Re: Volume ratio selection
Quote:
Originally Posted by rosetennis
Hi all,
We recently purchased high stage screw compressor that has ability to manually change volume ratio while stationary (2.2 to 5.0) Our contractor informed us they will set the volume ration for 5.0. This does not seem right when you run the numbers out for our operating conditions (28 psig suction, 110 to 150 psia discharge depending on season) Based on my calculations the volume ration should be 2.2 to 2.8 depending on season.
With a volume ration is 5.0 I believe we are over compressing with an energy penalty. Please comment or confirm
Sounds a little high. You need the installation manual for the compressor to set the VI up;)
It's then a compromise, especially if the suction pressure varies such as load pull down.
28 PSIG is not LT on Ammonia, the figures you give are for an LT compressor.
A guess on my part would be about 3.5 VI, but again if the compressor is on load pulldown the 5 VI may be fine:)
Kind Regards Andy:)
Re: Volume ratio selection
Hi rosetennis,
Quote:
Based on my calculations the volume ration should be 2.2 to 2.8 depending on season.
If the refrigerant is ammonia (you didn't specify) I arrived at the same Vi range you stated.
If the suction pressure does not change from 28 psig (a few pounds one way or the other), then the only reason to change the Vi would be the change in discharge pressure.
One thing you might want to check is the motor amps when the compressor is operating at the two different operating conditions (28/110 & 28/150).
Set the Vi for 2.2 and run the compressor at 28/110psig. Check the motor amps.
Set the Vi for 2.8 and run the compressor at 28/150psig. Check the motor amps.
I suspect if you would dial in the Vi for an average of 2.5, and then run the compressor at the two conditions (fully loaded), the motor amps will not change significantly.
A Vi of 5 is certainly too high for this range of operation. Unless, the compressor is being used as a swing compressor to run at a lower (much lower) suction pressure. If this compressor is being used as a swing compressor for low temp and high temp, then the Vi control should have been automatic.
Re: Volume ratio selection
thanks all. the system is ammonia. The machine is only a high side machine. Will set the compressor for 2.2 in winter and 2.8 in summer
Re: Volume ratio selection
Just be sure to monitor the motor amps. What you really want is the lowest amps at any condition. The actual Vi might be a little higher or lower.
Remember, the Vi is just an estimate.
