Motors and Drives

Fig. 4. Capacity control.

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.

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 : 315 kW, 560 A @ 380V, cos ϕ = 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

Fig. 5. Actual power consumption measurements with slide valve and VFD. www.engineerlive.com 43

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. l

Enter 43 or ✔ at www.engineerlive.com/ede

Abdelhak Dhabi is Global Business Development Manager, Refrigeration Business, Danfoss VLT Drives, Graasten, Denmark. www.danfoss.com/drives

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