Directly heated boilers can suffer from the phenomenon of surge boiling. The more advanced fractionating and compound stills require a well-regulated and moderate heat input to operate correctly. Whatever type of boiler you have, and whatever kind of still you use it in, you do need to think about controlling the boiler.
1 kW of power will vaporize 1.6 kg of water each hour at its boiling point. That same kilowatt will vaporize 4.2 kg an hour of pure ethanol, and a mixture of water and ethanol will vaporize at a rate in between the pure products. Since fermented mixtures contain at most 20% ethanol, the quantity of liquid converted to vapor will be close to that of water, even though it boils at a temperature below 100° C. If you are re-distilling a product high in ethanol content, the rate of vaporization will be much higher.
Despite this considerable difference in weight, the volume of vapor produced each hour will be the same no matter what the proportions of the mix may be (chapter 2). For mixtures that are primarily water and ethanol, 45 liters of vapor are produced every minute per kilowatt of heat applied. A 2550mm (1-2inch) diameter tube can easily handle several times this amount of vapor.
When you place packing in a column, as required by both fractionating and compound stills, the vapor path is restricted, and you have to start thinking about boiler control. Experience shows that a loosely packed 50 mm diameter column performs well with a 1 kW boiler, but is operating close to the limits of its reflux capability. 36mm (1.5 inch) is also a common column size, which reduces the power requirement to 750 Watts.
The most common control problem is how to heat a boiler full of liquid up to its boiling point rapidly, then scale back to the proper controlled simmer. A 750 Watt element would take several hours to heat up 20 liters of liquid.
The simplest method is to fit two heater elements in the boiler, one rated at 750 or 1000 watts for the voltage you'll be using and the other of any size you need to help heat up quickly. Control then simply becomes a matter of switching the larger element off when the pot begins to boil. Be careful not to exceed the capacity of the supply circuit, though, or you'll blow a fuse. You might have to use two separate circuits for the two elements.
It is possible to provide further control by changing the way in which the heater elements are wired together, and this can give you up to 4 discreet power levels. Full details of how to do this are in Chapter 7.
It is also possible to alter the output of an element by lowering the voltage. Specifically, running a 240v element on 120v will reduce its power to a quarter of its rated value. A 4000W 240v element will provide 1000W of heat running at 120v. We do not recommend this method, because it could create confusion about what devices may safely be plugged in to which receptacles.
120v power, common only in North America, is not designed for heavy power draws. The standard 120v household receptacle is rated for 15 amperes, and the largest heating element that can be legally connected to a 120v circuit is 1500W.
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