How Beer Kegs Work

When a pub orders up its draught beer supply, usually what's delivered is a truckload of standard kegs known as "half barrels." The half-barrel is also the garden-variety keg commonly nestled in a big bucket of ice at a frat party. It's called a half-barrel because the "barrel" is a legal unit of measure used for import and export statistics. In the U.S., a barrel, by definition, is 31 gallons (117 liters) of fermented beverage. So, as you might have guessed, a half-barrel holds half that amount -- somewhere between 14 and 16 gallons (53-60 liters), or by a more agreeable measure, 124 pints (117 metric pints). This is slightly larger than the 50 liter (13 gallon) standard European keg. Fill either kind of keg with beer, and you've got an object weighing about 160 pounds (73 kilograms). By contrast, an empty half keg is a lonesome but hardly insubstantial object that weighs approximately 30 pounds (14 kilograms) [source: Wayland Works].

If you're at a smaller party where people aren't jumping naked into the pool and spraying shaving cream all over somebody's car, then you might encounter a more civilized vessel -- the quarter keg, aka the "pony keg." It's called a pony because, like ponies, the quarter keg is small -- but it can still hold 90 pounds (41 kilograms) of beer. [sources: Wayland Works, Word Detective].

For the sedate dinner party, there's the refined and aerodynamic "torpedo keg" (or the "sixth barrel") which holds a little more than 5 gallons (19 liters). And for an evening with friends, there's the very restrained "mini-keg," affectionately known as "bubba," which provides a pleasant 10 and a half pints (10 metric pints) with which to while away the football game. Last but hardly least is the Beer Ball, a plastic number cradling 5 gallons (19 liters) in its svelte interior [source: Micromatic].

There are other, more arcane vessels lurking in the margins of beer culture. The Cornelius, or "corny" keg, is the basic soda canister that servers are always changing at fast-food joints. Capable of holding 5 gallons (19 liters) of liquid, the corny keg was long ago identified by home-brewers as the ideal vessel for their purposes. [source: Brew Ware].

All kegs, no matter their size or style, must deal with the same vital issue: how to get the beer out. On the next page we'll talk about the complex challenges this seemingly simple problem presents.

Steel kegs are built to withstand pressure, both from the outside and the inside. With some beer containers (casks, mini-kegs, torpedoes and beer balls) the spout is often at the bottom of the vessel, and gravity forces the beer out of the tap when it's opened. But when it comes to the average keg, the tap is at the top. So what makes that beer defy gravity and rise out of the tap? In a word, pressure. But creating that pressure is a tricky business [source: Wayland Works].

When people talk about pressure in beer kegs, they mean the force exerted down onto the surface of the beer, forcing it up through the spear. What exerts that pressure may differ. The average party keg uses, appropriately enough, a "party pump." Party pumps are simple and cheap. All you do is pump them a few times, and that builds up enough pressure to force a pint of beer up and out the tap. But what you're pumping into the keg is air, and one of the things that makes up air is oxygen. Our lungs love oxygen, but beer really doesn't. Oxygen tends to destabilize the complex flavors and aromas in a beer while at the same time absorbing the carbonation. This means that in the space of a day or two, the contents of the keg will be both tasteless and flat. Not a problem for a well-attended party, but also not an option for a pub that might take weeks or months to go through a single keg [source: Brew Ware].

That's why some clever beer lovers came up with the idea of maintaining the pressure inside a keg by feeding pressurized CO₂ or nitrogen into the keg at a constant rate. Of course, this means that you have to have a tank of the stuff handy. Carbon dioxide alone works pretty well, and for some beers, CO₂ combined with nitrogen is even better. It's all about maintaining the natural carbonation in a beer -- and carbonation is … CO­₂. So when pressurized CO₂ dissolves into the beer, it's just adding more of what's already there.

But there can be too much of a good thing. Excess carbonation can make turn beer into a big glass of fizz. That's why nitrogen can be so handy: Like CO₂ it's more or less tasteless and odorless, but unlike CO₂ it doesn't dissolve easily into beer. A good mix of the two gases can maintain the natural carbonation in the beer without upsetting the balance of its flavors and aromas [source: Wayland Works].

Ok, but how do you keep the contents of the keg (the beer and the gas) under pressure and still allow the beer to come out without losing that pressure? This is where taps and couplers come in.

A tap is a one-way valve that, when opened, permits beer to escape but allows nothing back in. Of course, a certain amount of pressure is lost with the exiting beer, and this is why the keg has to be re-pressurized with either air (via a party pump) or one of those fancy gases. But if you're using CO₂ or nitrogen to pressurize your keg, not just any old tap will do. You need a coupler.

Couplers are taps designed for kegs that are pressurized with gas, and because different beers need different levels of pressure, there are seven different kinds of taps. American beers follow the D system, German beers follow the A system, Bass and Anchor Steam follow the G system, and so forth [source: Micromatic]. Different beers require different amounts of pressure because of differences in aroma and flavor. The more aromatic and flavorful beers are less carbonated so they need less pressure to maintain that low carbonation level [source: Wayland Works].

Then there's the matter of temperature. The warmer a beer gets, the faster it goes flat. That's because heat causes the carbon bubbles to expand and pop out of the beer more rapidly. This is especially bad news for highly carbonated lagers like Coors or Molson. In bars where the kegs are kept far from the taps, the beer must travel through long lines to the tap. To keep the beer from warming up as it travels, a glycol system keeps the lines safely insulated and cooled [source: Wayland Works].