Abstract:
A scroll compressor system having a variable speed drive is utilized. By providing the economizer and/or bypass functions along with the variable speed drive, precise capacity adjustment between the discrete steps is achieved to exactly match load demands at a wide spectrum of operating conditions.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a variable speed scroll compressor that is operable in a refrigerant system with an economizer function and other means of capacity modulation. 
         [0002]    Refrigerant systems are utilized in many applications to condition an environment. In particular, air conditioners and heat pumps are employed to cool and/or heat a secondary fluid such as air entering an environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the building. 
         [0003]    Thus, refrigerant systems can be provided with sophisticated controls, and a number of optional components and features to adjust cooling and/or heating capacity. Known options include the ability to bypass refrigerant which has been at least partially compressed by a compressor back to a suction line. This function is also known as an unloader function. This additional step of operation is taken to reduce system cooling capacity. 
         [0004]    Other options include a so-called economizer cycle. In an economizer cycle, a refrigerant heading to an evaporator is subcooled in an economizer heat exchanger. The refrigerant is subcooled by a tapped refrigerant that is expanded and then passed through the economizer heat exchanger to subcool a main refrigerant. This tapped refrigerant is then returned to an intermediate point in the compression cycle. Thus, the economizer cycle provides a step in operation to vary system capacity by switching between economized and other modes or steps of operation. 
         [0005]    In the prior art, controls can be programmed to optionally actuate any one of these various functions. However, the capacity provided by these functions is increased or decreased in steps. It would be desirable to provide the ability to vary the capacity while the system is operating during any of the above described steps (modes) of operation in a continuous fashion in order to exactly match external load demands. 
         [0006]    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 and circulated throughout the system changes, and thus the system capacity can be changed accordingly. 
         [0007]    One increasingly popular type of compressors is a scroll compressor. In a scroll compressor, a pair of scroll members orbits relative to each other to compress an entrapped refrigerant. One design configuration of a scroll compressor utilizes both economizer and unloader functions. Further, this scroll compressor may employ a single port to provide both functions alternatively or simultaneously. This scroll compressor is disclosed in U.S. Pat. No. 5,996,364. However, this type of scroll compressor has not been utilized in combination with a variable speed drive for its motor. 
       SUMMARY OF THE INVENTION 
       [0008]    In a disclosed embodiment of this invention, a scroll compressor is provided in a refrigerant system with an economizer circuit. The scroll compressor has a motor that is driven by a variable speed drive. By selectively utilizing the economizer circuit, and/or the optional unloader function the controller can increase or decrease the capacity of the refrigerant system. Further, by varying the speed of the motor, capacities in each mode of operation can be additionally adjusted to provide essentially continuous stepless control. 
         [0009]    A controller identifies a desired capacity level, and then achieves this desired capacity level by first actuating the economizer cycle if increased capacity is desired, or not actuating the economizer cycle if extra capacity is not required, (or providing additional means of unloading to reduce the capacity even further) and then determining a desired motor speed for achieving the exact capacity level. Since the refrigerant compressor provides efficient and reliable operation only within a certain speed range, additional steps of capacity reduction, such as the unloader function, with or without the economizer circuit engaged, may be desired and similarly utilized with the corresponding compressor motor speed adjustment to precisely control the capacity level or achieve more efficient unit operation. In one simplified method, the variable speed is adjusted incrementally within a particular mode of operation (conventional, economized, unloaded, etc.), and the capacity provided is monitored. When the desired capacity is reached, then the system operates at that new speed. If the capacity still needs to be adjusted, then the speed is adjusted in another incremental step. Similarly, if capacity needs to be reduced, the optional unloaded mode of operation can be engaged either in conjunction with closed or open economizer line. Additionally, the controller may monitor the system efficiency level and select the most desirable mode of operation and motor speed. In this case, both capacity and efficiency considerations can be taken into account to establish the optimum unit operation. One more mode of unloaded operation can be added to the system operation, where both the economizer circuit and unloader are engaged simultaneously. 
         [0010]    By providing the variable speed drive in combination with the capacity adjustment options mentioned above, the present invention allows an end user to exactly tailor the system capacity and/or efficiency or combination of these two parameters to a desired level. The method of operation described above would be especially suitable for a transportation refrigeration applications, such as for example container refrigeration units, tractor-trailer units or buses, where a wide operating range of capacity is desired, while at the same time a precise capacity level control is also needed to maintain the cargo or the cooled environment within a narrow temperature range. As also common in these refrigeration applications, an additional throttling device, often called suction modulation valve (SMV) is provided to further reduce the capacity to the level below the level that would be normally achievable through unloading mechanisms and reduction in motor speed. The application of the variable speed drive can diminish or even in certain instances eliminate the need for an additional SMV. 
         [0011]    In other features, the scroll compressor is preferably provided with a single entry port into the compressor for injecting the refrigerant into the intermediate compression port, and wherein this single port is also utilized to route refrigerant to the suction line when the unloader function is actuated. 
         [0012]    In a second embodiment, the scroll compressor is a two-stage compressor, with the intermediate port located between the two stages. 
         [0013]    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 
         [0014]      FIG. 1A  shows a first embodiment refrigerant cycle. 
           [0015]      FIG. 1B  shows a detail of the scroll compressor of  FIG. 1A . 
           [0016]      FIG. 2  shows another embodiment refrigerant cycle. 
           [0017]      FIG. 3A  shows a graph of the capacity provided by the prior art. 
           [0018]      FIG. 3B  shows a graph of the capacity provided by the prior art. 
           [0019]      FIG. 4A  shows the capacity provided by the present invention. 
           [0020]      FIG. 4B  shows the capacity provided by the present invention. 
           [0021]      FIG. 5  shows a more precise view of the actual capacity provided by the typical existing variable speed controls. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    A refrigerant system  20  is illustrated in  FIG. 1A  having a single stage compressor  22 , a controller  42 , a variable speed drive  44  and other components as illustrated in this Figure. As is known, a motor  24  for the compressor  22  can be driven at a variety of speeds such that the amount of refrigerant compressed by the compressor  22  can be varied. The compressor  22  is a scroll compressor having an orbiting scroll member  26  and a non-orbiting scroll member  28 . As is known, a number of compression chambers are defined between the two scroll members to compress an entrapped refrigerant when the orbiting scroll member  26  is driven to orbit by the electric motor  24 . As can be seen, a suction tube  30  leads refrigerant into a suction chamber  31  surrounding the motor and leading into the compression chambers. Once the refrigerant is compressed, it is driven into a discharge chamber  33  communicating with a discharge port  32 . The structure of a scroll compressor is known. As also shown, an injection line  34 , to be disclosed below, communicates with a port  51  that is positioned at an intermediate compression point. As shown in  FIG. 1B , the port  51  may actually be a plurality of ports such as is disclosed in U.S. Pat. No. 5,996,364. 
         [0023]    Refrigerant compressed by the compressor  22  is discharged from the discharge port  32 , and then to an outdoor heat exchanger  46 , which would be the condenser in a cooling mode. Fan  47  moves air over the heat exchanger  46 . Downstream of the condenser  46  is an economizer heat exchanger  48 . As is known, the economizer heat exchanger receives a tapped refrigerant line  45  passing through an economizer expansion device  49 , and a main refrigerant line  41 . Although the two flows are shown flowing in the same direction in  FIG. 1A , this is merely to simplify the illustration. In practice, it is generally preferred to have the two flows flowing in counter-flow arrangement. 
         [0024]    The tapped refrigerant in the tap line  45  subcools the refrigerant in the main line  41 , such that after passing through an expansion device  52 , it will have a higher cooling potential prior to entering an evaporator  54 . Fan  55  moves air over the evaporator  54 . From the evaporator  54 , the refrigerant returns to a suction line  30  leading back to the compressor  22 . Variable or constant speed drives  110  are shown associated with fans  55  and  47 , and can be utilized to vary the speed of these fans to achieve system control, as known. An optional suction modulation valve  61  can be positioned in the suction line  30  between the compressor  22  and evaporator  54 . The tapped refrigerant from the tapped line  45  passes through the return injection line  34  to enter the intermediate compression point or injection port (or plurality of ports)  51  in the compressor  22 . A bypass line  19  may selectively bypass refrigerant from the compressor  22  back to the suction line  30  when a bypass valve  40  is opened. It should be understood that the economizer expansion device  49  also preferably incorporates a shutoff feature, or a separate shutoff device  36  is provided. When the bypass valve  40  is opened, the shutoff device  36  is preferably closed, and when the shutoff device  36  is opened, the bypass valve  40  is typically closed; however, it is also possible to operate with both shutoff valve  36  and bypass valve  40  open. As shown, the same port of the injection line  34  can be used to deliver the refrigerant from the economizer heat exchanger as well as to bypass the refrigerant back to the suction line. Of course, if so desired the bypass and refrigerant injection functions can utilize different ports, instead of common point  51 . 
         [0025]    As is known, the bypass valve  40  is opened when less than the full capacity of the compressor  22  is desirable. Thus, partially compressed refrigerant is returned to the suction line  30 , and the cooling capacity of the refrigerant system is reduced. If a capacity increase is desired, then the bypass valve  40  is closed. If even further capacity augmentation is desired, then the bypass valve  40  is closed and the economizer expansion device  49  and/or shut-off device  36  are opened to provide the economizer function. An enhanced capacity is then provided. 
         [0026]    The outline  15  is illustrated in  FIG. 1A  to make clear that the refrigerant system  20  may be incorporated into various items such as a refrigeration container, a refrigerated tractor-trailer unit, a bus air-conditioner, etc. 
         [0027]    As shown in  FIG. 2 , a refrigerant system  60  has two stages of compression  62  and  64 . A variable speed drive  66  can vary the speed of the motors for either or both of the compressors  62  or  64 . A third compressor stage  161  is illustrated and could also be controlled by a variable speed drive  66 , as could a fourth, etc. A downstream discharge line  68  leads to a condenser  70 , and to an economizer heat exchanger  72 . A tap line  74  passes through an economizer expansion device  76 , and back to a return intermediate pressure line  78 . The return line  78  is shown entering at an intermediate point  80  between the two compression stages  62  and  64 . If the expansion valve  76  is not electronically controlled, then an additional flow device (normally a solenoid valve) needs to be installed to selectively engage and disengage the economizer circuit. The bypass line  82  passes through a bypass valve  84  back to a suction line  86 . Downstream of the economizer heat exchanger  72 , the main refrigerant flow passes through a main expansion device  88 , and an evaporator  90  before passing back to the suction line  86 . The compressor stages  62  and  64  are both provided by scroll compressors. 
         [0028]    An additional, or alternate bypass valve  100  may communicate the discharge line  68  back to the intermediate line  78 . This would allow further control of unloaded or bypass operation. Further, while two stages of compression  62  and  64  are possible, it would be within the scope of this invention to provide additional stages. 
         [0029]    Again, a suction modulation valve  61  is placed downstream of the evaporator  55  to provide additional throttle into the suction flow in this embodiment as well. 
         [0030]    A control for either refrigerant cycle  20  and  60  is able to identify a desired cooling capacity, and operate the bypass function and/or the economizer function as necessary. Thus, as shown in  FIG. 3A , the prior art system provides varying stages A, B, C, D of capacity. Stage A corresponds to operation in economized mode, stage B corresponds to operation in economized and bypass modes engaged at the same time, stage C corresponds to non-economized mode, and stage D corresponds to bypass mode of operation. If there is an additional SMV, then, as shown in  FIG. 3B , by throttling the SMV between the modes of operation mentioned above the capacity can be adjusted between these modes. However, the SMV operation is inefficient, and in general should be avoided if possible. 
         [0031]    When the systems of  FIG. 1A  and  FIG. 2  include a variable speed drive for their compressor motors then there can be a stepless capacity control between the base values A, B, C, D, with or without the use of SMV. Thus, as shown in  FIG. 4A , if the system was operating at maximum capacity at point E 1  (which would normally correspond to economized circuit engaged and the compressor running at maximum speed) by reducing the speed of the compressor the capacity can be reduced to point E 2 . If further reduction is desired the compressor speed is adjusted and the switch is made to economized mode with bypass engaged. Further, the system capacity can be adjusted by varying the compressor speed along the line connecting points EB 1  and EB 2 . If further capacity reduction is desired, the speed can be adjusted once again and the system will move to the next mode of operation, which would be a non-economized mode. Now, the system capacity can be adjusted by varying the compressor speed along the line connecting points N 1  and N 2 . If even further reduction in capacity is desired, the speed can be changed once again and the system will move to the next operating mode, which would be a bypass mode. Now, the system capacity can be adjusted by varying the compressor speed along the line connecting points B 1  and B 2 . System operation shown in  FIG. 4B  is similar to operation in  FIG. 4A , except that abrupt changes in speed are avoided by engaging SMV shortly before the change in mode of operation. Also, even though four major modes of operation are shown in  FIGS. 3A ,  3 B,  4 A, and  4 B, the actual number of modes can be reduced. For example, the system can be operated only in a single economized mode, and the capacity in this mode can be varied by engaging a variable speed drive. As another example, it would be possible not to implement an economized/bypass mode of operation. An extension of operational modes can be achieved by selectively opening and closing the optional valve  100  that can be positioned between the discharge and intermediate compression lines in  FIG. 1A  and  FIG. 2  arrangements. It should be pointed out that additional modes of operation are possible for controlling capacity of the two-stage compressor arrangement where each or both of these compressor stages can be driven by a variable speed drive. It also should be noted that what is shown in the  FIGS. 4A and 4B  is only an illustration on how the switch between the modes is made, the decision on when to make the switch, how to adjust the speed and how to engage the SMV would depend on a specific operating condition, load characteristics, efficiency and power considerations. As an additional improvement to the system operation, either the condenser fan or the evaporator fan (or both) can be provided with a variable speed drive. 
         [0032]    While varying the speed of the compressors provides desirable benefit, there are upper and lower limits imposed on the actual operating compressor speed range that would be available to the end user. Typically, a lower limit is defined by reliability requirements to maintain adequate lubrication of compressor components such as bearings and compression elements. On the other hand, an upper limit is determined by undesirably high power consumption or excessive noise and resultant inefficient operation as well as safety considerations. These limits can be utilized at the system design stage to define times when it would be desirable to switch between modes of operation. The upper and lower speed limits may vary from one application to the other and be condition dependant during the system operation. 
         [0033]      FIG. 5  shows how the ramps would typically be achieved. As shown in  FIG. 5 , once a particular mode of operation is selected, the speed can be varied within that mode and within the speed limits mentioned above. This iterative change is how variable speed drives work in the prior art. If change beyond the speed limits is needed, then the system switches to a different mode of operation. 
         [0034]    In further aspects, it is known to make the economizer and unloader functions continuously adjustable. Still, providing a variable speed drive for the compressor will allow even more flexible, reliable and efficient operation to be achieved. 
         [0035]    Although preferred embodiments of this invention have 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.