This is a column or continuous still, perfected by Aeneas Coffey in around 1830 to get around the problem of having to load, distill and clean a traditional whisky pot-still. The spirit that it produces isn't as complex as pot-distilled spirit, but it can produce much higher purity alcohol at a massively increased rate of up to 1000 litres per hour at major plants. The use of continuous distillation has revolutionised spirit production around the world and has made spirit production far more affordable.
It consists of two, tall, interlinked copper and stainless steel columns, the analyser and the rectifier, which sit side-by-side.
Pressure-fed steam enters the analyser at the base and rises up through a series of compartments, separated by perforated sieve plates. As it does so, cold wash - essentially low alcohol beer, created by fermenting a wide range of fermented sugar-starch grains - is fed in at the top of the analyser and descends through the compartments.
The rising steam strips the alcohol from the wash and carries it over into the base of the rectifier, where it again ascends though another series of compartments. As it does so, it comes into contact with the cold wash supply pipe, which is routed through the rectifier in a series of loops and coils.
This acts as a surface on which the alcohol vapour condenses, and the strength of the condensate increases as it rises up the rectifier until it is gathered on top of the unperforated spirit plate in the topmost compartment, the spirit chamber.
Any uncondensed vapour is redistilled via the wash charger, as is the fluid known as the hot feints, which are piped away from the bottom of the rectifier and pumped back into the upper section of the analyser and redistilled.
Nowadays the wash is fed pre-heated into the analyser, and a cold water pipe acts as the condensing surface in the rectifier.
The spirit this process produces has less flavour than pot still malt spirit, but is of a very high purity at about 96% abv, and is perfect for onward processing into vodka, or further distillation with botanicals to make gin.