We mentioned several types of boiler in chapter 3, but a simple pot on a hotplate is by far the most common in amateur distilling. The pot on a hotplate approach can range from a standard kitchen pot or pressure cooker on a kitchen stove up to large pots (eg a 55 gallon drum) set on an industrial cooker. This design can also be used to provide semi-indirect heat by placing the actual boiler inside a larger pot, filling the space between the pots with a heat-transfer liquid (Water, salt water, glycerin and propylene glycol are all popular choices), and heating the outer pot with the hotplate.
Cheap hotplates are usually controlled by thermostats, which can result in uneven, or surge, boiling. You can overcome this by using a heat diffuser. A thick cast-iron skillet, or a sheet of steel about 1cm (1/2 inch) thick should be fine. The metal plate acts as a heat reservoir to smooth out temperature swings. Should hot spots persist, adding a wire screen between the metal plate and the pot introduces a thin layer of air that will further smooth out the distribution of heat. Wire mesh is often used in laboratories to spread the heat from a flame, and the combination of metal plate and trapped air is used in good kitchen pots whose thick bottoms have concentric grooves machined in them.
Sand beds are another simple and effective way of controlling heat flow, and are often used in laboratories. They are used to provide physical support and close contact with objects that are not flat, like round-bottomed flasks. Just fill a flat-bottomed, shallow metal container with 2 to 4 cm (1 to 2 inches) of fine, dry sand, and sit the boiler into it. CAUTION! The sand will get extremely hot, and can cause nasty burns in an instant.
A hotplate with a heat spreader is the simplest and least expensive way of heating a boiler in a controlled manner.
Standard cooking pots will work well, but often require some extra work to make a vapor-tight seal, as described earlier in this chapter. Pressure cookers are often used, especially for simple pot stills, because they come with both a tightly sealing lid and a vapor outlet (the nozzle that the pressure control dingus sits on).
Used stainless steel milk cans and beer kegs are often available, and can be an attractive choice because they are very rugged and well sealed. Milk cans have a tightly fitting, removable lid about 18cm (7 inches) in diameter. Beer kegs have a much smaller opening, and this makes filling, emptying and cleaning more difficult.
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