REFRIGERANT SYSTEM WITH VARIABLE SPEED COMPRESSOR IN TANDEM COMPRESSOR APPLICATION

BACKGROUND OF THE INVENTION This invention relates to a variable speed motor for driving a compressor that is incorporated into a refrigerant system with tandem compressors.

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.

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. 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.

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.

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 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. 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.

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.

Embodiments are disclosed which incorporate economizer cycles and unloader cycles into the schematic along with the variable speed drive.

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

Figure 1 shows a first embodiment refrigerant system. Figure IA shows other possible circuit schematics. Figure IB shows other possible circuit schematics. Figure 1C shows other possible circuit schematics.

Figure 2 shows a second embodiment refrigerant system. Figure 3 shows the capacity control provided by the prior art. Figure 4 shows the capacity control provided by the present invention. Figure 5 is a flowchart of a control algorithm according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant system 20 is illustrated in Figure 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.

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'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.

As also shown, an economizer circuit can be incorporated into the Figure 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.

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. 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.

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.

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.

Figure IA 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.

Figure IB 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.

Figure 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.

Figure 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.

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 Figure 3, a prior art system that incorporated the Figure 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 Figure 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 Figure 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 Figure 2 embodiment would have many other levels of capacity control available as well.

Figures 3 and 4 are an oversimplification of the Figure 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.

Figure 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

Figure 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 Figure 4. However, Figure 4 does provide a good illustration of the power of the present invention to provide varying capacity.

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. 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.