Fixed speed vs variable speed compressors

Most compressors in the past have been fixed speed. Meaning that they are either running at full speed or they are stopped. They cant run, for example, at half speed

Variable speed compressors work in a different way. They are designed to run at any speed between about 25% of full speed and 100% of full speed.

The volume of compressed air a compressor makes is directly related to the speed it is running at. So a fixed speed compressor is either making 100% of the design air volume or no air. In contrast, a variable speed compressor is able to control its speed, so it only makes enough air to exactly match demand.

 

Fixed speed compressors running unloaded

The situation with fixed speed machines, however, more complex than this. All fixed speed screw machines have a ‘unloaded’ running mode. When running in an unloaded condition the motor is turning at full speed but the compressor isn’t making any compressed air. It is effectively running at full speed but the inlet valve is closed so the compressor isn’t compressing any air. Fixed speed electric motors need to run unloaded since there are limits to how frequently electric motors of this size can be started every hour. As an example if a motor is running and then stops it cant start again for several minutes. This is all to do with overheating and putting too much stress into the motor and potentially damaging the motor.

Typical running profile of a fixed speed compressor

To illustrate the effect of the unloaded condition it is best to look at an example of how a fixed speed compressor will run.

In this example lets assume the compressor maximum pressure is 8 bar and the minimum set pressure is 7 bar.

 

Low air demand

The compressor turns on and runs at full speed until the receiver is up to full pressure (8 bar). If air demand is very low or zero the compressor will unload and run unloaded for usually about the next 3 – 5 minutes. If air demand is still low the compressor will stop running. When the receiver pressure drops to the minimum set pressure (7 bar) the compressor will start running again, charge up the receiver to maximum pressure (8 bar) and then run in unloaded condition for 3-5 minutes and then stop. This pattern then carries on as long as air demand remains low / zero

 

Moderate air demand

The compressor turns on and runs at full speed until the receiver is up to full pressure (8bar). If air demand is moderate the compressor will unload and run unloaded until the pressure drops to 7 bar and the compressor will then load up again. This will then charge up the receiver to maximum pressure (8 bar) again. It will then run in unloaded condition until the pressure drops to 7 bar and will reload again.

 

Maximum air demand

The compressor turns on and runs at full speed until the receiver is up to full pressure (8bar). If air demand is high the compressor will unload and run unloaded until the pressure drops to 7 bar. In this scenario this unloaded running may be for only a few seconds, before the compressor loads up again. Alternatively if the compressor supply and demand are equal the compressor will never reach maximum pressure. So the machine runs at full speed exactly matching demand until demand falls.

 

In all these scenarios the compressor provides enough air to meet demand but in all but the maximum air demand scenario the compressor is running unloaded for a period of time. When running unloaded the compressor is using approx. 50 – 60% of maximum power but not making any air. Effectively in unloaded running energy is completely wasted.

 

Fixed speed machines are therefore quite in-efficient when not running at full speed. The lower the air demand compared to the compressor size the bigger the amount of energy wasted.

 

Variable speed compressors

Variable speed machines solve this problem by slowing down to meet demand. So there is no need for an unloaded scenario. Taking the same 3 demand patterns again

 

Low air demand

The variable speed compressor runs up to full speed to reach maximum pressure (8bar). It then slows down until it reaches a speed that matches demand. If the demand falls below approx. 25% then the compressor stops. When demand picks up again the compressor runs up until it meets exactly demand

 

Medium and maximum air demand

The variable speed compressor runs up to full speed to reach maximum pressure (8bar). It then slows down until it reaches a speed that matches demand. When demand picks up or demand reduces the compressor changes its speed to exactly demand again

 

Variable speed compressor are obviously more efficient than fixed speed machines since they only absorb the energy they need to supply the air demanded, there is no ‘unloaded’ running wasting energy.