Based on calculations and with the assumption that all other process parameters (e.g. MW, pressure ratio, mass flow) are fixed , it is apparent that power consumption will reduce with temperature reduction.
However the assumption, that all of other process parameters namely suction pressure (Ps), discharge pressure (Pd) and mass flow (or molar flow rate, N) will remain fixed with change in suction density - is not true in my opinion.
Consider for example, suction temperature reduction for following two of the possible scenarios for a fixed RPM machine ( Assumption : Fixed composition & MW, nearly constant polytropic efficiency) :Scenario-1
: Fixed suction and discharge pressure -> Lower Polytropic head -> higher ACFM (actual cubic feet per minute, as per compressor curve) i.e. higher mass flowScenario-2
: Fixed mass / molar flow and suction pressure -> Lower ACFM at higher density -> higher polytropic head (as per compressor curve) -> higher discharge pressure
Power consumption is prooprtional to ( mass flow rate X Polytropic head)
So, in scenario-2, power should increase due to higher head at fixed mass flow.
In Scenario-1, power may reduce or increase depending on the operating point on compressor curve. In flatter part (near surge point - which is the normal operating range), power will increase with reducing temperature where as in verticle part (near choke flow) power will reduce.
However, with variable RPM machine, power will reduce since for given mass flow polytropic head can be reduced.
Please correct me if I'm wrong.
Edited by narendrasony, 11 May 2012 - 11:19 AM.