Two-phase flow

What is the range of velocities for thermo-syphon return risers that is acceptable in order to carry the liquid?

Have you tried the IIAR piping handbook? I believe there are some values listed in there. I don't remember what they recommend.

I looked there and based off my load my pipe size is correct, but they do not list the acceptable velocity, range only pressure drop, I suppose I could back out the acceptable velocity range from the acceptable pressure range using the Darcy-Weisbach Equation. Thanks for the idea, it got my brain unstuck.

Darch-Weisbach will only produce a value that may not be at all accurate. The ideal flow regime in this riser is annular flow, which is a vapor flow stream rising through a pipe with liquid being pulled up the pipe.

Two-phase flow stuff is a lot more complicated than single phase calc's. The best you can do is find the full & part load heat rejection of the oil cooler and use an average value for velocity when sizing the pipe.

What you can do is work backwards from the table data. Assume a superficial vapor velocity (based on the mass flow of the vapor only) through the pipe. Then use this find the median gas velocity for each load condition.

A little smaller is better than a little larger...;)