It's all going down the tubes

It's all going down the tubes

Ian Wisniewski compares the merits of worms against those of shell and tube condensers

Production | 23 Feb 2004 | Issue 37 | By Ian Wisniewski

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If only I’d been taught chemistry on the basis of how malt whisky is distilled, I wouldn’t have spent so many years ignoring the teacher. Always an arts swot and never a scientist, I’ve got a lot of homework to do now that malt whisky has added meaning to my life.Although I still consider malt whisky to be a work of art, I have to concede that it can’t happen without some science. And one of the most intriguing, not to mention challenging, topics is the influence of copper.Investigating copper tends to focus on its role in the stills, with copper absorbing sulphur compounds, and converting them into other, less organoleptically active compounds (ie. less sulphur character), while also acting as a catalyst that helps to manipulate the ester character.As sulphur compounds are conveyed in the form of struck matches, sulphurous, rubbery, meaty and sweaty socks, not to mention cabbage and vegetal notes, they can easily ‘conceal’ other characteristics within the new make spirit.While a certain level of sulphur character can be highly desirable, according to the house style, lowering the level of sulphur compounds allows the congeners, including esters, to show more readily.But the copper debate isn’t complete without considering the type of condenser, which also influences the new make spirit.A shell and tube condenser provides a greater surface area of copper, and consequently an increased degree of copper contact, compared to a worm.Consequently, the copper content of new make spirit using worms is typically around a quarter of that when using condensers.As the influence of copper continues to reduce the level of sulphur compounds and sulphury pungency within the cask during maturation, the challenge lies in controlling the level of sulphurous characteristics, to yield malt with a particular persona.Shell and tube condensers are now a standard choice in the industry, with the lye pipe (or lyne arm) conducting vapours from the still to a vapour chamber at the top of the condenser. This contains numerous copper pipes set in an interior, copper-lined jacket, with cold water circulating through these pipes cooling their exterior. Vapours condense as they come into contact with the pipes, and the distillate continues to flow over a copper surface before reaching a low wines or spirit receiver.The size of a condenser reflects the size of the stills and still charge, but the dimensions are also an important element in establishing the distillery character.Whether a condenser is small and short, long and thin, vertical or even horizontal (the latter being in a minority), the key is the amount of surface area contact, together with the efficiency of the condenser (ie. the temperature differential between the hot distillate and the cooling water).The amount of tubes within the condenser can vary enormously, from 150-400, depending on the size of the stills. However, a larger total of tubes doesn’t automatically mean greater surface area contact, as it also depends on the volume being condensed.As the traditional type of condenser, worms are an al fresco feature with the lye pipe extending through the still house wall, so that the worm and worm tub are exposed to the ambient temperature.Comprising a coiled copper tube of decreasing diameter, worms are set within a tub of cold running water.The diameter, length and configuration of a worm varies considerably among distilleries. An entire worm may comprise a series of 15-20 ‘turns’ or coils, with the diameter of a typical coil around three and a half meters. Meanwhile, the tube typically tapers from around a 30 cm bore to about 8 cms.Although most worms have a circular configuration, they may also comprise a series of continually decreasing rectangles. Technically the configuration makes no difference, and the usual reason for this variation is the available space.Supported by a frame, worms are set in tubs that were traditionally constructed from wood such as larch, before the introduction of longer-lasting cast iron, and latterly stainless steel.Cold water flows in at the base and rises within the tub, which means the water is colder at the bottom and warmer at the top, due to contact with the worm.Draining through an overflow at the top of the tub, this water can also be sent to a cooling tank and reused.Control valves set the flow rate of the water, which is a principal means of controlling water temperature.Depending on the distillery, the water temperature may show an insignificant fluctuation of three to four degrees centigrade between winter and summer.However, in extreme cases it can range from as much as two to 20 degrees centigrade.This is an important factor as distillers control quality through the temperature of the distillate. Consequently, if the water is warmer and less able to cool the worm, the distillation rate may have to be slowed down.This in turn provides a slower rate of condensation, and compensates for warmer water by allowing more time for the job to be done.What may seem like ‘worm minutia,’ can nevertheless provide a feast of speculation for those of us who are so inclined. Is it significant, for example, that in some instances the junction between the lye pipe and the worm is above the surface of the water (so that part of the worm has no contact with water).Other worms meet the lye pipe just above the surface, enabling the entire worm to be submerged.Some degree of condensation occurs as soon as vapours enter the worm, with the process completed at a relatively early stage in the worm.Being any more specific is difficult, if not impossible, particularly as varying water temperatures mean that the point at which condensation is complete isn’t fixed.“Certain stages of the condensation process seem to create more wear. The greatest wear is between one-third and halfway down the worm, so I’d take it that there’s greater interaction between the distillate and copper at that stage. This could be the vapour-liquid phase,” says
Alastair Robertson of Talisker.Once the vapours have been converted, the condensate flows in a small channel along part of the tube (ie. without actually filling the entire tube), which limits surface area contact between the condensate and the copper.Once the only choice for distillers, few new worms are now being made, and there are various reasons why. Apart from comparing the degree of copper contact between worms and condensers, comparing prices reveals further disparity.A shell and tube condenser costs around £15,000, compared to a worm offering the equivalent surface area at around £50,000-60,000.Shell and tube condensers are also less expensive to operate and replace than worms.“A worm is inefficient compared to a condenser which will give you the same surface area within a much smaller size, and it is also more time and energy efficient,” says Richard Forsyth, distillation maestro at Forsyth’s, which designs, manufactures and services pot and column distillation plants.“The rate of wear of a condenser attached to a wash still is greater, the thickness of tubes would be around 3mm, lasting around eight to 10 years, compared to a life-span of around 20-25 years for a condenser attached to a spirit still with tubes around 2.5 mm thick, as the vapours are less aggressive,” adds Richard Forsyth. “The life-span of worms attached to a wash and spirit still are similar to a shell and tube condenser, and the same thickness too.”Worms may be an expensive accessory but tradition is priceless in numerous distilleries.“Worms allow for slower condensing from vapours to liquid, whereas a condenser is a fairly short burst through, and there must be some benefit from this,” says Springbank’s Frank McHardy. Alastair Robertson adds, “The way we run our worms is crucial to determining the character of the new make spirit.”Talisker, together with other Diageo distilleries including Benrinnes, Glen Elgin, Mortlach, Dalwhinnie, Glenkinchie and Cragganmore, actually comprise the largest collection of worms in the industry.“Some people seem to think that Diageo is not concerned about quality, but everything we do in malt distilling is aimed at producing, from our 27 distilleries, spirits that are highly differentiated in character and flavour, but also of the highest possible standard,” says Nicholas Morgan, marketing director, Classic Malts.“Maintaining worms with all the incumbent additional costs that this brings, is one way of ensuring diversity, complexity of character, and of course a unique quality that few of our competitors can match.” 
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