Large, low speed machines are more efficient than smaller (cheaper) high speed units. Also, valve and bearing life is longer giving reduced maintenance cost.
Traditional compressor control is step -wise through the use of suction valve unloaders. Capacity is therefore typically 0% (at start-up) ~25%, ~50%, ~75% and 100%. Where the process requires some intermediate flow, it is normally achieved by spilling flow back from discharge to suction. This wastes energy. Further, operators often do not make best use of the step controls, needlessly increasing spillback and energy wastage. There are two solutions:
• Variable volume clearance pockets at the outboard end of the cylinders - manual or automatic.
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• Rapid acting hydraulic suction valve unloaders which operate at a variable percentage of each piston stroke.
It is often found that the energy saved when using valve unloaders is far less than expected. This is usually due to excessive pressure drop across the valves even when being held open. The solution is to select valves properly engineered for the service.
For existing machines, check the effect on motor amps at the various unloader settings. If the reduction falls short of expectations, investigate the valve design.
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