There are generally three ways that brewers can carbonate their beverage: Bottle conditioning, spunding, and forced carbonation. There is another, more advanced technique out there called kräusening (see Mr. Wizard to read more on that), but most every brewer uses one of those three techniques. If you’re looking to learn more on bottle conditioning, check out: https://byo.com/article/priming-with-sugar/. For more on spunding visit: https://byo.com/article/advanced-brewing-9/. But in this piece we’re talking about forced carbonation, which relies on an outside source of carbon dioxide to push the gas into solution.
Forced carbonation should be done in an appropriate pressure-rated vessel (as should all carbonation techniques). Corny kegs are the perfect containers to force carbonate your beverages since they are capable of handling high pressures. But nowadays there are also stainless steel conical fermenters available to homebrewers that are pressure-rated so homebrewers can force carbonate prior to packaging/kegging their beer (please note that the use of a spunding valve is highly recommended when filling Corny kegs with a carbonated beer from a pressurized tank). Once the beer is force carbonated, brewers then have the option of either serving on draft or bottling the beer using a counter-pressure filler.
Homebrewers need a couple pieces of specialty equipment to force carbonate. First is a vessel to carbonate in; as mentioned Corny kegs are the perfect container for this. Second is a tank of carbon dioxide with a good regulator. Carbon dioxide tanks are found in various sizes with the 5-lb. (2.3-kg) tank the most popular for homebrewers. You can source these from many homebrew shops or gas suppliers. When it comes to regulators, I’m a proponent of the “buy once, cry once” mentality as cheap-o regulators do and will fail. Also you do need to purchase a regulator specific to carbon dioxide.
A tool everyone who force carbonates their beverages should have is a carbonation chart: https://byo.com/article/master-the-action-carbonation/. The chart dictates the fact that the carbonation level you are looking to achieve in your beer (or other beverage) is controlled by two factors: Liquid temperature and pressure in the container. Most brewers will use the term “volumes of CO2” when talking about the carbonation level in their beer. What this term “volumes” refers to is how much space the gas would take up under normal atmospheric pressure. For example, the gas in a 5-gallon (19-L) Corny keg with 2 volumes of CO2 would expand to 10 gallons (38 L) if the equivalent gas were collected in a large balloon.
Most often homebrewers will force carbonate at refrigeration temperatures. But there are many times when the kegerator may not have room or the brewer is carbonating in a stainless conical, where temperatures may be quite a bit warmer. The warmer the temperature, the more pressure that is needed to achieve the same volumes of gas (think back to science class . . . cold gasses are much more dense than warmer gasses). Modern beers are often carbonated in the 2.0–2.5 volumes CO2 range. So to carbonate a beer to 2.4 volumes of CO2, you need to set your regulated pressure to 11 psi at 40 °F (4 °C), but need to crank it up to 27 psi to get the same carbonation level at 65 °F (18 °C). If you subsequently cool the fully carbonated beer from 65 °F (18 °C) down to 40 °F (4 °C) though, the carbonation level will not change (so long as no more CO2 is administered).
Forces At Play
The speed at which you force carbonate is something that can be toyed with. On one end of the spectrum is the “set and forget it” method. In this scenario the brewer hooks up their gas to their filled Corny keg and adjusts the regulated pressure to the level desired to obtain a certain level of carbonation . . . then waits. I have found that roughly two weeks are required to carbonate a full Corny keg. This is a great method for those with an abundance of beer already on tap or in queue . . . or for patient brewers.
To speed things up, there are two decent alternatives. First is the burst carbonation technique. The basic idea is a short period (usually 24–36 hours) at an elevated pressure (say ~30 psi) to get the beer most of the way to fully carbonated, then the regulator is turned down to what the regulator should be set at and the final push to fully carbonate the beer takes another 1–3 days. Another method is to get a carbonation stone. These carbonation stones greatly speed up the time to get the gas diffused into the beer.
Bring Balance to the Force
Once you have chosen an appropriate carbonation level for the beer, the final step in this equation is selecting a serving pressure. The serving pressure is 100% dependent upon the lines and faucets you are moving your beer through. Longer, narrower, more resistant lines require higher serving pressures, while shorter, wider-diameter, less resistant lines require lower pressure. Learn more about this concept here: https://byo.com/article/balancing-your-draft-system-advanced-brewing/. The ideal situation is to have the serving pressure match the forced carbonation pressure. This minimizes foaming issues or slow pours.