Motors and Drives
Variable speed control reduces energy consumption in refrigeration applications
Abdelhak Dhabi discusses the advantage for using frequency converter technology in refrigeration applications to control compressors.
A
s shown through theory and case study, speed control of refrigeration components provides maximum flexibility, control and energy efficiency. Most refrigeration systems spend most of their operating hours at
reduced capacity. Screw compressors back off slide valves, reciprocating solenoid valve control and other compressor types use on/off. Unfortunately, these control methods do not provide the maximum attainable reduction in brake horsepower as refrigerant capacity is reduced. In standard system design, electric motors are intended
to operate at a fixed speed. This speed is determined by the frequency of power supplied by the utility and motor design (number of poles). The shaft load on the motor is determined by the product
of shaft speed and torque. With a fixed speed, motor power is determined by the torque of the load. With a change in speed, motor load will not only benefit from the speed reduction, but also any reduction in torque with speed. Two types of motor loads exist; constant torque and variable torque.
Positive displacement compressors
Positive displacement compressors (eg, screw, reciprocating, rotary vane) are constant torque devices. That is, the twisting force required to turn the shaft is constant, regardless of speed. Therefore, the shaft power is determined by operating conditions (pressures) and method of capacity control, which both effect torque. In general, a reduction in 50 percent speed would provide a proportional 50 percent reduction in shaft power (Fig 1). Using frequency converters to control cold storage
capacity provide improved control and efficiency, whether for compressors, fans or pumps.
There are several incentives for using speed control on screw compressors:
l Drive control will reduce the power penalty associated with slide valve, poppet valve, or throttling capacity control. On compressors with no capacity control, speed control will eliminate other poor control strategies.
l Drive control will reduce wear and tear associated with slide valve action.
l Drive control allows a precise suction pressure to be maintained. With slide valve, a broad dead band is often maintained to avoid excessive wear.
l Drive speed control provides compressor size reduction with the same system capacity demand.
Drive operation of screw compressor
Almost every rotary screw compressor uses a slide valve for unloading. The slide valve moves along the length of the rotors, reducing the compression length within the rotors. Although this control method is infinitely adjustable and
provides reasonable suction pressure control, there can be a substantial power penalty associated with slide valve control. As the compressor unloads, there is not a proportional reduction in power. A typical screw compressor part load curve is shown in Fig. 2. In general, part-load performance degrades with deeper suction or higher discharge pressure. Also, economised compressors typically lose economiser operation at approximately 75 percent slide position. Below this position, the compressor operates non-economised. Most screw compressors can operate down to 50 percent
speed, as rated by the factory. Below 50 percent speed, the slide valve must be used for further capacity reduction. The improved part-load power curve is shown in Fig. 3. Note
Fig. 1. Torque speed curves (constant curves). 42
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Fig. 2. Typical screw compressor part load.
Fig. 3. Improved screw compressor part load.
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