Centrifugal Compressor Surge Condition

[Hippo]

I have what I suppose is a basic question about the surge point of a centrifugal compressor.

If I'm given the estimated surge in ICFM of a machine, will the surge flow be different if the inlet conditions change?

I've always considered a centrifugal compressor as a volumetric device that is not really impacted by inlet composition, temperature or pressure. I thought I only needed actual volumetric flow (inlet) to get what I needed from a supplied curve. I have a vendor completed datasheet and compressor curves that indicate reasonably different surge flows for different inlet conditions as well as different polytropic head for the same ICFM. I'm baffled.

Thanks for any replies.

Mark

 

[pmover]

a centrifugal compressor surge is the point at which the compressor cannot add enough energy to overcome the system resistance for a specified speed (rpm).

icfm is stated at some std flow condition specified/used by the compressor mfg, but the buyer or responsible engineer will specify the "std condition" (usually 14.7-psia and 60°F if in USA). so, changing inlet conditions will result in a change of outlet condition.

the short answer is yes, but it depends upon how significant the change of inlet conditions are. also, a variable speed compressor will have a different surge point at each operating speed.

is the compressor a variable speed unit or a single speed unit?

are there any inlet guide vanes to change the geometry of the compressor's performance?

hope this helps.
-pmover

 

[TPL]

To keep things simple, ignore efficiency terms,and consider the following:

(i) mass flow = density * volume flow

(ii) head = amount of energy added to a unit mass of gas passing through the compressor

(iii) Gas Power = mass flow * head
= density*volume flow*head

For a given volume flow, changing the density at the inlet (pressure, temperature or composition) will change the required gas power

For a given head, changing the inlet density will change the volume that can be delivered

This is true for any point on the curve including surge points.

Depending on the capacity of the driver to deliver varying amounts of power, changing the inlet conditions, will require a change in speed or a change in volumetric flow rate.

 

[wimple]

Thanks for keeping things simple.

Rgds
Wimple