Balancing A Draught System
by Wortgames on 03-09-2007 at 02:02 am

'Balancing' a draught system isn't exactly 'voodoo' - but it is a bit 'zen'. Actually it is quite a simple process, providing you don't allow yourself to be tripped up by misunderstanding, bad habits and impatience. If you are having trouble grasping the principles, then clear your mind, forget everything you know, and let us try to enlighten you from the ground up.


ACHIEVING CORRECT CARBONATION

Let's forget for a moment the business of moving the beer around or pushing it through lines. Let's just concentrate on carbonating the beer itself. The first thing to understand is that beer will carbonate to a specific fizziness based on only two things - temperature and pressure. Cold liquids absorb and hold gas better than warm ones (which is why warm beer will go flat very quickly); and higher pressures force more gas into solution than low pressures (obvious really). So we know, for example, that beer stored at 10C will need significantly more gas pressure to achieve the same level of fizziness as beer stored at 4C. The second thing to remember (and remember it well) is that the carbonation level in your beer will only remain stable as long as the conditions remain stable. Change either the temperature or the pressure, and the beer will try to expel or absorb gas accordingly. Keep the temperature and pressure constant, and the beer will ultimately reach a state of perfect equilibrium and stay there indefinitely.

We usually describe the fizziness of drinks in terms of the 'volumes' of CO2 they contain - most commercially available beers are served with around 2.5 volumes of gas (ie one litre of beer contains 2.5 litres of gas dissolved into it). This can vary though from some British ales (for example, stout) that may have as little as 1 volume, to German Weisse beers that can have over 4 volumes. But lets master the walking before we try to run, and work on 2.5 volumes as a nice middle ground. To achieve this desired level of carbonation, we must control our temperature and our pressure. Temperature is usually one of those set-and-forget variables - based on our personal taste and refrigeration capabilities - which means that once we've decided what temperature our beer will be stored at, we need to dial in the appropriate pressure to give us the carbonation level we want.

Ideally different styles of beer are served at different temperatures - 'correct' storing and serving temperature for a British ale is actually around 11C (known as 'British Cellar Temperature'), and for lagers it is closer to around 6 degrees. Unless we have multiple fridges though we need to be realistic and pick a single temperature we are happy with. Here in Australia a lot of folks like their beer icy cold, but most 'real' beer lovers would probably opt for something around 6 degrees, which is still cool enough for a refreshing lager without being so numbing to the tastebuds. It is a little cool for ale, but better too cool than too warm - the beer can always be allowed to warm in the glass if necessary, and the cooler temperature will tend to delay any spoilage.

So in order to correctly carbonate our keg of beer, we decide how fizzy we want it to be (again, different styles... but let's aim for our nice average figure of 2.5 volumes for now). Then we need to decide (or discover) what temperature our beer will be held at - just measure the temperature of a liquid that's been in your fridge for a few days and use that as your guide. Then we consult a chart like this one (look at the purple section at the bottom)...

reckoner.pdf (44.3kB)

...or we can use a calculator (such as the excel worksheet attached to the end of this article) to find out how much pressure we need in order to maintain our desired level of carbonation for our chosen temperature. It's as simple as that.

In our example (6 degrees C) we can see from the chart that we need 14psi (97kpa) to maintain 2.5 volumes of carbonation. If we leave the keg connected to the gas at this setting, it will gradually reach that level of carbonation all by itself over a week or so. Once it has got there it won't get any fizzier, and it won't use any more gas - it has reached its equilibrium. If we are impatient we can accelerate the process quite significantly by shaking or rolling the keg (increasing the surface area of both gas and beer), and/or by increasing the pressure temporarily to force the gas in faster. One popular way to speed up the carbonation process is the Ross method. Remember though that this is not in itself an essential step in carbonating beer - it is simply a way to accelerate the carbonation process that would otherwise occur all by itself.


BALANCING THE SYSTEM

So now we've got our numbers dialled in, the keg has been sitting at the correct temperature and pressure for several days, it has reached equilibrium and everything is nice and settled. Now we need to get at the beer. If we attach a tap directly to the keg, we will almost certainly get a high-pressure jet of beer in our lap. We need to slow down the flow, and we do this by adding some resistance between the keg and the tap to give us a nice controlled rate of pour. In other words we need to 'balance' our draught system.

There are a number of factors that affect serving resistance - the type of beer line, the design of the tap or gun, and the head (height difference between the keg and the tap) will all affect the real-world speed of the pour. The easiest and cheapest thing to control is the length and diameter of the line - so that is what we do. This is the reason you'll often see a small coil of beer line inside somebody's beer fridge - the line does more than just get the beer from A to B, it also helps to slow it down on the way.

Technically, we would use a different line length for each style of beer, as in an ideal world they would all have their own regulators and be under slightly different pressures. Few brewers go this far, but a number will run separate ale and lager taps. Most though simply settle on a happy medium that suits their normal range of beers in their normal range of conditions. In this case a low-carbonation ale will simply pour a lot slower than a highly-carbonated lager on the same line, because the ale will be under a lot less pressure.

For most homebrewers, we are generally talking about fairly short distances, so it probably makes sense to choose nice thin beer line as that will give us more resistance per metre than larger line. It can be tricky to push narrow line onto a barb, but nothing some boiling water (for softening), needlenose pliers (for stretching), and a pair of gloves (for skin retention) can't solve. For obvious reasons, it is better to start with a line length too long, and then shorten it if the flow rate is too slow. Beer line is pretty cheap, so buy a bit more than you think you'll need. As a suggestion, try starting with something like 3-4 metres of 4mm ID line, and shorten it as necessary.

A recent innovation is taps with built-in flow restrictors. Usually this is in the form of a small lever on the side or a rotating collar on the shaft, that moves a cone in or out of the flow, increasing or decreasing resistance. These taps allow convenient adjustment of the flow rate to tame higher line pressures, but they may be prone to slightly more foaming than a properly set up, non-adjustable tap.

It is worth mentioning here that professional draught system installers rarely deal in the same level of accuracy as home keggers. They may have multiple taps connected to a single keg, but they also have the luxury of being able to run their systems at higher-than-equilibrium pressures, as a keg of commercial beer is usually emptied within a few days. The beer is gradually becoming more and more carbonated, but it's contents are gone before this becomes a problem. Occasionally a cellarman may have to vent the excess pressure from a slow-moving keg that becomes a problem and let it rest for a couple of days before reconnecting it and starting the process all over again. As homebrewers, we make very fine adjustments to our systems to allow a keg to remain connected and pouring perfectly for sometimes weeks on end.

Ideally, our system will be free of any gas leaks (use soapy water around the joins and look for bubbles - but rinse well afterwards and make sure you keep the soap out of the beer!). This way we can leave the gas permanently connected at our equilibrium pressure, and the system is essentially maintenance-free. If you think you may have leaks, and you are worried about losing your entire bottle of gas, you can turn the valve off overnight - but the cost of solving a leak this way is fluctuating pressures and unnecessary inconvenience. A sealed, leak-free, and perfectly balanced system is a wonderful thing.

One final note: remember that any time you make an adjustment to the temperature or pressure of your system, it can take several days for it all to reach equilibrium again - so once you have a system which is workable, it is generally far better to be patient and make small tweaks over time rather than try to get it perfect all in one afternoon. Patience is most definitely a virtue when balancing a draught system.


FOAMING PROBLEMS

If you've followed these pointers, you really shouldn't have problems with excessive foaming. Foaming is caused when the dissolved gas comes out of the beer, forming bubbles in the process. This happens to a certain extent whenever we pour a beer, which is why we get a head - but if we're not careful we can cause it to happen too dramatically and we get far too much head and a flat beer. This most often happens when the beer warms up (for example, the first beer coming through a warm tap), or if the pressure drops (for example, someone has reduced the keg pressure to try and slow the rate of pour). It can also happen if the tap you are using was not designed for beer, or if it wasn't open all the way (beer taps are either open or shut - there is no in between!) It can also happen if the beer encounters turbulence on its way through the line - for this reason it is best to keep any joins or line diameter changes to a minimum, leave the fridge thermostat and the regulator alone, and insulate well any line that is outside the fridge.

If you have inadvertently overcarbonated a keg, then you will nearly always get excessive foaming unless you can increase the pressure to a level which can keep the gas in solution, but then you must also increase the serving resistance to a level that allows a controlled pour. In this case you will simply get a very gassy beer (and probably a large head), but it should be drinkable. Generally though, it is better to vent an overcarbonated keg and allow the excess gas to escape from the liquid. This can be done by simply disconnecting the gas, releasing the pressure valve, letting it rest for a while, then releasing the valve again and so on. Each time you do this you will be reducing the headspace pressure and allowing the beer to release more gas. Shaking the keg between each venting increases the surface area of the liquid and encourages gas to pass out of the beer into the lower pressure of the headspace, but it will create foam in the keg which you'll need to allow to settle before attempting to vent it. Shaking also helps when trying to get gas into the beer, of course, the only difference is whether the gas pressure in the headspace is higher or lower than the beer's current 'equilibrium' point. If higher, the beer will carbonate, if lower it will decarbonate. If the system is at equilibrium, then sloshing the keg around will have little effect.

Ultimately you want to reduce the carbonation level to just BELOW your target, so that when you reconnect the gas at your chosen pressure the beer will be in 'absorbing' mode rather than 'expelling' mode. It is, of course, better to avoid overcarbonating the keg in the first place, and the best way to do this is to heed the home keggers mantra:

"DON'T SCREW WITH THE REGULATOR!"

Well, grasshoppers, here endeth the lesson - now go forth and carbonate...
http://www.aussiehomebrewer.com/uploads/ineo/crozdog_balancing_table.xls