Kegeragtor balance equation is garbage?

[scruff311]

*Sorry for the typo in the thread title. Can't change it after posting apparently*

So when looking to balance your draft system, there is this simple equation that can be used to calculate how long your liquid lines should be for a given serving pressure.

http://beersmith.com/blog/2011/07/14/keg-line-length-balancing-the-science-of-draft-beer/

Length of hose = [Pressure - (Height from center of keg to faucet in feet * 0.5) - 1] / Resistance of Keg Line

I have a standard dual tap converted mini-fridge kegerator (essentially something like this). Some time ago I swapped out the standard ~5' liquid lines with 12' 3/16"ID Bevlex 200. I had been carbing/serving at about 12 psi @ 48F for a while, but I feel as though over time my beers start pouring too foamy.

I rearranged the above equation to solve for pressure:
Pressure = (Resistance)(Length) + Height/2 + 1
(3)(12) + 3/2 + 1 = 38.5 psi!!!!!

How is it then that I can get beer to start coming through the faucet at a mere 4 psi? It literally takes nothing to get beer to push through my lines. If this equation is based on physics (friction, gravity, pressure), then my draft system is defying physics.

 

[Homercidal]

You're not determining how much pressure you need to push beer through the lines, your trying to figure a pressure that will serve beer AND keep it at a specific Volume of atmosphere (how carbed it is.)

Almost no pressure at all is required to overcome line resistance and height. But it will push much slower.

The formula isn't designed for how fast you pour, it's designed for a good pour at the proper volumes of atmosphere.

Beers that pour too foamy over time may be overcarbed, or there may be a kink in the lines, dirty lines, cold/frozen spots, etc. I suspect that the serving pressure is just a bit high and it's finally creeped up to where it's causing foaming again.

 

[JuanMoore]

*Sorry for the typo in the thread title. Can't change it after posting apparently*

So when looking to balance your draft system, there is this simple equation that can be used to calculate how long your liquid lines should be for a given serving pressure.

http://beersmith.com/blog/2011/07/14/keg-line-length-balancing-the-science-of-draft-beer/

Length of hose = [Pressure - (Height from center of keg to faucet in feet * 0.5) - 1] / Resistance of Keg Line

I have a standard dual tap converted mini-fridge kegerator (essentially something like this). Some time ago I swapped out the standard ~5' liquid lines with 12' 3/16"ID Bevlex 200. I had been carbing/serving at about 12 psi @ 48F for a while, but I feel as though over time my beers start pouring too foamy.

I rearranged the above equation to solve for pressure:
Pressure = (Resistance)(Length) + Height/2 + 1
(3)(12) + 3/2 + 1 = 38.5 psi!!!!!

How is it then that I can get beer to start coming through the faucet at a mere 4 psi? It literally takes nothing to get beer to push through my lines. If this equation is based on physics (friction, gravity, pressure), then my draft system is defying physics.

Because that entire article is garbage. The equation above makes several assumptions and ignores some basic laws of fluid mechanics. Line resistance for a given line is not a constant like the article suggests, it's highly dependent on the fluid velocity (the faster the flow, the higher the resistance). The resistance figures provided are for a flow rate of ~1gal/min, which is quite fast. Why? Because that's the fastest a beer stored very cold and carbed to 2.7 vol (commercial bar conditions) can be served, and commercial bars want to serve beer as fast as possible without losing too much beer to foam.

The warmer and more highly carbed a beer is, the slower the flow needs to be to prevent excessive foaming. Unless you're serving under commercial bar conditions (which most of us aren't) then the resistance figures, as well as the rest of the equation, is completely useless. Since line resistance decreases as flow rate decreases, you need much much longer lines to slow the flow down just a little bit. This is why people serving at 2.5 vol and 40° find that 10' lines work well, even though the equation says you need something closer to 3'.

One of the biggest misconceptions that equations like this create, is that there's some magic length that produces a good pour, and anything else will cause problems. The only "problem" caused by extra long lines is a very slightly slower pour, but they also allow a lot of flexibility in terms of serving temperature and carb level.

 

[day_trippr]

The only non-garbage line length calculator I have ever seen.

http://www.mikesoltys.com/2012/09/17/determining-proper-hose-length-for-your-kegerator/

Cheers!

 

[thekraken]

The only non-garbage line length calculator I have ever seen.

http://www.mikesoltys.com/2012/09/17/determining-proper-hose-length-for-your-kegerator/

Cheers!

Bookmarking that one. Thank you.

 

[fixitoscar]

The only non-garbage line length calculator I have ever seen.

http://www.mikesoltys.com/2012/09/17/determining-proper-hose-length-for-your-kegerator/

Cheers!

this one works perfect for me. first time kegger and first time success

 

[scruff311]

The only non-garbage line length calculator I have ever seen.

http://www.mikesoltys.com/2012/09/17/determining-proper-hose-length-for-your-kegerator/

Cheers!

Fantastic calculator! Thanks! That empirical simplified equation I referenced earlier should be shot.

 

[JuanMoore]

Fantastic calculator! Thanks! That empirical simplified equation I referenced earlier should be shot.

Yep. Only thing you have to determine is the proper flow rate for your beer temp and carb level (measured in pint fill time). FWIW a 7 sec pint fill will give the same results as the majority of the other crappy calculators/equations found online. IME about 10 works well with average carb levels and temps up to 40°.