Patent Publication Number: US-2006000596-A1

Title: Multi-zone temperature control system

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to temperature control systems, and particularly to multi-zone temperature control systems. More particularly, the present invention relates to multi-zone temperature control systems for movable compartments.  
      Refrigeration systems are commonly employed to cool compartments such as truck trailers, cargo containers, and the like. These systems are well suited to maintaining the compartment temperature below a predetermined value.  
      In some applications, it is desirable to maintain the temperature of the compartment within a predefined range rather than below a maximum temperature. These systems often include a second heat exchanger or second flow path adapted to heat the compartment. A refrigeration system such as a vapor-compression cycle or cryogenic cycle cools the compartment using one heat exchanger or flow path, while a heating cycle operates to heat the compartment using the second heat exchanger or flow path. Many applications use engine coolant as the heat source.  
      In another application, it is desirable to maintain the temperature of two or more compartments or zones within two or more different ranges. Often, two separate refrigeration cycles are employed including two separate compressors and condensers. Alternatively, a single compressor is used. However, the complexity of the system limits the choice of compressors. Additionally, a second cycle is required if heating of one or more of the compartments is needed.  
     SUMMARY OF THE PREFERRED EMBODIMENTS  
      Accordingly, the present invention provides a dual-zone temperature control system operable to control the temperature in a first and second compartment. The system includes a compressor operable to compress a flow of fluid and first and second heat exchangers each of which is operable to control the temperature within one of the first and second compartments. The first heat exchanger is positioned adjacent the first compartment and the second heat exchanger is positioned adjacent the second compartment. A condenser selectively receives the flow of fluid and is operable to cool the flow of fluid. The system also includes a flow control system that is operable to selectively direct the flow of fluid to the condenser or to bypass the condenser such that the first and second heat exchangers operate to maintain their respective compartments within a first and second temperature range.  
      In another embodiment, the invention provides a multi-zone temperature control system operable to control the temperature within a plurality of compartments. The system includes a compressor operable at a speed to compress a flow of fluid and a condenser, operable to cool the flow of fluid. The system also includes a plurality of heat exchangers. Each heat exchanger is associated with one of the plurality of compartments and is operable to maintain the temperature of the compartment within a desired range. A plurality of valves are operable to direct and vary the amount of the flow of fluid from the compressor to the condenser and the plurality of heat exchangers. The valves are configurable to allow each of the heat exchangers to heat or cool their associated compartment. The system also includes a controller operable to control the valves to maintain the temperature of each compartment within its desired range.  
      In yet another embodiment, the invention provides a method of maintaining the temperature in a plurality of compartments, each compartment having a desired temperature range. The method includes operating the compressor at a speed to compress and heat a flow of fluid and determining which compartments require heating, cooling, or are within their desired temperature range. The method further includes directing the flow of fluid from the compressor to heat exchangers associated with compartments that require heat and using the heat of compression to heat the compartments and condense the flow of fluid. The method also includes directing the flow of fluid from the heat exchangers that are heating their respective compartments to heat exchangers of compartments that require cooling.  
      Additional features and advantages will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The detailed description particularly refers to the accompanying figures in which:  
       FIG. 1  is a schematic representation of a dual-zone temperature control system embodying the present invention;  
       FIG. 2  is a schematic representation of the system of  FIG. 1  configured to provide cooling to both zones;  
       FIG. 3  is a schematic representation of the system of  FIG. 1  configured to provide cooling to one zone and heating to the other zone;  
       FIG. 4  is a schematic representation of the system of  FIG. 1  configured to provide heating to both zones;  
       FIG. 5  is a schematic representation of the system of  FIG. 1  configured to provide cooling to one zone while the second zone is providing neither heating nor cooling;  
       FIG. 6  is a schematic representation of the system of  FIG. 1  configured to provide heating to one zone while the second zone is providing neither heating nor cooling. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Before describing the figures in detail, it should be noted that the figures illustrate a dual-zone temperature control system  10  for the sake of simplicity. However, the invention is envisioned as operating with a plurality of zones with the only limit being the flow capacity of the compressor. Therefore, while the invention will be described in detail as it relates to the dual-zone system  10  illustrated, the invention should not be limited to two-zone systems.  
      The system  10  as illustrated in  FIG. 1  includes a controller (not shown), a compressor  15 , a condenser  20 , two heat exchangers such as the first evaporator  25  and the second evaporator  30 , and a plurality of valves and pipes interconnecting the aforementioned components. It is envisioned that the system  10  will be most useful with mobile storage compartments in which temperature control is needed, such as a truck trailers, cargo container, train cars and the like. However, the present invention should not be limited to applications involving temperature control of moving compartments, as it will function to control the temperature within stationary compartments as well.  
      The controller is a micro-processor based programmable control that receives inputs from various sensors located throughout the system (e.g., pressure transducers, thermocouples, thermistors, RTD&#39;s, flow meters, pressure switches, etc.). The controller uses the inputs and the information programmed into the controller to determine how to configure the system  10 . Generally, each evaporator  25 ,  30  may be operated in one of several modes including, heating mode wherein the evaporators  25 ,  30  operate as heat exchangers and heat their respective compartments, cooling mode wherein the evaporators  25 ,  30  operate as evaporators and cool their respective compartments, inverted heating wherein one or more of the evaporators  25 ,  30  operate as condensers while the remaining evaporators  25 ,  30  operate as evaporators, and null mode wherein there is no flow through the evaporator  25 ,  30 . The actual configuration of the system will be discussed in detail below with regard to  FIGS. 2-6 .  
      The condenser  20  is a heat exchanger adapted for the exchange of heat between compressed refrigerant and air. The refrigerant generally flows within the tubes of a fin-tube type heat exchanger as air is forced across the fins. The refrigerant is cooled and condenses within the condenser  20 . In most constructions, fans move the air across the fins of the condenser  20 . However, other constructions may rely on natural airflow through the condenser  20 . For example, systems installed on moving vehicles can direct the moving air stream generated by the movement of the vehicle through the condenser  20 .  
      The compressor  15  operates to draw in refrigerant at an inlet  35  and discharge the refrigerant at an outlet  40 . While many types of compressors  15  will operate with the system (e.g., reciprocating, screw, centrifugal, etc.) the preferred compressor is a scroll compressor. One such compressor is Model Number TF22KL2E-42C marketed by Copeland Corporation of Sidney, Ohio. Scroll compressors are more efficient then reciprocating compressors and generally have fewer moving parts.  
      An engine or motor (not shown) drives the compressor  15  at a desired speed to compress the refrigerant. In constructions that are cooling compartments within moving vehicles, the vehicle engine itself is typically used to power the compressor  15 . The compressor  15  can be directly or indirectly connected to the engine. In another construction, the engine powers an alternator that in turn drives an electric motor that is coupled to the compressor  15 . The controller determines the desired speed of the compressor  15  and adjusts the motor or engine to achieve that speed.  
      The evaporators  25 ,  30  are similar to the condenser  20 . Refrigerant flows through the tubes of the evaporators  25 ,  30 , while air from the temperature-controlled compartment is forced over the fins of the evaporators  25 ,  30 . Variable speed fans disposed adjacent each of the evaporators  25 ,  30  operate to move compartment air through their respective evaporators  25 ,  30 . The fans are powered by variable speed electric motors to allow the controller to vary the mass flow rate of compartment air through the air side of the evaporators  25 ,  30 . In another construction, single speed fans are employed. The controller pulses the fans on and off to control the mass flow rate of compartment air through the evaporators  25 ,  30 .  
      Also included in the system  10  of  FIG. 1  are a receiver tank  45 , a dryer  50 , an accumulator  55 , and two additional heat exchangers  60 ,  65 . The receiver tank  45  is positioned downstream of the condenser  20 . The receiver tank  45  receives and stores refrigerant when the system  10  is operating in a configuration in which a full charge of refrigerant is not required. In addition, the receiver tank  45  acts to remove any bubbles (e.g., air) entrained in the flow of liquid refrigerant.  
      The dryer  50  receives a flow of liquid refrigerant from the receiver tank  45  and filters out any particles entrained in the flow. In addition, the dryer  50  absorbs any moisture trapped within the refrigerant flow.  
      The accumulator tank  55  is disposed in the suction line upstream of the compressor  15 . The accumulator tank  55  receives the flow of used refrigerant and assures that no liquid refrigerant passes to the compressor inlet  35 . During transient operation (i.e., transitioning between operating modes) liquid refrigerant may surge into the accumulator tank  55 . The accumulator tank  55  provides sufficient volume to allow the refrigerant to boil off before entering the compressor  15 .  
      In addition to the evaporators  25 ,  30 , each compartment also includes one of the additional heat exchangers  60 ,  65  or second heat exchanger. The second heat exchangers  60 ,  65  are used when the particular compartment is in the cool mode to improve the overall performance of the system. The second heat exchangers  60 ,  65  are plate heat exchangers having a liquid refrigerant flow path on one side of the plate and a suction or vapor flow path on the second side. The second heat exchangers  60 ,  65  improve system performance by pre-cooling the liquid refrigerant before it enters the evaporator  25 ,  30 . When the refrigerant exits the condenser  20 , it is no cooler than the ambient air that passes through the condenser  20 . When the refrigerant exits the evaporators  25 ,  30  it is typically cooler than the liquid refrigerant exiting the condenser  20 , thereby allowing it to pre-cool the refrigerant in the second heat exchangers  60 ,  65 .  
      The remaining components in the system  10  comprise valves, transducers, solenoids, switches, or regulators and will be described in conjunction with the operation of the system  10  and  FIGS. 2-6 .  
      Turning to  FIG. 2 , the system  10  is illustrated in a cool/cool mode. To arrive at this configuration, the controller determined that the temperature within each compartment is above a predetermined level, thus requiring cooling. The temperature measurements can be made using any suitable method with resistance type sensors (e.g., thermistor or RTD) being preferred. Other constructions may use temperature switches or other measuring devices (e.g., thermistors, infrared detectors, resistance temperature detectors (RTD), etc.).  
      In  FIGS. 2-6 , suction lines  70  are shown solid, hot gas lines  75  are shown dotted, and liquid lines  80  are shown dashed. Also, any components that are isolated and receive no flow are omitted from the figures for clarity. For example, when in the heat/cool mode illustrated in  FIG. 3 , the condenser  20  is not used and is thus omitted from the drawing. It should be understood that the component remains in place no matter the mode of operation.  
      Returning to  FIG. 2 , operation of the compressor  15  produces a flow of high-pressure refrigerant. The act of compression also produces significant heating, resulting in a flow of hot refrigerant. A discharge pressure transducer  85  (DIS) measures the discharge or outlet pressure of the compressor  15 . A diaphragm and strain gage type pressure transducer is used in the illustrated construction with other pressure measuring devices also functioning with the invention (e.g., capacitance pressure transducers, potentiometric pressure sensors, resonant-wire sensors, etc.).  
      The hot refrigerant also flows through a Schrader valve  90 , a condenser inlet solenoid  95 , and a condenser inlet check valve  100 . The Schrader valve  90  provides a convenient port for charging (adding refrigerant) to the system  10  and is not necessary for the performance of the system  10 .  
      The condenser inlet solenoid  95  (CIS) closes to prevent refrigerant flow to the condenser  20 . In the cool/cool mode illustrated in  FIG. 2  and the cool/null mode illustrated in  FIG. 5  the CIS valve  95  is open, thereby allowing refrigerant flow through the condenser  20 . In the remaining modes, illustrated in  FIGS. 3-4  and  6 , the CIS  95  is closed and no flow passes into the condenser  20  from the compressor  15 .  
      The condenser inlet check valve  100  (CICV) prevents fluid flow from the condenser  20  toward the compressor  15 .  
      A high-pressure cut-out switch  105  (HPCO switch) is disposed in the flow path between the compressor  15  and the condenser  20 . The HPCO switch  105  measures the pressure of the hot refrigerant exiting the compressor  15 . If the HPCO switch  105  detects a pressure in excess of a predetermined value, it will act to shut down the system  10 . The HPCO switch  105  is hard-wired directly into the system power supply to allow it to act independent of the controller to shut down the system  10 . In other constructions, the HPCO switch  105  sends a signal to the controller and the controller initiates a system shut down. In the construction illustrated herein, the pressure at which the HPCO switch  105  initiates a shut down is 450 PSIG with higher or lower pressures being possible.  
      The hot refrigerant flows through the condenser  20  and is condensed to produce a flow of cool liquid refrigerant. The flow of liquid refrigerant passes through a relief valve  110  and a condenser check valve  115  before entering the receiver tank  45 . The relief valve  110  operates to vent refrigerant to the atmosphere. The relief valve  110  opens to protect system components from damage when the internal system pressure exceeds a predetermined value. In preferred constructions, the relief valve  110  is set to open when the pressure reaches 500 PSIG or higher. With higher and lower settings being possible depending on the specific system components being used.  
      The condenser check valve  115  is positioned to prevent refrigerant flow from passing in a reverse flow direction (from the receiver tank  45  to the condenser  20 ) when operating in modes in which the condenser  20  is not used (heat/cool, heat/heat, and heat/null).  
      The flow exits the receiver tank  45 , passes through a receiver tank service valve  120  (RTSV), and passes through the dryer  50  to a distribution manifold  125 . The RTSV  120  is a valve that can be closed manually to service the system  10  and is not necessary for system function. In addition, the valve  120  includes a charging port that may be used to add or remove refrigerant from the system  10 .  
      At the distribution manifold  125 , the flow splits and flows toward the two compartments. Because both flows are identical, only one flow will be described. It should also be noted that in systems having more than two compartments, more flows would exit the distribution manifold  125 .  
      From the distribution manifold  125 , the flow passes through a liquid line solenoid (LLS)  130 , the second heat exchanger  60 , and a thermal expansion valve (TXV)  135 . The LLS  130  opens to allow liquid refrigerant to flow to the evaporator  25  when in cooling mode. In addition, the LLS  130  allows refrigerant to bleed to and from the receiver tank  45  as conditions require during other operating modes.  
      The thermal expansion valve  135  meters refrigerant to the evaporator  25  to maximize cooling capacity. The TXV  135  also includes a bleed port that allows refrigerant to flow to and from the receiver tank  45  when the evaporator  25  is operating in a mode other than cooling.  
      The inlet to the TXV  135  is a high-pressure region, while the outlet is a low-pressure region. Thus, the refrigerant at the inlet is a liquid, while the refrigerant on the outlet side has either completely, or partially evaporated and is a vapor or a vapor-liquid mix. The process of flowing through the TXV  135  reduces the temperature of the refrigerant. Thus, the exit of the TXV  135  is the lowest temperature point in the cycle.  
      After passing through the TXV  135  the low-pressure refrigerant passes through the evaporator  25 , the second heat exchanger  60 , a suction line solenoid  140  (SLS), and a suction line check valve  145  (SLCV) before it is collected at a vapor collection manifold  150 .  
      The SLS  140  is a control valve that remains open during cooling to allow the free passage of refrigerant therethrough. The SLS  140  closes during inverted heating to redirect refrigerant through a liquid return check valve  155  (LRCV) which will be discussed with reference to  FIG. 3 .  
      The suction line check valve  145  (SLCV) prevents reverse flow in the suction line and reduces the amount of liquid refrigerant that pools in the suction line during inverted heating.  
      From the SLCV  145 , the low-pressure refrigerant flows to the collection manifold  150  where refrigerant from the other compartments that are operating in a similar mode collects. From the collection manifold  150 , the flow proceeds through the accumulator tank  55 , a suction service valve  160 , and a mechanical throttle valve  165  before returning to the compressor  15  at the compressor inlet  35 . The suction service valve  160  (SSV) is a manually actuated valve that isolates the system  10  during maintenance and is not necessary for system function. The SSV  160  remains open during all normal operating modes.  
      The mechanical throttle valve  165  (MTV) restricts the pressure of the refrigerant at the compressor inlet  35 . The MTV  165  is set at a predetermined position to prevent overloading the compressor  15  or the prime mover driving the compressor  15 . A suction pressure transducer  170  (SUC) measures the suction or inlet pressure at the compressor  15 . A diaphragm and strain gage type pressure transducer is used in the illustrated construction with other pressure measuring devices also functioning with the invention (e.g., capacitance pressure transducers, potentiometric pressure sensors, resonant-wire sensors, etc.). After exiting the MTV  165 , the flow reenters the compressor  15  and the cycle continues.  
      Turning to  FIG. 3 , the system  10  is illustrated with one compartment operating in inverted heating mode and the second compartment in cooling mode. When operating as illustrated in  FIG. 3 , the controller closes the condenser inlet solenoid  95  (CIS) to prevent refrigerant flow into the condenser  20 . Instead, the high-pressure refrigerant flow passes through the discharge pressure transducer  85 , a discharge pressure regulator  175  (DPR), and a hot gas solenoid  180  (HGS) before entering the evaporator  25  in the compartment being heated.  
      The discharge pressure regulator  175  (DPR) increases the discharge pressure of the compressor  15  during heating or inverted heating, thereby increasing the discharge temperature to improve the heating capacity of the flow of refrigerant. The DPR acts as a controllable flow restriction downstream of the compressor  15 . The flow restriction acts to resist the flow of refrigerant and increase the discharge pressure of the scroll compressor  15 . Without the DPR, the scroll compressor  15  would simply move the refrigerant through the system  10  without adding significant heat.  
      The hot gas solenoid  180  (HGS) opens to allow flow from the compressor  15  to the evaporator  25  to heat the compartment. When in cooling mode, the HGS  180  closes to prevent flow of hot gas from the compressor  15  to the evaporator  25 .  
      The high-pressure vapor exits the HGS  180  and flows through the evaporator  25 . The vapor condenses to form a flow of high-pressure liquid that exits the evaporator  25  and flows through the second heat exchanger  60 . The air flowing through the evaporator  25  is heated by the flow of hot refrigerant, thereby heating the compartment. The liquid exits the second heat exchanger  60  and passes through the liquid return check valve  155  (LRCV) to the distribution manifold  125 . The LRCV  155  prevents reverse flow of high-pressure liquid when in cooling mode and allows the flow of high-pressure liquid when the SLS  140  is closed and the compartment is operating in heating mode as illustrated in  FIG. 3 .  
      From the distribution manifold  125 , the cycle is identical to that described above with regard to  FIG. 2 . In addition, excess refrigerant is free to flow into the dryer  50  and to the receiver tank  45  from the distribution manifold  125 . Alternatively, if additional refrigerant is required, it can flow from the receiver tank  45  through the dryer  50  and into the distribution manifold  125 . Thus, the first evaporator  25  operates as a condenser and heats its respective compartment using the heat generated by the compressor  15 , while the second evaporator  30  cools the second compartment in the manner described above with regard to  FIG. 1 .  
      With reference to  FIG. 4 , the system  10  is illustrated in heat/heat mode. Both compartments are calling for heat and the controller has configured the system to provide heat substantially as described above with regard to  FIG. 3 . The flow of hot high-pressure refrigerant exits the compressor  15  and flows through the DPR  175  to a distribution node  185  where the flow is distributed to the different compartments requiring heat. From the distribution node  185 , each flow passes through one of the hot gas solenoids  180  before entering one of the evaporators  25 ,  30 . Once the flow exits the evaporators  25 ,  30 , it follows a path that is similar to that described above with regard to  FIG. 2 .  
      The condenser check valve  115  prevents flow from the receiver tank  45  into the condenser  20  during operation. However, excess refrigerant can flow to the receiver tank  45  from the inlet of the evaporator  25 ,  30  through the thermal expansion valve  135 . Alternatively, additional refrigerant can flow from the receiver tank  45  through the thermal expansion valve  135  and into the evaporator  25 ,  30  as needed by the system  10 .  
      Turning to  FIG. 5 , the system is illustrated in cool/null mode. In this mode, one of the compartments is being cooled, while the other compartment is within its desired temperature range and thus requires no heating or cooling. In this mode, the refrigerant follows the path described above with regard to  FIG. 2  through only one of the evaporators  30 . The liquid line solenoid  130  and hot gas solenoid  180  of the second compartment are closed to isolate the evaporator  25  from the system  10 . Thus, the system  10  is able to cool only one of the compartments if necessary.  
       FIG. 6  illustrates the system  10  configured in heat/null mode. Like the configuration of  FIG. 5 , one of the compartments is operating to control temperature, while the second compartment is idle. The flow through the compartment being heated is similar to that described above with regard to  FIG. 4 . The liquid line solenoid  130  and hot gas solenoid  180  of the second compartment are closed to isolate the evaporator  25  from the system  10 . Thus, the system  10  is able to heat one compartment, while the second compartment remains idle.  
      Returning to  FIG. 1 , several flow paths are illustrated that function not to heat or cool a compartment but rather to protect the system  10  from conditions that may cause damage to system components or may prevent the system  10  from operating properly.  
      The controller monitors the pressure ratio between the compressor outlet  40  and the compressor inlet  35 . The pressure values are transmitted by the discharge pressure transducer  85  and the suction pressure transducer  170  to the controller. If the pressure ratio exceeds a predetermined value, a hot gas bypass solenoid  190  (HGBS) opens to reduce the pressure ratio. Alternatively, the HGBS  190  is opened when a suction pressure is detected that is below a predetermined value, regardless of the measured pressure ratio.  
      The HGBS  190  is an orificed solenoid that controls the flow through a high-pressure line that interconnects the compressor outlet  40  with the compressor inlet  35  as illustrated in  FIG. 1 . When open, high-pressure gas flows back into the low-pressure flow path, thereby increasing the suction pressure at the compressor inlet  35 . The hot gas bypass protects the compressor  15  from damage caused by operating at an excessively high-pressure ratio or operating with a suction pressure that is too low.  
      A second compressor protection system protects the compressor  15  from excessive heating. The system  10  routes cool refrigerant from the receiver tank  45  back into the compressor  15  to cool the compressor  15 . The refrigerant is injected into the compressor  15  at a point in its compression stroke between the inlet  35  and outlet  40  to assure that the liquid leaving the receiver tank  45  flashes to vapor before it enters the compressor  15 .  
      The line connecting the receiver tank  45  to the injection point of the compressor  15  includes a liquid injection solenoid  195  (LIS) and a liquid injection check valve  200  (LICV). The LICV  200  prevents reverse flow out of the compressor  15  and into the receiver tank  45  under operating conditions when the receiver tank  45  is at a lower pressure than the refrigerant at the injection point.  
      The LIS  195  is an orificed solenoid that is operated by the controller in response to a high compressor temperature. The LIS  195  allows for the admission of cold refrigerant vapor into the compressor  15  for cooling purposes.  
      During modes in which the condenser  20  is idle, it is desirable to evacuate the refrigerant from the condenser  20  so that it may be used in the system  10 . The present system  10  includes a purge solenoid  205  (PS) and a purge check valve  210  (PCV) disposed within a line that interconnects the outlet of the condenser  20  and the accumulator tank  55 . The purge check valve  210  prevents reverse flow of refrigerant from the accumulator tank  55  into the condenser  20 .  
      The purge solenoid  205  opens in conjunction with the closure of the condenser inlet solenoid  95  to evacuate the condenser  20 . When the purge solenoid  205  is open, the high-pressure liquid line exiting the condenser  20  is in fluid communication with the suction line entering the accumulator tank  55 . The purge solenoid remains  205  open throughout operation in modes in which the condenser  20  is idle. While the purge solenoid  205  remains open throughout operation, it is generally effective only during the transient period as the system  10  switches between modes.  
      When the unit is offline, the receiver tank  45  pressure is reduced by bleeding refrigerant through a receiver tank check valve  215  (RTCV) that interconnects the receiver tank  45  and the compressor outlet  40 .  
      In addition to the aforementioned hot gas bypass system, the system  10  includes two other systems that are operable to protect the compressor  15  against low suction pressure.  
      In the first system, the controller reduces the speed of the compressor  15  to reduce the system capacity. This can be done by slowing the engine or motor that drives the compressor  15 . In the second system, the speed of the fans moving air through the evaporators  25 ,  30  is increased to increase the effectiveness of the evaporators  25 ,  30 . This has the desirable effect of increasing the suction pressure at the compressor inlet  35 . Furthermore, the three methods described herein can be used in combination to enhance their effectiveness.  
      During the transient period when the system is switched from one mode to another it is possible for several operating parameters to stray out of their desired ranges. In many cases this could result in a system shut down or other undesirable action. One particularly troublesome transition is one that involves transitioning an evaporator  25 ,  30  from cooling to heating. To reduce the likelihood of unwanted shut down, the present system pre-cools the compressor  15  and pre-heats the evaporator  25 ,  30  before switching to the inverted heating mode.  
      To pre-cool the compressor  15 , the purge solenoid  205  is open to admit cool liquid refrigerant into the accumulator tank  45 , thereby lowering the temperature of the refrigerant entering the compressor  15 , thus cooling the compressor  15 . Alternatively, the liquid injection solenoid  195  is open. This allows for a flow of cold refrigerant from the receiver tank  45  to the compressor  15  to pre-cooling the compressor  15 .  
      To preheat the evaporator  25 ,  30 , the system maintains a flow path between the evaporator  25 ,  30  and the suction line. During the transition to one of the configurations in which an evaporator  25 ,  30  provides heating or acts as a condenser, hot refrigerant is cycled through the evaporator  25 ,  30 . The CIS  95  is closed to redirect refrigerant from the condenser to the evaporator or evaporators  25 ,  30 . During a predetermined transition period (e.g., two minutes) the SLS  140  remains open to allow hot refrigerant to pass through the evaporators  25 ,  30  and back into the accumulator tank  55  rather than to an evaporator  25 ,  30  where the refrigerant would be evaporated. Thus, the hot refrigerant cycles only through the compressor  15  and any evaporators  25 ,  30  operating in a heating mode for a predetermined time period to preheat the evaporators  25 ,  30 . In another construction, an electrical heating element is positioned adjacent the evaporator  25 ,  30 . The electrical heating element operates to preheat the evaporator  25 ,  30 .  
      It should be noted that the term “refrigerant” as used herein encompasses any fluid that can be used as a working fluid (e.g., ammonia, freon, R-12, etc.).  
      Furthermore, the drawings illustrate several configurations of the system  10  but by no means illustrate all possible configurations. For example,  FIG. 3  illustrates a heat/cool mode. It should be clear that the system  10  is capable of operating in a cool/heat mode wherein the cooling and heating regions are reversed. Therefore, the invention should not be limited to the modes described herein.  
      Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.