Balancing: Correct Serving Pressure Vs Beer Line Length

Question

How long should my beer lines be in my kegerator or keezer for the perfect pint?

Answer 1

Original Source: BYO.com Balancing your Draft System: Advanced Brewing

With:

• 3/16" beer lines
• Serving tap 2ft above the keg
• 5 PSI CO2 serving/dispensing pressure (high for some Homebrewers)

A 2ft beer line would be a good starting place (but start longer you can always cut some off but you can't put back on).

A matter of balance

Calculating the correct dispensing pressure and making changes to the system is known as “balancing” and is critical to pouring a perfect beer. Balance is not only dependent on the carbonation level and the temperature of the beer, but several other factors also enter into the equation. These include the overall height difference between the keg and the tap, the length and diameter of the dispensing line and the type of tap being used. Changes to any one of these will change the balance of the system.

Between the keg and the tap, there is resistance to the flow of the beer. Gravity (the difference in height) accounts for 0.5 PSI per foot (11.3 kilopascals per meter), a positive value if the tap is located above the keg, negative if the tap is below it. A standard beer faucet has a resistance of 2 PSI (13.8 kPa); the shank adds another 1 PSI (6.9 kPa). A picnic or “cobra” tap has a resistance of about 0.5 PSI (3.4 kPa). Additionally, the beer line itself offers the following resistance based on the inside diameter. (These figures are for flexible vinyl beverage tubing):

3/16 in. (4.75 mm) inside diameter (ID): 3.0 PSI/ft. (67.9 kPa/m) 1/4 in. (6.35 mm) ID: 0.8 PSI/ft. (18.1 kPa/m) 5/16 in. (7.94 mm) ID: 0.4 PSI/ft. (9.0 kPa/m) 3/8 in. (9.53 mm) ID: 0.2 PSI/ft. (4.5 kPa/m)

Finally, some additional pressure is necessary to achieve a proper flow rate. The generally accepted desirable pour rate for beer is considered to be 1 US gallon (3.8 L) per minute or 1 US pint (473 mL) per 7–8 seconds. For most systems, a value of 5 PSI (34.5 kPa) is sufficient for balancing calculations.

Assuming that the other values remain the same, the easiest way to balance the system is to adjust the line length so that the total resistance of the system equals the carbonation pressure minus the required 5 PSI (34.5 kPa) for a proper flow rate. Round the result to the next highest foot (0.3 meter).

For example, for a pale ale that is carbonated to 2.3 volumes of CO2 at 46 °F (8 °C), the correct carbonation pressure (from the force carbonation formula) is 13 PSI (89.6 kPa). The beer is dispensed through a standard shank and beer faucet at a height of 2 ft. (60.9 cm) above the center of the keg. Here are the calculations for the required length of 3/16 in. (4.75 mm) diameter beer line in order to balance the system:

Gravity resistance: +2 ft. (60.9 cm) * 0.5 PSI/ft (11.3 kPa/m) = 1 PSI (6.9 kPa)

Shank resistance: 1 PSI (6.9 kPa)

Faucet resistance: 2 PSI (13.8 kPa)

Fixed resistance of the system (not including the line): 2 + 1 + 1 = 4 PSI (13.8 + 6.9 + 6.9 = 27.6 kPa)

Carbonation pressure of the beer (2.3 volumes of CO2 at 46 °F/8 °C): 13 PSI (89.6 kPa)

Pressure required to dispense beer at 1 gallon (3.78 liters)/minute: 5 PSI (34.5 kPa)

Pressure needing to be balanced: 13 - 5 = 8 PSI (89.6 - 34.5 = 55.1 kPa)

Resistance to be supplied by the line: 8 - 4 = 4 PSI (55.2 - 27.6 = 27.6 kPa)

Resistance of 3/16 in. (4.75 mm) ID beer line: 3 PSI/ft. (67.9 kPa/m)

Length of 3/16 in. (4.75 mm) ID line required to achieve 8 PSI (55.1 kPa) resistance: 4/3 = 1.33 ft. (40.5 cm)

Rounded to next highest foot (0.3 meters): 2 ft. (61 cm)

Therefore, 2 ft. (61 cm) of 3/16 in. (4.75 mm) ID diameter tubing will balance this system for the example beer.

(Note: This length seems short by homebrew standards because 5 PSI is a higher “overpressure” than most homebrewers use. Lowering the dispensing pressure to 0.5–1.0 PSI will result in a line length more in line with usual homebrew setups. Experiment with flow rates to find one you like.)