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Calculate discharge pressure

Q: I have the complete data sheet of a compressor that compresses hydrogen from pressure P1 to P2. How do I calculate the discharge pressure if I compress nitrogen of pressure P3 by the same compressor? Will the result vary for centrifugal and reciprocating machines? If so, how?

A: The discharge pressure attained from the use of a hydrogen compressor when operated on nitrogen can be calculated first by using the parameters from the hydrogen service and calculating the polytropic head. Then use the head equation set equal to the head value calculated from the hydrogen and solve for the discharge pressure. The result will vary if a reciprocating compressor is used. The centrifugal compressor is a dynamic compressor generating head as a result of speed while the reciprocating compressor is a constant volume positive displacement compressor. The reciprocating compressor discharge pressure is a function of the system resistance.