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I don't agree with the simultaneous starts advocated below. The current draw is highest when a compressor is started from rest. Why would you want to rob volts (electrical pressure) from both machines at that moment? That condition causes amps to rise and tripped breakers. The standby compressor, that is reserved to augment CFMs, should have its cut-in and cut-out set below the everyday shop compressor. When the auxillary compressor is not needed, it is not permitted to run by putting its breaker on off. When starting a sandblasting job, the breaker is activated. The compressor pressure switch will keep the auxillary compressor shut down until the main compressor air receiver loses pressure due to consumption. At that point both compressors will be running as the second one is started. The sandblast nozzle should be sized so that both compressors run continuously. When stopping to refill the pot, the auxillary compressor will shut down first. This is called "lead/lag". It is used in industrial situations, such as sewage pumping, air compressing, HVAC, and , for all I know, electrical generating. The lead/lag units are often pregramed so that the machines take turns running the base load. I was recently in a hospital where four 103 CFM reciprocating compressors were pumping into one air receiver. They were tied into a panel that automatically started, stopped, and scrambled the hierarchy of each unit. The controller could sense the draw and fire up as many compressors, in turn. as needed to meet the load. The off/on signals were progressive. I believe this compressor management feature is more energy efficient than running one large continuous run compressor fitted with unloader valves. It may also be more efficient than running a screw type compressor with a throttle.
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