By Ron Marshall for the Compressed Air Challenge
A manufacturer of computer hard drives recently designed and installed a cutting edge compressed air system made up of premium efficiency components. The company installed six lubricant-free screw compressors, including one VSD. For air quality, premium efficiency heated blower style dryers were selected and came equipped with dew point controls that reduced the power required for the regeneration cycles to only that necessary. The dryers were installed in parallel, with dual inlet filtration installed to minimize pressure drop.
Compressed air piping was oversized, so that there was minimal pressure differential across the complete system. All the compressors were controlled by a sophisticated sequencing control system that communicated with the compressors and attempted to select the correct units for the load.
Some significant funds were spent in putting this system together—yet the system did not run at optimum efficiency. Why? Because the designer of the system did not understand the concept of control gap when VSD compressors are installed in systems of multiple compressors.
The system was made up of five large 300 kW compressors and one 250 kW VSD. The 300 kW unit put out 1530 cfm at full load. The 250 kW unit produced 1375 cfm but only had a turndown of about 66% or 915 cfm. Unfortunately, when this combination of sizes is put together, there will be a control gap equal to the difference in size between the fixed speed compressors and the VSD turndown. Any time the plant load is within the control gap, the VSD compressor will fight for control with the fixed speed units. Within this gap, with this combination of sizes, even a sophisticated controller gets confused.
During a plant visit, the system was observed to be within the control gap. Observation of the operating hours and the compressor history showed many hours of poor operation. The VSD was spending most of its time alternately ramping up to full speed, an inefficient point for VSD compressors, or rapidly ramping down to minimum speed, another less efficient point. While doing this, one fixed speed compressor was loading and unloading rapidly. These inappropriate operations accelerate the wear and tear on the compressors and consume more energy. This is a symptom of control gap.
What the designer should have done was ensure the VSD compressor’s variable range—the difference between its full load flow and minimum speed flow (915 cfm in this case)—was equal to or larger than the fixed speed base compressors(1530 cfm). If these conditions exist, there is no control gap and the system will run optimally. Buyer be aware of this sizing rule for VSDs: Unfortunatelym some compressor salesmen do not know this rule or choose to ignore it to complete the sale.
Learn more about VSD compressor control in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information
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