Talking Tech

Pumping beer – from cellar to glass

One thing that is guaranteed to set the lifelong cask ale drinker’s alarm bells ringing is when the bar server pulling their pint of cask tells them – “oh, I’ll just have to go and change the gas”. Having spent all their drinking lives believing that cask ale is unsullied by dreaded CO2, a pub that requires gas to serve their cask ales is surely up to no good?

Well usually, they aren’t, it’s just part of the modern pub cellar. In this piece, we’ll look at how your cask ale gets from the pub cellar to the bar.

The first thing to remember is that not all pubs are the same. How the beer gets to the pump in somewhere like Manchester Arndale’s Micro Bar is going to be very different from its route from one of the three cellars in JD Wetherspoon’s Moon Under Water.

While pins of Old Tom have seasonally appeared on the bar of Robinsons’ pubs, the recent growth of the ‘micro-pub’ has seen a revival of this most traditional method of serving – your beer poured directly from a cask. Micro-pubs like Stalybridge’s Bridge Beers have their casks on display on a rack behind the bar and use nothing more than gravity to fill your glass.

In the 70s, cask ales were regularly served by metered electric pumps but since the 1980s, the bar mounted handpump has become synonymous with cask ale.

The simple syphon pump, also known as a beer engine, was first patented in 1691 by a Dutch inventor called John Lofting. The principle of operation is simple – an airtight chamber sits between the line from the cask and the pump’s nozzle. A piston in the chamber is connected to the pump’s handle. When the server pulls the handle, the piston is pulled up, drawing beer into the chamber via a one-way valve. When the handle is returned, another one-way valve allows the beer to pass through the piston. On the next pull, the beer is pushed out of the chamber and through the nozzle while more beer is pulled into the chamber.

©John O’Donnell

As beer may be sat in the cylinder for some time between pulls, pumps are typically fitted with a cooling system which circulates chilled water through a jacket surrounding the cylinder.

The amount of beer dispensed on each pull can be a quarter, a third or half a pint. The larger the volume dispensed with each pull, the larger the effort required. With casks located in a traditional cellar, the beer engine must create enough suction to lift the beer from the cask. It must also overcome the natural resistance to flow of the beer line – the longer the line, the more effort required.

Flojet pump (©John O’Donnell)

Where the length of line and/or height between cellar and bar is too long, the handpump must be assisted with an additional pump in the pub cellar. While electric pumps can be used, the most common type of pump is a gas driven diaphragm pump – usually known as a Flojet pump, the trade name of the most commonly seen model.

In a diaphragm pump, two flexible diaphragms oscillate back and forth, creating chambers which suck in and then push out the beer. The diaphragms are connected by a shaft so as one sucks, the other pushes. The movement of the diaphragms is driven by compressed gas which does not come into contact with the beer.

On the first stroke, the gas moves one diaphragm to push beer from the first chamber via a one-way ball valve. At the same time, the second diaphragm is sucking beer into a second chamber. At the end of the stroke, the gas flow is diverted to push the second diaphragm, pushing out the beer drawn in on the previous stroke, while more beer is drawn into the first chamber. The cycle then repeats.

©John O’Donnell

Flojet pumps allow pubs to serve cask ale from cellars some distance from the bar and allow smaller diameter lines to be used, reducing the amount of beer in the lines at any given time. As they reduce the effort required to operate handpumps and reduce wear on the pump seals, they are regularly fitted in lines even where they aren’t strictly necessary.

Although electric powered flojet pumps are available, as pub cellars usually have a ready supply of gas, the gas-powered models are the most common – which leads to that unfortunate situation where the gas running out does stop cask ale flowing.

The Flojet is also the secret behind cask ale service from back bar taps such as those seen at the The Oast House and Stubborn Mule’s tap room. When the tap is opened, the flojet sets to work pumping the beer through the tap. They can easily generate enough pressure to force beer through a cask sparkler.

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