Trying to understand the relationship between foam and line length

[Dr_Horrible]

I've seen the popular equation that helps you calculate line length, and generally it is calculated to leave a residual pressure of 1 at the faucet. Is this extra 1 psi what helps generate the foam and resultant head? If you equalized it at 0psi, does that mean that there would be no extra pressure and the beer wouldn't travel out of the faucet?

If the line is too long, then the beer will trickle out. If you continue to cut it down a short length at a time, it will eventually start coming out quicker due to less resistance in the line. Does it then reach a point where it starts foaming more at the tap due to the higher rate of flow?

If you had a theoretical situation where your line was balanced, and slowly added 1 foot of line at a time, which would decrease first, the flow rate or the foam at the faucet? Or are they directly related?

Trying to understand some of this conceptually in my mind, thanks!

 

[porterpounder]

All correct. If there is no pressure at the end of the line, the system is in equilibrium and no beer will be pushed out of the faucet. The relative change in pressure of the solution from 1 to 0 when it exits the faucet encourages a controlled release of CO2 from solution to bind to proteins and create foam. Check out Dr. Charles Bamforth's book "Foam".

The physical resistance, depending on the diameter of the tubing and gravity (about 1/2 psi per vertical foot), keeps the CO2 in solution until it reaches the point where it can release, so the shorter the line, the less resistance to keep the CO2 in solution, thus a more vigorous release of gas when it comes out of the faucet. Think of letting gas out of a balloon slowly and popping it. You start with the same pressure in the balloon, but the results of slow vs. rapid gas release are very different.

If you added a foot at a time to a balanced system I don't know what would decrease first.

 

[SpeedYellow]

I've seen the popular equation that helps you calculate line length, and generally it is calculated to leave a residual pressure of 1 at the faucet. ...

Nope. The pressure at the faucet is necessarily zero (atm. pressure). Roughly speaking, halfway up the line, the pressure will be midway between zero and your keg pressure. That's how it works.

If the line is too long, then the beer will trickle out. If you continue to cut it down a short length at a time, it will eventually start coming out quicker due to less resistance in the line. Does it then reach a point where it starts foaming more at the tap due to the higher rate of flow?

Correct.

If you had a theoretical situation where your line was balanced, and slowly added 1 foot of line at a time, which would decrease first, the flow rate or the foam at the faucet? Or are they directly related?

Generally the flow rate and the foam will decrease together when you add line length. But there's another critical variable: line temperature. When you first start pouring, the lines are typically a bit warm if they're up top in the keezer. So the beer warms up thus causing foam, and the longer the lines are, the longer it takes to warm them and the more foam you get!