How IGV Work in Centrifugal Chillers

Inlet Guide Vanes (IGV) on centrifugal compressors don't work the way many would intuitively think. 

They are used for capacity control but they don't directly restrict refrigerant volume to the compressor once running. 

I say once running intentionally because during the startup cycle most centrifugals keep the IGV closed to minimize load through the impeller. This helps with unloaded starts and lowering starting current. So in this example, they are technically restricting refrigerant from getting to the impeller. 

Once the compressor is running IGV's job transitions to preswirl/prerotation. 

I think of this as adding turbulence to the refrigerant before it makes it to the impeller. The more the IGV is opened the less turbulence is created so the impeller can move more through it. 

I originally thought an IGV acted like a ball valve or butterfly valve. I thought they were being used to restrict the actual volume that makes it to the impeller. 

While on it's most basic levels the ball valve example can help give a very fundamental understanding. The actual mechanics of the process are different as described. 

Keep in mind this is for a variable IGV that's used for capacity control. If they are fixed IGV then they take a bit of a different function. 

MTT

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