A direct draw beer dispensing system is a self contained, thermostatically controlled refrigeration unit, in which one or more kegs of beer are stored at an optimum temperature of 38° F. This system is equipped with beer dispensing hardware and 3/16" inside diameter (I.D.) beer line that can be used exclusively between the keg coupler and faucet.
A direct draw beer system may be contained within a walk-in cooler, a commercial keg box, or within a kegerator or converted refrigerator. These units are available for purchase commercially and for home use. The information that follows will describe the process and procedures to successfully balance any direct draw beer system.
The purpose of this document is to define what a balanced beer system is, and introduce the reader to the components that make up a direct draw draft beer system. Additionally, this document will explain the procedure and illustrate the calculations required to properly balance this type of beer system.
What is a Balanced System – Two Objectives
Balancing a direct draw draft beer system involves calculations and measurements to meet two main objectives:
- Protecting the integrity of the beer eliminating waste
- Acquiring an industry standard flow rate of approximately 128 oz/min or approximately 1 gallon/min
Success in meeting these objectives depends on balancing two separate pressures: applied gauge pressure and restriction pressure. Note: System balance does not have to entail calculations reserved for rocket science. Acquiring accurate measurements and using simple math will be sufficient.
Balancing Gauge and Restriction Pressure
There are two distinct pressures in a draft beer system:
- Gauge Pressure, is the actual pounds per square inch gauge (PSIG) pressure of 100% CO2 applied to the keg. This propels (pushes) the beer through the system. The volumes of CO2 in the beer and its temperature determine PSIG. A domestic lager with 2.6 CO2 volumes at 38˚ F requires 14 PSIG @ sea level to maintain product CO2 levels while eliminating foam issues.
- Restriction is defined as the pounds per square inch pressure (PSI) of resistance the beer encounters as it flows through the system. This restriction comes from three sources: system hardware, gravity and beer tubing.
Each variable has a known psi value and is measurable. When identified and accumulated, this total restriction value works to oppose the gauge pressure.
When these two pressures are equal, a flow rate of approximately 128 ounces per minute is acquired, creating a balanced system. An example would be a draft beer system that requires 14 PSIG applied pressure to the keg with restriction consisting of hardware, beer tubing and gravity that amount to 14 PSI. The beer is pushed through the system with the same amount of resistance it encounters along the way resulting in approximately 128 oz/min.
The hardware components in a draft beer system consist of tailpieces, unions, faucets, shanks, couplers, and more. All of these components restrict the beer as it passes through. Most pieces impart so little restriction that they can be ignored.
One piece of hardware that cannot be ignored is a shank and tube assembly that is utilized in the towers of some kegerators. It consists of 1/4" outside diameter (O.D.) stainless tubing with threads on one end that protrudes down the tower into the refrigerated box itself. The beer line with fittings is easily attached to these threads. When this shank is present, beer encounters 3 PSI of restriction and must be included in the restriction calculations.
Gravity is referred to as either vertical lift or drop. The faucet is either above or below the bottom of the keg. The easiest method of measuring is to visualize two horizontal planes in the system, one at the faucet and the other at the bottom of the keg, then the vertical distance between them. This vertical distance is measured in feet and has a PSI value of .45 per foot. This is rounded to .5 PSI for ease of calculating. A typical direct draw system has a vertical lift distance between these two planes of approximately four feet thus 2 PSI of restriction in gravity. (4 X .5 = 2)
Since it is easily cut to length, beer tubing is the restriction component utilized to balance the system after hardware and gravity are determined. Vinyl beer tubing with a 3/16" inside diameter (I.D.) has 3 PSI of restriction per foot and is used exclusively in direct draw systems. Note: Kegs positioned in a walk-in cooler where the calculated length of 3/16" beer tubing will not be long enough to reach between the coupler and faucet is considered a remote system, and more advanced balance principles apply.
Establish the Applied Gauge Pressure
The first step to balancing a beer system is to establish the applied gas pressure gauge (PSIG) of 100% CO2 required to propel and maintain the beers carbonation to the brewers' specification. This is based on the beer's CO2 volumes and the temperature of the beer. Refer to the brewer's specifications for specific CO2 volumes. Note: Correct for altitude by adding one pound of psi gauge pressure for every 2000' above sea level.