Starting Compressors at Low Ambients

[apatjr]

We're working on a project where some 225kW rotary compressors are operating outside. The ambient temperature will vary from 21 to 93 degrees F throughout the year. When operating the compressors will be operating at approx. 205 degrees F. The units have freeze protection and will not be enclosed.

The question is, will the temperature changes affect the brake-horsepower requirements of the drive motor with the different mass flowrates or will the mass flowrates remain fairly constant as any air entering the compressors will quickly reach the 205 degrees.

Thanks in advance for any comments.

 

[JRLAKE]

apatjr,

I know you said rotary but I just wanted to clarify, is this a rotary screw or centrifugal compressor?

 

[JRLAKE]

Assuming you are using a rotary screw, the effect on bhp will be negligible. I have installed many machines in similar environments. The variation in inlet filter pressure drop has a much greater effect on bhp.

I would be more concerned with over heating on hot days. You should be fine at 93 degrees if your coolers are clean. Dirty coolers and a 105 degree day may cause a shut down. Also, on cold days, if the machine is over sized and runs unloaded for extended periods of time, you can experience a condensation problem in your oil. Not all freeze protection packages offered by compressor manufacturers prevent this from happening.

 

[apatjr]

JRLAKE,
Thanks for the input.

Yes, the rotary is an oil flooded screw. We will be running two 50% units so they should be loaded most of the time.

My thoughts on the cold ambient air is that on entering the air-end's compression chamber, the cold air will absorb the heat from the lubricant and start to expand, so that the volume of free air entering the compressor is fairly constant. The question is: Have any studies been done to quantify or address this issue.

 

[JRLAKE]

Not that I know of but I am sure there is one out there. Check the CAGI website:

http://www.cagi.org

The difference in mass flow will not affect the horsepower rating of the compressor, unless the motor is sized very very close to the compressor's referenced condition power draw. The differential pressure and altitude have a much larger affect than variation in temperature. Is this your only concern or are you just curious?

 

[apatjr]

JRLAKE,

The concern is from an engineer reviewing our equipment. He has experience with recips and it trying to get me to mathematically prove what the horsepower increase is going to be.

I've told him its a waste of time that rotaries and recip do not operate the same, but I'm obliged to try and answer his request if I can find the information.

Thank you for your insight.

 

[JRLAKE]

I guess he has to justify his job. In that case, do you have altitude and average humidity conditions for the area? Also include the pressure band that you intend to operate. It's a pretty simple calculation with some psychrometrics.

 

[apatjr]

Yes,

The altitude is sea level.

The humdity will vary from 40% to 100% with 77% being the mean.

The pressure band will be to unload at 155 psig and reload at 145 psig.

The barometric pressure will vary from 990mbar to 1037 mbar with 1015 mbars being the mean.