Line Resistance is Not Futile from BeerSmith

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.

*A simpler, practical 3/16 direct draw example would be 10# at 35 degrees F for a carbonation volume of 2.52. So with the same equation, (10-1)/3= 3′ of 3/16 tubing, which is normal in a standard beer meister or converted fridge. Lets see what happens when we go a bit longer, because that wont hurt anything right? Well working backwards with the same equation would look like this: 5=(x-1)/3 so x= 16 pounds of keg pressure! If adding length then perhaps going to bigger tubing is the answer. So 5=(x-1)/.85 so x would only be 5 pounds…not quite right either, at that point your beer will be FLAT in no time! So how much tubing do we need for 1/4″? Well with the same formula we have x=(10-1)/.85 or 9/.85 with is 10.59 feet, which you can curl up on the top of the keg & have great dispense volumes for filling pitchers. Who said 8th grade math class was useless?

*edit -C

Note that some authors leave out the 1 psi tap pressure (i.e. use zero tap pressure) and simplify the equation to L= (keg_pressure/Resistance) which makes the math even easier (the simplified equation would give you 4 feet of tubing vs 3.66 ft). The truth is that you can target anywhere between zero and 1 psi at the tap and still be in balance – the difference is relatively small, though a slight positive keg pressure will give you a better flow rate.

The four foot example with 3/16″ ID vinyl is great if we only have a few feet to go (i.e. in a fridge) but what if one needs to go further? A simple switch to 1/4″ ID vinyl tubing will get us there – looking at the same 12 psi keg system we get: L = (12-1)/0.85 = 12.9 feet. So with the larger tubing we can deliver our beer to just under 13 feet. For other applications we can consider polyethylene or stainless. However if going a long distance one needs to also consider refrigeration – as you don’t want a large volume of warm beer in the lines.

Beer Line Length and Elevation

Changes in elevation also come into play if you design a more complex serving system. The rule of thumb is that your beer loses 0.5 psi/foot of elevation gain. So if your tap is 1 foot higher than the keg it loses 0.5 psi, and conversely if it is lower than the keg it will gain 0.5 psi per foot of elevation.

So if we roll this into our equation, we get the following for a given height (Height – in feet) of the tap above the keg itself:

L = (keg_pressure – 1 – (Height/2)) / Resistance

So lets go back to our original example of a 12 psi keg pressure, 3/16″ ID vinyl tubing and this time put the tap 2 feet above the keg itself. We get L=(12-1-(2/2))/3 which is 10/3 or a line length of 3.33 feet.

Another example with longer lines: 12 psi keg pressure, 1/4″ ID vinyl and a tap four feet above the keg gives: L=(12-1-(4/2)/0.85 which is 9/0.85 or 10.6 feet of line length.