This is from the latest Beersmith newsletter:
Line Resistance is Not Futile
So how does one design a draft beer system to maintain proper balance at the tap? The pressure drop depends on resistance in the beer line. Beer lines have two types of resistance - one due to elevation change (i.e. the keg being higher or lower than the tap), and a second due to the beer lines themselves which generate friction as the beer flows through the lines.
Lets look at resistance first to keep things simple. Here are some sample resistance ratings for various popular beer lines:
3/16" ID vinyl tubing = 3 psi/ft
1/4" ID vinyl tubing = 0.85 psi/ft
3/16" ID Polyethylene tubing = 2.2 psi/ft
1/4" ID Polyethylene tubing = 0.5 psi/ft
3/8" OD Stainless tubing = 0.2 psi/ft
5/16" OD Stainless tubing = 0.5 psi/ft
1/4" OD Stainless tubing = 2 psi/ft
Generally plastic tube of smaller than 3/16" ID is not recommended - it provides too much resistance for practical use!
So now that we have the resistance factors how to we go about designing a keg system that is in balance? For the purpose of our example lets assume that you have pressurized your kegging system at a nominal 12 psi, which at a 40F refrigerator temperature represents a mid range carbonation level of about 2.5 volumes of CO2 - typical for an average American or European beer.
At the tap end of our balanced keg system we want a slight positive pressure to push the beer out, but not enough to foam. Generally this would be between less than 1 psi. So let's target a tap end pressure of 1 psi. The math from here is pretty easy to calculate the balanced line length (L):
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.