Compressed air dryers at high altitudes

[aclaux]

I have a compressed air application in a mine at 15,000 ft asl. I got a table from Sullair with derating factors for screw compressors at different altitudes. At 15,0000 ft asl a compressor looses 46,8% of its capacity.
What I need to know is how to derate a refrigerant dryer for this application. Would the altitude affect the refrigeration circuit?, how can I understand what happens with the R134a refrigerant in the expansion, evaporation, compression and condesation processes?.
Your advise would be very much appreciated.

 

[zdas04]

The arithmetic underlying the description of compressor performance uses a concept of "compression ratios" or the ratio of discharge pressure (in absolute units) to suction pressure (also in absolute units). As elevation goes up, an air compressor has to do a more compression ratios to reach a given discharge pressure (because suction pressure is low). For a refrigeration compressor, the suction pressure is much higher than atmospheric pressure, so the derate is less. For example if local atmospheric pressure is 10 psia at elevation and 14.7 psia at sea level:
[ul]
[li]For the air compressor if required discharge is 125 psig, then at sea level you are doing 9.5 ratios and at elevation it is 13.5 ratios [/li]
[li]For the refrigeration compressor if the suction is 35 psig and the discharge is 150 psig then at sea level you are doing 3.3 ratios and at elevation you are doing 3.5 ratios, not nearly as dramatic a difference[/li]
[/ul]

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[DLite30]

It's a sealed system. So, the atmospheric pressure will only affect what you see in the system's indicated gauge pressures (i.e. compressor discharge and suction gauge pressures will be higher than they would be at sea level). The internal thermodynamics of the refrigerant system will remain unchanged. However the heat rejection capability of the heat exchangers would probably change a little at the higher altitude, but I'm not sure how to quantify that.

 

[ChEMatt]

Basically you're going to find that the condenser fan (assuming you're not using cooling water) and exchanger are going to get bigger due to less mass moving across the coils. Other than that, I would think that the capacity wouldn't change much. As DLite30 mentioned, it's a sealed system. I'm going to assume it's driven by an electrical motor.