Can Centrifugal Air Compressors Adopt Variable Frequency Control?

Most people are familiar with energy-saving effects achieved by variable frequency control on screw and piston compressors, which creates a mindset that centrifugal compressors can work the same way. 

In fact, variable frequency control is inapplicable, or extremely difficult and unnecessary for conventional centrifugal compressors.

Working Principle of Centrifugal Air Compressors

Centrifugal air compressors belong to dynamic compressors. The impeller drives gas to rotate at high speed and generates centrifugal force. 

Gas undergoes diffusion and pressure rise inside the impeller, so the velocity and pressure of gas increase after passing through the impeller, continuously generating compressed air.

Centrifugal compressors boost gas pressure by converting kinetic energy. As the bladed rotor (impeller) rotates, blades drive gas and transfer energy to it to gain kinetic energy. When the gas flows into the stator, 

the diffusion function of the stator converts velocity head into required pressure energy: flow velocity drops while pressure rises. Guided by the stator, 

the gas flows into the next-stage impeller for further pressure boosting, and finally discharges through the volute.

Why Variable Frequency Control Is Not Feasible or Unnecessary

Variable frequency control delivers excellent performance for screw compressors. It adjusts rotating speed according to pressure feedback. 

For screw compressors, rotating speed variation has no impact on discharge pressure. In contrast, for centrifugal compressors, discharge pressure changes along with rotating speed.

That is to say, discharge pressure declines as the rotating speed reduces. Accordingly, variable frequency cannot be adopted for capacity regulation of centrifugal compressors when stable pressure is required at the end-use point.

Capacity regulation of conventional centrifugal compressors mainly relies on adjusting the opening of inlet guide vanes to control intake air volume, 

together with blow-off or recirculation schemes. Normally, the adjustable operating range of centrifugal compressors is approximately 70%–100% of full load. Even with variable frequency control, 

the flow rate cannot drop below the minimum stable flow of the compressor (or the backpressure will exceed the maximum discharge pressure). Otherwise, surge will easily occur.

Surge causes severe damage to centrifugal compressors and cannot be eliminated merely via mechanical design. The fundamental method to prevent surge is maintaining sufficient flow rate. Therefore, 

even if variable frequency speed regulation is applied, the effective adjustable range remains roughly 70%–100%.

Magnetic Levitation Variable Frequency Centrifugal Air Compressors
Magnetic levitation structures are mechanically suitable for variable frequency speed regulation.
Even for the latest magnetic levitation variable frequency centrifugal air compressors, the adjustable range is still limited to 70%–100%. The magnetic levitation design optimizes the unit structure for variable frequency operation.
The main purposes of variable frequency configuration for magnetic levitation centrifugal compressors:
Reduce wasted power under blow-off conditions (saving around 2.5% of power consumption);
Eliminate slow loading response.
With these two advantages, magnetic levitation variable frequency centrifugal compressors achieve improved load regulation performance.