Abstract:
A pulse width modulation control is provided for a suction valve located on a suction line. When the flow rate through a refrigerant system needs to be reduced, the suction valve is rapidly cycled from an open position to a closed position. A bypass line connecting compressor discharge to compressor suction with a bypass valve and a discharge valve positioned on the discharge side of the compressor are also provided. When the control closes the suction valve, it also closes the discharge valve to prevent the refrigerant to backflow into the bypass line, and, at the same time, the control opens the bypass valve. Opening of the bypass valve reduces discharge pressure, leading to reduction in compressor power consumption and subsequent operating efficiency gain.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a control for a refrigerant system wherein pulse width modulation technique is utilized to improve refrigerant system control and wherein a discharge bypass is operated in conjunction with the pulse width modulation to reduce compressor power consumption. 
         [0002]    Refrigerant systems are utilized in many applications to condition a climate controlled environment. In particular, air conditioners and heat pumps are employed to cool and/or heat air entering the climate controlled environment. The cooling or heating load in the environment may vary with ambient conditions, occupancy level, and changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment. 
         [0003]    Various features are known for providing adjustments in refrigerant system capacity. One approach which has been utilized in the prior art for reducing the capacity of a refrigerant system is the use of pulse width modulation technique to control a fast acting solenoid valve on a compressor suction line. By rapidly cycling this valve utilizing pulse width modulation techniques, additional and accurate capacity control is provided. 
         [0004]    The goal of the pulse width modulation control is to efficiently compress the refrigerant at reduced mass flow rates. This is done when the thermal load demand on the refrigerant system is lower than would be provided with a compressor that is fully loaded. 
         [0005]    However, this technique does not always achieve the goal of desired efficiency improvement, because even though the suction pressure is reduced substantially when the suction valve is closed (or almost closed), the discharge pressure still remains high causing a compressor power consumption to be higher than desired. Moreover, the compressed refrigerant on the discharge side can backflow into the compression chambers, further increasing compressor power consumption due to this backflow refrigerant re-compression. This problem is particularly acute in compressors that are not equipped with a dynamic discharge valve (as is often the case for compressors used in standard air conditioning applications). The absence of the dynamic discharge valve causes the compressed refrigerant at the discharge pressure to flow back into the compressor compression pockets, promoting increased power consumption. However, the problem also exists in compressors with a dynamic discharge valve, where the refrigerant still needs to be compressed to the discharge pressure. Refrigeration type compressors would normally be an example of compressors used with a dynamic discharge valve. 
       SUMMARY OF THE INVENTION 
       [0006]    In the disclosed embodiment of this invention, a compressor is associated with a refrigerant system. The refrigerant system has a valve capable of rapid cycling. The valve is installed on a suction line, and a pulse width modulation control is provided for that suction valve. The pulse width modulation control is operable to rapidly cycle the valve from an open position to a closed position to change the capacity of the refrigerant system by controlling the amount of refrigerant delivered to the compressor. 
         [0007]    A bypass line is provided to connect the compressor discharge side to the suction side; this bypass line also includes a bypass valve. When the suction valve is moved to a closed position by the pulse width modulation control, the bypass valve is opened. In this manner, the compressed refrigerant is returned to the suction line of the compressor. In a disclosed embodiment, the bypass line returns the refrigerant to a location downstream of the suction valve. Since the compressor discharge is now directly connected to the suction line, the refrigerant is not compressed to as high a pressure, and compressor power consumption is significantly reduced. 
         [0008]    Although, for illustrative purposes, this invention is described in relation to refrigerant systems incorporating scroll compressors, it is applicable to other compressor types as well. 
         [0009]    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 
         [0010]      FIG. 1  is a schematic view of a refrigerant system incorporating the present invention. 
           [0011]      FIG. 2  shows a pressure versus volume graph for the compressor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    A refrigerant system  19  is illustrated in  FIG. 1  having a scroll compressor  21  incorporating a non-orbiting scroll member  22  and an orbiting scroll member  24 . As is known, a shaft  26  is driven by an electric motor  28  to cause the orbiting scroll member  24  to orbit. An oil sump  32  and an oil passage  34  in the shaft  26  supply oil to various moving elements in the compressor  21 , as known. 
         [0013]    A condenser  36  is positioned downstream of the compressor  21 , an expansion device  38  is located downstream of the condenser  36 , and an evaporator  40  is positioned downstream of the expansion device  38 , as known. As is also known, the compressor  21  is driven by the electric motor  28  to compress a refrigerant and to drive it throughout the refrigerant system  19 . 
         [0014]    The control  30  may be a microprocessor or other type control that is capable of providing pulse width modulation control to a suction modulation valve  210  positioned on a suction line  212 . It should be understood that the control  30  includes a program that accepts inputs from various locations within the refrigerant system, and determines when the pulse width modulation of the suction modulation valve  210  needs to be initiated. Controls capable of performing this invention with such suction modulation valves are known in the art. The valve itself may be a solenoid type valve, again as known. 
         [0015]    Now, when the control  30  determines that it would be desirable to reduce capacity of the refrigerant system  19 , the suction modulation valve  210  is rapidly cycled from an open position to a closed position (with a cycle rate typically in the 3 to 36 second range) using a pulse width modulation control. For the pulse width modulation cycle, a closed position for the suction modulation valve  210  does not have to be a fully closed position and an open position for the suction modulation valve  210  does not have to be a fully open position. 
         [0016]    As is known, the compressor housing shell is sealed such that, when compressor is running, there is a suction pressure in a chamber  121 , and there is a discharge pressure in a chamber  123 , after the refrigerant has been compressed by the orbiting movements of one of the scroll members  22  and  24  in relation to the other. 
         [0017]    As shown, a discharge valve  200  is positioned in a discharge tube  202  (the valve can also be positioned in the discharge line  206 , which connects the discharge tube  202  to the condenser  36 ). The discharge valve  200  may be a solenoid type valve, or may be a mechanical check valve. In the illustrated embodiment, the discharge valve  200  is a solenoid valve, controlled by the control  30 . Notably, when the compressor does not run in the pulse width modulation mode, this valve is normally open, such that refrigerant can flow through the discharge tube  202  and to the condenser  36  relatively unimpeded. A bypass line  204  selectively bypasses the refrigerant from the discharge tube  202  (or the discharge line  206 , or the discharge pressure chamber  123 ) hack to the suction chamber  121 . A bypass valve  216  is positioned on the bypass line  204 . The bypass valve  216  typically needs to be open within the time interval of 0 to 0.2 seconds of (before or after) the closing of the pulse width modulation valve  210 . 
         [0018]    When the control moves the suction valve  210  to a closed position, the discharge valve  200  is also closed and the bypass valve  216  is opened. In this manner, the refrigerant is returned from the discharge chamber  123  to the suction chamber  121 . At the same time, the closed discharge valve  200  blocks the backflow of refrigerant from the discharge line  206  into the discharge chamber  123 . Therefore, the pressure in the discharge chamber  123  can now be maintained at the same or nearly the same low pressure as the pressure in the suction chamber  121 . This reduces power consumption of the compressor motor  28 , because the refrigerant no longer needs to be compressed to the pressure, corresponding to the high pressure in the condenser  36 . The discharge valve  200  typically needs to be open within the time interval of 0 to 0.2 seconds of (before or after) the closing of the pulse width modulation valve  210 . The discharge valve  200 , if it is a solenoid type valve, can be typically closed within the range of 0 to 0.2 seconds of the closing of the valve  210 . If the discharge valve  200  were, for example, a mechanical check valve, it would shut close automatically, as the refrigerant from the condenser  36  would begin to move into chamber  123  closing the discharge valve  200 . 
         [0019]      FIG. 2  shows a so-called PV diagram that represents compression process in the compressor  21 . In this diagram, P is changing pressure within the scroll elements and V is changing compression volume within the scroll elements for the compressor  21 . The area covered by the PV diagram is indicative of the power consumed by the compressor  21 . As shown in  FIG. 2 , the cross-hatched area (ABC) is indicative of the power consumed by the compressor  21  incorporating the invention when the pulse width modulation valve  210  is in the closed position and the inventive bypass arrangement is present. The non-cross hatched area (DEFG) is indicative of the power consumed by the compressor  21  without the inventive bypass line when the pulse width modulation valve  210  is closed. As can be appreciated, the present invention can save substantial amount of energy, as shown by comparison of the above two areas in  FIG. 2 . It should be understood that this graph is an illustration, and actual results will vary for any given compressor and operating conditions. As also shown in  FIG. 2 , the point G indicates pressure within the compressor suction cavity  121  without the inventive bypass arrangement when the suction modulation valve  210  is in the closed position. As known, this pressure needs to be maintained above a certain threshold for compressors with hermetically sealed motors (if this pressure decreases below a certain value, the motor terminal pins can be damaged by a so-called “corona discharge” effect, which occurs at near vacuum conditions in the compressor suction cavity  121 ). Normally, this pressure is kept at about 1 psia level. Without the bypass arrangement, the pressure in the discharge chamber  123  will be at the discharge pressure indicated by point F. 
         [0020]    When the bypass arrangement is employed, the pressure will be relieved to the pressure approaching the suction pressure, as indicated by the point C. Since in the inventive arrangement, the discharge pressure is reduced from F to C, the motor would consume less power, due to reduced amount of work required to compress the refrigerant. Also, it has to be noted that, for this inventive bypass arrangement, the suction pressure would increase somewhat from the pressure indicated by the point G to the pressure indicated by the point C. This occurs as some of the refrigerant trapped on the discharge side is re-expanded back into the suction chamber  121 , causing the pressure in the suction chamber  121  to rise above the pressure indicated by the point G, which was the pressure level in the prior art pulse width modulation arrangement. 
         [0021]    It should be understood that although this invention is described in relation to refrigerant systems incorporating scroll compressors, it is applicable to various compressor types, including screw compressors, reciprocating compressors, rotary compressors, etc. It is can also be applied to different refrigerant systems, including residential air conditioning applications, container and truck-trailer applications, heat pump application, supermarket applications, rooftop applications, etc. The refrigerant systems can also include additional features, such as economized circuit, employing a compressor having a vapor injection line. The compressor can also have bypass line, which bypasses refrigerant from an intermediate compression point to suction. If the intermediate to suction line bypass line is employed, then the connection between the discharge bypass, described in this application, and compressor suction can also be established via the intermediate to suction bypass line. Of course this invention would apply to various types of refrigerants, such, for example, R410A, R134a, R22, R407C, R744, etc. 
         [0022]    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.