Two different types of condenser are used. A worm is the most historic, though still used at distilleries such as Pulteney, Cragganmore, Talisker and Mortlach. Shell and tube condensers were developed in the late 19th century, and are now used by most distilleries, including Highland Park, Glenfiddich and The Balvenie.
A shell and tube condenser comprises a large vertical chamber, with long copper pipes (typically up to 100 pipes) running the length of the chamber.
Cold water is continuously pumped into these pipes, at the base of the chamber, cooling the pipes while rising through them, with the water discharged from the top of the chamber.
Meanwhile, vapours are conducted from the still to the top of the chamber, and then descend through the chamber (being ‘pushed down’ by more vapours arriving from the still). The vapours are cooled as they come into contact with copper pipes (carrying cold water) and condense on the surface of the pipes. The resulting liquid descends along the pipes to the base of the chamber, where it drains from the condenser and collects in a separate vessel.
An essential difference between a shell and tube condenser and a worm, is that with shell and tube the vapours condense on the exterior of numerous copper pipes that carry water.
Meanwhile, a worm comprises a single copper pipe. The vapours enter and then condense within this pipe, while water cools the exterior of the pipe.
A worm is usually arranged in a spiral configuration of increasingly smaller coils, with the pipe being up to 100 metres, or longer. Additionally, the diameter of the pipe gradually decreases, from around several inches or more, to a couple of inches at the end. A worm is set in a worm tub (large vessel) with cold water pumped continuously into the base of the tub. This water rises and cools the worm, then exits from the top of the worm tub.
The key event in the condenser is when the vapours condense, as the resulting liquid then has a significant interaction with the copper. This interaction sees the copper absorbing sulphur compounds which are present in the liquid, and so lowering the level of sulphur compounds which the liquid contains.
Sulphurs (which are formed during fermentation) include rubbery, meaty and vegetal notes, so reducing the level has a significant effect on the profile of the resulting spirit.
Moreover, being so assertive, sulphur compounds ‘conceal’ other congeners (flavour compounds). This means that reducing the level of sulphur compounds also ‘unmasks’ various congeners, particularly esters (fruity notes), which can then show through in the resulting spirit. Whichever type of condenser is used, a key issue is the point at which the vapours condense, which is referred to as moving from the ‘vapour phase’ to the ‘liquid phase.’ The sooner this happens, the longer the liquid is in contact with copper on its journey through the rest of the condenser. This means more interaction with copper, and a greater reduction in the level of sulphur compounds.
A key factor determining when condensation occurs is the temperature of the ‘cooling water’ (ie. the water used to cool the condensers). This is drawn from the surrounding environment, rather than turning on a tap.
Cooling water drawn from a deep river or underground source has the most stable temperature throughout the year, while shallower rivers can change from around 15 degrees centigrade in summer to a few degrees in winter. If water at such different temperatures was used, the rate at which the vapours condensed would vary, depending on the season. Using colder water would mean reaching the liquid phase sooner compared to using relatively warmer water.
This in turn would alter the degree of interaction between the liquid and the copper, and so promote differences between spirit distilled in summer compared to winter. As consistency is crucial, the temperature of the cooling water is carefully monitored, and can be adjusted as required.
A typical temperature for water entering the condenser is around 15 degrees centigrade.
Another key factor is that a shell and tube condenser provides a far greater surface area of copper than a worm. When vapours enter a shell and tube condenser they hit a strike plate (resembling a curved shield), which disperses the vapours across numerous vertical pipes. When the vapours condense they form a ‘film’ of liquid all around each pipe, with this liquid descending along the pipes to the base.
In a worm the vapours are in contact with the entire surface area of the pipe, but when the vapours condense the liquid gravitates to the lowest part of the pipe (the coils in a worm being effectively a horizontal configuration). This forms a small stream of liquid that trickles along the ‘base’ of the pipe, and so the liquid is only in contact with a fraction of the pipe’s total surface area (the volume of liquid is never such that it would ‘fill’ the pipe).
The greater surface area of copper utilised in a shell and tube condenser means greater interaction between liquid and copper, and a greater reduction in the level of sulphur compounds compared to a worm.
However, worms do not automatically result in higher levels of sulphur compounds compared to shell and tube condensers. This is because the influence of a worm depends on the temperature of the cooling water, and how long it takes to reach the liquid phase, as well as the length of the pipe, with some worms being extremely long, and arranged in a series of very tight coils. Moreover, a certain level of sulphur compounds can be desirable, depending on a distillery’s house style, and the challenge lies in controlling the level of these characteristics to achieve a complex whisky.