Draft beer balance....reality vs. calculated line length

Kegging
1

I’ve wondered about this for a long time. Before I discovered system balance “calculators” and knew little about the experience of others, I had balanced my chest freezer setup through trial and error. Typically this meant the following: temp at 38F, CO2 pressure at 10-12 psi over the beer, and the requirement for 8-10’ of 3/16" beer line. Reading about the findings of other posters regarding their systems, this seems to be quite common. Yet, when using the calculators or information from various draft quality manuals and FAQ’s, suggested beer line lengths are about half of what most of us have found to give the “perfect pour”. Just before Christmas, I setup my new double tap kegerator which came with 5’ beer lines. To achieve decent pour, as my experience might have predicted, I have to set CO2 pressure at 4-5 psi.

What’s missing here? Why do the calculators and so many draft beer FAQ’s predict the use of beer lines about half what we find in practice to be sufficient? I know how to balance my system, just curious why this discrepancy exists? I have read through many searches here confirming my findings, but haven’t seen an explanation as to why the math does not seem to work here.

2

Keg newbie here, but I recently found this about beer lines. don’t know if the info is accurate or myth.

[color=#FF0000]Your kegerator system’s restriction controls the flow rate of the beer from the beer faucet. Essentially most of the restriction comes from the beer line and gravity.

Now if you system has a lot of vertical rise or fall (up and down) gravity has a .5 lb/ft of restriction. The best way to determine the right length of line for your kegerator is to take the ideal pressure (call the manuractorer of the beer), add 5 then divide by the line’s restriction value per foot. Then if the beer pours too slow for you, shorten the line 6" at a time.

Example: Molson Candaian

Molson recommends keeping thier beer at 36 degrees F.
(You can call the brewery and ask for any of this information)
Molson recommends 10 psi at 36 degress for the beer you are serving.
Typically your line wil have 3lb/ft of restriction.
10psi plus 5 (to slow) = 15.
15 divided by 3 (line restriction value) = 5 ft of line.
The kegerator needs 5ft of line at 10psi.[/color]

:cheers:

3

My guess would be that people are just repeating what they’ve heard. I don’t think there’s any theoretical basis for the 2-3 psi/ft number. From a fluid dynamics standpoint, it works out to about 0.7 psi/ft: http://seanterrill.com/2011/11/11/a-mor … balancing/

4

+1
also the calculations are for professional bars where they want to pour beer fast.

I like 8-10 feet of 3/16" ID hose even thoough it pours more slowly.

cheers

5

I have been using 10’ lines and 40-44F 11psi for years. It makes the perfect pour … perhaps a bit slow for some people. It really helps the NTAC’s who can’t seem to pour a beer most places without getting half a glass of foam. Here is a video of mine if you need to take a look at what I deem the perfect pour.

By the way, in case you were wondering (NTAC=NoTalentAssClowns)

6

My guess would be that people are just repeating what they’ve heard. I don’t think there’s any theoretical basis for the 2-3 psi/ft number. From a fluid dynamics standpoint, it works out to about 0.7 psi/ft: http://seanterrill.com/2011/11/11/a-mor … balancing/[/quote]

I think that’s it…the restriction numbers often quoted are too high. If I run 5’ of beer line in either my chest freezer or kegerator at CO2 pressure necessary for correct carbonation levels flow is much too fast and far too much CO2 is knocked out of solution.

Good point MullerBrau, 40F at 10 psi with 10’ line will get you close to the perfect pour without hassle…nice pour, by the way!

7

My guess would be that people are just repeating what they’ve heard. I don’t think there’s any theoretical basis for the 2-3 psi/ft number. From a fluid dynamics standpoint, it works out to about 0.7 psi/ft: http://seanterrill.com/2011/11/11/a-mor … balancing/[/quote]

a10t2…went back with the calculators after reading the article you linked to and using the .7 line resistance came up with beer line lengths much more realistic and in-line with practical experience.

8

what about the gas line? is there a particular optimum length (same as beer line) or short as possible so as to not waste money or keezer space.

:cheers:

9

[quote=“StormyBrew”]what about the gas line? is there a particular optimum length (same as beer line) or short as possible so as to not waste money or keezer space.

:cheers: [/quote]Gas line does not matter at all.

10

[quote=“MullerBrau”][quote=“StormyBrew”]what about the gas line? is there a particular optimum length (same as beer line) or short as possible so as to not waste money or keezer space.

:cheers: [/quote]Gas line does not matter at all.[/quote]

Sweet!

thanks.

11

I’ve used a lot of different calculators, and agree that using a lower resistance corrects the formulas for real-world serving with most common setups. That has resulted for us in 6-7ft of 3/16" ID line at 36-38F and standard serving towers on commercial draft boxes.

12

Thanks, Dean. I’ve even run 20’ of 3/16 line for a hefe I kegged and carbed to the correct volumes of CO2.

13

So if the calculator states that the optimum length is 5 ft, and I have 8 ft lines, what difference does it make? Does it make the beer pour more slowly or not dispense as well as it should?

14

It only matters when the pour becomes extremely slow that it is just not usable. Nobody wants to stand at the tap forever for each pour.

The old standard of 5ft of 3/16" line is a minimum, and works when serving conditions fit the formula, and it works for many many people who serve a constant flow of typical commercial/macro products with C02 pressure and temps right where they need to be. These places will want a fast pour anyway for service reasons and you’ll see waste in a lot of these places that us geeks won’t tolerate :slight_smile:

I think after many years of dealing with serving systems in commercial establishments, at festivals with all kinds of rigs, and in homes and club situations where we may be serving a huge variation of products that there is a true need for lines that may calculate out to be a bit longer. It just gives more freedom with no penalty until you go overboard in length and it starts to affect the pour.

You can’t really say there is a standard, because someone who prefers to serve above the accepted temps of 36-38F will need longer lines just because of the tendency of C02 to come out of solution faster as temps warm. The devil is in each variable, so you’ll learn what works for what YOU want when you serve your product. The key here is that we can do whatever we prefer for the result we want.

Keep in mind that a line that is a bit longer than the math tells you will easily adapt to a much larger variation in serving pressures with no appreciable change in the results. As such I have a line in the setup that is probably 8 or 9 feet, and that is reserved for the higher carbonation levels, but most of the time you can serve lower carbonation beers on it and never know it is the longer line.

15

I did some research on this a while back after noticing the same issue. The fundamental flaw with all the calculators is that they ignore flow rate and assume a fixed line pressure drop/ft. This number, however, is not fixed, and is goverened by flow rate, fluid density, pipe roughness, and pipe cross section. The pressure at the tap is always going to be atmospheric regardless of line length. As the line length is decreased, the flow rate increases so that the line resistance dissipates the gauge pressure. A 100’ line and a 1’ line will dissipate the same amount of pressure, but it’s the flow rate that changes and, consequently, the pressure dissipated per foot of line.

The equation to use for this type of flow is called the D’Arcy-Weisbach Equation for Pressure and Head Loss (link

).

Δp = λ (l / d_h)(ρ V^2 / 2)

Δp = pressure drop in pipe (this is the gauge pressure!)
λ = friction coefficient (based on Re, hydraulic diameter, pipe roughness)
l = pipe length
d_h = hydraulic diameter (just the ID of the pipe since this is circular)
ρ = fluid density
V = linear fluid velocity

Since we commonly set the gauge pressure based on the desired CO2 and temperature, I re-arranged this equation to solve for fluid velocity as a function of line length. The dip tube in the keg will add about 2.5’ of line to the equation, but the diameter isn’t precisely the same so these numbers would need to be fudged a bit to exactly match reality. Either way they seem to match my own observations WRT pour time much better than the online calculators and ‘2-3psi/ft’ rule of thumb.

Also keep in mind that a more highly carbonated beer needs to be poured even slower to keep CO2 in the liquid. I like 100 oz/min as a general rule, but dropping down to 80 or even 60 can help with really highly carbonated stuff. Unfortunately the required line length is ridiculiously long in this case, so I usually use the epoxy mixing tubes.

16

Does the temperature matter?

My corny diptubes are 1/4" ID. I have thought about replacing it with 5-7’ of 3/16" vinyl or silicone hose inside the keg. Then the hose at the output would only need to be 1-3 feet.

Anyone try this?

17

Temperature affects the dynamic viscosity, which is used to determine the Reynold’s number and ultimately the friction factor λ. The difference between 4°C and 10°C is about 5%, and a little over 10% at 20°C.

18

[quote=“nyakavt”]I did some research on this a while back after noticing the same issue. The fundamental flaw with all the calculators is that they ignore flow rate and assume a fixed line pressure drop/ft. This number, however, is not fixed, and is goverened by flow rate, fluid density, pipe roughness, and pipe cross section. The pressure at the tap is always going to be atmospheric regardless of line length. As the line length is decreased, the flow rate increases so that the line resistance dissipates the gauge pressure. A 100’ line and a 1’ line will dissipate the same amount of pressure, but it’s the flow rate that changes and, consequently, the pressure dissipated per foot of line.

The equation to use for this type of flow is called the D’Arcy-Weisbach Equation for Pressure and Head Loss (link

).

Δp = λ (l / d_h)(ρ V^2 / 2)

Δp = pressure drop in pipe (this is the gauge pressure!)
λ = friction coefficient (based on Re, hydraulic diameter, pipe roughness)
l = pipe length
d_h = hydraulic diameter (just the ID of the pipe since this is circular)
ρ = fluid density
V = linear fluid velocity

Since we commonly set the gauge pressure based on the desired CO2 and temperature, I re-arranged this equation to solve for fluid velocity as a function of line length. The dip tube in the keg will add about 2.5’ of line to the equation, but the diameter isn’t precisely the same so these numbers would need to be fudged a bit to exactly match reality. Either way they seem to match my own observations WRT pour time much better than the online calculators and ‘2-3psi/ft’ rule of thumb.

Also keep in mind that a more highly carbonated beer needs to be poured even slower to keep CO2 in the liquid. I like 100 oz/min as a general rule, but dropping down to 80 or even 60 can help with really highly carbonated stuff. Unfortunately the required line length is ridiculiously long in this case, so I usually use the epoxy mixing tubes.[/quote]

Thanks, great info!

19

[quote=“Westy”][quote=“nyakavt”]I did some research on this a while back after noticing the same issue. The fundamental flaw with all the calculators is that they ignore flow rate and assume a fixed line pressure drop/ft. This number, however, is not fixed, and is goverened by flow rate, fluid density, pipe roughness, and pipe cross section. The pressure at the tap is always going to be atmospheric regardless of line length. As the line length is decreased, the flow rate increases so that the line resistance dissipates the gauge pressure. A 100’ line and a 1’ line will dissipate the same amount of pressure, but it’s the flow rate that changes and, consequently, the pressure dissipated per foot of line.

The equation to use for this type of flow is called the D’Arcy-Weisbach Equation for Pressure and Head Loss (link

).

Δp = λ (l / d_h)(ρ V^2 / 2)

Δp = pressure drop in pipe (this is the gauge pressure!)
λ = friction coefficient (based on Re, hydraulic diameter, pipe roughness)
l = pipe length
d_h = hydraulic diameter (just the ID of the pipe since this is circular)
ρ = fluid density
V = linear fluid velocity

Since we commonly set the gauge pressure based on the desired CO2 and temperature, I re-arranged this equation to solve for fluid velocity as a function of line length. The dip tube in the keg will add about 2.5’ of line to the equation, but the diameter isn’t precisely the same so these numbers would need to be fudged a bit to exactly match reality. Either way they seem to match my own observations WRT pour time much better than the online calculators and ‘2-3psi/ft’ rule of thumb.

Also keep in mind that a more highly carbonated beer needs to be poured even slower to keep CO2 in the liquid. I like 100 oz/min as a general rule, but dropping down to 80 or even 60 can help with really highly carbonated stuff. Unfortunately the required line length is ridiculiously long in this case, so I usually use the epoxy mixing tubes.[/quote]

Thanks, great info![/quote]

Yes, but only before homebrew-time. Afterwards, when I read it my brains says, “don’t taze me bro.”

:cheers: