Essentials of the Rhodesian Boiler

The essential feature of the Rhodesian boiler is the use of a single 200 liter (44 imperial gallon) drum as both hot water storage tank and air/water heat exchanger. In the days before WWII, I presume other types of drums were used, but thereafter they are mostly constructed with a standard international 200 liter (tight head) drum.

The 200 Liter Drum

A standard international 200 Liter drum (also known as a 55 US gallon drum or  44 imperial gallon drum) is a cylindrical container with a nominal capacity of 200 Liter. Standard rolled steel drums have inside dimensions of 22.5 inches (572 mm) diameter and 33.5 inches (851 mm) height. These dimensions yield a volume of about 13,320 cubic inches, but are commonly filled to 200 Liters.
The outside dimensions are typically 23 inches (584 mm) diameter at the top or bottom rim, 23.5 inches (597 mm) diameter at the chimes (ridges around drum), and 34.5 inches (876 mm) height.

The sides and ends are generally constructed from 18 gauge (1.214mm) sheeting, and are fitted with one 2" and one 3/4" bung. The safe operating pressure is generally 5 psi (0.34 atm.), which is equivalent to a 3m head of water.

They were first developed by the Axis powers (Germany and Italy) during WWII, but were quickly adopted by Allies.

Basic Principle

When lying on its side over a wood fire (with the bungs placed one above the other), cold water may feed into the drum from the bottom 3/4" bung, and leave from the top 2" bung. As colder denser water is heated by the fire, the warmer and less dense water rises from the bottom of the drum toward the top. When overheated, steam must be vented from the upper bung and a 'stand pipe' must be provided close to the hot water outlet. An example of this basic design (with the bungs upside down) may be found on this blog.

Here is another very basic drum donkey boiler, set up to be fed with cold water from a header tank via the lower 3/4" bung, with hot water drawn from the upper upper 2" bung (I hope.) This one is stoked from the end, as in the case of the traditional Rhodesian. The corrugated iron sheeting placed over the top will assist the hot flue gases contacting with the surface of the drum. In this case, the open 2" bung acts as an open vent for steam, and the stand pipe is not required:

 Materials

The other feature of the classic Rhodesian donkey boiler, is the mud brick surround and chimney. This enabled the construction of the boiler from local materials, provided there was a disused 200 liter drum, a few pipe fittings and a couple of fencing standards lying around: