The wash is heated in the boiler to boiling point and kept at boiling point throughout the distillation process.
The vapour boiled off from the wash rises up the column, in which the vapours of the heavier liquids (water and heavier undesirable molecules) condense and fall back down. The alcohol vapour, being lighter than water vapour, rises to the top of the column and condenses back in the condenser into a liquid alcohol. This cycle of evaporation, rising and condensing, is continuous and is described as refluxing.
The saddles in the column provide a large surface area to maximise the contact between the liquid and vapour flows in the column, which accelerates the refluxing action.
The T500 has been carefully engineered to provide a continuously compensating reflux rate. As the distillation process progresses, the alcohol concentration in the wash reduces. To maintain the high purity of alcohol being produced, the refluxing ratio will increase to compensate and the alcohol production will slow. This slowing is most noticeable after two hours of alcohol production.
Towards the end of the distillation, the alcohol purity remains high, but output will slow until it stops altogether. At this point, the water vapour will keep rising and condensing part way up the column, but nothing will reach the top of the column to pass into the condenser. The alcohol extraction is finished at that point.