Patent Abstract:
A refrigerant system incorporates at least two compressors that act in tandem to provide variable control over the refrigerant system performance. The tandem compressors can be of conventional or economized type and are configured for maximum performance utilization. Further, a reheat circuit can be incorporated into the refrigerant system at several different locations. The reheat cycle provides additional control over sensible and latent capacity of the refrigerant system, and is particularly advantageous when utilized in combination with the tandem compressors. As a result, multiple steps of unloading can be implemented in all operation regimes, the external load demands are satisfied with much greater precision, eliminating undesirable variations in temperature and humidity, system efficiency and reliability are augmented and equipment life-cycle cost is reduced.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to refrigerant cycles incorporating a reheat coil, and wherein a tandem compressor arrangement is utilized. The resulting cycles provide an enhanced control over both humidity and temperature in the conditioned space as well as improve efficiency, reliability and life-cycle cost of the equipment.  
         [0002]     Refrigerant cycles are utilized to change the temperature and humidity, or otherwise control the environment, in the conditioned space. In particular, a refrigerant cycle will typically include a compressor delivering a compressed refrigerant to a condenser, and from the condenser to an expansion device. The refrigerant exchanges heat with an outdoor environment at the condenser, and is expanded in the expansion device to a lower pressure and temperature. From the expansion device, the refrigerant continues to an evaporator at which it exchanges heat with an indoor environment, or an environment to be conditioned. From the evaporator, the refrigerant returns to the compressor.  
         [0003]     The amount of cooling that can be supplied by a refrigerant cycle is known as its “capacity.” There are numerous ways to provide control over the capacity for a refrigerant cycle. One way is to substitute multiple compressors acting in tandem for a single compressor. In such an arrangement, the compressors can be selectively operated or shut down in response to an external heat load demand. Also, tandem compressors of different sizes can be utilized such that the various steps in the total capacity can be achieved. Moreover, both economized and conventional compressors can be employed in the tandem configuration, allowing for switching between various compressor modes of operation and further increasing a number of the unloading steps. Lastly, the tandem compressor configuration can be selectively chosen for any of the independent circuits of a multi-circuit system.  
         [0004]     Another way of controlling an environment in the conditioned space with a refrigerant cycle is the incorporation of a reheat coil. Typically, a reheat coil is provided in the path of air that has been blown over the evaporator. The air passes over the reheat coil to regain heat from a refrigerant that is at a temperature hotter than the temperature of air leaving the evaporator. The reheat coil is thus able to raise the temperature of the air leaving the evaporator. Hence, the air dehumidified and overcooled in the evaporator is reheated back to a comfortable temperature level in the reheat coil. In other words, in the system with the reheat coil, the humidity in the conditioned space is mainly controlled by the evaporator and its temperature—by the reheat coil.  
         [0005]     The reheat coil has not been utilized in a combination with tandem compressors, however. Thus, the refrigerant cycles with tandem compressors have not had as complete control over temperature and humidity levels as may be desired. It has to be noted that this invention is not related to any particular reheat concept or tandem compressor configuration, but rather provides advantages, that could not be obtained before for a refrigerant system, by integrating both design features in a single cycle. Consequently, a system with any reheat schematic or tandem compressor configuration can take advantages from the invention.  
       SUMMARY OF THE INVENTION  
       [0006]     In disclosed embodiments of this invention, a refrigerant system incorporates tandem compressors. At least two compressors each separately compress a refrigerant and deliver it into the refrigerant cycle. A reheat coil is also incorporated into the refrigerant system, and receives air having passed over the evaporator to reheat the air to a desired temperature when required. In one embodiment, the reheat coil is positioned in the refrigerant cycle to receive a hot gas. Other embodiments may position the reheat coil such that it utilizes a two-phase mixture of refrigerant, or a liquid refrigerant. Also, some embodiments allow the condenser to be bypassed by at least a portion of refrigerant flow, when desired. Further, the reheat cycle may be incorporated in a system utilizing either economized or conventional tandem compressors, or a combination of both, in other embodiments. Also, in one embodiment, at least one compressor may be unloaded to provide even greater capacity control.  
         [0007]     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a first embodiment of the present invention.  
         [0009]      FIG. 2  shows a second embodiment of the present invention.  
         [0010]      FIG. 3  shows a third embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]      FIG. 1  shows a refrigerant system  20  incorporating a pair of conventional tandem compressors  22  and  24  for compressing a refrigerant and passing the refrigerant downstream to a discharge manifold  30 , and then to condenser  36 . The compressors  22  and  24  may or may not have oil equalization line  12  and pressure equalization line  13  connecting the two compressors for the purpose of potentially improving the oil management in the connected compressors. Furthermore, compressors  22  and  24  can be of different sizes and may have shutoff or check valves  32  located on the compressor discharge line to enhance system performance upon shutdown of one of the compressors.  
         [0012]     A three-way valve  34  selectively communicates at least a portion of the refrigerant to a reheat coil  44 . The valve  34  can be of a fixed orifice design, a regulating device, or can be substituted by a pair of solenoid valves. The refrigerant passes to a line  39  downstream of the reheat coil  44  through the check valve  46 , and rejoins the main refrigerant circuit at point  41 . Point  41  is upstream of condenser  36 . A main expansion device  38 , and an evaporator  40  are downstream of condenser  36 . Thus, refrigerant flows from the main expansion device  38  to the evaporator  40 , and then is returned to a suction manifold  26 , communicating through lines  28  to the suction port of each compressor  22  and  24 .  
         [0013]     As is known, an indoor airflow  45  is driven over the evaporator  40 . As the air is cooled, the moisture content in the air stream is typically reduced, and, thus, the air supplied to the conditioned space has been dehumidified. If the temperature of air leaving the evaporator is lower than desired for the conditioned space, reheat coil  44  is utilized to reheat the air stream  45  to a required temperature level. As shown, air moving device  42  drives air stream  45 .  
         [0014]     A control  19  for the refrigerant system  20  can operate the system to achieve various goals. In particular, one or both of the compressors  22  and  24  can be operated by controlling motors and closing shut-off valves  32 . Thus, the total amount of refrigerant being compressed and circulated throughout the refrigerant system can be controlled to achieve different capacity levels. Moreover, the three-way valve  34  may be controlled to direct at least a portion of refrigerant into the reheat coil  44 . Under many conditions it is not necessary to circulate refrigerant through the reheat coil. However, when it is desirable to achieve dehumidification to the extent that the air stream  45  would otherwise be cooled below the desired temperature, then circulating at least some refrigerant through the reheat coil  44  will allow the temperature of the air stream  45  to be raised to a comfortable level, while the moisture content had been reduced in the evaporator  40 . Thus, system sensible and latent capacity can be controlled in both cooling and dehumidification modes of operation to a much greater extent by selectively operating tandem compressors  22  and  24 .  
         [0015]     As known, each of the tandem compressors  22  and  24  may be provided with a means of unloading, where part of the compressed refrigerant is by-passed back to the compressor suction (internal or external of compressor) and allow for additional steps in capacity control. This is particularly true when the capacities of the two compressors are selected to be different. Control  19  can close either shut-off valve  32  or leave both valves open (in conjunction to controlling the respective compressor motor shutdowns) to achieve several capacity steps. Further, at each capacity step, there is greater humidification control by selectively opening valve  34 , utilizing the reheat coil  44 .  
         [0016]     Also, more than two compressors can be configured into a tandem arrangement offering multiple steps of unloading and control over various system operation parameters.  
         [0017]      FIG. 2  shows another embodiment  50 . There are tandem economized compressors  22  and  24  delivering refrigerant to a discharge manifold  30 . Downstream of this manifold is a condenser  36 , a main expansion device  38 , and an evaporator  40 . An air moving device  42  blows air over evaporator  40 . Refrigerant is returned to a suction manifold  26 , and through lines  28  back to the individual suction ports of compressors  22  and  24 . Compressors  22  and  24  also have intermediate pressure, or economizer, ports communicating through lines  61  and economizer manifold  57  to the refrigerant system. Economizer lines  61  may incorporate shutoff valves  59  in order to switch between economizer and conventional modes of operation for each individual compressor. Bypass valves  51  allow the compressors to be unloaded, such that either or both of the compressors can be operated at a reduced capacity.  
         [0018]     An economizer loop is incorporated in the refrigerant system  50  downstream of the condenser  36 . In the economizer cycle, an economizer heat exchanger  52  receives a tapped refrigerant flow  54 , and a main refrigerant flow  55 . As can be seen, the tapped refrigerant flow in this embodiment is tapped from the main refrigerant flow  55  downstream of condenser  36 . The tapped refrigerant passes through an economizer expansion device  56 . After having passed through the economizer expansion device  56 , the tapped refrigerant is at a lower pressure and temperature, and is able to cool the main refrigerant flow  55  in the economizer heat exchanger  52 . In a preferred embodiment, the flow of the tapped refrigerant through the economizer heat exchanger  52  is preferably in the reverse direction to that illustrated (that is in the opposed direction to the flow  55 ). However, the flows are illustrated in the same direction to simplify the drawing.  
         [0019]     The tapped refrigerant is typically returned as a vapor to be injected into the compressors  22  and  24  through the economizer manifold  57  communicating with separate economizer return lines  61 , each having a shut off valve  59 .  
         [0020]     In this embodiment, the control  19  has the ability to open the valves  59  and adjust expansion device  56  to control the economizer loop. This control strategy is employed in combination with the abovementioned control methodology for operating the valves  32  to utilize either or both of the compressors. In addition, the bypass or unloader valves  51  provide further ability to reduce the refrigerant flow into the cycle. A worker of ordinary skill in the art would recognize the times when a controller would like to utilize each of these options. Valves  32 ,  51  and  59  may be either shutoff valves or regulating flow control devices, allowing for more flexibility in the refrigerant flow control.  
         [0021]     The economizer loop may or may not be engaged. To turn off the economizer loop, the economizer expansion device  56  may be closed down such that no refrigerant is tapped. Furthermore, to disengage an economizer cycle for each individual compressor, a respective valve  59  needs to be closed. Similarly, to turn off the reheat coil  60 , which is now positioned downstream of the condenser  36  and utilizes a two-phase mixture or liquid refrigerant for the reheat purpose, the three-way valve  58  may be moved to such a position that no refrigerant is tapped through the reheat coil  38 . It has to be noted, that the illustrated position of the reheat coil  60  and the economizer heat exchanger  52  is not critical to take advantage of the invention benefits, and various possible arrangements become obvious to a person ordinarily skilled in the art.  
         [0022]     Further, as is shown, a bypass line  64  with a valve  65  selectively bypasses refrigerant around the condenser  36 . The valve  65  preferably allows a metering of flow around the condenser  36 . Now, when the system is utilized under conditions such that humidity control is desirable, but no significant temperature change is necessary, a significant portion of the refrigerant may be bypassed through line  64 , through valve  65 , and around the condenser  36 . This refrigerant is not cooled in the condenser  36 , and when mixed with the refrigerant passing through the condenser has more heating capacity to be realized in the reheat coil  60 , reheating refrigerant overcooled in the evaporator  40 . On the other hand, if greater temperature reduction is desired, then more or most of the refrigerant passes through the condenser  36 , and the system operates as in a standard cooling and dehumidification cycle, utilizing liquid refrigerant in the reheat coil  60 . Thus, any of these reheat loop controls may be employed independent of each other, in combination with the economizer loop controls and tandem compressor control strategy, or none of them need be used. The present invention is mainly directed to providing the ability to use all techniques in combination with each other, while providing better control over the humidity and temperature, while enhancing system efficiency by matching latent and sensible load demands more closely and improving component reliability by reducing a number of start-stop cycles. Also, it has to be understood that the three-way valve  58  can be substituted by a pair of conventional valves. If the expansion device  56  is of such a type that it cannot be closed down completely, an additional shutoff valve may be placed on the tap line  54 .  
         [0023]     When relatively low humidity and temperature levels are desired in the air stream  45 , or the capability to provide a significant amount of sensible and latent capacity is required, both economizer expansion device  56  and the three-way valve  58  are moved to an open position to operate both the economizer heat exchanger  56  and the reheat coil  60  and both tandem compressors  22  and  24  are controlled to provide maximum refrigerant flow with valves  32  and  59  in open positions and valves  51  closed. Refrigerant passing through the main line  55  will be subcooled by the refrigerant from the tap  54 . Thus, that refrigerant will have a higher cooling capacity (both sensible and latent) when reaching the evaporator  40 . Consequently an air stream  45  leaving evaporator  40  can be cooled to a lower temperature. At this lower temperature, more moisture can be removed from the air. Then, refrigerant in the refrigerant cycle  50  passes through the reheat coil  60 , where its temperature is reduced further during the heat transfer interaction with the indoor air stream  46  leaving the evaporator  40 . As a result, the refrigerant cooling capacity is boosted even further, allowing for even more dehumidification in the evaporator  40 . This drier air then passes over the reheat coil  60 , which will have a higher temperature refrigerant, as it is positioned upstream of the main expansion device  38 . An air moving device  42 , shown schematically, drives air over the evaporator  40  and reheat coil  60 . This hotter refrigerant in the reheat coil will reheat the air stream  45  such that the desired temperature is reached. Moisture has already been removed from this air stream in the evaporator  40 . Thus, by utilizing the combination of the economizer cycle, tandem compressor configuration and the reheat coil, a refrigerant system designer is able to achieve both desired temperature and humidity levels, especially in hot and humid environments. Moreover, the higher efficiency levels are achieved due to implementation of the economizer cycle concept. Obviously, other external load demand scenarios can be considered along with corresponding control strategies for the refrigerant cycle  50 . These scenarios are well known to a person ordinarily skilled in the art.  
         [0024]     Furthermore, this invention offers additional steps of unloading. Turning a tapped refrigerant flow in the economizer heat exchanger  26  on and off, the system capacity can be correspondingly increased or decreased, depending on the external load requirements. Also, one or both compressors  22  and  24  can be operated in the conventional, economized or unloaded modes. Again, this provides several distinct capacity control steps. This will allow matching the desired temperature and humidity levels with a greater precision as well as improve system reliability through the reduction of the start-stop cycles. Also it has to be understood that various configurations of this embodiment are possible including (but not limited to) more than two compressors, and more than one economizer heat exchanger.  
         [0025]      FIG. 3  shows yet another embodiment  70 . In embodiment  70 , the reheat coil  74  communicates with the main cycle refrigerant by means of a three-way valve  72  downstream of the economizer heat exchanger  52 . Thus, there would be a warm liquid refrigerant circulated through the reheat coil. Further, the compressors  22  and  24  are provided with a bypass line through valve  51  on only one of the two compressors (here compressor  24 ). Further, the economizer fluid is returned through line  28  to only one of the two compressors (here compressor  24  as well). While this embodiment does not provide the extreme number of steps of control provided by  FIG. 2  embodiment, it does provide additional control when compared to a system that does not have tandem compressors, an economizer cycle or a reheat coil. The two tandem compressors  24  and  22  can still be operated independently, or in combination. The economizer cycle would only be utilized in combination with operation of compressor  24 . Any number of economized and conventional compressors can be employed in this embodiment and will function and communicated to the refrigerant system  70  in the manner described above.  
         [0026]     It should be understood that a refrigerant cycle designer would be able to identify many different options that would flow from the several embodiments in this invention. Additionally, an equivalent approach can be applied to each independent circuit of a multi-circuit system to obtain similar benefits.  
         [0027]     Thus, the invention presents a significantly enhanced control over temperature and humidity precisely satisfying latent and sensible capacity demands, while reaching superior efficiency and reliability levels and reducing life-cycle cost of equipment.  
         [0028]     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

Technology Classification (CPC): 5