Refrigerant Suction Riser & Oil Return Calculator
Determine minimum entrainment vapor velocity (fpm), vertical suction riser pipe sizing, and double suction riser necessity under part-load compressor unloading.
Refrigerant & Suction Conditions
Velocity & Oil Return Results
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ASHRAE Refrigerant Suction Riser Design Rules
Compressor crankcase lubricating oil continuously circulates into the refrigeration system. In horizontal lines, gravity and fluid shear drag the oil film along; however, in vertical upward suction risers, gas velocity must exceed a critical entrainment threshold ($1,000 - 1,200 ext{ fpm}$) to drag oil droplets upward against gravity.
Double Suction Riser Arrangement
When an unloaded compressor drops to $25% - 50%$ capacity, a large single pipe experiences low vapor velocity ($< 700 ext{ fpm}$), causing oil to settle and pool at the base of the riser until the compressor seizes from crankcase oil starvation. A double suction riser with an inverted P-trap automatically seals off the larger leg at part load, forcing all vapor through the smaller leg to maintain high velocity.