The Correct Pressure for your Beer Dispense Lines: The Practice
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Part 1 of this blog series tackled the science behind obtaining the correct pressure for your beer dispense line. Part 2 (this blog) addresses the practice. Of course, all of us in the brewing industry are obsessed with pouring the right beer in the right beer-clean glass to obtain the proper presentation. In this blog, we expose the variables which play a role in achieving (or not!) the perfect pour.
As we all know, generally, and we emphasize generally, most beer is dispensed (tapped) at 10-14 PSI. If the account is getting too much foam, then they are told they should have a longer beer line but it is a tad more complicated than that, and often, when this happens, the beer is too warm.
So, let’s break it down:
Beer temperature.
The colder the beer, the more CO2 is absorbed into the beer. In general, the lower the temperature of the beer, the lower the pressure which is required to provide a specific number of volumes of CO2 in the beer.
For example, most American light lagers have a specification of 2.7-2.8 vols CO2. To ensure this CO2 stays at the correct level, this is one reason why these type of beers are often served ice-cold, sometimes as low as 30 degrees F. But not all beers or beer styles need to have this level of CO2. British ales might be as low as 1.5 vols CO2, so one piece of information required is to know the design carbonation level for the beer being served.
Keg Pressure.
Together with the beer temperature, the keg pressure will control the level of CO2 staying in solution. The beer line length must be balanced with the amount of pressure in the keg used for serving. If the system is balanced, then the beer should pour from the tap at about 2 oz per second.
Beer Line Temperature.
The beer line temperature should be kept cold to prevent CO2 release from the beer. For most, this should not be an issue. However, if draft beer towers are used, then it is a good idea to ensure that those lines are refrigerated AND the tower should also be carefully insulated to minimize CO2 release from the beer.
Resistance.
There are several examples where resistance plays a significant role in successful dispense. Line resistance is sometimes called the “Restriction Value.” The beer line length is important. The longer the beer line, the more resistance will be experienced. The beer line material is also essential. Vinyl beer lines exhibit more resistance than stainless steel beer lines per foot of line. The inside diameter of the beer line has a restriction value, i.e., the larger the inside diameter, the less the restriction. Resistance values were listed in Part 1 of this blog as was the need to know the rise of the beer line from the center of the keg to the tap (so-called gravity value). The general rule of thumb is that the beer loses 0.5 PSI/foot of elevation. So, if the tap is 2 ft higher than the center of the keg, then the loss is 1 PSI.
Armed with all this information, we can now calculate the pressure to be applied to the system.
Pressure = (length of beer line in feet x line resistance) + (Gravity x 0.5)
One can also use this information to calculate the proper length of what the beer line should be with:
L = (P - (H x 0.5) - 1)/R
where:
L = length of beer line in feet
P = pressure set of regulator
H = height in feet from the center of keg to tap
R = resistance of the line referring to the table in Part 1
1 = residual pressure remaining at the tap (this can be increased to 2 if the dispense rate needs to be increased
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