrigerant travels through the distributor  82  into the evaporator coil  84  to defrost the evaporator coil  84 . The blower  86  is preferably turned off during operation in the defrost mode so that heat is not transferred from the evaporator coil  84  into the air-conditioned space  14 . 
     After passing through the evaporator coil  84 , the refrigerant travels along the defrost circuit  126  through the vapor portion  90  of the heat exchanger  76  and the second check valve  92  before entering the tank  57 . Additionally, as mentioned above, during operation in the defrost mode, the third solenoid valve  56  is opened, the fourth solenoid valve  94  is closed, and the adjustable throttling device  96  is closed. Therefore, the refrigerant flows out of the tank  57  along the defrost circuit  126  past the first throttling device  55  into the compressor inlet  26 . 
     If the discharge pressure rises above the desired pressure range during operation in the defrost mode, the controller  100  is programmed to open the first solenoid valve  38 , thereby diverting at least some of the refrigerant from the defrost circuit  126  and into the condenser  42  where the superheated gas refrigerant is condensed into a liquid and stored. If after opening the first solenoid valve  38  the controller  100  determines that the discharge pressure is still above the desired pressure range, the controller  100  is programmed to close the third solenoid valve  56  and open the adjustable throttling device  96 . Thus, the suction pressure can be reduced, which causes a drop in the discharge pressure. If the discharge temperature rises above the maximum allowed discharge temperature, the controller  100  is programmed to inject liquid refrigerant into the compressor  22  to cool the compressor  22  and/or the compressor motor  24 . The controller  100  is programmed to inject liquid refrigerant into the compressor  22  through the liquid injection line  108  by opening the liquid injection valve  109  and/or by opening the fifth solenoid valve  110 , as described above with respect to operation in the heating mode. 
     During normal operation in the cooling, heating, and defrost modes, oil used for lubricating the compressor  22  commonly mixes with the refrigerant. Over time a substantial quantity of oil is mixed with the refrigerant and is removed from the compressor  22 . During operation in the cooling mode, the oil discharged from the compressor  22  is mixed with the refrigerant and follows the same path as the refrigerant. 
     However, during operation in the heating and defrost modes, because the refrigerant is in a mostly saturated vapor state and the oil is in a liquid state as the mix enters the tank  57 , the refrigerant and oil are separated as the mixture enters the tank  57 . More specifically, the vaporous refrigerant remains at the top of the tank  57  while the liquid oil settles to the bottom of the tank  57 . During operation in the heating and defrost modes, oil is periodically removed from the bottom of the tank  57  through the second opening  60  and is returned to the compressor  22  via the second branch  68  through the throttling device  69 , the third solenoid valve  56 , and the inlet  26 . 
     FIG. 2 shows a refrigeration system  10  according to a second embodiment of present invention, which is substantial similar to the previously described embodiment. For simplicity, like parts have been labeled with like reference numbers and only differences between the first and second embodiments will be described in detail hereafter. 
     In the second embodiment of the present invention, the fifth solenoid valve  110  is located along a defrost/heating bypass  130 , which extends between the internal heat exchanger  76  and the pan heater  120 . When the fifth solenoid valve  110  is closed, refrigerant flows from the internal heat exchanger  76  through the expansion valve  80  and into the distributor  82 . When the fifth solenoid valve  110  is open, the refrigerant flows out of the internal heat exchanger  76  into the defrost/heating bypass  130 , the pan heater  120 , and into the distributor  82 , bypassing the expansion valve  80 . 
     With reference to operation in the cooling mode, the refrigeration circuit  20  extends from the compressor  22  through the condenser  42 , the tank  57 , and the internal heat exchanger  76 . From the internal heat exchanger  76 , the refrigeration circuit  20  extends through the expansion valve  80 , the distributor  82 , the evaporator coil  84 , the vapor portion  90  of the internal heat exchanger  76 , the adjustable throttling device  96 , and back into the compressor  22 . 
     As mentioned above with respect to the first embodiment, the discharge pressure of the compressor  22 , the cooling capacity, and the compressor load of the refrigeration circuit  20 , are controlled by opening and closing the adjustable throttling device  96 , opening and closing the second solenoid valve  40  and adjusting the compressor speed. In some cases, particularly during startup and when the ambient temperature is relatively high, the adjustable throttling device  96  is incremented to reduce the flow of refrigerant through the refrigeration circuit  20 . However, when the refrigerant flow is reduced beyond a minimum flow rate, the cooling capacity of the refrigeration circuit  20  may not be sufficient to adequately cool the compressor  22  and the compressor motor  24 . In these cases, to prevent the compressor  22  and compressor motor  24  from overheating, the controller  100  is programmed to open the fifth solenoid valve  110 . When the fifth solenoid valve  110  is opened, at least some of the refrigerant bypasses the expansion valve  80  and flows through the distributor orifice  82 . The small pressure reduction caused by the distributor orifice  82  causes the refrigerant to flood the evaporator coil  84 . Once the evaporator coil  84  is flooded, liquid refrigerant flows out of the evaporator coil  84 , through the vapor portion  90  of the internal heat exchanger  76 , through the adjustable throttling device  96 , and back into the compressor  22 , cooling the compressor  22  and the compressor motor  24 . 
     After the compressor temperature or the discharge pressure drop below the maximum allowable temperature or pressure values or after a predetermined time period, the controller  100  is programmed to close the fifth solenoid valve  110 . The refrigerant again flows through the expansion valve  80 , the distributor  82 , and the evaporator  84 , cooling the air-conditioned space  14 . If either or both of the compressor temperature or discharge pressure again rise above the maximum allowable values after the fifth solenoid valve  110  is closed, the controller  100  is programmed to wait a predetermined time period and reopen the fifth solenoid valve  110 . This process is repeated until the compressor temperature and discharge pressure drop below the maximum allowable values. 
     The second embodiment of the refrigeration system  10  is also operable in a heating mode. During operation in the heating mode, refrigerant is directed through the heating circuit  118 . However, as with the first embodiment, the controller  100  is programmed to evacuate residual refrigerant from the condenser coil  48  before directing refrigerant through the heating circuit  118 . After a substantial quantity of refrigerant has been evacuated from the condenser coil  48 , the controller  100  directs refrigerant through the heating circuit  118 . The heating circuit  118  (represented by arrows) extends from the discharge  28 , through the branch  36 , the second solenoid valve  40 , the bypass  98 , the liquid line  78 , the fifth solenoid valve  110 , the defrost/heating line  130 , the pan heater  120 , the evaporator coil  84 , the vapor portion  90  of the internal heat exchanger  76 , the second check valve  92 , the tank  57 , and back into the compressor  22 . 
     With reference now to operation in the defrost mode, the controller  100  is programmed to evacuate residual refrigerant from the condenser coil  48  prior to directing refrigerant through the defrost circuit  126  as described above with respect to the previous embodiment. Once the condenser coil  48  has been evacuated, refrigerant flows out of the discharge into the defrost circuit  126 . The defrost circuit  126  (represented by arrows  126 ) extends through the branch  36 , the bypass  98 , the liquid line  78 , the fifth solenoid valve  110 , the pan heater  120 , the evaporator coil  84  where hot gas melts the frost or ice that has accumulated on the evaporator coil  84 , the vapor portion  90  of the internal heat exchanger  76 , the second check valve  92 , the tank  57 , the fixed throttling device  55 , and through the third solenoid valve  56  back into the compressor  22 . 
     The embodiments described above and illustrated in the drawings are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art, that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention as set forth in the appended claims. 
     For example, the present invention is described herein as being used to pull down and maintain the temperature in a truck having a single air-conditioned space  14 . However, one having ordinary skill in the art will appreciate that the present invention could also be used in trucks or trailers having multiple air-conditioned spaces  14 . Similarly, the present invention can also be used to pull down and maintain the temperature in buildings, containers, A/C applications and the like. 
     Similarly, the present invention is described herein as including a first, second, third, fourth, and fifth solenoid valves. One having ordinary skill in the art will appreciate that in other applications stepper motors and other valve controls could also or alternatively be used. Also, one having ordinary skill in the art will appreciate that adjustable valves, pulse valves, expansion valves, or the like could also or alternatively be used to provide additional mass flow rates and additional modes of operation. 
     As such, the functions of the various elements and assemblies of the present invention can be changed to a significant degree without departing from the spirit and scope of the present invention.