Refrigerant Pipe Velocity and Oil Return

Return to Engineering Main Menu

Email from Tim Morris to TIS on 12/14/18 

Subject: Refrigerant Pipe Velocity and Oil Return

 

*** This discussion took place during the VBalance software rewrite, so I thought the info may be of benefit to others. ***

Here is what I have learned (or re-learned) on refrigerant pipe velocities and oil return. I will continue to look around for more or better data, but this attachment has good info for R-410A and R-22.

 

H:\EngMstr\General Piping Recommendations (For Reference Only).pdf

 

Suction Piping

• Suction pressure losses affect capacity, so it is important to minimize pressure drop thru the suction line. Capacity lose is 0.6% per 1 psi (for R-410A). (Page 3). We may not account for it properly.
• Larger suction pipe sizes help reduce pressure drop, but oil return is critical. There is a minimum refrigerant velocity for the suction line in order to have proper oil return. This value is different for horizontal runs versus vertical runs. 800 FPM (min) on horizontal runs. 1000 FPM (min) on vertical risers.
• Seasons-4 rule has always been the range of 1500 – 2500 FPM. If we set our hard minimum limit to 1000 FPM for the validation warning, we should be okay on oil return, regardless of vertical or horizontal pipe.
• Velocity of unloaded of compressors needs to be examined to make certain the FPM is still adequate.

Liquid Piping

• Liquid pipe pressure lose affects subcooling. For R-410A, 1° of SC is lost for every 5 psi drop.
• Maximum recommended velocity is 400 FPM.
• Minimum velocity is 100 FPM.
• Seasons-4 rule has always been the range of 150 – 250 FPM.

Discharge Piping

• Seasons-4 velocity rule has always been < 2900 FPM.
• Pressure drop on the discharge line has little effect on the overall system.