When installing compressed air storage to help with compressor control of lubricated screw compressors, the location of the tank makes a difference. Having significant receiver capacity of at least 5 gallons per cfm of trim compressor capacity serves to slow down compressor cycles and make the compressors run more efficiently when operating in load/unload more.
If the tank is installed at the compressor discharge, on the wet side of the dryer, the receiver tank is very effective in slowing the cycles (it also helps cool and dry the air). If installed after the dryer and filters the tank is less effective, the “effective” size of the tank, as seen by the compressor, reduces in proportion to the pressure loss across dryer and filters. For example, if the compressor pressure band is 10 psi, and the loss across the dryer and filters is 7 psi, then the tank is 7 tenths (70%) less effective in reducing compressor cycles.
The receiver in the accompanying photograph is at the far end of the plant, and connected with very small plastic tubing. Because of the pressure loss across the system, and the poor connections, this receiver is useless in helping reduce compressor cycles.
Some tips about control storage:
- Bigger is better, between 5 to 10 gallons per cfm of trim capacity (for example a 200 cfm compressor would need 1,000 to 2,000 gallons)
- Minimize all pressure loss between the compressor and the receiver by properly sizing or over-sizing components
- If the plant load has large peaks, put most of the storage on the dry side of the air dryer so as not to overload the dryer during the events
- Be sure to install proper drains on the receivers, especially wet receivers
- Receivers need to be installed as per local pressure vessel codes for safety, with proper mounting, pressure relief, gauges, impact protection, and testing schedule.
- Generally, unless the pipes are very large and long (3 in. and above), the compressed air piping in a plant does not provide good control storage.
- Widening the pressure band of the compressor(s) can make storage more effective. A pressure band of 20 psi will provide up to double the effectiveness of a 10 psi band for reducing compressor cycle frequency.
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