Refrigerant Line Velocity and Oil Return
Line velocity from mass flow, inside diameter, and specific volume.
Example
You enter
- Mass flow (lb/hr) 600
- Line inside diameter (in) 0.75
- Specific volume (ft^3/lb) 0.5
- Orientation riser
You get
- Velocity 1630 fpm
Checked against published worked examples from: ASHRAE Refrigeration Handbook (line sizing / oil return).
Details, formula, and sources
ScopeWith an oil-return verdict against the riser / horizontal minimum and the ~4000 fpm noise ceiling. Per the ASHRAE Refrigeration Handbook.
Formula or tableV_fpm = (mass_flow_lb_hr * specific_volume_ft3_lb) / area_ft2 / 60, where area = (pi/4)(ID_in/12)^2. Oil return needs a minimum velocity (higher in a suction riser); above ~4000 fpm the line is noisy.
Source and editionASHRAE Refrigeration Handbook line-sizing and oil-return guidance, by name.
Free accessASHRAE licensed; refrigerant specific volume is read from the manufacturer's P-T / property data (user-supplied).
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesmass_flow_lb_hr, line_id_in, specific_volume_ft3_lb, orientation, velocity_fpm
- Oil-return window horizontal min ~700 fpm, suction-riser min ~1500 fpm, noise ceiling ~4000 fpm (advisory)ASHRAE Refrigeration Handbook line-sizing guidance
- Specific volume refrigerant specific volume at the line condition is strain-of-refrigerant- and state-specific and user-suppliedmanufacturer refrigerant property data