Doublers are continuously operating potstills. These are only known from large Bourbon distilleries. The low wines, at around 60 %abv, are evaporated and condensed a further time in the doubler, which removes impurities and also gives the product a somewhat higher alcohol strength. Not too high, because Bourbon may not be distilled to more than 80 %abv, but of course at least to the desired alcohol strength for ageing in barrels, often 62.5 %abv, which is also the maximum permitted. The goal is usually to reach just under 80 %abv.
The doubler is a perfect instrument for exactly this purpose. You can control the target alcohol strength very well with the speed of the “feed” (the supply of low wines) and the heating power. And, of course, its continuous distilling method is very economical.
The disadvantage of a doubler is its low alcohol recovery rate.
For example, the calculator's preset:
200 ml/min of low wines with 55 %abv are continuously pumped into the doubler. The 2000 W heating power produces 75.7 ml/min of distillate at 79.1 %abv. In the doubler, 40 %abv has been established, of which 125 ml/min must be continuously drained so that the level in the doubler remains the same, i.e. neither overflows nor runs dry. This results in an alcohol recovery of only 54.4 %.
If this “backset” were to be poured away, it would be a very uneconomical system. However, the large bourbon distilleries pump it together with the fermented mash into the column ("stripping column"), i.e. to the first distillation stage, which, in contrast to the doubler, has a very high alcohol recovery rate. The backset with 40 %abv becomes low wines with 55 %abv again. Or to put it another way, its high alcohol strength helps to allow 55 %abv to be achieved at all. In the end, hardly any alcohol is lost. And finally, most alcohol is distilled more than twice.
Another disadvantage of the doubler is that, as with other continuous systems, it is very inefficient at cutting the foreshots. With a doubler, this is done by not completely condensing the vapor and either simply allowing the remaining vapor, which now has a higher concentration of foreshots, to escape into the air or condensing and collecting it separately.
And as with other continuous systems, it is probably not easy to start the process with the doubler either. The alcohol strength required for the targeted distillate strength in the doubler only becomes established over time if you heat and feed constantly.
The good controllability of the doubler: If you want to increase the %abv of the distillate in the example above, you simply have to reduce the heating power or increase the feed. However, both also have an effect on the other values.
And of course, not every combination of feed and heating power works: If you increase the heating power, the distillate quantity increases and the backset quantity decreases. At some time, the point comes where no more backset needs to be pumped out. And then the doubler will empty until it runs dry. Conversely, if you continue to reduce the heating power, at some point no more distillate is produced at all, so the feed runs out of the doubler unused as backset. In both cases, the calculator displays a message.
You can also use the calculator to simulate a doubler with a column on top. To do this, enter the number of real plates. For example, four physical plates means five real plates (because the boiler itself is a plate). And you enter the % reflux. The calculator then calculates the rectification according to the McCabe-Thiele-Method. You might think that this would increase the alcohol recovery rate. But this is not the case. To increase the alcohol recovery rate, you would have to get the alcohol from the backset to the top, not further concentrate the alcohol that has already come to the top. In other words, the feed would have to be introduced into the top of the column so that the alcohol-rich liquid flowing downwards is leached out further with each plate further down and the backset therefore contains almost no alcohol at the bottom. In other words, just like a stripping column does. This setup with feed from the bottom and column on top is therefore quite pointless. In addition, the energy consumption of the doubler rises sharply as soon as a lot of reflux is used, as can be seen from the calculated "kWh per lpA", i.e. kilowatt hours per liter of pure alcohol in the distillate.
This calculator function is rather intended to be able to look at 1.5 real plates with 10 % reflux, for example. In other words, passive rectification through condensation on the inner walls of the vapor area.
Heating power losses in the lower liquid-filled area do not contribute to the reflux and are therefore not taken into account by the calculator, or are not included in the % reflux or watt heating power sliders. As there is no such thing as a perfectly insulated still or a perfectly insulated doubler, in reality you always have to use a little more power than is theoretically necessary.
You can also set different feed temperatures with the calculator. However, this is actually quite uninteresting: A higher temperature means that the feed needs to be heated less. This results in slightly higher vapor development in the still and therefore more distillate, but slightly weaker. Exactly the same as if you increase the heating power a little.
The air pressure entered in this calculator is not particularly important. The slightly different calculation results are mainly due to the fact that the boiling points are lower at underpressure and therefore the feed has to be heated a little less and therefore more heating power is available for vaporization. This is also the same as changing the heating power slightly.
Since, as far as I know, there is no homedistiller that uses a doubler, this calculator is probably only of theoretical value.

Information about our boiling point data and about the influence of atmospheric pressure