The method I use for evaporative condensers is to install a pipe the same size as the drain line connection. This is typically a 4" pipe. I do not use velocity because the pipe will only have liquid across the full cross-section of the pipe above the trap.

Liquid drop legs as long as possible is a good general answer, but... what is long enough?

The operating conditions change throughout the year so what works in summer may not be enough in winter. Secondly, if you have multiple condenser installed at different elevations you have a more complex problem to solve. One answer for a single condenser is different than an answer for multiple condensers!

The easiest way to sum this up is: the trap height (distance from outlet connection to the top of the trap) is dependent on the worse case pressure loss. Some people use 2-4 feet, some manufacturers will say to double this for winter time operation, while others may say you need 8-12 feet or more.

Generally, more is better but you need to know what you need exactly before the piping is installed.