Abstract:
A refrigerant system is provided with tandem compressors. As is known, tandem compressors operate in parallel to provide a refrigerant system designer with the ability to achieve a stepped capacity control of the refrigerant system. At least one of the tandem compressors is provided with a variable speed drive. Further, at least one of the tandem compressors may be provided with the economizer and/or unloader functions. System configurations with multiple compression stages and multiple injection ports are disclosed. In this manner, the stepless capacity control can be achieved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a variable speed motor for driving a compressor that is incorporated into a refrigerant system with tandem compressors.  
         [0002]     Refrigerant systems are utilized in many air conditioning and heat pump applications for cooling and/or heating the air entering an environment. The cooling or heating load on the environment may vary with ambient conditions, and as the temperature and/or humidity levels demanded by an occupant of the building change.  
         [0003]     In some refrigerant systems, a single compressor is utilized to compress the refrigerant and move the refrigerant through the cycle connecting indoor and outdoor heat exchangers in a closed loop. However, under many circumstances, it would be desirable to have the ability to vary the capacity, or amount of cooling or heating provided by the refrigerant system. Thus, known refrigerant systems may be provided with tandem compressors. Tandem compressors are essentially at least two compressors operating in parallel, where the compressors are interconnected with each other via common suction and/or discharge manifolds. For instance, a control for the two-compressor system may actuate both of the compressors or either one of the two compressors. The two compressors may have different sizes to provide distinct stages of capacity during part-load operation. Rather than having a single level of capacity, a refrigerant system provided with tandem compressors would have several discrete levels of capacity.  
         [0004]     In the prior art, controls can be programmed to optionally actuate the tandem compressors. However, the capacity control provided by the tandem compressors is increased or decreased in large discrete steps. It would be desirable to provide the ability to improve system control capability to continuously vary capacity between these discrete steps to precisely match external load demands at a wide spectrum of environmental conditions.  
         [0005]     Variable speed drives are known for driving compressors at a variable speed in a refrigerant system. By driving the compressor at a higher or lower speed, the amount of refrigerant that is compressed per unit of time changes, and thus the system capacity can be adjusted.  
         [0006]     Variable speed drives have not been utilized in refrigerant systems incorporating tandem compressors, where a selected number of the tandem compressors is driven by a variable speed drive, for the purpose of varying the system capacity to control temperature and humidity levels within the conditioned space.  
       SUMMARY OF THE INVENTION  
       [0007]     In the disclosed embodiment of this invention, a variable speed drive is provided into at least one compressor in a refrigerant system having tandem compressors. By selectively controlling this one compressor, capacity adjustment between the discrete steps provided by tandem compressor operation can be achieved.  
         [0008]     A control identifies a desired cooling capacity, and then achieves this desired capacity by first actuating the tandem compressors to accurately approximate the necessary capacity in the most efficient and reliable manner. Then, the speed of the at least one compressor provided with variable speed is changed incrementally. The capacity is then monitored. When a desired level is finally achieved, the at least one compressor is operated at that new speed. If the capacity still needs to be adjusted, then the speed is again adjusted incrementally, and the resulting condition is again monitored.  
         [0009]     In disclosed embodiments, one of the tandem compressors may be provided with the variable speed drive while the other is not. In other embodiments, plural compressors are provided with a variable speed drives.  
         [0010]     Embodiments are disclosed which incorporate economizer cycles and unloader cycles into the schematic along with the variable speed drive.  
         [0011]     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  
       [0012]      FIG. 1  shows a first embodiment refrigerant system.  
         [0013]      FIG. 1A  shows other possible circuit schematics.  
         [0014]      FIG. 1B  shows other possible circuit schematics.  
         [0015]      FIG. 1C  shows other possible circuit schematics.  
         [0016]      FIG. 2  shows a second embodiment refrigerant system.  
         [0017]      FIG. 3  shows the capacity control provided by the prior art.  
         [0018]      FIG. 4  shows the capacity control provided by the present invention.  
         [0019]      FIG. 5  is a flowchart of a control algorithm according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]     A refrigerant system  20  is illustrated in  FIG. 1 . A compressor  22  is provided with a variable speed drive  24 . A second compressor  26  is not provided with a variable speed drive, and operates in tandem with the compressor  22 . As shown, a shut-off valve  28  may allow the compressor  26  to be isolated from the discharge manifold, should a control for the system determine that only the compressor  22  is necessary for achieving a given capacity. As is known, the compressors  22  and  26  deliver refrigerant to a common discharge line  30  leading to a condenser  32 . While the system  20  is illustrated as an air conditioning system, it should be understood that the present invention would also apply to heat pumps and chillers.  
         [0021]     As is known, the two compressors  22  and  26  may preferably be provided with distinct capacities such that varying total levels of capacity can be achieved by operating one or the other, or both of the compressors  22  and  26 . In this case, it is at the system designer&#39;s discretion to select whether a larger or smaller compressor is provided with a variable speed drive. The decision will depend on many factors including (but not limited to) application requirements, cost, system operation efficiency, etc. An expansion device  34  is positioned downstream of the condenser  32 , and an evaporator  36  is located downstream of the expansion device  34 . A common suction line  38  leads to distinct suction lines  39  for returning refrigerant to the compressors  22  and  26 .  
         [0022]     As also shown, an economizer circuit can be incorporated into the  FIG. 1  schematic. An economizer heat exchanger  40  receives a tapped refrigerant from a line  42  having passed through an economizer expansion device  44 . As is known, by passing the tapped refrigerant through the expansion device  44 , its pressure and temperature are lowered. Thus, in the economizer heat exchanger  40 , this tapped refrigerant subcools a refrigerant in a main liquid line  45 , which also passes through the economizer heat exchanger  40 . The economizer function is known in the prior art, and allows increased capacity and/or efficiency of the refrigerant system  20 .  
         [0023]     As shown, the tapped refrigerant is returned through a line  46  to an intermediate compression point  48  in at least one of the compressors, here illustrated as compressor  22 . While refrigerant in the tap line  42  is shown flowing through the economizer heat exchanger  40  in the same direction as refrigerant in the main liquid line  45 , it should be understood that in a preferred embodiment, the two flows would actually be in counter-flow arrangement.  
         [0024]     A bypass line  50  is also incorporated, and allows a portion of refrigerant from the intermediate compression point  48  in the compressor  22  to be returned to the suction line  39 . When it is desired to have unloaded operation, a valve  52  is opened while the expansion device  44  is preferably (but not necessarily) closed. In this way, refrigerant that has been partially compressed by the compressor  22  will be returned to the suction line  39 , thus providing the unloading function.  
         [0025]     It has to be understood that the economized compressor  22  may have more than one injection port  48  and more than one associated economizer heat exchanger  40 . Also, as known, the economizer heat exchanger arrangement can be substituted by a flash tank. Further, multi-stage compression system may be employed instead of a single economized compressor. In such multi-stage compressor system, one or several of the stages may be provided with a variable speed drive.  
         [0026]     As shown, electric motors  200  are associated with fans for blowing the air over the condenser  32  and evaporator  36 . One or other of these electric motors  200  may be provided with a variable speed drive  202 . A worker of ordinary skill in the art would recognize when the variable speed control of the fan, or other components such as a secondary loop pump, motors associated with the refrigerant system might be desirable.  
         [0027]      FIG. 1A  shows another circuit schematic  100  wherein one of the two compressors, e.g. compressor  22 , is replaced by two compressor stages  104  and  106 . While both of the compressor stages  104  and  106  are shown connected to the variable speed drive  102 , only one stage or the other could be connected instead. As shown, the return line  108  from the economizer heat exchanger extends simply between the two stages, rather than into compression chambers in either of the stages.  
         [0028]      FIG. 1B  shows another embodiment  110  wherein there are three compressor stages  112 ,  114  and  116 . The variable speed drive  118  controls both stages  114  and  116 . Each of the stages is shown associated with an unloader valve  120 . Two separate economizer heat exchangers  122  selectively deliver refrigerant through lines  124  back to points between the compressor stages. It is well known to a person ordinarily skilled in the art that a number of compression stages (as well as a number and particular position of compression stages operating at variable speeds), a number of unloader valves and a number of economizer heat exchangers are at a designer freedom and depend on a particular application.  
         [0029]      FIG. 1C  shows another embodiment  130  wherein a first stage of the compressor is provided by a pair of tandem compressors  134  and  136  feeding a second compressor stage  138 . As shown, an intermediate pressure refrigerant return line  140  extends between the stages. A variable speed drive  132  is associated with the compressor  134  only. Of course, many other schematics would come within the scope of this invention, including (but not limited to) a varying number of tandem and variable speed compressors.  
         [0030]      FIG. 2  shows a distinct embodiment  60 , wherein the two tandem compressors are replaced by a bank of four compressors. As shown, compressors  64  are each provided with a variable speed drive  62 . Shut-off valves  66  are placed on the discharge lines for three compressors  64 ,  68  and  70  to isolate those compressors when they are stopped by the system control. A common discharge manifold  72  leads to a condenser  74 , an expansion device  76 , and an evaporator  78 . A control for this refrigerant system  60  is configured to operate the two compressors  64  at variable speeds, and the two compressors  68  and  70  at fixed speed to achieve desired capacity.  
         [0031]     A control for either refrigerant system  20  and  60  is able to identify a desired cooling capacity, and operate the tandem compressors and/or the economizer and unloader functions as necessary. Thus, as shown in  FIG. 3 , a prior art system that incorporated the  FIG. 1  schematic without the variable speed drive could provide at least three stages A, B, and A+B of capacity control. In fact, the schematic shown in  FIG. 1  would have even more stages, in that the operation of the unloader valve and economizer function would provide additional capacity steps. However, for purposes of understanding the remainder of this invention, the simplified schematic of  FIG. 3  will suffice. As can be seen, there are several values between values A, B, and A+B that cannot be provided by this prior art system. This is, of course, an oversimplification of the system, yet this does provide a good basis for understanding the present invention. The  FIG. 2  embodiment would have many other levels of capacity control available as well.  
         [0032]      FIGS. 3 and 4  are an oversimplification of the  FIG. 1  embodiment and the capacity levels it can provide. As mentioned, by operating the unloader valve and economizer function, additional capacity steps can be achieved. However, a control for this system would operate one of the compressors (e.g., compressor  26 ) that may be smaller than the compressor  22  to provide the level A. The other compressor  22  can be operated to provide the level B, with the compressor  26  stopped. By operating both compressors  22  and  26 , the level A+B can be achieved. Within each of these levels, by increasing the speed of the motor for the compressor  22 , a ramp R above the step A, B, or A+B can be achieved. On the other hand, by slowing the speed, the opposite can occur to move a ramp downwardly from these values. A decision of switching between the compressor speed adjustment and moving to a different mode of operation is usually based on the amount of required cooling, efficiency and reliability considerations. For instance, it may be unsafe to operate the compressor below certain speed due to inadequate lubrication provided to compressor elements. On the other hand, running compressor at a relatively high speed may be inefficient in comparison to switching to an economizer mode of operation.  
         [0033]      FIG. 5  shows how the ramps would typically be achieved with a standard variable speed motor control as is known in the prior art. Ramps R as shown in  FIG. 4  are an oversimplification. In fact, the control typically moves in incremental steps, and then monitors the operation of the refrigerant cycle after that incremental change. Thus, there would be a plurality of step changes along each ramp R, rather than the infinite number of changes as is illustrated in  FIG. 4 . However,  FIG. 4  does provide a good illustration of the power of the present invention to provide varying capacity.  
         [0034]     It has to be noted that variable speed tandem compressors can be utilized in conjunction with other system components such as fans or pumps also operated at variable speeds.  
         [0035]     Although preferred embodiments 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.