Abstract:
A method and system for controlling refrigerant pressure in an HVAC system. The method includes providing a compressor, a condenser and an evaporator connected in a closed refrigerant loop. The condenser has a header arrangement capable of distributing refrigerant to a plurality of refrigerant circuits within the condenser. The header arrangement also is capable of selectively isolating at least one of the circuits from refrigerant flow. Refrigerant pressure is sensed at a predetermined location in the refrigeration system. At least one of the circuits is isolated when the refrigerant pressure is less than or equal to a predetermined pressure.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to heating, ventilation and air conditioner HVAC systems. In particular, the present invention is related to methods and/or systems that control HVAC system refrigerant pressure.  
       BACKGROUND OF THE INVENTION  
       [0002]     An HVAC system generally includes a closed loop refrigeration system with at least one evaporator, at least one condenser and at least one compressor. As the refrigerant travels through the evaporator, it absorbs heat from a heat transfer fluid to be cooled and changes from a liquid to a vapor phase. After exiting the evaporator, the refrigerant proceeds to a compressor, then a condenser, then an expansion valve, and back to the evaporator, repeating the refrigeration cycle. The fluid to be cooled (e.g. air) passes through the evaporator in a separate fluid channel and is cooled by the evaporation of the refrigerant. The cooled fluid can then be sent to a distribution system for cooling the spaces to be conditioned, or it can be used for other refrigeration purposes.  
         [0003]     One type of air conditioner system is a split system where there is an indoor unit or heat exchanger, which is generally the evaporator, and an outdoor unit or heat exchanger, which is generally the condenser. Often, the outdoor unit is placed outdoors and is subject to outdoor ambient conditions, particularly temperature. When the outdoor ambient temperature falls, the amount of heat being removed from the refrigerant in the condenser increases. The increased heat removal in the condenser can result in a decrease in the refrigerant pressure at the suction line to the compressor, commonly referred to as head pressure. The decrease in head pressure results in a lowering of the temperature of the refrigerant at the evaporator. When the temperature of the refrigerant at the evaporator becomes too low, icing of the system can occur. Icing is a condition when the temperature at the exterior of the evaporator is sufficiently low to freeze water present in the atmosphere. The ice formed by the water frozen on the surface reduces the available heat transfer surface and eventually prevents the proper operation of the HVAC system by inhibiting heat transfer and/or damaging system components.  
         [0004]     Some attempts to address the problem of icing have utilized the control of system pressure. In one approach, a variable speed condenser fan or a plurality of condenser fans having independent controls are used to control airflow over the condenser coil. As the amount of air passing over the coil decreases, the amount of heat transfer taking place at the coil decreases. Therefore, the temperature of the refrigerant in the condenser and the pressure of the system increase to allow the indoor coil to cool the air without icing problems. The use of the variable speed condenser fan or a plurality of condenser fans having independent controls has the drawback that it is expensive and requires complicated wiring and controls.  
         [0005]     An alternate approach for the problem of low system pressure or icing is a parallel set of condensers in the refrigerant cycle, as described in U.S. Pat. No. 3,631,686. The parallel set of refrigerant condensers allows for two modes of operation. One mode of operation allows refrigerant to flow from only one of the refrigerant condensers. During this mode of operation, the condenser that does not permit the flow of refrigerant fills with liquid refrigerant. Because of this flooding, there is a reduction in the effective surface area of the condenser. The reduced surface area thereby reduces the ability of the condenser to remove heat from the refrigerant. Therefore, the temperature of the refrigerant in the condenser and the head pressure of the system increase allowing the indoor coil to cool the air without icing. The use of parallel refrigerant condensers has the drawback that it requires an additional condenser coil and additional piping, thereby increasing the space and cost required for installation. Another drawback associated with refrigerant flooding of the condenser coil is the resultant decrease in system capacity. Refrigerant normally available in a properly operating system is trapped in the condenser coil and not available to the compressor, thereby decreasing system capacity.  
         [0006]     An additional alternate approach for the problem of low system pressure is the use of a valve that controls the discharge or flow of liquid refrigerant from the condenser to a receiver vessel downstream of the condenser to maintain control of the amount of condensing surface exposed to the outside temperature, as described in U.S. Pat. No. 2,874,550. The discharge of refrigerant from the condenser is controlled by a pressure-response valve that mechanically opens to allow the flow of liquid refrigerant from the condenser to the receiver vessel reducing the level of liquid inside the condenser, thereby lowering the system pressure. Alternatively, the valve is closed to stop the flow until the level of refrigerant rises in the condenser in an amount that reduces the effective cooling surface of the condenser. The reduced surface area thereby reduces the ability of the condenser to remove heat from the refrigerant, thereby raising the pressure of the system. The use of a pressure-response valve and a vessel downstream of the condenser to maintain control of the amount of condensing surface has the drawback that it includes a specially designed valve and additional piping, thereby increasing the required space and cost. As discussed above, another one of the drawbacks with refrigerant flooding the condenser coil is decreased system capacity. Refrigerant normally available in a properly operating system is trapped in the condenser coil and not available to the compressor, thereby decreasing system capacity.  
         [0007]     An additional alternate approach for the problem of low system pressure is the use of a refrigerant bypass around the condenser, as described in U.S. Pat. No. 3,060,699 and U.S. Reissued Pat. No. Re. 27,522. If the temperature and pressure of the refrigerant in the condenser are sufficiently high, a valve will close on a condenser bypass and the flow of refrigerant will be directed to the condenser. If the temperature and pressure of the condenser are not sufficiently high, the valve will open on a condenser bypass and at least some of the flow of refrigerant will be directed away from the condenser. The result of the bypass is an increase in pressure through the pipe leading to the evaporator downstream of the compressor. The use of a bypass has the drawback that it includes a specially designed valve and additional piping, thereby increasing the required space and cost.  
         [0008]     What is needed is a method and system for controlling the system refrigerant pressure without the drawbacks discussed above.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention includes a method for controlling refrigerant pressure in an HVAC system. The method includes providing a compressor, a condenser and an evaporator connected in a closed refrigerant loop. The condenser has a header arrangement capable of distributing refrigerant to a plurality of refrigerant circuits within the condenser. The header arrangement also is capable of selectively isolating at least one of the refrigerant circuits from refrigerant flow. Refrigerant pressure is sensed at a predetermined location in the refrigeration system. At least one of the refrigerant circuits is isolated when the refrigerant pressure is less than or equal to a predetermined pressure.  
         [0010]     The present invention also includes a method for controlling refrigerant pressure in an HVAC system. The method includes providing a closed loop refrigerant system comprising a compressor, a condenser and an evaporator. The condenser has a header arrangement capable of distributing refrigerant to a plurality of circuits within the condenser. The header arrangement is also capable of selectively isolating at least one of the circuits from refrigerant flow. Refrigerant pressure is measured at a predetermined location in the refrigeration system. At least one of the circuits is isolated from refrigerant flow when the measured pressure is equal to or less than a predetermined pressure. The number of circuits isolated within the condenser varies with the measured pressure with respect to the predetermined pressure. The isolation of the refrigerant circuits continues until the measured pressure is greater than the predetermined pressure.  
         [0011]     The present invention also includes a heating, ventilation and air conditioning system. The HVAC system includes a refrigerant system having a compressor, an evaporator, and a condenser connected in a closed refrigerant loop. The HVAC system also includes a refrigerant pressure measuring device for sensing refrigerant pressure disposed at a predetermined location within the refrigerant system. The condenser includes a plurality of refrigerant circuits, a first valve arrangement and a second valve arrangement. The first valve arrangement is arranged and disposed to isolate one or more of the refrigerant circuits from flow of refrigerant when the refrigerant pressure is below a predetermined pressure. The second valve arrangement is arranged and disposed to draw refrigerant into or out of the isolated circuits of the condenser in response to the refrigerant pressure sensed by the refrigerant pressure measuring device.  
         [0012]     The present invention provides an inexpensive method and system to control head pressure. The method and system requires little or no additional piping in order to implement the method and system. The system requires less in materials and therefore costs less. Additionally, the method and system of the present invention does not require the use of variable speed or multiple stage fans to control air flow across the heat exchangers of the HVAC system.  
         [0013]     The lack of additional piping also allows retrofitting of the system into existing HVAC systems. Because, little or no additional piping is required, the system occupies approximately the same volume as existing HVAC systems. Therefore, the method and system of the present invention may be used in existing systems whose piping has been arranged according to the present invention or as a new system.  
         [0014]     Another advantage of the present invention is that the air conditioning or heat pump unit can operate at lower ambient temperatures. The method and system of the present invention provides an increase in system pressure, thereby allowing the system to operate at lower ambient temperatures without icing of the system components.  
         [0015]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates schematically a refrigeration system.  
         [0017]      FIG. 2  illustrates schematically a condenser piping arrangement in one embodiment where the isolation valves are positioned inside the header.  
         [0018]      FIG. 3  illustrates schematically a condenser piping arrangement in another embodiment where the isolation valves are positioned on the piping connected to the headers for the individual circuits.  
         [0019]      FIG. 4  illustrates schematically a refrigeration system according to another embodiment including a pressure switch for controlling the isolation valves.  
         [0020]      FIG. 5  illustrates schematically a refrigeration system according to another embodiment including a drain line for the isolated portion of the condenser.  
         [0021]      FIG. 6  illustrates a control method according to one embodiment of the present invention.  
         [0022]      FIG. 7  illustrates an alternate control method according to one embodiment of the present invention. 
     
    
       [0023]     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 1  illustrates an HVAC, refrigeration, or chiller system  100 . Refrigeration system  100  includes a compressor  130 , a condenser  120 , and an evaporator  110 . The compressor  130  compresses a refrigerant vapor and delivers it to the condenser  120  through compressor discharge line  135 . The compressor  130  is preferably a reciprocating or scroll compressor, however, any other suitable type of compressor can be used, for example, screw compressor, rotary compressor, and centrifugal compressor. The refrigerant vapor delivered by the compressor  130  to the condenser  120  enters into a heat exchange relationship with a first heat transfer fluid  150  and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid  150 . Suitable fluids for use as the first heat transfer fluid  150  include, but are not limited to, air and water. The first heat transfer fluid  150  is moved by use of a fan  170 , which moves the first heat transfer fluid  150  through the condenser  120  in a direction perpendicular the cross section of the condenser  120 . In a preferred embodiment, the refrigerant vapor delivered to the condenser  120  enters into a heat exchange relationship with air as the first heat transfer fluid  150 . The refrigerant leaves the condenser through the condenser discharge line  140  and is delivered to an evaporator  110  after passing through an expansion device (not shown). The evaporator  110  includes a heat-exchanger coil. The liquid refrigerant in the evaporator  110  enters into a heat exchange relationship with a second heat transfer fluid  155  to lower the temperature of the second heat transfer fluid. Suitable fluids for use as the second heat transfer fluid  155  include, but are not limited to, air and water. The second heat transfer fluid  155 , preferably air, is moved by use of a blower  160 , which moves the second heat transfer fluid  155  through evaporator  110  in a direction perpendicular the cross section of the evaporator  110 . Although  FIG. 1  depicts the use of a blower  160  and fan  170 , any fluid moving means may be used to move fluid through the evaporator and condenser. In a preferred embodiment, the refrigerant vapor delivered to the evaporator  110  enters into a heat exchange relationship with air as the second heat transfer fluid  155 . The refrigerant liquid in the evaporator  110  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the second heat transfer fluid  155 . The vapor refrigerant in the evaporator  110  exits the evaporator  110  and returns to the compressor  130  through a suction line  145  to complete the cycle. It is to be understood that any suitable configuration of evaporator  110  can be used in the system  100 , provided that, the appropriate phase change of the refrigerant in the evaporator  110  is obtained. The conventional refrigerant system includes many other features that are not shown in  FIG. 1 . These features have been purposely omitted to simplify the figure for ease of illustration.  
         [0025]      FIG. 2  illustrates a condenser  120  according to one embodiment of the invention. Condenser  120  includes a plurality of heat transfer circuits  210 . The heat transfer circuits  210  are preferably partitioned into a first condenser portion  220  and a second condenser portion  230 . The first and second condenser portions  220  and  230  may be sized in any proportion. For example, the first condenser portion  220  may be 60% of the size of the condenser  120  and the second condenser portion  230  may be 40% of the size of the condenser  120  or the first condenser portion  220  may be 40% of the size of the condenser  120  and the second condenser portion  230  may be 60% of the size of the condenser  120  or the first and second condenser portions  220  and  230  may each represent 50% of the size of the condenser  120 . When the first and second condenser portions  220  and  230  are different sizes, e.g., 60%/40% split, the refrigerant flow may be directed in any manner that provides efficient condenser  120  operation. For example, the first condenser portion  220  may constitute 60% of the size of the condenser  120  and the second condenser portion  230  may constitute 40% of the condenser  120 . When desirable, the flow may be directed to either the 60% portion or the 40% portion and the designation of the first and second condenser portions  220  and  230  may be alternated to the isolated portion that provides the desired condenser  120  operation.  
         [0026]     In addition to the various ratios of the first condenser portion  220  to the second condenser portion  230 , the locations along the face of the condenser, perpendicular to the air, of the first and second condenser portions  220  and  230  may be selected to provide a greater efficiency in heat transfer when a condenser portion is isolated. In one embodiment, the first condenser portion  220  is arranged and disposed to isolate heat transfer circuits  210  that are positioned along the face of the condenser  120  in locations having a decreased overall heat transfer efficiency. Suitable locations for the isolated first condenser portion  220  in this embodiment include the heat transfer circuits  210  at the edges of the condenser, where the flow of heat transfer fluid is lower. The heat transfer circuits  210  on the outer edges of the condenser  120  typically receive less heat transfer fluid flow and have a lower heat transfer efficiency. Isolating the heat transfer circuits  210  having a lower efficiency and allowing the flow of refrigerant in heat transfer circuits  210  having a higher efficiency, such as the heat transfer circuits  210  near the center of the condenser  210 , permits the condenser  120  to operate at a higher overall efficiency, while controlling the head pressure of the system. The isolation of the heat transfer circuits  210  may take place with each of the condenser portions in a single continuous area along the face of the condenser, or may be discontinuous, such that the heat transfer circuits of a single condenser portion may be split into two or more sections to provide increased heat transfer efficiency for the condenser  120 . In this embodiment, the first condenser portion  220  may be arranged and disposed along the face of the condenser such that the less efficient heat transferring edge portions may be isolated in discontinuous portions of the face of the condenser, leaving a continuous second condenser portion in the more efficient heat transferring center portion of the condenser  120 .  
         [0027]     As shown in  FIG. 2 , inlet flow  250  includes vaporous refrigerant from the compressor  130 . Inlet flow  250  enters the condenser  120  travels through the heat transfer circuits  210 , where the heat transfer circuits  210  can enter into a heat exchange relationship with a heat transfer fluid such as air or water. The condenser  120  preferably has two condenser portions; however, the present invention is not limited to two condenser portions. The present invention may include more than two condenser portions. Where more than two condenser portions are present, the flow may be regulated to each of the portions. For example, in the embodiment where the condenser is split into three portions, two of the three portions include valve arrangements that allow independent isolation of each of these portions. One or both of the two portions with valve arrangements may be isolated, dependent on a signal from a controller and/or sensor. In  FIG. 2 , isolation valves  240  are positioned in the vapor header  290  and liquid header  292  of the condenser  120 . When isolation valves  240  are closed, the refrigerant is prevented from flowing into the second condenser portion  230 . When isolation valves  240  are open, refrigerant is permitted to flow to both the first condenser portion  220  and the second condenser portion  230 . The outlet flow  260  leaving the condenser comprises liquid refrigerant resulting from the heat exchange relationship with the heat transfer fluid and the resultant phase change. The outlet flow  260  is then circulated to the evaporator  110 .  
         [0028]      FIG. 3  illustrates a condenser  120  according to alternate embodiment of the invention. Condenser  120  includes a plurality of heat transfer circuits  210 . The heat transfer circuits  210  are partitioned into a first condenser portion  220  and a second condenser portion  230 . Although  FIG. 3  shows two condenser portions, the present invention is not limited to two condenser portions. The present invention may include more than two condenser portions. Inlet flow  250  is vaporous refrigerant from the compressor  130  that is split into two refrigerant streams. The two refrigerant streams enter the condenser  120  through two vapor headers  293  and  294  and travel into the heat transfer circuits  210 . Heat transfer circuits  210  can enter into a heat exchange relationship with a heat transfer fluid such as air or water. The two refrigerant streams then exit the condenser  120  through two liquid headers  295  and  296 . Isolation valves  240  are positioned on the piping to the vapor header  294  and on the piping from the liquid header  296  of the condenser  120 . When isolation valves  240  are closed, the refrigerant is prevented from flowing into the second condenser portion  230 . When isolation valves  240  are open refrigerant is permitted to flow to both the first condenser portion  220  and the second condenser portion  230 . The outlet flow  260  leaving the condenser  120  includes liquid refrigerant resulting from the heat exchange relationship with the heat transfer fluid and the resultant phase change. The outlet flow  260  is circulated to the evaporator  110 .  
         [0029]      FIG. 4  illustrates a refrigeration system  100  according to an alternate embodiment of the present invention. The refrigeration system  100  includes a compressor  130 , a condenser  120 , and an evaporator  110 . The condenser  120  is a partitioned condenser having two partitions, shown as the first and second condenser portions  220  and  230 . Although  FIG. 4  shows two condenser portions, the present invention is not limited to two condenser portions. The present invention may include more than two condenser portions. The piping to the condenser  120  includes isolation valves  240  on the inlet side and the outlet side of the second condenser portion  230  inside the condenser  120 . Closing the isolation valves  240  prevents the flow of refrigerant to the second condenser portion  230 . The isolation valves are controlled by a pressure switch  410  that senses pressure on the refrigerant line from the evaporator  110  to the compressor  130 . When the pressure on the compressor suction line  145  from the evaporator  110  to the compressor  130  reaches a predetermined level, the isolation valves  240  can be closed to the second condenser portion  230 . For example, the predetermined pressure may include a pressure of from about 160 to about 200 psi, preferably about 180 psi. However, the predetermined pressure is not limited to about 180 psi. and may be any suitable minimum pressure for the system. In particular, the suitable minimum pressure may be a minimum pressure utilized for a particular type of compressor  130  present in the system. Once isolation valves  240  are closed, the refrigerant is only permitted to flow through the first condenser portion  220 . Because the refrigerant is only permitted to flow into first condenser portion  220 , the heat transfer area and the corresponding amount of heat transfer occurring in the condenser  120  is reduced. Therefore, less heat is removed from the refrigerant. Likewise, less heat is transferred to the first transfer fluid  150 , thereby maintaining a higher refrigerant temperature. Additionally, because the temperature of the refrigerant is higher, the corresponding pressure of the refrigerant is also higher. Therefore, the refrigerant pressure of the system is increased.  
         [0030]      FIG. 5  shows an alternate embodiment according to the invention.  FIG. 5  has substantially, the same piping arrangement as  FIG. 4 .  FIG. 5  further includes a drain line  505  and a drain valve  510 . The refrigerant remaining in the second condenser portion  230  after isolation valves  240  are closed may be stored in the second condenser portion  230  or may be drawn into the refrigeration system  100 . Drain line  505  connects condenser portion  230  with the suction line of the compressor. Opening drain valve  510  allows the refrigerant to be drawn from the isolated portion of the condenser into the active system. Drawing refrigerant into the refrigeration system provides additional refrigerant per unit volume of the system, thereby further increasing the refrigerant pressure. Alternatively, refrigerant may also be drawn out of the active portion of the refrigerant system  100  to reduce the pressure of the refrigerant, when a reduced refrigerant pressure is desirable.  
         [0031]      FIG. 6  illustrates a flow chart detailing a method of the present invention relating to head pressure control in a HVAC system. The method includes a determination of the minimum system head pressure, Pf, at step  601 . The minimum head pressure is set to the desired operating pressure of the refrigeration system  100 . The minimum head pressure is preferably greater than the pressure corresponding to temperature of evaporator icing. Evaporator icing occurs at refrigerant evaporation temperatures of about 25° F. to about 32° F. The actual refrigerant temperature corresponding to frost build up will depend on numerous heat transfer factors specific to a given coil. Pf is preferably the refrigerant pressure that corresponds to greater than about 27° F. A suitable minimum system head pressure includes, but is not limited to about 180 psig. Subsequent to determining the minimum system head pressure, Pf, the actual system head pressure, Pm, is measured at step  603 . Any pressure measurement method is suitable for determining Pm. Preferably, the measurement takes place at or near the outlet of the evaporator. Subsequent to the measurement taken at step  603 , a determination of whether the pressure of the refrigerant measured is below the pressure corresponding to minimum system head pressure, Pf, at step  605 . If the measured pressure of the refrigerant, Pm, is below the pressure for evaporator freezing, which correspond to Pf, (i.e. “YES” on the flowchart show in  FIG. 6 ), isolation valve(s)  240  are closed and refrigerant flow is blocked to one or more of the refrigerant circuits inside of the condenser  120  in step  507 . If the measured pressure of the refrigerant, Pm, is not below the minimum system head pressure, Pf, (i.e. “NO” on the flowchart shown in  FIG. 6 ), isolation valves  240  either opened, if previously closed, or remain open, if previously open. The opening of the valves  240  in step  609  allows refrigerant to flow to all refrigerant circuits within the condenser. When the refrigerant flows through all the circuits  210  of the condenser the heat transfer to the first heat transfer fluid  150  from the refrigerant is at a maximum. If the isolation valves  240  are closed in step  607 , the refrigerant is only permitted to flow through a portion of the condenser  120 . Each portion has a predetermined heat transfer surface area. Because the refrigerant is only permitted to flow into a portion of the condenser and some portions are isolated, the heat transfer area and the corresponding amount of heat transfer is reduced. Therefore, less heat is removed from the refrigerant. Likewise, less heat is transferred to the first heat transfer fluid  150 , thereby maintaining a higher refrigerant temperature. Additionally, because the temperature of the refrigerant is higher, the corresponding pressure of the refrigerant is also higher. Therefore, the refrigerant pressure of the system is increased.  
         [0032]      FIG. 7  shows an alternate method according to the present invention with a refrigerant pressure reset to provide less cycling of the isolation valve(s)  240 . The method includes the determination step  601 , the measuring step  603 , the valve operation systems  607  and  609 , as shown as described with respect to  FIG. 6 . However,  FIG. 7  includes a reset determination step  703 . In the method describe in  FIG. 7 , subsequent to the measurement taken at step  603 , a determination of whether the measured refrigerant pressure is less than the minimum system head pressure, Pf, is made at step  701 . If the measured pressure of the refrigerant, Pm, is less than the pressure for evaporator freezing, which corresponds to Pf, (i.e., “YES” on the flowchart show in  FIG. 7 ), isolation valve(s)  240  are closed and refrigerant flow is blocked to one or more of the refrigerant circuits inside of the condenser  120  in step  607 . If the measured pressure of the refrigerant, Pm, is greater than the minimum system head pressure, Pf, (i.e., “NO” on the flowchart shown in  FIG. 7 ), a determination of whether the measure head pressure, Pm, is less than the system reset pressure, Pr as shown in step  703 . If the measured pressure, Pm, is greater than the system reset Pressure, Pr, (i.e., “YES” on the flowchart shown in  FIG. 7 ), the isolation valves  240 , if closed, will be opened. If the measured pressure, Pm, is less than the system reset pressure, Pr, (i.e. “NO” on the flowchart shown in  FIG. 7 ), then no action will be taken regarding the isolation valves  240 . If open, the isolation valves  240  will remain open. If closed, the isolation valves  240  will remain closed. The value Pr−Pf represents a pressure buffer for the system so that the isolation valves  240  will not be inclined to open and close rapidly. The opening of the isolation valves  240  in step  609  allows refrigerant to flow to all refrigerant circuits within the condenser.  
         [0033]     In the HVAC system according to the present invention, when the pressure in the suction line  145  to the compressor  130  falls, the temperature of the refrigerant in the evaporator  110  likewise falls. When the pressure falls to a certain level, the evaporator  110  operates at temperatures that may result in icing of the evaporator  110 . Icing is a condition when the temperature at the exterior of the evaporator is sufficiently low to freeze water present in the heat transfer fluid. In particular, in a residential system, the heat transfer fluid is typically air and the water that freezes is water present in the air in the form of humidity. The ice formed by the water frozen on the surface eventually prevents the proper operation of the HVAC system by inhibiting heat transfer and/or damaging system components. This icing generally begins at temperatures of from about 25° F. to about 32° F. In order to prevent the freezing of the evaporator, the pressure in the suction line  145  is preferably maintained above the temperature that corresponds to the freezing point of the evaporator  110 .  
         [0034]     The method and system for controlling the refrigerant pressure of an air conditioning or heat pump unit according to the present invention includes an HVAC unit that can operate at lower ambient temperatures. The present invention involves a piping arrangement that partitions the circuits within the condenser of a refrigeration system. The piping arrangement includes valves positioned so that one or more of the circuits within the condenser may be isolated from flow of refrigerant. The piping arrangement may be applied to a new system or may be applied an existing system. Applying the piping arrangement to the existing system has the advantage that it allows control of the refrigerant pressure without the addition of expensive piping, equipment and/or controls.  
         [0035]     When the temperature around the condenser coil falls (e.g. when the outdoor temperature falls), the system refrigerant pressure falls proportionally. To help build head pressure, the present invention uses the valves connected to the circuits of the condenser to isolate a portion of the condenser from flow of refrigerant. The portion of the condenser that is not isolated remains in the active circuit and receives refrigerant. Because the refrigerant is only permitted to flow into a portion of the condenser  120 , the heat transfer area and the corresponding amount of heat transfer is reduced. Therefore, less heat is removed from the refrigerant. Likewise, less heat is transferred to the first heat transfer fluid  150 , thereby maintaining a higher refrigerant temperature. Additionally, because the temperature of the refrigerant is higher, the corresponding pressure of the refrigerant is also higher. Therefore, the refrigerant pressure of the system is increased.  
         [0036]     In one method according to the invention, the pressure of the refrigerant is measured and compared to a predetermined pressure. The pressure measurement may be taken from any point in the system. However, the preferred point of measurement of refrigerant pressure is on the suction line  145  to the compressor. The suction line  145  to the compressor also corresponds to the outlet of the evaporator  110 . The outlet of the evaporator  110  represents a low pressure point in the system, due the phase change of the refrigerant to a vapor resulting from the heat exchange relationship existing between the refrigerant and the second heat transfer fluid  155  in the evaporator  110 . The lowest pressure point where liquid refrigerant is undergoing evaporation also corresponds to the lowest temperature in the system. The predetermined pressure is preferably a pressure that is greater than or equal to the pressure that corresponds to a temperature that results in icing at the evaporator  110 .  
         [0037]     The piping arrangement of the condenser  120  of the present invention includes piping sufficient to isolate the two or more heat transfer circuits  210  within the condenser. In one embodiment, the isolation valves  240  are positioned inside the vapor header  290  of the condenser  120 . In an alternate embodiment, the isolation valves  240  are positioned on piping upstream from the vapor headers  290  of the condenser  120 .  
         [0038]     In an alternate embodiment according to the invention, refrigerant stored in the isolated portion of the condenser  120  after isolation valves  240  are closed may be drawn out of the isolated portion of the condenser  120  into the active system by suction pressure. Because the refrigerant from the isolated portion of the condenser adds to the amount of refrigerant per unit volume of the refrigeration system  100  not isolated, the pressure of the refrigerant is increased. Therefore, this addition of refrigerant into the system from the isolated portion of the condenser further assists in raising the system pressure. Alternatively, refrigerant may also be drawn out of the active portion of the refrigerant system  100  to reduce the pressure of the refrigerant, when a reduced refrigerant pressure is desirable. Drawing refrigerant out of the isolated portion of the coil provides additional control of the refrigerant pressure that provides a decrease in refrigerant pressure, particularly during times of unexpected, temporary or small refrigerant pressure increases. For example, the isolated condenser portion may not be opened during a particular pressure increase and the refrigerant may be drawn into the system. This operating condition may be desirable during times such as when the system is subject to gusting wind, changes in sunlight intensity or other temporary change in ambient conditions.  
         [0039]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.