Dispensing your beer with a properly balanced draft system is a delight. By "properly balanced" I mean dispensing your beer without getting a glass full of foam, or a glass full of flat beer. To understand what balance is, consider the components of a draft system. The keg is under the amount of pressure which will carbonate your beer to the correct value at the temperature inside the kegerator. As an example, our beer is under 8 psi of pressure at 38°F (3°C) which will equalize at 2.2 volumes of CO2. The shank on the faucet will cause about 1 psi of pressure loss on the system. The faucet will supply about 2 psi of resistance to the flow of beer. Say the faucet is 2 feet above the center of the keg, gravity will cause 0.5 psi per foot in resistance to the flow of your beer. The only thing we have control over is the length and diameter of the beer line from the keg to the faucet. Changing either the length or diameter will change the dispensing pressure at the faucet.
So, for a simplified calculation to balance your draft system, all we need to know is the resistance of the most common beer lines used in your homebrew draft system. A 3/16 in. (4.75 mm) inside diameter (ID) beer line has 3.0 PSI/ft. (67.9 kPa/m) resistance. A 1/4 in. (6.35 mm) ID beer line has 0.8 PSI/ft. (18.1 kPa/m) of resistance. And a 5/16 in. (7.94 mm) ID beer line has 0.4 PSI/ft. (9.0 kPa/m) of resistance to flow. Now we have all the information we need. We will assume that we have a 3/16" ID beer line because it offers the most resistance per foot. And we will assume a 1 psi overbalance will serve our beer without any foam. Lets add up all the resistance numbers except the beer line.
Faucet is 2 feet above keg so 0.5 psi/ft x 2 ft. = 1 psi + 1 psi for shank + 2 psi for faucet = 4 psi total resistance in our system without including the length and diameter of our beer line. To figure out the length of our 3/16" ID beer line we need to attain the proper pour rate of 1 US pint per 7-8 seconds, we adjust the length so that the total resistance of our system (4 psi + ? psi from beer line length) equals the carbonation pressure inside the keg (8 psi in this case) minus the 1 psi overpressure required for the proper flow rate. So what we need to find is the 8 psi pressure in the keg minus the 4 psi supplied by gravity,the shank, and the faucet (equals 4 psi) minus the 1 psi overpressure for a proper pour equals 3 psi of resistance that must be supplied by our 3/16" ID beer line. To find out how long the line should be to get this 7 psi of resistance, divide 3psi by 3 psi/foot and we get 1 foot of 3/16" ID hose. Switching to the 1/4" hose would require 3.75'or 4 feet of beer line for dispensing your beer with the correct pour and still maintain the carbonation level in the kegs.
In the real world, you will have to experiment with the pressure and temperature before dispensing your beer. Try starting at 2 psi and gradually increasing the pressure until you get foam in your glass, then back off just a little. This will be very close to your optimum serving pressure. If I get too much foam, sometimes I will release the head pressure on the serving keg, then back the regulator pressure down to 5 psi or so, then try again, it usually works. Just remember to increase the pressure that sits on the keg back to the 8 psi setting so the CO2 doesn't come out of solution and give you an under-carbonated beer next time.
Information for this article Dispensing Your Beer was taken from my own experience with kegging, and the article by Ed Westemeier called A Bottler's Guide to Kegging which appeared in Zymurgy Magazine Summer 1995 (vol. 18, no. 2), and the article from Brew Your Own magazine entitled Balancing your Draft System: Advanced Brewing from the Jan/Feb 2006 issue.
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