I have been fielding a lot of questions about size of boiler, mostly people asking if they can use a particular still to produce a particular product. Often people look at the Tom Thumb Still and see that price is in their range and they wonder if they can do vodka or rum or whisky in it. Let’s have a look:
Consider you’re making whisky and you’ve done a great mash with 100% fermentable sugars (not many people achieve this) and you’ve an ABV of 7% and you want to use the 4L Tom Thumb Still. So, (4 x 7)/100 = 0.28 litres of pure alcohol. After 2 distillations you will have an ABV of around 65% depending on where you made your cuts and you will have lost some alcohol to feints, say 40%, meaning that you recovered 60%. So final yield is (0.28L x 60)/100 = 0.168 litres. At 65% ABV you will have a final volume of (0.168 * 100)/65 = 0.258 litres. This is not a lot of whisky and it is probably not a good idea to use a Tom Thumb Still to make whisky. Try doing the calculation for a 500L still.
The calculation is a bit more complex as you would be adding your feints to your stripping run and your starting ABV would be a little higher, but this gives a good idea of where you will come out.
Let’s look at a sugar wash at 16% ABV and go into a little more detail. You do your stripping run first, no reflux and you are going to get about 40% ABV by the time you are done. Assume you have a 500L boiler, therefore total yield should be (500 x 16)/100 = 80 litres. At 40% ABV the volume should be (80*100)/40 = 200 litres. What this means is that you will have 300 litres left in your boiler and it will contain some alcohol. Let’s assume you did your stripping run until the boiler was 99°C. On this chart VLE Chart you can see the ABV of the liquid in the boiler at 99°C is 1%. Therefore, the volume of alcohol remaining in the boiler is (300×1)/100 = 3 litres. Your yield from the stripping run is therefore 80-3 = 77 litres in a perfect world.
So you have 77L at 40% ABV = (77*100)/40 = 192.5 litres and you are going to run this through a reflux column and aim to get 94% ABV out of it. In theory your yield would be (77 x 100)/94 = 81.9 litres. You will be left with 192.5 – 81.9 = 110.6 litres in the boiler. Again, assume you ran your boiler until the temperature was 99°C so you have 1% alcohol in the boiler, or (110.6 x 1)*100 = 1.1 litres. You total alcohol would be 77 – 1.1 = 75.9 litres. At 94% ABV the volume of this would be (75.9 * 100)/94 = 80.7 litres.
There are other system losses that are determined by the method of distillation and vapourisation of alcohol during the initial heating period, and these will definitely affect your yield, but the calculation gives you a theoretical value to aim for and you can measure your efficiency and use it to predict future yields.