By Ron Marshall
A sawmill installed a VSD compressor in their compressed air system to save energy and provide better pressure control. The site had two 50-hp fixed speed compressors and a new 75-hp VSD controlled compressor. The system was supposed to be set up so that the VSD compressor would provide the variable flow and the fixed speed units would run base duty, either fully loaded or off.
After the installation, a compressed air auditor placed data loggers on the system to see how much energy was being saved. He was disappointed to find that the system was running poorly and the VSD compressor was not running as a true trim machine. The compressors appeared to be fighting each other, reducing the system efficiency due to excessive unloaded run time.
An analysis of the data showed that the VSD target pressure had been incorrectly set up by the supplier. Instead of the target pressure being nicely “nested” between the control range of the two fixed speed compressors, the setting was right near the load point of the other compressors. Further to this, both fixed speed compressors had exactly the same set point, making both compressor start instead of one when the VSD compressor needed help. These settings caused system pressure instability because the fixed speed compressors and VSD would fight for control and inefficiency because the fixed speed units could not automatically turn off when they were not required.
There was another problem, a timer driven blowing load, where some air conditioning coils were cleaned of sawdust was firing every 5 minutes, consuming a large amount of air for a short period of time. When this operation happened, even at light loads when only the VSD compressor was required, the plant pressure dropped causing the one or more fixed-speed compressors to start and run unloaded, wasting energy for a period of time.
The auditor recommended the installation of dedicated storage with metered recovery to help solve the problem. A dedicated storage tank was installed to feed this flow, but the input to the tank was restricted, so the tank would fill slowly. When the blowing load fired, a large pulse of air was used from stored air for a short period of time, but the air was replaced slowly with a low flow over a long period of time. This reduced the flow low enough so that no additional compressors would start due to the transient flow. This reduced the power consumption substantially and also improved plant pressure stability. Work is proceeding to supply this blower with something other than compressed air.
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Ron Marshall is a compressed air energy efficiency expert at www.compressedairaudit.com.