Variations and Efficiency of the Rhodesian Boiler

Those of you from colder climes will be wondering how these open air, wood  burning monsters survived for so long. The design of the Rhodesian boiler must, however, be evaluated in the context in which they evolved, and from the origins of its most primitive ancestor:

• They were a huge improvement on the open air kettles they replaced - these often made from a half 44 gallon drum cut across, standing over a three stone fire.
• Construction was rapid - if bricks were already made, one could be built in a couple of days.
• They were constructed from local materials, provided a used drum, a few piping parts and a couple of fencing standards were available.
• They required only limited building skills.
• Wood in those days was plentiful - in most cases sufficient fuel for the boiler could be collected with a wheelbarrow from the bush surrounding the homestead. Lands and roads were being cleared - where firewood was scarce around the homestead, firewood could easily be brought home.
• The wood collected, in these tropical climes, is hardwood and is dry throughout the year other than during and shortly after a downpour. 
• The cold feed often came from a borehole and the water was often full of lime - the boiler would function even with a considerable build up of lime in the drum.
• A leaky drum could we whipped out and replaced in half an hour (this was the reason for the open front.)

All of them did the job - some of them just consumed far more firewood than others and were, therefore, subsequently smoky.

Efficiency

The efficiency is a little difficult to estimate, but the useful power output can be deduced from the rate of heating of water and the input power from the size of the firebox:

•  A bath may be drawn after 30-45 minutes from cold, and baths may be drawn in succession at 30 minute intervals once up to temperature. A bath requires 60-80 liters of hot water at 60-70 degrees C, and assuming that the temperature of the incoming cold water is 10-20 degrees C (remember, this is the tropics), gives a useful power output of about 10 kW.
• The useful volume of the firebox is about 2-3 cubic feet, which gives an output of 20-30 kW from the fire.
• Efficiency is thus 30-50%
• The remainder of the heat goes straight up the flue - this is especially evident when the boiler is first fed, and the fire is situated close to the back of the firebox.

Heat transfer is primarily via heat radiation from the ash bed to the drum and walls of the firebox - this is the reason for the drum sized firebox and the massive brick construction. Heat transfer from hot flue gases to the sooty surface of a drum is poor, and would require a larger surface area to effect sufficient heat transfer by direct conduction. The whole structure reaches a working temperature which is hot to the touch - steam can be seen rising from the walls during a rain shower. Most of the heat in the flue gases, derived from volatiles in the wood, is lost up the chimney. This is only acceptable owing to the nature of the fuel - abundant supplies of tropical hardwoods.

Design Details

 The construction should be massive - considerable heat is transferred to the drum via radiation from the brickwork. Mud bricks should be burnt, water from a leaky drum and unburnt clay do not go too well together. The walls should be  two courses thick (230mm) up to the midpoint of the drum. These same walls can be extended to the top when a flat concrete roof is used, however, in the case of a vaulted masonry roof, the thickness may be reduced to a single course. The chimney may also be built with a single course of masonry.

The height and cross sectional area of the chimney effects the draft - sufficient updraft may be obtained from a 6" square (150 by 150mm) chimney rising 3 feet (1000mm) above the top of the boiler housing. If the chimney is too tall, the excessive draft draws heat up the chimney, both whilst the boiler is fired up, and during the cooling down cycle.

The position of the flue into the chimney is critical - when placed too high (opposite the upper rim of the drum), warm air rises up the chimney during the cooling off cycle, and heat retention is too poor to last overnight for morning ablutions. The optimum height places the top of the flue opposite the center line of the drum. The annulus around the front of the drum should also be bricked up with adobe from the top to at least the half way mark, to prevent similar convective cooling from the front. This positions half of the drum of water above the convective flow of cooling air.

The drum should be positioned with a 2-3" (50-75mm) gap from the wall of the chimney - this allows adequate draw whilst forcing the flue gases into a re-burn zone before entering the chimney - this reduces smoke. A similar gap should be allowed around the top of the drum, both to allow easy removal of the drum, and to provide an insulating layer of hot air between the drum and the roof.

The firebox should not be more than 12" (300 mm) deep. Most of the heat transfer is radiative, and the optimum distance for transfer is in the order of 8-9" (200-230mm) from the smoldering coals to the surface of the drum. Excessive size of the front opening also results in considerable radiative heat loss from the front of the firebox.

If a fire grate and ash box is provided, the ash box should be no more than 9" (225mm) deep, and the grate should not be too large nor extend to the front of the firebox (a sheet of iron placed over the front gratings to half way usually suffices.)

The plinth should be sufficiently tall to keep the length of the stand pipe under 10 feet (3000 mm) - and should exceed 2 feet (600mm) in order to prevent rain-splash from reaching the ash bed (unless the boiler is placed under a roof.)

The outside of the construction should be plastered with lime plaster to prevent wetting of the masonry during rain storms and from the geyser of water that often spouts from the standpipe when the boiler is overfed (the standpipe may be directed into the header tank when this is placed on top of the boiler.)

The 2" bung must be placed uppermost for the hot water outlet, and should not be reduced in size before the standpipe take-off. The most common point of failure is obstruction of the hot water off-take from excessive lime (this also blocks the pressure release mechanism, with sometimes disastrous consequences!) This portion of pipe should, therefore, have the largest diameter possible, and should be protected from direct heat - a boiler with the off-take to the rear in the stream of flue gases will not last long. This also minimises the volume of air or steam trapped in the drum, above the hot water outlet which reduces 'bumping'.

All other plumbing may be reduced to 3/4", including the standpipe. A stop cock should be installed in the cold water feed to facilitate maintenance of the boiler.

A fairly common variation, for larger establishments, was to add a second fully enclosed hot water storage drum above the heat exchanger drum. In this way, the entire hot water storage is kept out of the convective stream of cooling air and easily remains hot overnight. The upper drum does nor need frequent replacement, and can be fully built in - construction of a vaulted masonry roof is thus simplified. This scheme has the disadvantage of taking longer to reach working temperature from cold:

I am unsure of the best plumbing arrangement - as shown, the flow when water is being drawn, will substantially short circuit the upper drum.