"Heat exchange technology has evolved significantly with heat exchangers becoming far more efficient, and absolutely intrinsic to a distillery," says John Ross, technical area leader, William Grant & Sons.
Heat exchangers are used, for example, to pre-heat the charge (ie. liquid being distilled) before it enters the pot still. The significance of this is that the hotter the charge the less energy is required to distill it and so less gas or oil is used to run a boiler, creating the steam that heats pot stills.
Prior to the first distillation, the wash (ie. fermented liquid) is already at a temperature of around 30-34 degrees centigrade. This is a natural consequence of fermentation, when adding yeast which converts sugars in the liquid into alcohol. The temperature of the wash can subsequently be increased by passing it through a particular type of heat exchanger, called a plate heat exchanger, which is essentially a series of metal plates.
The traditional method is to pipe wash into one end of the plate heat exchanger, while pot ale is piped into the opposite end. Pot ale is the residue liquid in the pot still at the end of the first distillation, with pot ale leaving the pot still at up to 98-99 degrees.
The plates have a 'corrugated' surface on both sides. This is a narrow pipe which conducts the liquid around the surface of each plate, before the liquid continues to the neighbouring plate. Wash circulates on one side of each plate, with pot ale on the other side.
"The laws of thermodynamics ensure that heat from a hotter liquid always transfers to a cooler liquid, through the plate. Plates are typically stainless steel, which is a very effective conductor of heat," says John Ross.
As the wash continues from plate to plate it becomes progressively hotter, and the pot ale cooler, until each liquid exits from opposite ends of the plate heat exchanger. The wash reaches a temperature of around 70 degrees.
Similarly, prior to the second distillation, the low wines (ie. result of the first distillation) can have a temperature of around 15-20 degrees centigrade when entering a plate heat exchanger. Meanwhile, spent lees (ie. residue liquid in the pot still after the second distillation) can be introduced at the opposite end. Spent lees have a temperature up to around 99-100 degrees centigrade, which results in the low wines leaving the plate heat exchanger at temperatures that can be around 70 degrees.
"The heat transfer to the low wines is far greater than it is to the wash. This is because the low wines are a thinner liquid than the wash, which contains sediment and yeast particles, for example," says Allan Logan, Bruichladdich's production director.
The significance of temperature is that once the charge enters the pot still it needs to be heated to 78 degrees to start vaporising and ascend the neck of the still. The temperature continues to rise during distillation, with the charge eventually peaking at almost 100 degrees centigrade.
"Heat exchangers probably save us around 25 minutes of boiling time in the still, with the first distillation taking about six hours, and the second distillation around 7.5 hours. Saving time is important for the production schedule, but also represents a saving in fuel costs," says Allan.
Meanwhile, the benefits of a heat exchanger depend on maintenance."The heat exchanger needs to be cleaned once a week to ensure the maximum heat exchange. Otherwise, if the surfaces become fouled this would compromise efficiency. An automated CIP (Cleaning In Place) system keeps the heat exchanger in good condition. Additionally, every two-three months, we open up the plate heat exchanger to conduct a physical inspection," says John Ross.