An important component in any compressor installation is the receiver tank. Having the correct amount of storage volume in any system acts will make the compressors more efficient and can stabilize the pressure, reducing or eliminating low-pressure events. But a common problem is “effectiveness”: Sometimes there is enough volume, but it is not being used effectively to do its intended job.
For lubricant-injected screw compressors running in load/unload mode, having large and effective storage acts to reduce the frequency of load and unload cycles per minute. Having long cycles reduces the number of unload cycles where the compressor dumps air from its air/oil sump. Long cycles of at least three to four minutes give the compressor input kilowatts time to settle to the lowest value, and sometimes running long cycles even allows the compressor to automatically turn off. The desired effective storage capacity to do this is between 5 and 10 gallons of storage volume for every cfm output of the largest trim compressor (the trim compressor is the one that takes partial load). So, for a typical 100 hp compressor, this would be a volume of between 2,000 and 4,000 gallons. Yes, I know, that is a big tank. But I always say that if your tank does not make people do a double-take when they first see it, then it is probably too small.
How do you know if you have enough effective storage? If you have load/unload compressors, you can time the load and unload cycles of your trim compressor. The best time to do this is when the compressor is around 50% loaded and where the shortest cycles happen. Listen for the load/unload cycles and time both the loaded and unloaded run time. If both are only a few seconds, that is a bad sign; if they're four minutes or more, that is best in class.
OK, say you have large storage, but your cycle times are very short. What’s wrong? There are likely some pressure restrictions causing the storage to become less effective in reducing cycle time. For example, a locally controlled compressor might have a load/unload pressure setting that is 10 psi wide, but in the path of the compressed air flow, there is 4 psid filter pressure drop, and 3 psi air dryer drop subtracting from the pressure setting. The pressure swing at the storage receiver then is 10 psi minus the total pressure differential, or in this case only 3 psi. This makes the storage receiver only 30% effective – not what you paid for at all.
How do you fix this? Here are some tips:
- Regularly change filters and/or oversize for lower pressure differential
- Widen the compressor load/unload pressure band
- Place large storage right at the compressor discharge (wet storage)
- Implement remote sensing or a central compressor controller that controls the compressor at the receiver
- Purchase dryers with lowest pressure differential or oversize
- Correctly size compressed air headers and piping for lowest pressure drop.
Don’t have load/unload compressors? If your compressors are running in modulation mode, I would suggest that you have work to do to improve your efficiency, but even modulating compressors need storage. Have variable-speed compressors? These still need storage to work efficiently, but smaller receivers can be used.
Ron Marshall is a compressed air energy efficiency expert and a compressed air trainer with website at www.compressedairaudit.com
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