A good check to determine if your oil injected rotary screw compressor is running efficiently is to do a timer test of the load/unload cycles. A rapidly cycling compressor — one that loads and unloads in a cycle that last less than a minute — is inefficient. A best case compressor would be one that has cycles that last for 4 minutes or longer.
Cycle frequency is a factor in the efficiency of oil-injected screw compressors due to slow oil sump blowdown times. Internal to the compressor is an air/oil separator that removes the lubricant from the air so that it doesn’t pass downstream. This separator is blown down to low pressure when a compressor unloads, reducing the power the compressor consumes when it is producing no air.
But blowing down instantaneously would cause a problem with the compressor lubricant in the sump, an oil reservoir below the air/oil separator used to collect and store compressor lubricant before recirculating it through the compressor. Because the compressor lubricant is full of pressurized air, quickly reducing its pressure would cause lubricant foaming, which would overwhelm the separator — not unlike shaking up a pop can and releasing the pressure. Therefore, the sump blow must be done slowly, typically taking about 20 to 60 seconds (and even longer for large compressors), during which the compressor consumes more power than compared to an instantaneous blow down.
The concept is simple: the more blowdowns in a given time, the less efficient the compressor will be. The key to making the compressor more efficient is to somehow reduce the cycle frequency.
Some ways to slow down the cycles:
- Increase the size of the main storage receiver tank; the math is simple, double the receiver size, cut the cycle frequency in half.
- Reduce the pressure drop between the compressor and the main storage receiver; cutting pressure drop by 5 psi on a 10 psi wide pressure band doubles the effective storage on any receiver downstream of the improvement.
- Widen the pressure band, using a 15 psi wide pressure instead of a 7 psi pressure band on a load/unload compressor reduces the cycle time in half.
- Operate a smaller compressor. Smaller compressors cycle less than larger ones when working with a similar sized storage tank. And a compressor at heavier load cycles less.