Speed Control

All methods that reduce refrigerant flow reduce the amount of compressor work done under low loads. However, compressor volumetric efficiency will decrease as output goes down. Reducing the speed (rpm) of the driver and compressor allows the compressor to operate at part load more efficiently with a lower degree of reliance on compressor unloading techniques. Most applications that use speed control have prime movers as drivers, although variable frequency drive (VFD) electric motors are also used and are becoming increasingly popular.

Without using other compressor unloading techniques, capacity control can be achieved by reducing speed. Once the driver reaches the minimum (idle) speed level, for stable operation of the driver and/or compressor, compressor unloading is required to further reduce capacity. Once unloading is required, compressor isentropic efficiency begins to decrease. However, this occurs at a lower load level than would occur under constant speed operation. Therefore, overall part-load efficiency is greater with speed control. Under very low loads (bottom 20% of the load curve), all compressors (electric- or prime mover-driven) experience a significant reduction in volumetric efficiency. Reducing speed in this range can reduce compressor volumetric efficiency degradation to some extent. However, under these low load conditions, the driver itself will begin to experience a fall off in thermal efficiency as well. This is often far more pronounced with a prime mover than an electric motor.

The amount that speed can be reduced depends on both the characteristics of the compressor and the driver. Generally, the minimum speed is 50 to 70% of the full load design speed with centrifugal compressors. The minimum speed is 40 or 50% of the full load design speed for screw compressors and somewhat lower with reciprocating compressors.

Speed control with positive displacement compressors is a relatively simple process. As speed is reduced, compressor volumetric efficiency remains fairly constant and the refrigeration effect improves as a result of improved condenser operation and other related factors.

Speed control is more limited with centrifugal machines than with positive displacement machines, but it is still effective. The inherent characteristics of the compressor are that head drops with the square of the speed. Speed must, therefore, be carefully controlled with centrifugal compressors to avoid surging. In some cases, as load decreases, speed must actually be increased to avoid surging. Typically, speed control is used down to about one-half or two-thirds capacity. In order to optimize overall system efficiency, a complex control sequence is often required.

Figure 37-25 shows a centrifugal compressor's response to load change by using speed control. Consider this example in which the vertical line on the right represents 100% capacity, and the compressor is selected to operate at 120% of full load design speed. At Point A, the head output matches the head required. At Point B, the compressor requires 120% of the nominal power. As the load drops off to the vertical line on the left, the compressor speed is reduced. In this case, reducing speed will allow the compressor to match the reduced head and flow requirements of the heat exchangers down to about 40% of full load. Below 40%, the head required by the heat exchanger performance is represented by Point D, while the compressor is able to only produce head equivalent to Point C. This disparity will force the compressor into surge. To prevent surge, the compressor must operate at a load corresponding to Point E. This may be accomplished by using hot gas bypass and increasing the speed to 80% of full load design speed. Under this condition, the cooler handles the load corresponding to the vertical line at the left, while hot gas is passed from the condenser to the bottom of the cooler to make up the difference in flow from

Compressors i20%

Compressors i20%

Fig. 37-25 Speed Control Effect on Centrifugal Compressor's Response to Load Change. Source: Carrier Corp.

Flow or Capacity

Fig. 37-25 Speed Control Effect on Centrifugal Compressor's Response to Load Change. Source: Carrier Corp.

Points D and E.

Speed control is a valuable efficiency tool, whether done simply or with complex integration of control functions. A computer module can be used to collect system operating conditions and process the information to optimize control of the driver speed and the compressor unloading. For each system, a control sequence must be developed to optimize performance under the entire range of operation. In either case, available microprocessor control technology allows for optimization of any type of system using speed control. Refer to Chapter 29 for additional detail on mechanical equipment system drivers and speed control.

Figure 37-26 shows the relative performance of various part-load control methods for a centrifugal compressor system. Performance is expressed as a percentage of full-load power versus a percentage of full-load refrigeration load. Power requirements are shown in kWe for several electric motor-driven compressor options and in bhp for a steam turbine-driven compressor option. Among the options shown, speed control using a steam turbine shows the best performance down to about 40% of full load. Next to that is constant speed operation with inlet guide vanes. This methods offers control down to very light loads without requiring hot gas bypass for stable operation.

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