Abstract:
A system and humidity control method is provided for a multi-stage cooling system having two or more refrigerant circuits that balances humidity control and cooling demand. Each refrigerant circuit includes a compressor, a vessel and an evaporator. A hot gas reheat circuit having a heat exchange coil is connected to one of the refrigerant circuits and is placed in fluid communication with the output airflow from the evaporator of that refrigerant circuit to provide additional dehumidification to the air when humidity control is requested. The hot gas reheat circuit bypasses the vessel of the refrigerant circuit during humidity control. Humidity control is performed during cooling operations and ventilation operations. During a first stage cooling operation using only one refrigerant circuit and having a low cooling demand, the request for humidity control activates the hot gas reheat circuit for dehumidification and activates a second refrigerant circuit to provide cooling capacity. The hot gas reheat circuit is sized to match the cooling provided by the evaporator so that air cooled by passing through the evaporator can be reheated. Excess refrigerant is passed into the inactive cooling circuit so that proper pressure and temperature can be maintained in the active reheat circuit and so that high head pressure that can damage the compressor can be avoided. During a second stage cooling operation using two or more refrigerant circuit and having a high cooling demand, the request for humidity control is suspended and is initiated only upon the completion of the second stage cooling demand.

Description:
RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 11/027,402, filed Dec. 30, 2004, the entirety of which is hereby incorporated by reference and U.S. patent application Ser. No. 10/929,757, filed Aug. 30, 2004, the entirety of which is hereby incorporated by reference and U.S. patent application Ser. No. 11/159,925, filed Jun. 23, 2005, the entirety of which is hereby incorporated by reference and U.S. patent application Ser. No. 11/165,106, filed Jun. 23, 2005, the entirety of which is hereby incorporated by reference and U.S. patent application Ser. No. 10/970,958, filed Oct. 22, 2004, the entirety of which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to controlling refrigerant flow into an air conditioning system having a hot gas reheat circuit, and specifically for controlling the amount of refrigerant flowing into the reheat circuit based on outdoor and indoor ambient conditions. 
   BACKGROUND OF THE INVENTION 
   Air delivery systems, such as used in commercial applications, typically are systems that can be used to cool or to accomplish dehumidification when ambient conditions are such that there is no demand for cooling. This demand for dehumidification can often occur on days when the temperature is cool and there is a high humidity level, such as damp, rainy spring and fall days. Under such conditions, it may be necessary to switch the operation of the air delivery system from cooling mode to dehumidification mode. 
   When switching an air delivery system, such as are used in commercial applications, from the cooling mode to the dehumidification mode in a reheat system that includes a reheat coil and a condenser coil configured in a parallel arrangement, some refrigerant will become trapped in the condenser coil. As the outdoor temperature falls, the amount of refrigerant that becomes trapped in the condenser coil will increase, resulting in a drop in the quantity of refrigeration available in the remainder of the refrigerant system to accomplish dehumidification. Without adequate refrigerant in the dehumidification circuit, operational problems will occur with the air delivery system. Some refrigerant can become trapped in a system that includes a reheat circuit even on warm days when dehumidification is required, but cooling is not required. The refrigerant can become trapped in the condenser coil, and if switching is required to the cooling mode, additional refrigerant can be trapped in the reheat circuit. 
   One of the problems is decreased system capacity as the refrigerant normally available in a properly operating system is trapped in the condenser coil and not available to the compressor. Associated with this problem is inadequate suction pressure at the compressor, since the gas refrigerant that normally is available to the compressor from the evaporator is trapped as a liquid in the condenser. A solution to the problem of refrigerant trapped as a liquid in a condenser or in a reheat heat exchanger is set forth in United Stated Patent Application No. U.S. 2004/0089015 A1, based on U.S. Ser. No. 10/694,316 to Knight et al., filed Oct. 27, 2003, now allowed, (“the Knight application”) and assigned to the assignee of the present invention, which allowed application is incorporated herein by reference. 
   The system described in the Knight application utilizes a system having a reheat circuit in which a hot gas reheat exchanger is coupled to an evaporator and a compressor, but which does not include a condenser. A separate cooling circuit utilizes a compressor, a condenser and an evaporator. The evaporator and compressor may be shared between the two circuits, when suitable valving is used to isolate the circuits. As discussed in the Knight application, the system may be combined with additional cooling circuits, as required. Thus, systems having more than one compressor are envisioned, and these compressors also may be coupled to additional reheat circuits. Although such complex systems are envisioned by the Knight application and the present invention, both the Knight application and the present invention are readily understood without reference to these more complex arrangements, as one skilled in the art can readily adapt the simpler concepts of the Knight application and the present invention to such complex arrangements. 
   In order for the reheat circuit to operate efficiently and properly, the hot gas reheat exchanger must be suitably sized in relation to the evaporator. Generally, the properly sized hot gas reheat exchanger is smaller than the condenser that is included in the cooling circuit that shares the same condenser and evaporator. The result is that when the cooling circuit is inactivated and the reheat circuit is activated to accomplish dehumidification, excess refrigerant can be directed into the reheat circuit. The Knight application, while implicitly recognizing the need to balance the size of the hot gas reheat coil against the size of the evaporator coil, explicitly addresses the problem of refrigerant, which is also shared by the cooling circuit and the reheat circuit, trapped in the inactivated circuit. However, it fails to address the problem of refrigerant being drawn into the activated circuit. Excess refrigerant drawn into a circuit can result in operational problems which should be avoided. One of these problems is unacceptable discharge pressures from the compressor, which can lead to decreased system efficiency. If the amount of excess refrigerant drawn into the activated circuit is too great, slugging can also be a problem. Slugging is a condition in which liquid refrigerant is drawn into the compressor. These operational problems can result in a severe reduction in compressor life, and in the worst circumstances, to premature compressor failure. 
   What is needed is a system that can readily and rapidly accommodate the difference in refrigerant capacity between the reheat circuit and the cooling circuit to avoid these operational problems without having to resize or otherwise reengineer the hot gas reheat coil or the condenser coil. 
   SUMMARY OF THE INVENTION 
   The present invention utilizes a system having an independent hot gas reheat circuit and a cooling circuit. The hot gas reheat circuit includes a compressor, an evaporator and a hot gas reheat coil. The hot gas reheat coil is engineered to work in conjunction with the evaporator to provide a sufficient rise in temperature of air that has been cooled after passing over the evaporator. The cooling circuit, which is isolatable from the reheat circuit, includes a condenser, and shares the compressor, the evaporator and refrigerant with the reheat circuit. The hot gas reheat coil is generally sized to accommodate sufficiently less refrigerant than the condenser. 
   The present invention controls the amount of refrigerant entering into a first circuit from a second circuit, wherein the first circuit is being activated and the second circuit is being inactivated. This control is of particular importance when the activated circuit has less refrigerant capacity than the inactivated circuit. 
   The present invention accomplishes the control of the amount of refrigerant entering a first circuit that is activated from a second circuit that is inactivated by utilizing a plurality of valves that operate in response to a monitored environmental condition. When the sensed, monitored environmental condition is outside of predetermined limits, the valves operate to move refrigerant into the inactivated circuit, thereby utilizing the inactivated circuit as a storage area. In this manner, the inactivated circuit can be utilized as a receiver for the excess refrigerant. 
   As noted above, the engineering of most systems results in a hot gas reheat coil that has less refrigerant capacity than the condenser coil. Thus, the system of the present invention, as a minimum, should, when the reheat circuit is activated, monitor an environmental condition, such as system pressure, and utilize the condenser coil as a receiver for excess refrigerant when the monitored pressure is outside of predetermined limits. Excess refrigerant is the difference in refrigerant capacity, the excess refrigerant being the amount of refrigerant that should be removed for the circuit having lesser capacity in order to maintain satisfactory and efficient operation of the circuit. However, the system is not so limited, and can be engineered so that when the cooling circuit is activated, the inactivated hot gas reheat coil can be utilized as a receiver for excess refrigerant if the monitored environmental condition is outside of predetermined limits, if required. The present invention moves the excess refrigerant out of the circuit, here the reheat circuit when activated. While an accumulator of the prior art stores refrigerant, the refrigerant is still present in the circuit. So the present invention, while eliminating the need for an accumulator, does not substitute the inactive circuit for the accumulator as a reservoir for excess refrigerant. The present invention physically moves the excess refrigerant from the active circuit to an inactive circuit. By doing so, the operating temperature of the evaporator, that is the evaporation temperature of the refrigerant in the evaporator, can be controlled for efficient operation. This is monitored and manipulated in the present invention by controlling the refrigerant pressure in the active circuit, although any other monitoring method may be utilized. 
   An advantage of the present invention is that refrigerant is not inadvertently trapped in the inactivated circuit, but is initially moved from the inactivated circuit into the activated circuit, and then is metered from the activated circuit back to the inactivated circuit based on sensed environmental conditions that exceed predetermined limits. In this way, the proper amount of excess refrigerant can be moved into the inactivated circuit. Stated alternatively, the proper amount of refrigerant is metered into the active circuit based on sensed environmental or operating conditions in the active circuit. 
   Another advantage of the present invention is that, by using the inactivated circuit for storage of the excess or unneeded refrigerant, the accumulator can be eliminated. 
   Another advantage of the present invention is that by adjusting the amount of refrigerant in the activated circuit, proper system control can be maintained. Specifically, the compressor pressure range for maximum capacity and efficiency can be maintained. The refrigerant evaporation temperature, which is related to the pressure, also can be controlled for efficient dehumidification. This translates to energy savings for the operator or owner. 
   A related advantage to maintaining compressor operational pressures within design pressures and avoiding the fluctuations in pressure that occur as a result of excess refrigerant is extended compressor life. Premature compressor failure as a result of events such as slugging can be avoided. 
   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 
       FIG. 1  illustrates a schematic of a prior art single compressor system having a cooling circuit and a reheat circuit. 
       FIG. 2  is a schematic illustration of the present invention depicting control of the circuits utilizing solenoid valves to channel excess refrigerant into the inactive circuit. 
       FIG. 3  is a schematic illustration of a second embodiment of the present invention with a second arrangement of solenoid valves to channel excess refrigerant into the inactive circuit. 
       FIG. 4  is a schematic illustration of a third embodiment of the present invention with a third arrangement of solenoid valves to channel excess refrigerant into the inactive circuit. 
   

   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 
     FIG. 1  illustrates a prior art single compressor circuit. This system is set forth in United Stated Patent Application No. U.S. 2004/0089015 A1, based on U.S. Ser. No. 10/694,316 to Knight et al., filed Oct. 27, 2003, now allowed, (“the Knight application”) and assigned to the assignee of the present invention, which allowed application is incorporated herein by reference. This system includes a reheat circuit and a cooling circuit which operate independently. In operation, the prior art system of  FIG. 1  includes the usual components of a cooling system circuit, a compressor  2 , connected by conduit to a condenser  6  which is connected by conduit to an evaporator  12 , which is connected by conduit to compressor  2 . In the cooling mode, refrigerant sealed in system  1  is compressed into a hot, high-pressure gas in compressor  2  and flows through conduit to condenser  6 . The condenser having a heat exchanger or coil  6 , includes a fan  10  which blows air across the condenser coil  6 . In the condenser, at least some of the hot, high-pressure gas refrigerant undergoes a phase change and is converted into a fluid of high-pressure refrigerant liquid or a fluid mixture of high-pressure refrigerant liquid and refrigerant vapor. In undergoing the phase change, the refrigerant transfers heat through the coils of the condenser to the air passing over the coils with the assistance of fan  10 . Additional heat, heat of condensation, is given off by the refrigerant as it condenses from a gas to liquid. The high-pressure fluid passes through a conduit to an expansion device  16 . As the fluid passes through expansion device  16 , it expands, flashing some of the liquid to gas and reducing the fluid pressure. The low-pressure fluid then passes to the evaporator  12 . In evaporator  12 , the refrigerant passes through the evaporator heat exchanger circuits where the liquid refrigerant undergoes a second phase change and is converted to a vapor. This conversion requires energy, provided in the form of heat, which is drawn from air passing over the evaporator coils. This airflow is assisted by a fan which forces air over the coils. As shown in  FIG. 1 , the air is drawn over the coils by an air circulating means, indoor blower  18  in  FIG. 1 . After passing over the evaporator heat exchanger circuits, the air, which is now cooler as heat has been absorbed from it to assist in the refrigerant phase change, can be supplied to the space that requires refrigeration. Of course, the ability of the cooled air supplied to the space to hold moisture in the form of humidity is reduced below its capacity when it passed over the evaporator heat exchanger circuits, so the air passing into the space is also dehumidified. The excess moisture is removed from the air as condensate as it passes over the heat exchanger circuits and is directed to a drain. The refrigerant gas, now at low-pressure and low temperature is returned to compressor  2 . As shown in  FIG. 1 , there is an accumulator  13 , which stores any excess liquid refrigerant and lubricant until a system demand calls for it. A suction line circuit  44  includes a bleed line  46 . The line  46  runs from suction line  42  to valve  29  to activate or inactivate valve  29  in response to a signal from a controller (not shown). The prior art system also includes a reheat circuit. 
   This prior art unit also includes a reheat circuit. The reheat circuit includes compressor  2 , reheat exchanger  32 , and an evaporator  12 . Although the reheat circuit shares the compressor  2  and the evaporator  12  with the cooling circuit, the reheat circuit and the cooling circuit are independent circuits. This means that the circuits run as independent loops. To that end, backflow valves or check valves  31 ,  34  are included to maintain the operation of the circuits as independent loops. In operation, valve  29  is switched to direct hot refrigerant gas from the compressor discharge to the reheat coil  32 . The refrigerant gas is directed through expansion device  16  where the refrigerant is expanded and the pressure is reduced. The refrigerant is prevented from back flowing to condenser coil  6  by check valve  34 . As when the cooling mode is activated, air circulated through the evaporator is cooled and dehumidified. However, to return the air to the space or room at the same temperature, it passes through the reheat coil  32  where it is reheated. As will be appreciated by those skilled in the art, in order for the reheat coil or reheat heat exchanger  32  to properly reheat the cooled air, it is engineered to the proper size so that the proper heat balance between the evaporator  12  and the reheat coil  32  can be achieved to properly reheat the cooled, dehumidified air. It will also be appreciated that if the reheat coil is too small, excessive head pressure can result, leading to compressor failure. 
   The prior art reheat circuit utilizes a reheat refrigerant recovery circuit  60  and a condenser refrigerant recovery circuit  60  to prevent unused refrigerant from becoming entrapped in the inactivated circuit. When unused refrigerant is trapped in the inactive circuit, system efficiency drops. If a large quantity of refrigerant is entrapped in the inactive circuit, damage to the compressor can result. To prevent this situation, the reheat circuit is connected to the suction side of the compressor by reheat refrigerant recovery circuit  50  and the cooling circuit is connected to the suction side of the compressor by condenser refrigerant recovery circuit  60 . When a circuit is inactivated, either the cooling circuit or the reheat circuit, a controller opens a valve ( 52  for the cooling circuit,  62  for the reheat circuit) and the suction of the compressor draws any refrigerant trapped in the evaporator  12  or the reheat coil  32  out of the inactive circuit. Any excess refrigerant is stored in an accumulator  13 . 
   Because of typical size constraints in the unit cabinet, the reheat coil  32  is rarely, if ever, the same size as the condenser coil  6 . The condenser coil  6  is typically larger than the reheat exchanger  32 . Thus, the cooling circuit may require more refrigerant to operate efficiently than does the reheat circuit. While this solution prevents the entrapment of refrigerant in the inactivated circuit, a problem exists when switching from the one circuit to the other. This problem exists when switching from the circuit having the larger heat exchanger, typically the cooling circuit having condenser coil  6 , to the system having the smaller heat exchanger, typically the reheat coil  32 . Specifically, upon switching, the system may place excess refrigerant into the circuit having the smaller heat exchanger. This in turn can result in the system operating at too high of a pressure, as discussed above. 
   Referring now to  FIG. 2 , the present invention prevents excess refrigerant from being supplied to a circuit that has just been activated. The present invention includes a first solenoid valve  52  between reheat circuit and the suction side of compressor  2 . The first solenoid valve  52  is controlled by a controller (not shown in  FIG. 2 ). A second solenoid valve  203  is positioned between the condenser coil  6  and the reheat coil  32  to allow refrigerant to by-pass check valve  34  after it has flowed through reheat coil  32 . Three-way valve  29  controlled by a controller, operates to direct hot, high pressure refrigerant discharged by compressor  2  to either the cooling circuit and condenser coil  6  or to the reheat circuit and reheat coil  32 . 
   When the system is switched by the controller to the cooling mode, hot, high pressure refrigerant is directed by three-way valve  29  into the cooling circuit  49 , three-way valve  29  closing to block the flow of the refrigerant into hot gas reheat circuit  26 . Substantially simultaneously with the opening of three-way valve  29  to permit flow of refrigerant into cooling circuit  49 , the controller sends a signal opening solenoid valve  52 , thereby activating reheat refrigerant recovery circuit  50 . The suction of the compressor  2  thus draws any refrigerant that may have been trapped in reheat coil  32  from coil  32  and into cooling circuit  49 . No accumulator is required for this operation, as cooling circuit  49  requires substantially all of the refrigerant in the system for efficient operation as condenser coil  6  is larger than reheat coil  32 . The cooling circuit  49  operates in the conventional way. The hot, high pressure refrigerant is pumped through a conduit to condenser coil  6 , where it is converted from a gas to substantially a high pressure liquid, and it transfers heat to air forced across it by fan  10 . The cooled, liquid refrigerant then flows through a thermal expansion device  16  which reduces the pressure of the liquid. Flow into reheat circuit  26  is prevented by check valve  31 . After the refrigerant passes through expansion device  16 , it then flows into evaporator  12  where it undergoes a second change of state from liquid to gas. Heat is absorbed from the air passing through the circuits of evaporator  12 . To facilitate this change of state, an air circulating means, such as a blower  18  utilized. The air is also dehumidified and the refrigerant then flows back to compressor  2  where the cycle is repeated. 
   When the controller switches to a reheat mode, three-way valve  29  is signaled to switch the flow of refrigerant from the cooling circuit  49 , which is closed, to hot gas reheat circuit  26  in the direction of the arrow shown between compressor  2  and reheat coil  32 . The controller makes this determination based on the temperature and humidity of the space that is being controlled. If the sensors in the room indicate that the temperature is sufficiently cool, but that the humidity remains above a preselected level, such a switch will be accomplished. In a system that utilizes more than one compressor or more than one cooling circuit, if the controller determines that maximum cooling is not required, but that additional cooling and dehumidification are required, the hot gas reheat circuit  26  will be activated simultaneously with the operation of one or more cooling circuits controlled by additional compressors in the system. Solenoid valve  203  is responsive to reheat high-pressure device  205 . When this device  205  indicates that the compressor discharge pressure is above a preselected level, device  205  causes second solenoid valve  203  to open. Since solenoid valve  203  bypasses check valve  34 , at least some refrigerant, after flowing through reheat coil  32 , is directed into condenser coil  6 . As the pressure initially increases in the reheat circuit, check valve  34  will be closed and refrigerant will flow through solenoid valve  203 . When reheat high pressure device  205  indicates that the compressor discharge pressure has stabilized at or below a preselected level, solenoid valve  203  is closed and further flow of refrigerant into condenser circuits  6  is blocked. The remaining refrigerant circulates through the hot gas reheat circuit  26 . Because the amount of refrigerant in the hot gas reheat circuit  26  is regulated by the compressor discharge pressure, problems arising from excess refrigerant in hot gas reheat circuit  26  are eliminated. Excess refrigerant is stored out of the reheat circuit and in condenser circuits  6 , which serves as a receiver, and the accumulator of the prior art may be eliminated. However, the condenser circuits are not merely a substitute for the prior art accumulator. When the accumulator is used, the refrigerant remains stored within the circuit, but is otherwise available to the compressor and can lead to excessive pressures. When the condenser circuits are used to store the excess refrigerant, the refrigerant is removed from the reheat circuit and is not available to the circuit while it remains in the reheat mode. Furthermore, when the cooling circuit is activated and the reheat circuit is inactivated, the additional refrigerant required by the cooling circuit is already located in the cooling circuit, being stored in the condenser circuits  6 . In addition to eliminating the accumulator, set forth in the prior art, the system also eliminates condenser refrigerant recovery circuit and its associated solenoid valve and conduits. 
   Reheat high pressure device  205  may be any control device that can control solenoid valve  203 . For example, reheat high pressure device  205  may be a switch that has settings. When a first preselected pressure setting is reached and is detected by switch  205 , the switch closes and sends a signal to open solenoid valve  203 . Solenoid valve  203  remains open until a second preselected pressure setting is reached. The second preselected pressure setting may be the same as the first preselected pressure setting or it may be a lower pressure. When the pressure drops below a second preselected pressure setting, the switch opens, removing the signal, thereby closing solenoid valve  203 . In another embodiment, reheat high pressure device  205  may be a sensor that senses the refrigerant discharge pressure from compressor  2 . The reheat sensor is in communication with the controller, not shown in  FIG. 2 , which is programmable. The preselected pressure can be programmed into the controller. When the pressure measured by the sensor  205  exceeds or falls below a preselected setting, the controller, which is constantly monitoring sensor  205 , sends a signal to change the status of solenoid valve  203 . For example, if the preselected pressure setting programmed into the controller is, for example 225 psig, and the pressure detected by sensor  205  exceeds this value, a signal is sent by the controller that opens solenoid valve  203 . The opened valve permits refrigerant to bypass check valve  34  and flow into condenser coil  6 . As refrigerant flows into condenser coil  6 , less refrigerant remains in reheat circuit  26  and the discharge pressure from compressor  2  begins to decrease. When the discharge pressure reaches a second preselected pressure, either at or below the first preselected pressure, the controller, which is monitoring the pressure at sensor  205 , sends a signal to close solenoid valve  203 . Thus if the second pressure is, for example, 180 psig, then the controller sends a signal that closes second solenoid valve  203 , stopping the flow of refrigerant fluid through solenoid valve  203  and around check valve  34 . The reheat circuit should now be stabilized, operating at a capacity that is within a pressure range that produces the required reheat for balancing temperature. If the system should become unstable, the reheat high-pressure device  205  will detect the change in pressure and adjust the pressure by channeling additional refrigerant into condenser  6  by repeating this process. 
   When the system is switched over to the cooling mode, such as for example, if there is a call for maximum cooling, three-way valve  29  switches to direct the compressor discharge into cooling circuit  49  in the direction of the arrow shown between compressor  2  and condenser coil  6 , stopping the flow of refrigerant into hot gas reheat circuit  26 . This circuit requires additional refrigerant to operate efficiently, but the refrigerant is already properly stored within condenser  6 . This refrigerant should already be condensed, and the pressure differential caused by compressor suction and compressor discharge will result in the flow of condensed refrigerant to the evaporator. 
   While the system illustrated in  FIG. 2  depicts reheat pressure device  205  positioned in the conduit of reheat circuit between three-way valve  29  and reheat coil  32  so as to monitor high pressure discharge from the compressor, it will be recognized by one skilled in the art that reheat pressure device  205  may be positioned anywhere in the active circuit to monitor the pressure of refrigerant at a preselected location within the circuit. 
   One skilled in the art therefore will understand that reheat pressure device  205  may be a low-pressure device. In this embodiment, the reheat pressure device is positioned in the suction line between the evaporator  12  and the suction port of compressor  2 , on the low pressure side of the compressor, to monitor the pressure of the refrigerant returning to the compressor. Second solenoid valve  203  is cycled based on preselected pressure settings as before. When the detected pressure is above a first preselected limit, the valve is opened until the pressure falls below a second preselected limit, which may be the same as, or lower than the first preselected limit. This arrangement is an equivalent arrangement to the arrangement using reheat pressure device  205  on the high pressure or discharge side of the compressor. When the reheat pressure device  205  is located on the discharge or high pressure side of the compressor, it may be referred to as a reheat high-pressure device, and when it is located on the suction or low pressure side of the compressor, it is referred to as low-pressure device, the designations simply indicating the location of pressure device  205  within the circuit. 
   In a second embodiment of the present invention, shown in  FIG. 3 , a conduit  301  connects the discharge side of compressor  2  to the discharge line  49  between three-way valve  29  and condenser  6 . In this embodiment, reheat high-pressure device  305  is positioned along the discharge line of compressor  2  between compressor  2  and reheat coil  32 . As shown in  FIG. 3 , reheat high-pressure device  305  is positioned on the discharge line of compressor  2  between the compressor and three-way valve  29 . In normal steady state operation, cooling circuit  49  acts in a conventional manner as described above. Similarly, reheat circuit  26  also acts in a conventional manner as described above. 
   In transition from the cooling mode to the reheat mode, three-way valve is switched to direct the refrigerant gas discharged from the discharge port of compressor  2  into reheat circuit  26 . When the pressure detected by reheat high-pressure device  305  is above a first preselected limit, reheat-high pressure device  305  acts to open solenoid valve  303 , thereby directing some refrigerant gas from the active reheat circuit to condenser  6 . The operation of solenoid valve  303  is as described above. As refrigerant gas flows out of reheat circuit  26  and into storage in condenser coil  6 , the discharge pressure from compressor  2  will decrease. When the pressure detected by reheat pressure device  305  falls below a second preselected limit, reheat high pressure device  305  acts to close solenoid valve  303 , thereby stopping the flow of refrigerant out of the reheat circuit and into the cooling circuit where it is stored in the condenser. As before, on reaching the second preselected limit, the amount of refrigerant in reheat circuit  26  will be self-regulated to a level to permit efficient operation, and the risk of too much refrigerant being supplied to the compressor in the reheat mode is reduced, if not eliminated. Inasmuch as the pressure in the reheat circuit is related to the refrigerant evaporation temperature, by controlling the amount of refrigerant in the reheat circuit, the operating temperature of the evaporator and hence the amount of dehumidification provided by the evaporator to the air flowing through it can also be controlled. 
   Also shown in  FIG. 3  is an alternate embodiment in which the reheat pressure device  305  positioned on the high pressure side of the compressor is replaced by reheat pressure device  307  which is located on the low pressure side, or suction side of the compressor. It will be understood that either reheat pressure device  305  or reheat pressure device  307  is operational in the system, as discussed for the embodiment shown in  FIG. 2  in paragraph [0033] above. Reheat pressure device, when located in the suction line of compressor  2  between accumulator  309  and the suction port of compressor  2  operates as described above for the alternate, equivalent embodiment described in  FIG. 2 . 
     FIG. 4  discloses yet another embodiment of the present invention. This embodiment is a system that utilizes independent reheat circuit  26  and an independent cooling circuit  49 , both operating in the steady state as set forth above. Three-way valve  29  directs refrigerant to either cooling circuit  49  or reheat circuit  26  depending upon whether there is a demand for cooling or dehumidification in the space. The system of  FIG. 4  includes a reheat refrigerant recovery circuit  50  that includes a solenoid valve  52  operating as described above to remove refrigerant from the reheat coil when the system switches from reheat to cooling. The system of  FIG. 4  includes a third solenoid valve  403  positioned to bypass check valve  34  to allow refrigerant to flow from reheat coil  32  to condenser coil  6 , as discussed above. The system also includes a fourth solenoid valve  413  positioned in a conduit that connects the low pressure or suction side of compressor  2  to condenser circuits  6 , here shown connected to line  49 , although a direct connection to condenser circuits is also within this embodiment. Valve  413  can be opened when three-way valve  29  is positioned to direct refrigerant flow to reheat circuit  26  and in response to pressure device  415 . If the pressure drops below a preselected value, valve  413  is opened in response to the low pressure reading from pressure device  415  allowing refrigerant to be drawn back into the circuit until the refrigerant pressure is raised above a preselected value, at which time valve  413  is closed stopping the flow of refrigerant into the circuit. 
   A first pressure device  405  controls the operation of solenoid valve  403 . The first pressure device  405  may be either a high pressure device positioned as shown in  FIG. 4 . Alternatively, pressure device  405  may be a low pressure device positioned on the low-pressure side of compressor  2 , causing valve  403  to open or close in response to the pressure sensed on the suction or low-pressure side of the compressor. A second pressure device  415  controls the operation of solenoid valve  413 . Again, second pressure device  415  may be either a low pressure device positioned as shown in  FIG. 4 , or alternatively a high pressure device positioned on the high pressure side of compressor  2  between three way valve  29  and expansion device  16 . Preferably, when first reheat pressure device  405  is a high pressure device, second reheat pressure device  415  is a low pressure device. When the pressure in the system is determined by the high pressure sensor to be too high, valve  403  is opened and refrigerant is transferred to the condenser circuits  6  until the pressure is at or below a predetermined set point, at which time valve  403  is closed. If the pressure in the reheat circuit drops below a predetermined set point, as measured by pressure device  415 , then solenoid valve  413  is opened and refrigerant is drawn by suction of the compressor from storage in the condenser circuits  6  back into reheat circuit. When the pressure rises to or above a preselected set point, as measured by device  415 , valve  413  is closed. 
   The first pressure device  405  controls third reheat solenoid valve  403  in the manner that pressure device  205  and second solenoid valve  203  operate in the embodiment of  FIG. 2 , which removes refrigerant from the circuit when a high head pressure is detected. Pressure device  415  and solenoid valve  413  are configured similarly to the circuit described in the Knight application. They differ operationally. The circuit in the Knight application is operates by opening a solenoid valve such as valve  413  when the circuit is initially switched from a first mode to a second mode, such as cooling to reheat, in order to prevent refrigerant from being trapped in the inoperative circuit. This is essentially automatically accomplished when the mode is switched. In this application, solenoid valve  413  is operated only in response to a drop in pressure below a preselected value or setpoint as monitored by pressure device  415  as the reheat circuit is operating in the reheat mode. Of course, a sophisticated controller can be programmed to allow valve  413  to operate when switching from one mode to another mode occurs, and when required by sensed pressures by pressure device  415 . In this circumstance, redundant hardware can be avoided. 
   Because the system of  FIG. 4  monitors both the high pressure side of compressor  2  and the low pressure or suction side of compressor  2 , the system of  FIG. 4  can respond rapidly to achieve steady state operation when the pressure is either too high or too low. When solenoid valve  403  or solenoid valve  413  is controlled by a high-pressure device, the remaining solenoid valve is controlled by a low-pressure device. Thus, when the system of  FIG. 4  switches from the cooling mode to the reheat mode, the system not only responds initially to remove refrigerant trapped in the condenser circuits  6 , but quickly responds to remove refrigerant from the reheat circuit  26  when the refrigerant pressure on the exceeds a preselected pressure valve. It also corrects the refrigerant pressure when the refrigerant falls below a preselected pressure valve. 
   By a slight modification to the control functions of  FIG. 4 , which is best set out in the following description, a different embodiment of the present invention is set forth. Controller  450  is shown in communication with the various components of the system, which is generally true of all of the previously described embodiments. Controller  450  is also in communication with sensors in the space that is to be controlled or with a controller in that space, which is shown by the dashed lines of  FIG. 4 . Other related elements, as required, may also be in communication with controller  450 , and the communications lines are not limited to the lines shown. The communications among the various components of the system of  FIG. 4  may be via hard wiring between the components and the controller, or it may be by wireless communication, such as RF communications, or some combination. Any known or new method of communication among the components and the controller may be used, as long as reliable communications exist. 
   As set forth in  FIG. 4 , controller  450  monitors pressure device  405  and pressure device  415  in order to control the operation of third solenoid valve  403  and fourth solenoid valve  413 , respectively. It will be understood by those skilled in the art that pressure device  405  and pressure device  415  alternatively may also be provided as switches to control the operation of valve  403  and valve  413 . 
   In this embodiment, communications are shown between controller  450  and solenoid valve  403  via line  454 ; between controller  450  and blower  18  via line  456 ; between controller  450  and condenser fan  10  via line  458 ; between controller  450  and solenoid valve  29  via line  460 ; between controller  450  and first device  405  via line  462 ; between controller  450  and compressor  2  via line  464 ; between controller and solenoid valve  413  via line  466 ; between controller  450  and second device  415  via line  468  and between controller  450  and solenoid valve  52  via line  470 . As shown, the controller  450  is also in communication with a controller or sensors in the space in which conditioned air is required. 
   When the space to which conditioned air is supplied requires cooling, a signal is sent to controller  450  from the space, either from a controller, or from a sensor, in which circumstance controller  450  includes programmable limits and the signal from the sensor indicates that the limits have been reached. This controller sends a signal along line  464  to activate compressor  2  if it is not already operating. Controller  250  sends a signal to three-way valve  29  to direct compressed refrigerant toward condenser  6 . Controller also sends a signal to solenoid valve  52  via line  470 , opening refrigerant recovery circuit  50 . The suction action of compressor draws any refrigerant from reheat coil  32  into the cooling circuit. Solenoid valve  52  may remain open while the cooling circuit is activated, or it may be shut, such as by a time delay and/or another signal from controller  450 . A signal is also sent to condenser fan  10  along line  458  and to blower  18  along line  456 . Controller  450  also closes solenoid valves  403  and  413 , if not already closed. The cooling circuit operates in a conventional manner supplying cooled, dehumidified air to the space. 
   When the space to which conditioned air is supplied indicates that the air exceeds a preselected humidity, such as when a cooling circuit is providing air that requires additional dehumidification, or when the cooling requirements have been met in the space, but the dehumidification requirements have not been met, a signal is sent from controller  450  to compressor  2  to activate compressor  2  if it is not already running. A signal is also sent along line  460  to three-way valve  29  to place it in a position to direct refrigerant gas into hot gas reheat circuit  26 . Controller  450  also sends a signal to solenoid valve  52  to close it, if it is not already closed, so that hot refrigerant gas is not cycled to the suction side of compressor  2 . Of course, a signal is sent along line  456  to activate blower  18  if it is not already running. 
   In the reheat mode, first device  405  and second device  415  control the operation of third solenoid valve  403  and fourth solenoid valve  413  respectively. As shown in  FIG. 4 , first device  405  and second device  415  are in communication with controller  450 . Controller  450  is programmed with set points that initiate an action when the set points are reached. However, as noted above, first and second device  405 ,  415  may include switches that are activated to control valves  403 ,  413  when the set points are reached. In this embodiment, when the reheat circuit is activated, one of first and second devices  405 ,  415  opens one of valves  403 ,  413  to open the valve to move refrigerant into condenser coil  6 . This valve is closed when a preselected pressure is reached. The remaining device monitors the pressure in the system and opens the remaining valve to maintain a preselected pressure in reheat circuit  26 . When the preselected pressure is reached, the controller, which is monitoring the sensor, sends a signal closing the remaining valve. 
   To further illustrate this example, on switching to reheat mode, a reheat high pressure sensor  405  sends a signal to controller  450  along line  462  indicative of the presence of high pressure refrigerant gas is flowing in reheat circuit  26 . Controller  450  sends a signal along line to solenoid valve  403 , opening it, allowing refrigerant to flow into condenser circuits  6 . Reheat sensor  415  provides a signal to controller  450  indicative of the pressure in the suction line of compressor  2 . If the pressure in the line is above a predetermined limit, the controller  450  maintains solenoid valve  413  in a closed position. However, if the pressure falls below a predetermined limit, as determined by controller  450  which is monitoring the signal from pressure sensor  415  along line  468 , a signal is sent by controller  450  to solenoid valve  413 , opening it and allowing refrigerant to be drawn from condenser circuits  6  into reheat circuit  26  by the suction of compressor  2 . When pressure sensor  415  indicates to controller  450  that the pressure has risen sufficiently, again to a predetermined limit, controller  450  sends a signal along lines  460  and  454  lines  454  to close solenoid valve  413 . 
   As may be clear, it is possible to control the operation of both valves  403  and  413  with a single sensor. In this case, the activation of the reheat circuit  26  (or inactivation of the cooling circuit  49 ) results in the opening of valve  403 . Valve  413  is controlled as set forth above. Either valves is closed by controller  450  in response to a signal from the sensor indicating that the pressure is within a preselected range. For example, if the sensor indicates a high pressure, the controller can send a signal to effect operation of valve  403  until the pressure is reduced, at which time valve  403  is closed. Similarly, if a low pressure is indicated by the sensor, the controller can similarly effect operation of valve  413 . 
   The arrangement of  FIG. 4  with the use of a sophisticated controller or control program can result in a very complex system operation, which is beyond the scope of the present invention. It must be considered, however, that a series of sensors can be set up to detect conditions of the air. These sensed conditions can be of the return air or the supply air or both, and can include, for example, humidity or temperature. The control program can assess the sensor signals to determine whether the air is being properly dehumidified and/or warmed after passing through the system. Based on the sensed conditions, the control program can determine the set points required for operation of valves  403 ,  413  in order to obtain proper dehumidification and reheat. The amount of refrigerant in the reheat circuit can be adjusted to provide the proper refrigerant evaporation temperature in the evaporator, which is related to system pressure. Although the operational details of such a complex system are beyond the scope of the present disclosure, the mechanical arrangement of  FIG. 4  make such a complex yet efficient system possible. 
   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.