You know the story. You’ve ordered a beer and you’re watching in horror as the bartender fills your glass with a serving of foam that shoots from the tap. They waste valuable beer by attempting to scrape off the excess foam, and you waste valuable drinking time waiting for the excess head to dissipate. At another bar, the beer is barely a trickle from the tap and you and other customers are lined up wondering if you’ll actually get to order your beers before happy hour finishes. Both situations are examples of unbalanced keg systems. It’s because of these experiences that most home brewers who are looking to build some form of keg system know that if it’s not done correctly, it’s a pain in the proverbial. The correct design of a beer dispensing line - from the keg to the tap - to ensure that foam doesn’t form in the line whilst maintaining a healthy flow rate is known in home brewing circles as balancing kegs.
Too much back pressure means a faster pour, but more foam. Give your beer a smooth ride. We want smooth beer lines with as few changes in cross-section as possible. You’ll see that increases in velocity caused by hose kinks, fittings with non-smooth corners, small tap orifices etc will create a rapid pressure drop and in turn, create foam. Keep it in solution. We also want to apply enough back pressure to keep the carbon dioxide in solution (balanced) as it travels through the beer line at the temperature of your kegerator. This back pressure must be applied very gradually, because rapid changes also cause foam (think of opening a soft drink that foams up). This back pressure should be applied by simply extending the length of the beer line to keep things smooth. The back pressure applied per unit length of hose is related to the hose diameter. This means that we can set the desired back pressure by selecting the most suitable combination of beer line diameter and length. The difference in height between the tap and the keg also affects back pressure, but unless you’re putting in a serving system in a large pub where there are often keg cellars underground, one can typically disregard this as it’s insignificant for kegerator setups. Keep it cool. The primary reason is to keep the beer in the line in equilibrium, as warm taps will cause foaming. Manage your flow rate with the beer line, not the keg pressure. The pressure in your keg should be kept at equilibrium with your desired carbonation level and kegerator temperature. Increasing the keg pressure to get higher flow will cause more rapid pressure drop and in turn, foam. Design for the worst-case. Like any engineering activity, you should design your system for the worst case scenario IE - highest keg pressure/carbonation you expect (note that keg pressure isn’t always the same as regulator pressure, particularly in a warm keg with check valves). Err on the side of longer beer lines. That way you can shorten them easily rather than having to install a whole new line (you could use joiners but that’s bad practice in sanitary lines).
Back pressure: BP Keg pressure: KP Over pressure: OP Specific back pressure (hose resistance): R (pressure drop per unit-length) 1. Determine your required back pressure. Since the pressure of the beer at the end of the tap is almost atmospheric (zero gauge pressure), the back pressure required is simply the keg pressure minus a value of about 6 kPa (1 PSI). This “over pressure” is to give a decent flow rate. To give you an idea of how much you can play with this over pressure, note that professional installations very rarely exceed 30 kPa (5 PSI). Less over pressure generally means a slower pour but a more easily stable system. BP = KP - OP 2. Select a beer line diameter, and note the “nominal” resistance/back pressure per unit length: 3. Calculate the beer line length (L) required: L = BP/R 4. Test and Adjust. If your calculated line length is too short to reach where you want to go, simply repeat steps 2-3 with a larger line diameter until it works out. Remember it’s easier to have a hose that’s too long that too short!
1. Determine required back pressure. I’ve determined that my beer needs about 2.3 volumes of carbon dioxide for carbonation. Therefore, my kegs are pressurized to 70 kPa (10 PSI) and held at about 4C (39 F). I’ll work to a 6 kPa (1 PSI) over pressure, and thus my required back pressure is 64 kPa (9 PSI). Since my kegs are held at 4C, I’ll maintain my beer lines at this temperature too. 2. Select a beer line diameter I’m selecting a 5mm ID (refer to table above) line because it fits to my existing hose barbs 3. Calculate line length required: L = 64kPa / 33kPa/m = 1.94 m It’s worth noting that this will be a balanced line up to a keg pressure of 70 kPa. Should I wish my system to be useful for styles with higher carbonation, I will need longer lines or a lower temperature to remain balanced. Is the calculated length long enough? Turns out that 1.94 m isn’t long enough for me kegerator. I could repeat my calculations for a larger hose, but then I’d need hose adapters which cause turbulence! I’ll elect to have a line 2.5m long so I can pour beers with slightly higher carbonation as well, albeit with a slightly slower pour.
If you’ve followed this article and still get foaming in your beer line, check for the following: • Is your keg pressure higher than your regulator pressure? Many CO2 manifolds have check valves on them, and therefore the keg may be at a higher pressure than you realize as the check valves may have prevented equalization • Check your fittings for smoothness of cross section changes and replace if possible. • Too long is better than too short! Try a wider diameter, but correspondingly longer beer line • Reduce the temperature of your kegerator to drive the carbon dioxide back into equilibrium
Being a geek myself, I like to understand why I’m doing something. For those who are similarly-minded, keep on reading. If you’re not interested in the science, thank you for reading! Why does foam form? If the temperature and pressure of your beer lines and keg aren’t right for your carbonation level, carbon dioxide will come out of solution inside the beer line and you’ll get foaming. On the other side of the coin, you could end up with a very slow pour. Carbonation is simply carbon dioxide that has been dissolved in the beer. How much carbon dioxide can be dissolved (solubility) depends on temperature, pressure and composition of the beer. Since we’re unlikely to be changing what’s in the beer after we’ve kegged it, we can ignore the last point. Understanding how solubility varies with pressure and temperature are key to designing a balanced beer line and keg system. The difference in height between the keg and tap also plays a role, but only a minuscule one on the homebrew scale. The higher the temperature of the beer, the less carbon dioxide can exist in solution (this may be counter-intuitive to those who are used to dissolving solids where it’s the other way around). The higher the pressure, the more carbon dioxide can exist in solution. Another important point is the rate at which carbon dioxide either dissolves into, or or out of, the beer. Essentially, the closer the conditions are to an equilibrium, the slower the dissolution of carbon dioxide. What does this mean? We don’t want to make any large and rapid changes to either temperature or pressure as the beer flows through the line. This is where the term balancing comes in. You need to balance the temperature with the pressure to keep the desired amount of carbon dioxide in solution to avoid foam. Let's look at a garden hose. The longer the garden hose, the more resistance it applies, and consequently, the less water flows through it. The same outcome occurs if you make the hose a smaller diameter or insert fittings such as taps and elbows in the hose. We can jack up the pressure on a garden hose to compensate, but with beer, we have to keep our carbon dioxide in solution by selecting a combination of pressure and temperature that maintains equilibrium. Well that sounds easy enough! Just play with the fridge temperature and gas regulator settings until it works! Unfortunately that would only reliably work when our beer tap is closed, and the pressure in the beer line is the same all the way from the keg to the tap. When we have our tap open, the pressure at the spout is nearly atmospheric (zero gauge pressure) and increases as we follow the line back to the keg. The beer line applies pressure drop through friction and other hydraulic effects. We want a very small amount of pressure remaining to give us a decent flow rate. Without going into the nitty gritty of fluid mechanics (dammit Kris, we’re here for brewing!), the faster a fluid flows, the greater the pressure drop (and lower its pressure) at that point. This is because mechanical energy in a flowing non-compressible fluid can take two forms - pressure and momentum (mass x velocity). High pressure = low momentum, and visa versa. This image shows the situation, and what’s going on with the pressures and temperatures as beer flows through your lines into your glass. You can see that any sudden change in velocity due to small orifices, such as those in typical Cornelius keg disconnects or hose kinks, cause sudden large pressure drops. These often can be the cause of foaming in an otherwise balanced line.
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