One of the hottest topics in the compressed-air energy-efficiency field is the variable-speed-drive (VSD) screw compressor. Since this unit’s acceptance in the industry several years ago, all major manufacturers have developed a line of compressors that control compressor output pressure by varying the speed of the compression element. This results in an almost linear reduction in power consumption between the compressor’s full load and minimum speed. Minimum speed is the slowest the compression element is allowed to spin while under variable-speed control.
There are other, less efficient, ways to control compressors so that they maintain a constant pressure under varying loads. Some compressors modulate, choking off the inlet flow to vary the compressor output.
Others load and unload to maintain pressure between two set points. Still others might use variable capacity, opening up ports in the compression element with a secondary control system to effectively vary compressor capacity. None of these modes of control is as efficient as variable speed through the full range of compressor output.
Setting up a VSD-controlled unit in a system with a single compressor is easy, but a VSD compressor, inserted into a multiple-compressor system, can generate problems. Most often, multiple fixed-speed compressors are set up in a cascaded arrangement where each successive compressor feeds varying air demand in a set order. On the other hand, a system using a combination of fixed-speed and VSD units must be carefully set up so each VSD always takes partial loads. This requires each VSD target set point to be within the overlap between the fixed-speed compressor set points. This is known as a nested arrangement.
There is also an important sizing rule when applying VSD compressors: The variable part of the VSD capacity must equal or exceed that of the fixed-speed compressors with which it must sequence. If the VSD variable range is smaller than the fixed-speed range, there will be a gap within which the compressors will inefficiently fight for control. This control gap is evident if the VSD is constantly ramping between minimum and full speed while another fixed-speed compressor is loading and unloading. This condition will exist only when the flow is within a control-gap region, defined as the difference between the fixed-speed capacity and the VSD control range (full load minus minimum speed).
Basically, when you mix compressor types, watch your control gaps.
More information about compressor efficiency can be found at the CAC website or in CAC’s Best Practices for Compressed Air Systems manual. Be sure to check the online calendar for scheduled training. MT
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