How a Draft System Actually Works

Most bartenders can pour a beer perfectly without knowing what's actually happening behind the wall. That's fine — until something goes wrong, and you need to describe the problem to whoever's fixing it. Here's the basic anatomy of a draft system, in plain language.

The Keg & Coupler

The keg is sealed until a coupler locks onto it. The coupler is the connector that does two jobs at once: it lets gas in to push the beer, and it lets beer out toward the line. Different keg types use different coupler styles (Sankey/D-system is the most common in the US) — using the wrong coupler on a keg simply won't seal.

The Gas Blend

Gas pressure is what actually pushes beer from the keg to the faucet. Most beer uses straight CO2, but some styles (nitro stouts, and often long-draw systems) use a blended gas — a mix of CO2 and nitrogen — because pure CO2 at the pressure needed for a long line run would over-carbonate the beer. Getting the gas blend and pressure wrong is a common, invisible cause of pour problems that has nothing to do with how clean the lines are.

The Line

The line (or "beer line") is the tubing beer travels through from the keg to the faucet. Its diameter and length aren't arbitrary — they're chosen to create the right amount of resistance against the gas pressure, so beer arrives at the faucet at a controlled rate instead of blasting out or dribbling. See our full breakdown in Why Line Size Matters.

The FOB Detector

FOB stands for "Foam On Beer." It's a small valve, usually mounted near the cooler, that automatically shuts off flow if a keg runs dry — without it, an empty keg would pull straight foam (and eventually air) through the line until someone noticed. It's a small part that saves a lot of wasted product and a lot of bad pours.

The Shank & Faucet

The shank is the fitting that passes through the tower or wall, connecting the line to the faucet. The faucet is the part everyone sees — but it's also one of the easiest places for residue and bacteria to build up, since it's handled constantly and rarely gets taken apart for a real cleaning.