Abdelhak Dhabi discusses the advantage for using frequency converter technology in refrigeration applications to control compressors.
As 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:
- 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.
- Drive control will reduce wear and tear associated with slide valve action.
- Drive control allows a precise suction pressure to be maintained. With slide valve, a broad dead band is often maintained to avoid excessive wear.
- 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 the substantial improvement in compressor performance throughout the entire range of loads.
The concept is based on intelligent frequency converters. These frequency converter types can handle the open-loop and close loop control tasks in the compressor pack. The main function of intelligent frequency converters is to maintain the suction pressure constant by continually adapting the speed of the variable speed compressor.
Cascade control compressors
The benefit by using cascade control compressors is to reduce the size and the cost and to keep the same or better capacity.
To use drives to control speed increases the COP (coefficient of performance) of the system and reduces the energy consumption. The comparison is shown in Fig 4.
Example 1: Cost saving comparison between drive operation and slide valve control on screw compressors.
Screw compressors used in refrigeration plants come in two types: a) with slide valve for capacity modulation and b) without any capacity control regime. Although the slide valve control gives reasonable suction pressure control, there is a certain energy consumption associated with the slide valve control. From the power consumption chart (Fig. 5) we can see that the slide valve method of control do not follow the capacity control. In the case of 60 per cent capacity, the slide valve control consumes approximately 80 per cent of the power. Where as, with drive control, at 60 per cent capacity, the power consumed is approximately 60 per cent.
Our experience with a customer in Canada with Mycom screw compressor is that we obtained the following results:
Compressor motor data: 315kW, 560A @ 380V, cos phi = 0.91
Slide valve @ 68% = 438 A = 260 kW
Slide valve @ 80% = 450 A = 267 kW
Slide valve @ 100% = 490 A = 291 kW
(based on actual measurements)
Assuming average annual capacity is 80 per cent of the installed capacity, 20 hours working per day and 365 days operation, the comparison of slide valve versus drive control is given below:
With slide valve: 267 kW x 365 x 20 = 1,949,100 units
With drive control: Energy consumption is 15% lower = 1,656,735 units
Savings = 292,365 units
Cost savings (@ Rs. 5 per unit) = Rs.1,461,825
Average installed cost a 315 kW VFD = Rs. 1,400,000
Payback period = 1 year.
Danfoss VLT Drives operates as a supplier of drives solutions to the HVAC, refrigeration, food and beverage and general industry. As a global player the company has supply chain and research and development centres in China, Denmark, United States and Germany and a global network of sales offices serving customers with high quality energy-efficient power electronics solutions.
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Abdelhak Dhabi is Global Business Development Manager, Refrigeration Business, Danfoss VLT Drives, Graasten, Denmark. www.danfoss.com/drives
