Efficiency of Load / Unload Capacity Control?
It is estimated by the DOE that the average compressor in industrial plants operates at approximately 70% of full load. This fact underlies the importance of air compressor capacity controls that operate efficiently at partial loads. Determination as to what compressor control is appropriate is not always easily determined. Literature from sales brochures usually present charts or graphs that are misleading at best, leaving out details that would be needed to achieve such efficiency levels. One of the air compressor capacity controls most commonly promoted is the Load/No Load control.
Oil injected rotary screw air compressor with load / no load controls are thought to be one the most efficient part load air compressor control schemes. This however is misleading, since an adequate receiver volume is essential to obtain real savings in energy at loads other than full load. Without adequate storage a linear drop in actual power requirements from full load to unload bhp is not realized. Storage in gallons verses compressor output in cfm would have to be approximately 20 /1 for the reduction in power to be almost linear.
The reason for this is that it takes time for the compressor to drop from 100% power to its lowest unloaded power level, the Transition Period. The Transition Period starts when the compressor reaches it's unload pressure, which is the highest pressure on the pressure switch differential setting. At this point the inlet valve is signaled to close and the air / oil receiver is blown down, either partially or completely to atmospheric pressure. Blow down of the reservoir will reduce the power requirement from 100% at full load/maximum pressure to 15% - 35% at the fully unload point depending on h.p. and manufacturer. This is accomplished in a controlled manner with a duration time of 20 - 60 seconds for oil flooded rotary screws from 10 - 350 h.p. The air / oil reservoir blow down is controlled mainly to reduce the oil carryover and oil foaming that would result with an instantaneous relieving of pressure. The rate of blow down and pressure decay occurs rapidly at first then gradually slows as the pressure is reduced. This corresponds closely with the power requirements which also drops rapidly then slows down until the lowest power requirements air achieved (20 -60 seconds). Without adequate storage the compressor could be forced to reload before the Transition Period is over thus keeping the compressor from ever attaining its lowest unloaded power. The longer the compressor is allowed to stay at the lowest unloaded power level, after the Transition Period, the more efficient the compressor will operate at partial loads.
The graph illustrates the percent of power at various storage levels reflecting the efficiency of compressor operation. As you can see the general rule of thumb of 1 -2 gallons of storage per cfm does not result in a very efficient part load operation. Whereas storage capacity at a ratio of 10/1 or greater will result in an almost linear drop in power making the load/no load control one of the most efficient part load capacity controller.
Potential application of a load / no load air compressor should always involve the question of storage capacity. Factors that need to be addressed should include the initial cost, installation and floor space for the size of reservoir needed to provide adequate storage for efficient operation of the compressed air system. Lastly, does the ROI support that level of investment?
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