Part Three: Screw compressor VFD application
Part One: Introduction
Part Two: Variable frequency drives in refrigeration systems
Screw compressors used in refrigeration plants are generally available in two types – one with a slide valve for capacity modulation and the other without any capacity control.
Nearly all rotary screw compressors use a slide valve for unloading. The slide valve moves along the length of the rotors, reducing the compression length within the rotors. This method of control is infinitely adjustable and provides reasonable suction pressure control. However, there can be substantial power penalties associated with slide valve control primarily because when the compressor unloads, there is no proportional reduction in power.
In general, part-load performance degrades with lower suction or higher discharge pressures. Economized compressors typically lose economizer operation at approximately 75% slide position. Below this position the compressor operates non economized.
Most screw compressors can operate satisfactorily down to 50% speed, as rated by the factory. Below 50% speed the slide valve is typically used for further capacity reduction.
Using a Variable Frequency Drive (VFD) to control the compressor will cause a substantial improvement in compressor performance throughout the entire range of loads. Here, the VFD maintains a constant suction pressure by continually adapting the speed of the variable speed compressor with respect to load fluctuations.
It is possible to set up a variable speed compressor controlled by a VFD and connected in parallel with several fixed speed compressors. VFDs like Danfoss’ VLT® drives effectively manage open-loop or closed-loop control tasks in such a compressor pack.
The benefits of using cascade controlled compressors are reduction in the number of compressors per system and a reduction of the cost to deliver the same capacity or better.
There are more control methods for compressors and compressor systems. But using a VFD to control the compressor speed offers the ability to match the capacity to the actual need based on measurements in the system. Cost are reduced both through energy savings when running at lower speed, but also through lower installation costs caused by optimization of the system and the compressor itself. Additionally fewer starts and stops will reduce mechanical wear. And when your VFD comes with a “skip resonance” function you easily identify for the drive which frequencies to bypass to avoid mechanical vibrations and any possible damage.
Compared to the slide valve method of control, applying a VFD for control will result in a higher efficiency and significant energy savings. Payback times of one to two years are no exception.
A good example of how you can save cost by using VFD’s in compressor applications is this Danfoss VLT Drives Kartarpur / India Cold Storage application.
Article by: Deepinder Singh Chani
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