Purpose and Implementation of IGV, and How Can We Trim its Values?

Some quick background - there are a number of ways of controlling a centrifugal compressor:

• variable speed - good - large control range
• discharge pressure control - poor, pushes machine towards surge and small control range - turn-down ratio small. No power saving at part load.
• suction pressure control - very common, lowers inlet pressure which increases inlet volume flow - power at reduced load is reduced.
• inlet guide vane control - also good, but lots of potential mechanical linkage problems.

Now to your question - the head developed by the impeller(s) depends on tip speed, but is influenced by the angle that the gas enters the impeller eye.

IGV's rely on this effect and modify the angle at which the gas enters the impeller.

If you turn the IGV's such that you create a rotation of the gas that is in the same direction as the rotation of the impeller the inlet angle of the gas will be lower than 90 degrees and the head will be lower.

The opposite is also true - if you adjust the IGV to give you a counter-rotation of the inlet gas then the inlet angle of the gas to the impeller will be greater that 90 degrees and the resulting head will higher. (This becomes clear if you study the inlet velocity triangle of an mpeller).

I can not comment on what is happening to your machines without seeing the whole control scheme.