Patent Publication Number: US-6705093-B1

Title: Humidity control method and scheme for vapor compression system with multiple circuits

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to vapor compression systems and, more particularly, to a vapor compression system with multiple circuits having humidity control. 
     Vapor compression systems are widely used in air-conditioning, chilling and refrigeration applications, and humidity control in such units is frequently an important concern. 
     In some instances, multiple circuit vapor compression systems are provided and, in such systems, the equipment utilized to provide humidity control must be duplicated as many times as there are circuits in the system. This leads to increased complexity and cost of the system. 
     It is therefore the primary object of the present invention to provide simplified and more efficient humidity control in multiple circuit vapor compression systems. 
     Other objects and advantages of the present invention will appear hereinbelow. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the foregoing objects and advantages have been readily attained. 
     According to the invention, a vapor compression system is provided which comprises a first circuit having a first compressor and a first condenser; a second circuit having a second compressor and a second condenser; an evaporator communicated with said first circuit and said second circuit for cooling a stream of air to provide a cooled air stream; and an air reheat circuit communicated with said cooled air stream and one circuit of said first circuit and said second circuit for exposing said cooled air stream to heat transfer interaction with refrigerant from said one circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of preferred embodiments of the present invention follows, with reference to the attached drawings, wherein: 
     FIG. 1 schematically illustrates one embodiment of a vapor compression system in accordance with the present invention; 
     FIG. 2 schematically illustrates an alternative embodiment of a vapor compression system in accordance with the present invention; and 
     FIG. 3 schematically illustrates a further alternative embodiment of a vapor compression system in accordance with the present invention incorporating three compressor circuits and two air reheat circuits. 
    
    
     DETAILED DESCRIPTION 
     The invention relates to vapor compression systems having multiple circuits wherein humidity control is provided in the form of an air reheat circuit which allows for re-heating of over-cooled air from the system evaporator, thus allowing air to be cooled beyond a desired temperature for enhanced moisture removal, and then re-heated to a desired temperature. In accordance with the present invention, and advantageously, the air reheat circuit is communicated with only a single circuit of the multiple circuit system, and serves to reheat the entire stream of air coming from the evaporator so as to provide for humidity control in an efficient and cost-effective manner, and further in a manner which allows for more continuous operation of the system, thereby avoiding frequent starts and stops which can lead to premature system failure. Moreover, overall system part- and full-load performance is improved, and system operating range is extended. 
     Turning now to FIG. 1, a system  10  in accordance with the present invention is illustrated. System  10  includes a first circuit  12  including a first compressor  14 , a first condenser  16  and first expansion device  18 . These components are connected by refrigerant lines such that refrigerant passes from compressor  14  to condenser  16 , from condenser  16  to expansion device  18 , and from expansion device  18  to an evaporator  20  which is preferably adapted to serve all circuits of the system. 
     It should be understood that evaporator  20  does not need to be a single unit as shown, and can instead be several units. 
     From evaporator  20 , refrigerant flows back to compressor  14 , and refrigerant is processed along this circuit as is well known to a person of ordinary skill in the art so as to provide the desired cooling of air through evaporator  20  for generating a cooled air stream into a conditioned space. 
     In further accordance with the present invention, a second circuit  22  is also provided and includes a second compressor  24 , a second condenser  26 , a second expansion device  28  and refrigerant lines communicating same so that refrigerant flows from compressor  24  to condenser  26 , from condenser  26  to expansion device  28 , from expansion device  28  to evaporator  20 , and from evaporator  20  back to second compressor  24  as desired. 
     In still further accordance with the present invention, system  10  is provided with an air reheat circuit generally indicated at  30 , which in this embodiment includes a refrigerant line  32  communicated with second circuit  22  and passing through an air reheat heat exchanger  34 , and from heat exchanger  34  back to refrigerant line  36  for feed through expansion device  28  to evaporator  20 . Air reheat circuit  30  advantageously serves to convey warm refrigerant liquid from condenser  26  to heat exchanger  34  for reheating of air as desired. 
     It should be appreciated that although the drawings show air reheat heat exchanger  34  extending across the entire stream of evaporator air, it may be desirable, and it is considered well within the scope of the present invention, to provide for reheat of only a portion of the evaporator air stream if desired, and such a configuration is illustrated in FIG. 3 discussed below. 
     In accordance with the present invention, air passing through evaporator  20  is cooled beyond a desired temperature, advantageously to a temperature selected to remove moisture or humidity from the air stream at a desired rate, so as to provide an over-cooled air stream which is then exposed to air reheat heat exchanger  34  as desired. Refrigerant flowing through air reheat circuit  30  advantageously re-heats the cooled air back to the desired temperature, thereby maintaining the desired temperature of air while nevertheless allowing for humidity control. 
     It is particularly advantageous in accordance with the present invention that a single air reheat circuit  30  is provided for treating the entire flow of air through evaporator  20 , and this circuit is communicated with refrigerant only from second circuit  22 . Of course, in embodiments having a plurality of evaporators, air reheat circuit  30  can be used to treat flow of air through all of them. 
     In accordance with the present invention, air reheat circuit  30  is preferably communicated with the circuit of the overall system which is last to unload in a partial load operation. In this manner, the humidity control function is continuously provided regardless of the level of load on the system. Further, the system typically will have different discharge pressures for each circuit due to various factors including non-uniform air flow, uneven heat exchanger surface split and the like. In accordance with the present invention, air reheat circuit  30  is advantageously communicated with the circuit having the highest discharge pressure in a conventional cooling mode of operation. 
     By communicating single air reheat circuit  30  with the circuit which has the highest discharge pressure, the discharge pressure in this circuit is reduced due to extra cooling obtained by heat transfer interaction in heat exchanger  34 , thereby increasing high ambient operation limit and providing more efficient arrangement and reduction in a number of start-stop cycles, and benefiting overall system full-load and part-load efficiency as well. 
     Turning now to FIG. 2, a further embodiment of the present invention is illustrated. FIG. 2 shows a system  10 ′ in accordance with the present invention having a first circuit  12  including first compressor  14 , first condenser  16  and first expansion device  18 . These components are connected by refrigerant lines which flow from compressor  14  to condenser  16 , from condenser  16  to expansion device  18 , from expansion device  18  to evaporator  20  which serves both circuits of system  10 ′ as described above, and from evaporator  20  back to compressor  14 . 
     System  10 ′ also includes a second circuit  22  including second compressor  24 , second condenser  26  and second expansion device  28  which are communicated by refrigerant lines such that refrigerant flows from compressor  24  to condenser  26 , from condenser  26  to expansion device  28 , from expansion device  28  to evaporator  20 , and from evaporator  20  back to compressor  24 . 
     As in the embodiment of FIG. 1, system  10 ′ in accordance with the present invention also has an air reheat circuit  30 . In this embodiment, however, air reheat circuit  30  flows from a discharge of compressor  24  through a heat exchanger  34 , such as an air reheat coil, and back to a line  40  for feeding evaporator  20  through expansion device  28  as desired. In similar fashion to the embodiment of FIG. 1, refrigerant in air reheat circuit  30 , in this case, in a gaseous state, is exposed to cold air from evaporator  20  and advantageously serves to warm this air or reheat this air to a desired temperature. 
     It should also be noted that in this embodiment, evaporator  20  is operated on one side in communication with first circuit  12 , and operated on the other side in communication with second circuit  22 . This is called a face-split configuration. In the embodiment of FIG. 1, refrigerant lines from first and second circuits  12 ,  22  cross so that both extend along the entire length of evaporator  20 . This is called an intertwined configuration. Evaporator  20  can also be a row-split configuration (not shown) which is similar to the intertwined configuration in that the refrigerant lines extend the entire length of the evaporator, but are not crossed as shown in the FIG.  1 . Of course, other types of evaporators could be used as well, well within the scope of the present invention. 
     It should be appreciated that although FIGS. 1 and 2 show two-circuit embodiments of the present invention, the teachings of the present invention can readily be incorporated into systems having three or more circuits, wherein some number less than the total number of circuits are communicated with air-reheat circuits as described in connection with the embodiments of FIGS. 1 and 2. Such an embodiment is illustrated in FIG.  3 . 
     FIG. 3 shows an embodiment of a system  10 ″ including three compressor circuits and two air reheat circuits. FIG. 3 shows first circuit  12  including compressor  14 , condenser  16 , expansion device  18  and evaporator  20 . Second circuit  22  includes compressor  24 , condenser  26 , expansion device  28  and evaporator  20 , while third circuit  44  includes compressor  46 , condenser  48 , expansion device  50  and evaporator  20 . In this embodiment, two air reheat circuits are incorporated, one communicated with circuit  12  and another communicated with circuit  22 . As shown in FIG. 3, the air reheat circuit communicated with circuit  12  is defined by a refrigerant line  52  extending from 3-way valve  42  and passing through a portion of air reheat heat exchanger  34  as shown. Discharge from this portion of air reheat heat exchanger  34  then passes through discharge line  54  and to main refrigerant line  56  for feeding expansion device  18 . 
     The air reheat circuit communicated with circuit  22  includes a refrigerant line  56  extending from 3-way valve  42  of circuit  22 , and passing through another portion of air reheat heat exchanger  34 , with a discharge line  58  from air reheat heat exchanger rejoining refrigerant line  60  for feed to expansion device  28  and evaporator  20 . 
     It should be noted that the embodiment of FIG. 3 shows the configuration of the present invention wherein the air reheat circuits draw refrigerant downstream of the condenser. This configuration of more than two refrigerant circuits and more than one air reheat circuits could of course be incorporated into configurations wherein refrigerant is drawn from compressor discharge as well. 
     It should also be noted that in this embodiment, air reheat heat exchanger  34  is positioned so as to reheat only a portion of air passing through evaporator  20 . 
     A configuration as illustrated in FIG. 3, with more than two refrigerant circuits and more than one air reheat circuit, advantageously provides for further flexibility in humidity control, additional configurations for unloading, and increased system redundancy, all as desired in accordance with the present invention. 
     FIGS. 1 and 2 show air reheat circuit  30  drawing refrigerant from the main refrigerant line through a 3-way valve  42  which can advantageously be a 3-way shutoff device for use in switching 100% of the refrigerant flow between conventional cooling mode and humidity control mode of operation. Alternatively, 3-way valve  42  can be provided as a regulating device for gradually controlling refrigerant flow between conventional cooling and dehumidification modes of operation. 
     Furthermore, as occupied space load changes over time, full cooling capacity is not required through the complete equipment life cycle. Under such circumstances, some circuits of conventional systems are shut down, thereby increasing the number of start/stop compressor cycles, and thereby potentially reducing compressor reliability. In accordance with the present invention, and particularly at low load operation, one or more circuits of the plurality of circuits can be completely dedicated to the air reheat humidity control function and thereby substantially improve compressor reliability while having nominal affect on system performance. 
     Finally, it should be readily appreciated that the system in accordance with the present invention advantageously provides vapor compression systems having multiple circuits wherein humidity control is provided in an inexpensive and efficient manner. 
     It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.