Compressor efficiency estimate? 1

[MeatWagon]

I am evaluating the savings for the installation of an auto-purger for a ammonia chiller refrigeration system. I know the suction and discharge pressure/temperature for the system. I have an estimate for the partial pressures of the air and ammonia, so I can find the entering and exit enthalpies of the compressor. I also have an estimate of the average kW that the system uses.

In order to find energy savings for purging the air from the system I want to calculate the mass flow rate of ammonia through the compressor. I'm trying to back-calculate the mass flow rate from the kW usage of the compressor.

I need to estimate the efficiency of the compressor (energy in/work on gas mixture). I don't have much information about the chiller except that it is a 350 HP screw type compressor. Can anyone suggest an average ballpark estimate for the efficiency of a compressor of this size at full load?

To further complicate the problem, the chiller system consists of several compressors. The 350 hp screw compressor runs continuously and reciprocating compressors stage on as needed. The recips are between 75 and 150 hp. What is a reasonable efficiency for recips of this size?

Thanks.

 

[pipehead]

Screw compressors are terribly inefficient. As the ratio across the unit increases, their effiecy decreases. At a 1.25 to 1 ratio, they may be about 75% and at 4 ratios as low as 50%. Recips also have some variance. At 1.25 ratios they are 85% and at 4 ratios closer to 78%.

Download the compressor sizing program from Ariel at arielcorp.com. You can select from their units or customize your own. It even has their non current screw units to give you a rough idea on the efficiency.

From this, I'd base load the recips and add the screw as needed.

 

[25362]

When speaking of the capacity of oil-flooded screw compressors refer to:

http://www.irc.wisc.edu/?/file&id=129

Pressing "open file".