Refrigeration and the role of variable speed drives
09 November 2014
Abdelhak Dhabi explains how variable speed control of compressors can be deployed to reduce energy consumption and wear, as well as increasing the overall system efficiency of refrigeration installations.
Refrigeration systems spend most of their operating hours at reduced capacity. Screw compressor back-off slide valves, reciprocating solenoid valve control and other compressor types are generally either 'on' or 'off'. Unfortunately, these control methods do not provide the maximum attainable reduction in brake horsepower as refrigeration capacity is reduced with respect to prevailing demand.
In standard system designs, electric motors are intended to operate at a fixed speed, which is determined by the frequency of the ac power supplied by the utility and the number of poles that the motor possesses.
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 (screw, reciprocating, rotary vane, for example) are constant torque devices. This means that 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 affect torque. In general, a reduction of 50 percent in speed would provide a proportional 50 percent reduction in shaft power.
General drive control
Using frequency converters to control cold storage capacity provides improved control and efficiency, whether for compressors, fans or pumps. Indeed, there are several incentives for using speed control on screw compressors.
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.
The use of drive control will reduce wear and tear associated with slide valve action, and will also allow a precise suction pressure to be maintained. With slide valve control, a broad dead band is often maintained to avoid excessive wear. Importantly, drive speed control may enable compressor size reduction without compromising the system’s ability to meet capacity requirements.
Drive operation of screw compressors
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 them. Although this control method is infinitely adjustable and provides reasonable suction pressure control, there can be a substantial power penalty associated with it. As the compressor unloads, there is no proportional reduction in power.
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 in a non-economised mode.
Most screw compressors can operate down to 50 percent speed, as rated by the factory. Below 50 percent speed, however, the slide valve must be used for further capacity reduction.
A compressor pack in which a master drive is connected in parallel with several fixed speed compressors will require intelligent frequency converters. These frequency converter types can handle the open-loop and closed-loop control tasks in the compressor pack, and their main function is to maintain constant suction pressure by continually adapting the speed of the variable speed controlled compressor.
The benefit of using cascade control of compressors is to reduce installation size and hence cost while maintaining (or even improving) the system capacity. The use of drives to control speed increases the coefficient of performance of the system and reduces the energy consumption.
Comparing drive with slide valve control
Screw compressors used in refrigeration plants generally come with a slide valve for capacity modulation, or may have no capacity control regime at all. Although the slide valve control method gives reasonable suction pressure control, there is a certain energy consumption penalty associated with it.
Slide valve method of control does not follow the capacity control. In the case of 60 percent capacity, the slide valve control consumes approximately 80 percent of the power, whereas, with drive control, at 60 percent capacity, the power consumed is approximately 60 percent.
To give some idea of the benefits of compressor drive control in terms of savings and investment payback, consider the following example of a customer’s installation in Canada using a Mycom screw compressor:
-Compressor motor data : 315kW, 560A/380V, Cos f = 0.91
-Slide valve at 68 percent = 438A = 260kW
-Slide valve at 80 percent = 450A = 267kW
-Slide valve at 100 percent = 490A = 291kW
(based on actual measurements)
Assuming average annual capacity of this refrigeration plant is 80 percent of the installed capacity, and that it works 20 hours per day, 365 days per year operation, the comparison of slide valve versus drive control with respect to power consumption is as follows:
With slide valve, the power consumption is 267kW x 365 x 20 = 1,949,100kWh. With drive control, we can conservatively assume that the energy consumption is reduced by 15 percent – ie: 1,656,735kWh. The resulting power savings are 292,365kWh. Based on a price of 5p per kWh, this translates into annual savings of £14,618.25.
With the average installed cost of a 315kW variable speed drive being £14,000.00, the above calculation indicates that the investment payback is of the order of one year, with savings accruing year-on-year thereafter.
Danfoss VLT Drives (now available in power range up to 1.4MW) has been mass producing frequency converters for over 40 years and operates in 120 countries world-wide.
Abdelhak Dhabi is global business development manager, Refrigeration Business, Danfoss VLT Drives
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