One key use of heat exchangers is to cool the wort (ie. sugary liquid resulting from mashing). During mashing hot water is added to the mash tun, to convert starches within the malted barley into sugars. The water temperature is initially 63.5 degrees centigrade, and becomes progressively hotter. This means that wort draining from the mash tun is too hot to begin the next stage, fermentation, as such heat would render yeast inactive.
Rather than allowing wort to cool naturally, using a heat exchanger is more time-efficient, while also cooling the wort to a specific ‘pitching temperature,’ ie. temperature at which yeast is added. This temperature varies among distilleries, depending on the ‘house style,’ because the pitching temperature determines the length of fermentation, which in turn influences the range of characteristics in the resulting wash (ie. fermented liquid).
A pitching temperature around 16-18 degrees centigrade promotes a slower start and rate of fermentation, compared to 20-22 degrees centigrade prompting a faster start and finish. Shorter fermentation, around 48 hours, yields a cereal, biscuity wash, while longer – around 100 hours – gives a fruitier wash (longer fermentation creates broader characteristics).
The pitching temperature is achieved by piping wort into one end of a heat exchanger, and cold water into the opposite end. The typical choice of heat exchanger, from the range available, is a plate heat exchanger. This is essentially a series of metal plates, typically stainless steel (a good heat conductor). Plates have a ‘corrugated’ surface on both sides, which is effectively a narrow pipe conducting liquid around the surface of each plate, before the liquid continues to the neighbouring plate.
Wort circulates on one side of each plate, with cold water on the other side. Thermodynamics ensure that heat from hotter liquids transfers to cooler liquids (through the plates), and the wort becomes progressively cooler as it continues from plate to plate, while the water becomes progressively warmer, until each liquid exits from the opposite end of the heat exchanger.
Cooling wort to a specific temperature depends on the rate of water flowing into the plate heat exchanger. This flow rate is adjustable (faster or slower) and depends on the temperature of the water.
“Water sources for cooling the wort include springs, wells, rivers and lochs. It has to be potable water, and there has to be plenty of it. Water from a spring has a similar temperature throughout the year, between 5-10 degrees centigrade, though surface water such as a river can range from five degrees centigrade in winter up to 25-30 degrees centigrade in summer. If necessary, warmer river water in summer can be cooled using a chiller unit before going into the heat exchanger,” says Douglas Murray, process technology manager, Diageo.
The colder the water is, the slower the flow rate into the heat exchanger. When the water is warmer, a faster flow rate is required to achieve the same degree of cooling, which also uses greater volumes of water. Adjusting the flow rate is an automated process.
“A temperature probe is located at the outlet where wort exits the plate heat exchanger. This is linked to a valve that controls the flow of water into the plate heat exchanger. And technology works out what the flow rate of water should be to achieve the desired temperature,” says John Ross, technical area leader, William Grant & Sons.
Stuart Robertson, The Dalmore’s distillery manager, provides an example of the statistics. “It takes between three and a half to four hours to pass the wort from one mashing cycle, comprising 48,500 litres, through the plate heat exchanger which contains 150 plates.”
From the heat exchanger, wort reaches the wash backs to be fermented. Meanwhile, warmed water exiting the exchanger can be used.
“The temperature of the water leaving the heat exchanger could vary from 70 up to 85-90 degrees centigrade, depending on the temperature of the wort and efficiency of the heat exchanger. This water is conducted to a holding tank and is hot enough to use in the next mashing cycle without any additional heating, saving a lot of energy,” says Douglas Murray.
Heat exchangers began to be used in distilleries during the 1970s, and the technology has been evolving ever since.“At the heart of Scotch is the spirit of innovation. Manufacturers are constantly coming up with ways of improving heat transfer, and the way heat exchangers operate is becoming even more efficient,” says Douglas Murray.
Another key example of heat exchangers being used is to increase the temperature of the charge (ie. liquid being distilled) before it enters the pot still for each distillation (see Whisky Magazine Issue 143).
“Separate heat exchangers are used for different stages of the process, and these can also be different models. The dimensions and technical specifications of each plate heat exchanger, including the number of plates, are worked out by an engineer, using statistics such as the distillery’s production capacity,” says John Ross.