serving pressure confusing - Home Brew Forums

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09-29-2009, 07:34 PM   #1
smizak
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Dec 2007
Buffalo, NY
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I'm a little confused here.

Some kegging tutorials I've read have suggested calculating the required hose length and vertical lift to equal the head pressure in your keg, effectively balancing the draft system with an open tap, then adding 2-3 psi for dispensing. The problem is that eventually the beer will absorb the gas and equalize the additional 2-3psi and over-carb your beer, so you would have to turn the regulator up while dispensing, then turn it back down again after.

What would stop me from calculating the hose length/vertical lift 2-3 psi below the head pressure, so no finnagling with the regulator is needed? That way there would always be an additional 2-3 psi between the head space and the line resistance/lift restriction.

This seems way too easy, so tell me how I've got it wrong.
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09-29-2009, 08:07 PM   #2
ajwillys

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May 2008
Holly Springs, NC
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The beer would absorb the exta psi but as long as you leave the tank open and don't fiddle with anything, it will still push it out. The formula is for balancing a system, ie how much pressure is required to overcome the resistance of height and line pressure. If PERFECTLY balanced, the beer will travel through the beer line, up to the faucet, and run out of the necessary pressure to counteract the resistance right at the tap, with no beer actually pouring out.

The extra 2 psi is so that it actually comes out. When determining the balance of the line, you also have to consider desired carbonation, like you mentioned. Since CO2 in a kegerator is used for two purposes, carbonation and propellent, its a delicate balance. The best way is to determine your desired carbonation, come up with a range of psi that will accomplish that (at the given temperature), usually in the 10-13 psi range, and then adjust your line length accordingly.

If you want the short answer, set your regulator at 11 PSI, your fridge at 36-38 degrees and use 10 feet of 3/16" tubing. This will get you pretty close on all variables. Then, you can tweak slightly hear and there to eliminate foaming from excess pressure but maintain the desired level of carbonation.

09-29-2009, 08:15 PM   #3
Bobby_M
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Aug 2006
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"balanced" in draft balancing is really just determining the line length/diameter to achieve enough pressure to dispense while NOT screwing with the carbonation equilibrium pressures in the keg. What they're trying to say is that you want your calculated line resistance to be about 2 psi less than your keg pressure so that you net a flow out of the tap. The calculations don't really work that well and you'd be better off starting with 10' of 3/16" ID tubing. Go with that for a while and if you find your pours too slow, hack off a foot at a time.
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09-29-2009, 09:08 PM   #4
smizak
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Thanks Bobby and aj. I was sure that the equations would be too good to be true. I'll start with 10' 3/16" and tweak from there.
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09-29-2009, 09:43 PM   #5
shortyjacobs
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Aug 2009
Twin Cities, MN
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Quote:
 Originally Posted by ajwillys If you want the short answer, set your regulator at 11 PSI, your fridge at 36-38 degrees and use 10 feet of 3/16" tubing. This will get you pretty close on all variables. Then, you can tweak slightly hear and there to eliminate foaming from excess pressure but maintain the desired level of carbonation.
This works perfect for me.