How to determine the best pressure and gas or gas mix to optimise your draught beer dispense and minimise losses and unnecessary extra costs.
It came to my impression that there is some misinterpretation about the use of nitrogen for non-nitro beer dispense and that beers aren’t always adequately pressurised.
Standard beers are carbonated with dissolved CO2 which turns almost entirely into carbonic acid.
Depending on the amount of carbonation decided by the brewer, more or less CO2 will be dissolved in the beer. This usually goes around 5g of CO2 per litre of beer (or 2,5 litres of CO2 per litre of beer, unit known as “volume of CO2” which goes: 1L of CO2 per litre of beer = 1,96g of CO2 per litre of beer).
These values can vary between around 3g/L for low carbonation English types of ale to almost 6g/l for weissbiers or lambic beers.
This amount of CO2 must remain dissolved into the beer to avoid carbonation losses (flat beer) or foaming. To allow that, top head pressure should be applied to the keg to prevent the gas from leaving the solution.
How much is needed? This is a function of 2 factors: beer temperature and beer carbonation.
The higher the temperature, the higher the pressure you’ll need to keep de gas in. And obviously, the more carbonated the beer, the more pressure you will need as well.
The right pressure will be determined by easily found charts or calculators. This is called “equilibrium pressure”, the pressure at which no CO2 will be forced into the beer and no decarbonation will occur. The beer remains stable and can be kept without changing.
We can therefore see that for a standard Lager carbonated at 2,5 vols and kept in a cold chamber at 3°C, the equilibrium pressure is 0,75 bar (at gauge/not considering atmospheric pressure).
For the beer to be dispensed at its equilibrium pressure, it is important to consider losses of pressure due to the tubulation, change in height and fittings (valves, John Guest etc.). This can precisely be calculated.
For a tap house where kegs are kept in a cold chamber around 3°C and connected to the taps directly on the other side of the chamber’s wall, a pressure of no more than 0,8/0,9 bar can safely be applied to maintain the beer at equilibrium and with enough force to arrive at the right speed at the tap.
For more complex set-ups where the cold chamber is far away from the bar, it is necessary to increase the pressure for the beer to arrive at the tap adequately.
This is where things get a little tricky. We can calculate the pressure loss between the keg and the tap and realise that a pressure like 1,5 or 2 bar need to be applied for the beer to arrive at the tap at its equilibrium, and therefore avoid foaming or excessive speed.
If CO2 is used alone, this pressure would be much higher than the equilibrium pressure and therefore would be forced into the beer and over carbonate it over time.
This is where N2 comes into play. Nitrogen being poorly soluble in beer, it can be used to give an extra pressure on top of the CO2 without affecting the carbonation.
This means that in the gas mix, the amount of CO2 will be used to maintain the beer at equilibrium and N2 to give extra pressure to counteract the longer distance to the bar.
If a keg needs 2 bar of pressure, a mix of 40%/60% CO2/N2 can be used. In this scenario, of 2 bar total pressure, 40% would be due to CO2 and 60% due to N2: 0,8 bar CO2 (equilibrium) + 1,2 N2.
When using a gas mix. It is important to remember that both gases will act independently. Therefore, if the total pressure is at the beer’s equilibrium pressure but a part of it is due to N2, the CO2 will leave the solution to find its own equilibrium and therefore decarbonate the beer.
A finely tuned system + clean pipes, taps and adequately cleaned glasses: this is what will give you the perfect pour. It is believed that that N2 being less soluble than CO2, it will contribute to finer bubbles and help with the foam and head retention. It is true for nitro beer but seems relatively irrelevant for CO2 carbonated beers poured with a CO2/N2 gas mix.
Nitrogen being more expensive than CO2, it is fair to consider whether its use is relevant or not. And if it is. How much of it.
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