Rotary Screw Compressor Control Systems and How they Effect Your Energy Bill

Rotary Screw Air Compressor Control Systems and How they Effect Your Energy Bill

Rotary screw air compressor control types are a major factor in the total energy efficiency of a compressed air system. These controls are used to match the output of a compressor (cfm) with the system demand.

Key Terms:

Full Load: the air demand exactly matches the total available capacity of the compressor.

Part Load: the air demand is less then the total available capacity of the compressor.

Did You Know:

  • All compressors run most efficient at full load.
  • All compressors run less efficient at part load.
  • Not all compressors are created equal and the efficiency loss at part load is related to the type of control system it utilizes.
  • In systems with multiple compressors, which utilize poor efficiency control methods, there are ways to minimize the efficiency loss without purchasing new compressors.

Four primary control system types are available in rotary screw compressors (listed in order of most efficient to least efficient at part load).

Variable Speed Drive (VSD): This type of control changes the speed of the motor and air end to exactly match the air demand.

Load/ No Load: This type of control is utilized with a compressor that has a constant speed motor. The control system will dictate three possible conditions to match the air demand:

  • Load: The motor and air end rotors are turning at a constant speed. The air inlet to the rotors is open, so the air end is making 100% of its rated flow.
  • No Load: The motor and air end are still turning at a constant speed, but the inlet valve is closed. The air end is still turning but producing ZERO flow. Therefore, the unit is still consuming energy (15-35% of full load), but not producing any flow.
  • Off: If the compressor runs unloaded for a determined amount of time without needing to load due to low air pressure, the compressor will turn off which consumes no energy.

Adjustable Rotor Length: This type of control system is utilized with a constant speed motor. The control system will match the compressor output to the demand by changing the length of rotor which is available to compress air.

Modulation: This type of control system is utilized with a constant speed motor. The control system will match the compressor output to the demand by starving air into the inlet of the air end by closing down the inlet valve.

Car Analogy – Sometimes the control types can be best understood by relating them to changing driving speed in an automobile. In all examples, imagine you are driving down the highway at 100 mph and you decide you are driving to fast and want to slow down. You could slow down using several different methods which can be related to the type of control system a compressor uses to make less air.


  • Variable Speed Drive: the accelerator is let up, which changes the speed the engine is turning, ultimately slowing down the vehicle. To speed back up, the accelerator is push back to the floor.
  • Load/No Load: the accelerator is on the floor, but the clutch is engaged. The engine will continue to turn at maximum speed, but it is not connected to the drive shaft, so the car will slow down.
  • Active Rotor Length: the accelerator is on the floor, but the vehicle is shifted to a smaller gear. The engine is turning at maximum speed, but the drive shaft will turn slower, resulting in a lower speed.
  • Modulation: the accelerator is again on the floor, but the brake is applied. This creates a resistance and the vehicle will slow down.

This graph details the type of control system and the corresponding energy efficiency at part load:



Let’s look at a 100 hp modulating compressor which has a rated full load capacity of about 400 cfm. If the air demand is 100 cfm, this compressor would be running at 25% part load. (100 cfm/400 cfm = 25%) From the graph, this compressor would be using approximately 77% of the loaded hp, or 77 hp.

Now let’s use the same scenario, but replace the 100 hp modulating compressor with a 100 hp Load/ No Load compressor. The part load percent would remain the same (25%), but the hp required to make the 100 cfm would only be 44 hp.

This graph should make it obvious that the type of compressor control has a huge impact on the total compressor energy efficiency at part load. Click on this graph for more part load results.


Single Compressor System: Replace the compressor with a compressor that uses a more efficient control system (VSD) or replace it with a compressor which is sized to run fully loaded as much as possible. Bigger is not always better!!!

Multiple Compressor System: Utilize a multiple compressor control (MCC) system. The result will be a substantial reduction in system pressure and energy consumption, in addition to minimal compressed air leakage and a more stable pressure across the network.

ManagAIR®is a control system that makes running your compressed air system easy and is designed to save you money. It includes the following features:

ManagAIR® will make sure a compressor is fully loaded before turning on another compressor to meet demand. This prevents multiple compressors from running part loaded. The fully loaded compressors are termed base load and the single part loaded compressor is commonly called the trim compressor.

ManagAIR® will select the best combination of compressors to run at any time to ensure the smallest possible compressor is running trim.

ManagAIR® will select the compressor with the most efficient control method to be the trim compressor. For example, if a compressed air system consists of a VSD compressor, this compressor will always be the trim compressor.

If you would like a free evaluation on your compressed air system, with recommendations, please call now.