A commonly recommended energy strategy for a lubricant-injected rotary screw air compressor is to add main storage receiver capacity to enable the switch from inefficient modulation control to the more efficient load/unload mode. A typical guideline was to add 1-2 gal of storage receiver volume for every cfm of main compressor capacity.
While this amount of storage is better than nothing, this rule represents old thinking that was used for large reciprocating compressors and might be draining your pocketbook. And even when systems have greater volumes of storage, it’s common to discover this measure hasn’t produced the expected results. Further, the cause might not be what you expect.
To understand the issue we must look inside a typical lubricated rotary-screw air compressor (Figure 1). Each unit has an air/lubricant separator and sump assembly that filters and collects the compressor lubricant that was injected into the main compression elements during normal operation.
When a compressor is loaded, this sump is fully pressurized. To gain power savings when a compressor unloads, the compressor inlet valve closes and the sump is vented to atmosphere. During this operation, the unit compresses only small amounts of air, and at a low pressure, so its power consumption is reduced to a much lower level, typically about 25% of the fully loaded power.
Figure 1. In a typical lubricated screw compressor system, the control pressure sensor is located at the outlet of the compressor package. (Draw Professional Services)
The main compressor control initiates this load/unload operation within an adjustable pressure band. The most commonly observed pressure band width is 10 psi, measured at the outlet of the compressor package. While it would be nice to be able to load and unload the compressor instantaneously, unfortunately, it takes a few seconds to pressurize the sump fully when the compressor loads, and significantly more time to release the sump pressure when the unit unloads. While these transitions are taking place, the compressor is consuming power over and above what it takes to produce the compressed air the system actually requires (Figure 2).
Figure 2. This typical load/unload power consumption curve reveals areas of inefficiency at the start and end of each cycle. Adding storage increases efficiency by reducing cycling. (Source: Manitoba Hydro)
Load/unload efficiency versus storage
Storage receiver volume affects how often a compressor loads and unloads (cycles). The more frequently a compressor cycles within a given time period, the less efficient it is. Figure 3 shows how storage capacity can affect the power vs. capacity curve of a lubricated-screw compressor. A perfect compressor, with instantaneous sump pressurization and depressurization, has a power curve characterized by a straight line between fully loaded and unloaded. The actual characteristic exhibits a definite hump, the less storage, the more pronounced the shape and the lower the compressor efficiency, especially at lower load levels.
Figure 3. Power/capacity curves for a lubricated-screw compressor show how efficiency increases with storage at part-capacity. (Compressed Air Challenge)
But there’s more to the story. Two other important factors affect screw-compressor load/unload efficiency: the time it takes to depressurize the sump and pressure band width. The sump depressurization time typically is fixed, a characteristic of the compressor make and model and the sump size. The longer that depressurization takes, the less efficient each compressor cycle becomes. It’s important to maintain the unload circuit in good working order, especially the blowdown vent.