You probably know that beer served from the tap gets there via large, pressurized containers called kegs. But how do kegs actually work? Allow us to explain.
We have here a standard half-barrel keg—more on what that means in a second—helpfully cut open so you can see its inner workings.
The tube you see extending through the middle of the keg is called the “spear;” it reaches to within a centimeter of the bottom of the keg, allowing almost all of its contents to be served. The keg connects to the draft system via a device called a “coupler.” When the coupler is attached to the top of a keg, a probe within the coupler depresses the spring and ball valve in the keg’s neck. This allows gas—usually a blend of carbon dioxide and nitrogen—to enter the top of the keg and apply pressure downward, forcing the beer up through the spear and out the attached beer line.
Although kegs have been made out of many materials over time, including wood, plastic, and aluminum, the vast majority are now made from stainless steel. The one you see here is the most common size in rotation: 15.5 gallons. It’s commonly called a “half-barrel” because it’s half the size of a 31-gallon “barrel,” the standard measurement of volume for beer production in the U.S. It can pour 124 sixteen-ounce pints. “Sixth-barrels”—slim kegs containing 5.16 gallons—are also common.