A lot of people ask me how long the sill takes to boil. The correct answer is: “It depends.” Sounds like a lawyer, doesn’t it? There a lot of factors involved but I will provide you with a method to estimate the time it takes.
First let us understand that it takes an input of energy to increase the temperature of your product, and this energy is delivered for most small to medium size stills by electrical heating elements which have a power rating. Remember that power is energy divided by time, so a 3.6kW heating element has the ability to deliver 2,200 joules (J) every second.
Another aspect of heating liquids is that each type of liquid requires a different amount of energy to heat it up. Water requires the most energy, 4200 J per kg to raise the temperature by 1°C. Alcohol requires 2,570 J/kg/°C. The energy you require for your mixture will depend on the alcohol %. So we first need to calculate the mass of water and mass of alcohol. Let’s say you have done a sugar wash of 100 litres and it has finished fementation with 15% ABV. So you have 85 litres of water and 15 litres of alcohol. We need mass here so mass of water is volume x density = 85 x 1 = 85 kg. Mass of alcohol is 15 x 0.789 = 11.84 kg. Total mass = 96.84.
Now we must work out the combined specific heat which is proportional to the mass of each component. So water will contribute (85/96.84) x 4200 = 3686 J/kg/°C. The alcohol will contribute (11.84/96.84) x 2570 = 314 J/kg/°C. The total is 4000 J/kg/°C.
Ok, so we have 100 litres, or 96.84kg, and we must calculate the energy required to get it to its boiling temperature, which happens to be about 90°C for a 15% ABV wash (there is a whole lesson on this one day but look at this: Equilibrian chart for alcohol). The starting temperature is going to be about 20°C, but it will be whatever it is on the day and will vary by about 10°C between winter and summer and this part of the reason I said “it depends” earlier. The total increase in temperature we want is therefore 90-20 = 70°C. The total energy required is mass x specific heat x temperature increase = 96.84 x 4000 x 70 = 34,862,400 J.
If we divide this by the power in Watts (J/s) we will get time is seconds. A standard 100 litre still will probably have 2 x 3.6kW heating elements which gives a total of 7.2kW, or 7,200W which is the number we need. Now let’s do the calculation: 34,862,400/7200 = 4,842 s. We ought to convert this to minutes to make it useful so divide it by 60 = 81 minutes.
There is another factor influencing the calculation which is the heat lost during the process and this will depend on the surface area of the still that is in contact with the wash and the ambient temperature. In winter the heat losses will be higher than in summer, but you can work on about 10% losses, so 81 minutes becomes almost 90 minutes. You can therefore predict when to come back to your still to start the condenser water running
How easy was that?