Air Compressor Control Methods

When is comes to Compressor Control Methods, there are more options than simply “Variable Speed” or “Fixed Speed.”

With Variable Speed (also called Variable Frequency Drives), the motor driving the airend is able to spin at different speeds, altering the (1) energy consumption, and (2) compressed air output. You only need 40% of the air? Then spin at 40% speed. Now need 85%? Spin at 85%. This is the most energy efficient option on the market, however, the units are far more expensive in terms of purchase price and maintenance, so it may not always be the most cost-effective method overall. Pros: most efficient; Cons: most expensive.

With Fixed Speed, the motor driving the airend always spins at the exact same speed. However, there are a few options for the airend. It can be a standard Load/Unload, meaning that it produces 100% of its capacity, or 0% of its capacity. There is no in between. Pros: least expensive; Cons: least efficient & prone to rapid-cycling if not properly sized and selected.

Another option is that the airend could utilize Inlet Modulation, in which the unit throttles off the air inlet to the airend as pressure rises above the setpoint pressure. This causes the compressor to draw in less air, matching compressor capacity with air usage for relatively steady pressure control. However, this also causes the compressor to draw a vacuum at the inlet, so that it is trying to make high pressure from a lower starting pressure. This causes part load performance to be very poor (a machine that modulates to 0 capacity still uses about 70% of its full load power). Pros: relatively low acquisition cost, more efficient than Load/Unload; Cons: Still consumes 70% of energy, even at 1% output.

Another option for a Fixed Speed compressor is to have Variable Displacement controls.  A compressor with Variable Displacement is able to change the compressor’s output by opening and closing ports, increasing and decreasing the effective rotor length.  For most rotary screw designs, this is done in discrete steps 50%, 62.5%, 75%, 87.5% and 100%. Other rotary screw designs are continuously adjustable from 50 to 100% capacity.  Efficiencies are typically good within the variable displacement range, but screw compressors sometimes sacrifice some full load performance to use this method, and screw compressors operating below 50% must resort to another control method, such as inlet modulation or variable speed. Pros: rather efficient; Cons: higher acquisition cost, still not as efficient as a Variable Speed.