Trying to serve a 2.8 vol beer at 42° is going to be a challenge. You're going to need pretty long lines to slow the beer way down, since the CO2 is really going to want to escape as soon as it's at atmospheric pressure.
Originally Posted by d_striker
I used the calculation on this page:http://beersmith.com/blog/2011/07/14...of-draft-beer/L = (keg_pressure – 1 psi) / Resistance
So starting with our example of 12 psi keg pressure, and some typical 3/16″ vinyl keg tubing (which loses 3 lb/ft) we get L= (12-1)/3 which is 3.66 feet. So a 12 psi kegging system would provide 1 psi of pressure at the tap with 3.66 feet of tubing.
What's the proper equation?
That is the "proper" equation, but that doesn't mean it's safe to use for everyone. It calculates the MINIMUM length that can be used without creating a foamy mess under assumed ideal conditions. Your actual conditions or idea of ideal conditions may be much different.
If you serve your beer a little warmer than the commercial industry standard of ~36-38° (like most of us homebrewers do), then you'll need longer lines to slow the pour down a little and keep the CO2 in solution. If you want to carb your beer over ~2.7 vol, you'll also need longer lines to keep the CO2 in solution. There's also a lot of variance in the actual resistance of the line between manufacturers or even between batches. The equation also assumes that the line resistance is a constant, when it's actually variable depending on the beer velocity. Sharp shoulders, restrictions, or expansions in the area the beer flows through from fittings and shanks that aren't ideally sized or designed can also knock CO2 out of solution, and longer lines help prevent this.
The only side effect of longer lines is a slightly slower pour. It's also much easier to trim lines that are a little too long than it is to get short lines to grow longer. There are a lot of variables involved, so calculating the shortest possible length that won't create foam isn't easy. Since extra long lines compensate for less than ideal real world conditions without any negative side effects, why not use them instead of trying to calculate and use the bare minimum length?