Five-step unloading means capacity control in approximately 25% steps from zero to 100%. Therefore, if two 50% compressor units are used, capacity control is available in 12.5% steps. Suction valve or plug-type unloaders can only deactivate each cylinder to zero or 50%. If the 25% intermediate step is desired, it is necessary to use clearance pockets. It is possible to obtain five-step unloading in increments other than 25%.
When the pressure ratio is quite low (less than approximately 1.7) the volume of the clearance pocket becomes very large. The actual volume is dependent on the piston displacement and “k” value of the gas. Refer to the formula for volumetric efficiency. Also, API 618 recommends that volumetric efficiency remain above 40% since performance prediction is generally unreliable below that value. (However, values as low as 25% have been used when less precise capacity control is acceptable.)
Note also that when the cylinder bore is not much larger than the piston rod, unloading the crank end of the cylinder results in a capacity reduction much less than 50%.
Three-step (100%, 50%, 0%) capacity control is used on some general purpose machines such as air compressors. Three-step control requires more cyclic actuation of the unloaders than five-step control. Therefore, three-step control is more detrimental to machine components (particularly valves).
When precise capacity turndown is required, a bypass with a control valve is necessary. Depending on the system requirements, the bypass may only be across the first stage. But, more often the bypass spills back across all stages unless the differential pressure is too high to be handled by a single control valve. The take-off point for the bypass must be downstream of a heat exchanger so that cooled gas will be spilled back to the suction. If there is no exchanger in the discharge, the bypass must branch into the suction line upstream of an exchanger. Alternatively, a cooler may be placed in the bypass line. In any case, the bypass should tie-in upstream of a suction knockout drum so that any condensate resulting from the expansion cannot enter the compressor.
When a bypass is used in combination with step control, five-step operation is more efficient than three-step operation.