Norm, why is it normally best to suction throttle, rather than to spill-back, to control my FCU wet gas compressor suction pressure in the fractionator reflux drum?
--Kumar, India
Kumar, I assume you have a motor driven, fixed speed compressor. If you had a variable speed compressor, you would simply slow the compressor. When we suction throttle to hold back-pressure on the reflux drum it will start to force the spill-back valve to close. This does two things:
1. Total number of moles of wet gas compressed will drop.
2. Compression ratio (due to the suction throttle valve delta P) will increase.
As long as you are on the flat portion of the flow vs. head compressor performance curve, the motor amps will decline.
--Norm Lieberman
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