Kegged beer: Draught system components

The “Kegged beer” series explores draught system set-up and operation, plus how to pour from and change a keg. While it’s likely most helpful for people working in bars, restaurants, and bottle shops with kegged beers on offer, it’s interesting for all of us to learn more about beer’s journey from keg to glass! New to the series? Start here.

Have you ever wondered how beer gets from a keg to your glass? You’re about to find out!

Here we’ll learn about four key elements in most draught dispense systems – the keg, coupler, Foam on Beer or FOB detector, and beer tap/beer faucet – and how they work.

What are the basics?

Have a look at the image below of a typical direct-draw system.

When a keg is tapped by a coupler, dispense gas (typically carbon dioxide or CO2) enters the keg and applies pressure to the beer. This then forces the beer out of the keg, via the coupler, and through the beer lines to the faucet. (FOB detectors are only used in specific systems, which you’ll read more about shortly.)

Let’s get to know each of these elements in a bit more detail below.

Source: Draught Beer Quality Manual
(with annotations)


Kegs are containers that enable beer to be transported in bulk and dispensed by the glass while maintaining its quality and integrity. Keg design protects beer from the damaging effects of both air and light, while enabling it to be easily and rapidly dispensed.

Kegs are available in a variety of sizes, as seen below. Most brewers use kegs made from stainless steel, but single-use plastic kegs are now becoming popular. Regardless of the material used, all kegs are pressurised vessels, so should be handled with care.


A keg is tapped using a coupler.

A coupler attaches to the keg valve, allowing gas to flow in and beer to flow out. (As a note, gas always flows in through the side, while beer flows straight up through the centre.)

When tapped, the keg’s valve admits dispense gas into the keg’s head space, applying pressure to the beer. This then forces the beer to travel up the keg’s spear, or downtube, and through the beer lines to the faucet, as seen in the illustration below.

Keg valves vary and each has its own coupler type as seen in the image above. It’s important for bars and pubs to have the correct coupler type for each keg they’ll be serving.

While many coupler designs function in the same way, there are some exceptions. (To see how to change a keg for various coupler types, check out this article here.)

Foam on Beer detector (FOB)

FOBs, or Foam on Beer detectors, are a common feature in long-draw draught systems in which beer has to travel through beer lines that are at least 25 feet or longer to reach the faucet. (Note that they are not used in direct-draw systems, like the one featured in the first image in this article.)

A FOB stops the flow of beer through the line once the keg empties.

How does it work? There is a small float ball inside the FOB detector chamber. Once the keg empties, and there’s no more beer traveling through the FOB, the float ball drops down – sealing the chamber.

This then keeps the beer line full of pressurised beer while the keg is changed and prevents the beer line from filling with gas and foam, reducing beer loss.

After a keg is changed, the FOB needs to be reset by releasing the float ball to (re)enable the flow of beer through the line. (To see a video on how to reset a FOB, check out this article on changing a keg.)

Beer tap/ beer faucet

Taps or faucets dispense beer to the glass. And, they often hold a tap marker to identify the type of beer being dispensed.

The two most common faucet types are vented rear-sealing faucets (the Standard or European faucets in the top left of the image above) and ventless forward-sealing faucets (in the top right of the image above).

Vented faucets are most common in the US. Their vent holes allow for smooth beer flow and permit the faucet to drain between pours. But this means that the interior of the faucet is susceptible to microbial growth, so the faucet and its vent holes need to be carefully cleaned and regularly inspected.

The ventless, or forward-sealing faucets, are easier to clean and have a lower susceptibility to microbial contamination, but may have a slightly more turbulent flow of beer.

As seen above, other faucet types exist, but the only additional one we’ll mention here is the nitro faucet (in the lower left of the image above). Nitro faucets are specifically used for nitro beers (beers that contain a blend of carbon dioxide and nitrogen gas, which changes their texture and appearance). These faucets contain a restrictor plate that forces the beer through a series of tiny holes to facilitate the breakout of both gases, giving nitro beer its cascading effect when poured and a smooth, creamy mouthfeel.

Dispense gas

Draught systems require the use of dispense gas, which is typically carbon dioxide (CO2) or CO2-nitrogen blend, and it must be used at the proper pressure setting.

It’s important that compressed air is not used to pressurise and dispense beer from traditional kegs (ones in which the dispense gas comes into contact with the beer), as this introduces air – and therefore oxygen – into the keg, making the beer go stale and leading to the development of oxidative off-flavours in our beer. (More on off-flavours in our next series here).

There are two exceptions though:

  • Single-use kegs with an internal bag-in-ball system can be served using compressed air because the gas does not come into contact with the beer itself. (Instead the gas compresses the internal bag within the keg, which pushes the beer out through the coupler and to the faucet.)
  • And for temporary dispense, a party pump can be used, but it limits the flavour stability of the beer to less than one day because oxygen is put in contact with the beer. (A party pump is a manually operated pump that attaches to the top of a keg. When pumped, it pushes air into the keg… and beer out.)

Now that we know more about the key components in a draught system, it’s time to learn about system operation.

Discovering Beer is not affiliated with or endorsed by the Cicerone® Certification Program.

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