November 23, 2007

Understanding Compressors Curves

Compressor Curves are generally misunderstood in day-today working of process engineers and many confusions arises out of discussion among cross functional teams. This becomes more confusing in case of Recycle loop systems for example in Ammonia synthesis loop OR Ethylene oxide Synthesis Loop. In view of this, I decided to write this article based on my recent experience.


In case of Compressors, first thing is to understand the Head Vs Flow Curve. Please do not get confused between Head and discharge pressure which are significantly different than pumps where usually fluid specific gravity is around 1.0.

Now Let us first note down the requirement, as below

  1. Curve Readings - Head & Flow from Design Curve.
  2. Speed at Which the Curve is applicable.
  3. Design Suction P & T.
  4. Design Molecular Weight of the Gas
These are the minimum requirements to simulate your compressor for any modification study.

Now if you need to consider any speed change it can be (As shown in attached file), otherwise new operating conditions can be specified. So the first step is to consider the new flow & head curve based on revised speed.

Then the program in attached sheet calculates actual suction volume at design conditions and then calculates developed head at new speed at specified flow. This head can be converted to new conditions in terms of discharge pressure.

Or if desired head is given new flow can be calculated by iterations from the given curve at revised speed.

In this way when we convert actual condition to design basis and consider converted suction volume the original curve is still applicable. This helps in avoiding any confusion due to change in suction condition in actual plant operation compared to design. The fundamental is simple - if you convert gas volume at design condition that will be understood by the compressor as if it is operating on the curve.

OR if compressor handles Actual M3 of flow at design condition then the developed head point will lie on the original curve or revised curve if speed is changed.

The assumptions in this sheet are
  1. No speed change beyond 10% of design value because errors are larger due to change in internal flow pattern.
  2. No change in efficiency. Though I have considered efficiency correction factor if you wish to apply it manually.
  3. No thermodynamic calculation of discharge temperature is done.
If you have any query related to any compressor problem kindly let me know. I have also developed a BC++ program for any high pressure real gas mixture compression system which uses equation of states for the calculation of thermodynamic properties of the gaseous mixture and its efficiency, which is tested for highly non ideal system also upto 220 bar pressure.

To Download the file , just click .... Compressor Evaluation

No of Downloads:-

Readymade pack is available for Hydrogen, Air, Nitrogen, Oxygen, Synthesis Gas of Ammonia plant, Recycle gas for EO plant, CO2. For any other gaseous mixture it can be easily modified.

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