Abstract:
A pulse width modulation control is provided for a suction modulation valve in a refrigerant system. An intentional small “leakage” path is maintained through the suction modulation valve to ensure that the pressure inside the compressor shell does not decrease below a safe reliability threshold but, at the same time, does not exceed a certain value, which would cause the refrigerant system to operate inefficiently, when the pulse width modulation control has moved the suction modulation valve to a closed position. The size of this minimum “leakage” path is continuously adjusted to ensure that the optimum pressure inside the compressor shell is maintained regardless of the evaporator pressure and other operating conditions.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a refrigerant system, in which a suction modulation valve (or other type of a valve which has a small controlled opening in the closed position) is provided with pulse width modulation control to adjust refrigerant system capacity. A minimum opening size of the suction modulation valve is maintained to ensure that suction pressure inside a shell of the compressor located downstream of the suction modulation valve does not decrease below a specified value. However, this minimum opening size is adjusted in response to system operating conditions to ensure that the suction pressure within the compressor is close to the allowable minimum, and is not undesirably higher. 
     Refrigerant systems are known, and are utilized to condition a secondary fluid. As an example, an air conditioning system cools and dehumidifies air being delivered into a climate controlled environment. Refrigerant systems generally include a compressor compressing refrigerant and delivering that refrigerant through a discharge line to a first heat exchanger. From the first heat exchanger, refrigerant passes through an expansion device and then through a second heat exchanger. The refrigerant is then returned to the compressor. 
     Under various conditions, a refrigerant system may provide excess of capacity to cool or heat a secondary fluid supplied to a climate controlled environment. A number of methods are known for reducing the capacity of the refrigerant system. 
     One known method of reducing capacity is to provide a pulse width modulation control for a suction valve located upstream of the compressor to control the amount of refrigerant moving from the second heat exchanger to the compressor. In pulse width modulation control for a suction valve, the valve is rapidly cycled (opened and closed) to limit the amount of refrigerant flowing to the compressor. This in turn limits the refrigerant amount compressed in the compressor and refrigerant flow circulating throughout the refrigerant system, resulting in a capacity reduction for the refrigerant system, and providing more efficient operation. 
     One challenge with regard to such operation is that the pressure within the compressor shell must not be reduced below a specified limit defined by compressor reliability considerations. As a rough guideline, it is desirable to maintain a pressure within the compressor shell of at least 1 psia. However, when the suction modulation valve is completely closed during pulse width modulation control cycle, sometimes, the pressure within the compressor shell can decrease below this specified minimum pressure. Under such circumstances, sparking can occur at the terminals for the compressor motor, which can lead to terminal damage. This phenomenon is known as a “corona discharge” effect, and is undesirable. 
     Thus, it is known in the prior art to provide a minimum “leakage” opening for the suction valve, while it would be otherwise closed during pulse width modulation cycle, to prevent compressor suction from entering a deep vacuum region. Also, in another approach, a branch bypass line, containing a small internal diameter capillary tube or a small orifice, around the pulse width modulation valve has been proposed in the past to prevent compressor suction from going into deep vacuum by providing an alternate small “leakage” path for refrigerant flowing into the compressor. While the prior art does provide good control of capacity, the “leakage” opening is typically sized to ensure that the suction pressure in the compression shell exceeds the specified minimum pressure at all operating conditions. 
     However, the downstream pressure inside the compressor shell, when the suction valve is in the closed position, changes substantially for a constant size opening, depending on the pressure upstream of the opening. The evaporator pressure can vary by at least an order of magnitude, depending on the operating conditions of the refrigerant system. Therefore, under high pressure operating conditions at the evaporator, in the prior art, the suction pressure inside the compressor would also be much higher then what can be considered desirable for the minimum pressure in order to avoid the “corona discharge” effect. Having the suction pressure well above this threshold is undesirable, since it decreases the efficiency of the refrigerant system operating in a pulse width modulated mode. Thus, the prior art could not effectively control the suction pressure inside the compressor to be just above the acceptable threshold for all operating conditions, while at the same time avoiding the “corona discharge”. 
     SUMMARY OF THE INVENTION 
     In a disclosed embodiment of this invention, a control for a suction modulation valve operates the suction modulation valve using pulse width modulation control to reduce refrigerant system capacity. When the valve is in the closed position, the control varies the size of the minimum or “leakage” opening in the valve, depending on the refrigerant system operating conditions. In a disclosed embodiment, the controlling refrigerant system operating condition would be a pressure upstream of the suction modulation valve. This pressure is typically associated with, and closely approximated by, the pressure inside the evaporator. The evaporator pressure can be measured by one of the sensors, and the registered value is related to a desired minimum opening of the suction modulation valve to achieve a minimum desired pressure within the compressor shell. As known, the smaller the opening of the valve, the larger the pressure drop through the valve, therefore, for the same upstream evaporator pressure, the downstream compressor suction pressure can be controlled by varying the size of this opening. In this manner, the prior art problem of having suction pressure far above the minimum threshold pressure within the compressor shell, under high evaporator pressure conditions, during periods of time when the suction modulation valve is in the closed position, is eliminated. 
     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 
         FIG. 1  is a schematic view of a refrigerant system incorporating the present invention. 
         FIG. 2  shows the operation of a pulse width modulation control in the prior art. 
         FIG. 3A  and  FIG. 3B  show a problem with the prior art systems. 
         FIG. 4  is a chart explaining the feature of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A refrigerant system  20  is illustrated in  FIG. 1 . The refrigerant system  20  incorporates a compressor  22  compressing refrigerant and delivering it downstream to a condenser  24 . Refrigerant from the condenser  24  passes through an expansion device  26 , and then to an evaporator  28 . Refrigerant from the evaporator  28  passes through a suction modulation valve  30  and back to the compressor  22 . As is known, a control  34  for the suction modulation valve  30  may provide a pulse width modulation control to rapidly change the size of the opening through the valve  30  between open and closed positions, in order to limit the amount of refrigerant passing from the evaporator  28  to the compressor  22 . In this manner, a reduced capacity during part-load operation for the refrigerant system  20  can be achieved. 
     As shown in  FIG. 2 , the refrigerant system capacity is cycled between a maximum (fully open suction modulation valve) and minimum value (suction modulation valve closed with a minimum opening) over time, such that the average capacity is less than the full-load capacity without the pulse width modulation control. 
       FIG. 3A  and  FIG. 3B  explain shortcomings in the prior art. As mentioned above, some “leakage” path is typically maintained across the suction modulation valve to ensure that a relatively small amount of refrigerant does reach the compressor  22 , and such that a minimum suction pressure is maintained within a compressor shell  52 . As explained above, a motor  50  for a compressor pump unit  51  is received within the compressor shell  52 . If the pressure within the compressor shell  52  becomes unduly low, then a “corona discharge” effect can occur, which is undesirable. For this reason, a refrigerant “leakage” path is typically provided to prevent the compressor from entering into a deep vacuum region. However, the size of this minimum “leakage” path has typically been designed to ensure that the pressure will never drop below the specified minimum pressure (e.g., 1 psia) for all operating conditions. For example, if the minimum expected upstream pressure, P UPSTREAM , is equal to 30 psia, then the size of the minimum opening is designed to be such that the downstream pressure, P DOWNSTREAM , at the suction modulation valve closed position, is at 1 psia, as shown in  FIG. 3B . However, at 100 psia P UPSTREAM  pressure value, for the same amount of opening for the suction modulation valve  30 , the P DOWNSTREAM  is about 6 psia, as shown in  FIG. 3A , even though, for the most efficient operation, it would have been desirable to also have 1 psia pressure downstream of the suction modulation valve. 
       FIG. 4  shows a chart of pressure downstream (P DOWNSTREAM ) of the suction modulation valve versus pressure upstream (P UPSTREAM ) of the suction modulation valve for three different minimum opening sizes through the pulse width modulation valve (e.g., opening A 1 , opening A 2 , and opening A 3 ) when the valve is in the closed position. The larger the opening, the larger is the P DOWNSTREAM  pressure for the same P UPSTREAM  pressure. As indicated in  FIG. 4 , A 1  is the largest minimum opening size, A 3  is the smallest minimum opening size, and A 2  minimum opening size falls between A 1  and A 3  opening sizes. As can be seen in  FIG. 4 , when the valve has the largest minimum opening size A 1 , the downstream pressure, P DOWNSTREAM , is equal to 1 psia, when the upstream pressure, P UPSTREAM , is equal to 30 psia. Further, for the same opening A 1 , P DOWNSTREAM  is equal to 6 psia, when P UPSTREAM  is equal to 100 psia. However, what is desirable is to have 1 psia downstream pressure, P DOWNSTREAM , regardless of the upstream pressure P UPSTREAM . This P DOWNSTREAM  pressure of 1 psia can be achieved by having the adjustable minimum suction modulation valve opening, namely the minimum suction modulation valve opening needs to be at A 1 , when P UPSTREAM  pressure is equal to 30 psia, and the minimum suction modulation valve opening needs to be at A 3 , when P UPSTREAM  pressure is equal to 100 psia. 
     As can be appreciated from  FIG. 1 , a pressure sensor  32  can be positioned upstream of the suction modulation valve  30  to measure the upstream pressure, P UPSTREAM . Another sensor  44 , can be positioned downstream of the suction modulation valve  30  to measure the pressure downstream of the suction modulation valve  30 , P DOWNSTREAM  (this downstream pressure corresponds to and typically closely approximates the suction pressure inside the compressor shell). From the graph in  FIG. 4 , a desired area “A” of the minimum suction modulation valve opening, which provides a desired 1 psia minimum downstream pressure, P DOWNSTREAM , while the suction modulation valve is in the closed position, can be selected. It has to be noted that exemplary  FIG. 4  only shows three curves for different area “A” openings, and a more precise graph is to be developed with a larger number of more closely spaced lines corresponding to areas “A”, such that the desired area “A” can be accurately selected by interpolating between the lines corresponding to areas shown on this graph. The control  34  thus not only drives the suction modulation valve  30  to have a pulse width modulation movement between opened and closed positions, but also adjusts the minimum opening for the suction modulation valve  30  depending on operating conditions (and the pressure upstream P UPSTREAM  of the suction modulation valve  30 , in particular) to maintain 1 psia P DOWNSTREAM  pressure regardless of the upstream pressure P UPSTREAM . Thus, the pressure within the compressor shell  52  can always to be maintained close to the minimum pressure (e.g., 1 psia), rather than being higher then desired, causing irreversible efficiency losses in operation of the refrigerant system  20 . 
     Instead of developing a graph as shown in  FIG. 4 , the refrigerant system  20  can have a feedback control, where the amount of minimum opening for the pulse modulation valve  30  can be adjusted based on pressure detected by a sensor  44 , that is measuring the downstream pressure P DOWNSTREAM . If the sensor  44  measures the value of P DOWNSTREAM  to be substantially higher than 1 psia, when the pulse width modulation valve  30  is in the closed position, then the minimum opening size for the pulse width modulation valve  30  is reduced. In case the downstream pressure, P DOWNSTREAM , is trending to drop below 1 psia, then the minimum opening size for the suction modulation valve  30  is increased. The control  34  can also operate in a learning mode, or in a mode when it learns what amount of opening is needed to maintain the downstream pressure P DOWNSTREAM  nearing the vicinity of 1 psia, with respect to the upstream pressure P UPSTREAM . 
     The graph presented in  FIG. 4  is exemplary and shown for illustration purpose only, as the exact shape of the curves would depend on the particular compressor size and type, refrigerant type, etc. In addition to relying on the measurement of upstream pressure, P UPSTREAM , other parameters can be measured to fine tune the establishment of the required minimum opening area of the pulse width modulation valve  30  in the closed position (such as temperature upstream and downstream of the valve, etc.). While a scroll compressor is used to illustrate this invention, other compressor types would fall within the scope of the invention, including, for example, rotary, screw, and reciprocating compressors. This invention can be applied to various types of systems and can include refrigeration container and truck-trailer systems, supermarket installations, residential air conditioning and heat pump systems, and rooftop units. Lastly, as mentioned above, other valve types capable to adjust minimum opening size would be within the scope and can equally benefit from the invention. 
     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.