Fluid Dynamics

Ian Wisniewski asks what is reflux, and what influence does this have?
By Ian Wisniewski
Reflux is a complex process during distillation, which significantly influences the character of the resulting spirit. This is because reflux determines the proportion of lighter and richer flavour compounds within the spirit, and the greater the level of reflux the higher the proportion of lighter flavour compounds. The level of reflux is determined by the size and shape of the stills, and the rate of distillation, whether faster or slower, which is set by each distillery. The level of reflux that occurs, and the influence this has on spirit character, is individual to each distillery.

Reflux begins once the still is heated and flavour compounds within the charge (i.e. liquid in the still) begin turning from a liquid into a vapour form. Different flavour compounds have their own volatility (boiling point). Lighter flavour compounds are generally the most volatile, due to a simpler molecular structure known as ‘short chain,’ as they typically comprise up to a couple of linked units. This means that lighter flavour compounds, including certain esters (fruity notes), evaporate at a lower range of temperatures than richer flavour compounds, such as some cereal notes. These have a heavier molecular weight, being either ‘medium chain’ (several linked units), or ‘long chain’ compounds (10 or more linked units), and so require a higher range of temperatures to evaporate.

Consequently, lighter flavour compounds evaporate first, rising from the boil pot (base of the still) into the neck. Vapours experience a range of temperatures while ascending the neck. It’s hotter at the beginning, then the temperature decreases along the length of the neck, with the coolest temperature at the end of the neck. However, the temperature remains hot enough along the length of the neck to keep these lighter flavour compounds in a vapour form. It’s only when these vapours are conducted from the neck into the condenser that the temperature is cool enough to condense them back into a liquid.

Richer flavour compounds only start to evaporate and rise up the neck once the temperature in the boil pot is that much higher, which means these compounds also require a higher temperature along the neck to remain in a vapour form. However, as richer flavour compounds ascend the neck and the temperature decreases, they start to condense back into a liquid on the side of the neck. Some of this liquid rolls down the neck and returns to the boil pot. Meanwhile, as the temperature rises while this liquid is descending the neck, it can become hot enough to make some flavour compounds evaporate again, and rise as a vapour – only to condense again once reaching a cooler temperature higher up the neck, and some flavour compounds are caught up in this repetitive cycle.

The length of the neck is a vital factor, with a shorter neck having a smaller decrease in temperature between one end and the other, which means less reflux. A longer neck has a greater decrease in temperature and so more reflux, resulting in a higher proportion of lighter flavour compounds in the resulting spirit.

‘Wider and taller necks can also give more reflux than a narrow neck, partly due to wider and taller necks being slightly cooler, while also offering a larger surface area of copper for the vapours to condense on. Our necks start very wide then taper up,’ says Russell Anderson, Highland Park’s distillery manager.

Meanwhile, conditions within the neck change significantly.

Dr Bill Crilly, technical support manager, The Edrington Group, provides a simplified explanation of a complex scenario. “As vapours condense onto the copper surface of the neck the copper absorbs the heat of the vapours, which raises the temperature of the copper, and the copper is constantly gaining more heat from vapours that condense. This results in the temperature rising along the neck, which means a lower level of reflux as the distillation run continues, and a higher proportion of richer flavour compounds reaching the condenser.”

Reflux can also be increased by adding ‘accessories’ to the still, such as a boil bowl, also known as a boil ball. This bulbous section, fitted between the boil pot and the neck, can vary in shape from mildly to acutely convex (i.e. broad).

When vapours rise from the boil pot they expand into this larger and slightly cooler area, causing richer flavour compounds to condense and return to the boil pot; and the broader the boil bowl the greater the level of reflux.
Fitting a lampglass between the boil pot and neck achieves a similar result. This feature is also known as a lantern, and a ‘pinched waist,’ because it looks as though a corset has been tightened around this part of the still.

The vapours are initially concentrated within the smaller area of the ‘pinched waist,’ before they reach the wider, relatively cooler area above, where richer flavour compounds condense.

The influence of still design must be considered in conjunction with the rate of distillation.

Applying heat gently to the boil pot results in a slower rate of distillation as vapours rise more gradually. This leads to a lower density of vapours in the neck, with the neck also remaining relatively cooler, which increases reflux and results in a higher proportion of lighter flavour compounds in the spirit.

Applying heat more rapidly increases the distillation rate by driving off the vapours faster, leading to a greater density of vapours, and a relatively hotter temperature in the neck.

This means less reflux, which increases the proportion of richer flavour compounds in the spirit.

‘How you heat the still is probably more significant than the still design, as a very slow distillation rate could result in a spirit that is much lighter compared to running it at the usual rate.

“Even a short neck will give more reflux if you slow down the distillation rate,” says Dennis Watson, of technical and scientific affairs director, Chivas Brothers.