The Wood Burning Stove - its Design and Operation.

Intro: This project began when a similar stove was seen at a wilderness camp here in Zambia. Although much larger than the one detailed here, it had two inherent faults:
a) the funnel-shaped chimney was not detachable, so fume condensation ran down onto the boiler and it was impossible to rod out the fire tubes without complete disassembly and

b) there was no tank insulation, so the water cooled rapidly after the fire had gone out.

However the concept of fire tubes within the tank appealed to me, as it meant greater efficiency.

In this project, a disused 48kg gas cylinder was used, providing some 100 lt water capacity. It required careful purging of gas residue and thorough cleaning.

The fire tubes (approx 35 mm ID) were welded directly to the cylinder walls but it was thought later that it would be better for collets to be welded into the cylinder first, then tubes inserted, so that it might be possible to replace the tubes at a later date (if necessary). Ensure that the diam “X” across the outsides of the tubes is less than the inside diam of the rim – else the tubes will be obstructed by it.

The cylinder might be welded onto the truck rim for security, although the metal pipework would support it adequately.

The fire box consisted of a steel 16” truck rim. The fire door in the truck rim was cut out at an angle (see the “cuts” made on the two views of the rim) so that the door resealed effectively when assembled. Hinges needed to be substantial and offset sufficiently so that the door swung clean out of the way for wood loading. A handle (hook/ring) was added to aid opening the door and an AIR/SPY hole drilled (say 10mm diam) near the door top. The rim sat directly on the ground but it may be better resting on a concrete base.
Pipe connections were ½” but ¾” might be preferable. The upper connection (on the cylinder side) was water outlet (HOT) and the lower was water inlet (COLD). Steel nipples were welded onto the cylinder and water pipe connected to them using copper fittings. The ¾” BSP threaded boss at the top should be connected to an expansion pipe leading steam/excess pressure away to a safe exit. This should be sufficiently higher than the pressure head of the system.

The Prototype:

The cylinder (perimeter) was wrapped with bubble-wrap as a crude form of insulation. Gaps between the cylinder and the rim were sealed with anthill clay but it was soon discovered that no smoke leaked out at all. (Though the stud holes did need to be welded closed) No funnel or chimney of any sort was placed on top of the cylinder – yet the fire tubes “drew” so well that it was reckoned that a chimney would be unnecessary, unless the smoke required to be led completely away from the site. (Using a chimney may then cause condensation to form and drip down on the cylinder, possibly spoiling the insulation.)
The prototype was fed with 4 Kg of rubbish (rotten) wood and the door closed after lighting the fire. The air hole limited the air intake (may need to be adjusted), allowing the door to be closed completely. This ensured a slight vacuum inside the fire box ensuring a clean environment around the stove. The 100 lts water was heated from ambient 20o C to 80o C in approx 2 hours (when the fire had died).
Heat retention in the cylinder would depend on the quality of insulation used. I would reckon to spray on an expanding foam to a thickness of 2cm.
In practice, a non-return valve would be needed to prevent feedback of the hot water into the cold feed system, unless this boiler fed into an (elevated) hot storage tank.

Cleaning: Apart from the obvious daily clearing out of ash, it will be necessary to clean out the fire tubes of soot/tar deposits. This chimney-less stove would permit the rodding out of the tubes with a steel bar, scraping deposits into the firebox, without need of disassembling the stove.

It is hoped that this design may produce a cheap product of reasonable longevity, with ease of repair and economical wood consumption.

Yours, Dusty.

The earth is the Lord’s and the fullness thereof; the world and they that dwell therein.
Ps 24:1
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Wood Burning Boiler - the economical way to heat water!

In this next blog, I wish to introduce a stove that burns wood in an efficient manner.
Rhodesian Boilers are commonly found in this country at campsite ablution blocks. They often consist of a 45 gallon drum supported horizontally on bricks with a fire underneath burning basically all day – the heat escaping in all directions! Where trees abound, this extravagance may not matter but deforestation and high consumptions of charcoal mean that the humble firewood is becoming scarce. Much is wasted in annual bush fires too.
However, with this boiler design, one 4kg fill of wood is enough to heat a tank to 800C in 2 hours.
Of course, if using it to feed an (elevated) hot water storage tank, one could run the stove continuously.
So have a look. To date, 2 boilers have been made. The second is heating water for a guesthouse.
Dusty.

Watchman's Stove - Six Years On

Six years of nightly service and this stove still boils some 4 kettles (some 8lts) of water each morning. The stove burns for about 11 hours, keeping our night guard (and Chocky, the Dachshund) warm.

These pics were taken with stove pulled out of the hut into the open. Notice the black stains around the chimney fittings. Lots of condensation runs down the chimney and must escape from a hole at the chimney base – else acid gets into the stove base causing serious corrosion. Occasionally the tube to the chimneys gums up with tar and needs chipping out. The need is indicated by smoke leaking out from around the hotplate.

Needless to say, this design is to be recommended.

The Watchman's Stove - its Design.

Intro: This project was the result of a concern about the wasted sawdust being dumped / burned that could instead be used for heating purposes.

This stove was purposed to give horizontal radiant heat, as well as a cooking surface, for the duration of a guard’s night duty. Tests proved that this stove could exceed 11 hours of burning.

Main Design: Air entering the airtube (through the small hole in the restrictor plate), proceeds to the bottom centre of the stove and is drawn up into the burning tube of the inner drum. The heat rises, striking the underside of the hotplate and is forced to spread outward then down the annular space between the inner and outer drums (giving a hot drum wall) before escaping up the chimney situated half way down the outer drum side. The chimney (when hot) provides a slight vacuum that not only maintains the incoming draught and combustion but ensures that no fumes escape from the stove into the watchman’s hut.

The Outer Drum: may be made from a 20kg gas bomb (OD of 30 cm), cut horizontally 40cm up from the base. Cutting from the side into the base of the cylinder, one can fit the air intake tube (75 x 40 mm), leaving some 50 mm outside (in which the restrictor plate can later be inserted). Half way up the “back”, one can cut a 50 diam hole for the chimney – a 2” galv “Tee” welded on the outside.

The Inner Drum: must be 2 cm smaller in diam than the outer drum, sit nicely on top of the air intake tube spigot and finish about 2 cm lower than the top of the outer drum. It may be made from a cut up 20 lt oil drum or from another gas bomb with some 6.3 cm cut from the circumference before re-welding it back into a 28 cm circle (a new base would need to be made). The base must have a central hole cut at approx 30mm diam. The drum must be removable (for fuelling and ash disposal).

The Chimney Tee: A spigot (10 mm diam) should be welded to the upper-facing exit, to support the 3” diam chimney stack. This spigot allows the chimney to be removable but to be reasonably sealed. In the lower-facing exit, fit a 2” bung from an oil drum, with a hole drilled in its centre. The hole allows chimney condensation – and there is a lot! – to drain out into a plastic cup placed below to catch it.

The Air Intake Tube: is made of 75 x 40 mm rectangular steel tube. The inboard end needs to be sealed up, but a rising 30 mm extension must be welded on to meet up with the bottom hole of the inner drum. Again a spigot is suggested, to ensure a reasonable seal with the inner drum and to lend support to it. At the outer end, a slot needs to be made from above to permit the restrictor plate to slide in neatly, effectively blocking the air tube (when inserted).

The Restrictor Plate: can be made of 2 mm sheet, having a 6 –7 mm hole drilled in the middle (to allow meagre amount of air in) and having a short length of chain welded to the top – the other end of which is welded to the outer drum (to prevent loss).

The Hotplate: is made from a disk of 2 mm thick steel sheet welded to a band of 5 mm thick bar. It must be a reasonable fit with the top of the outer drum. Handles should be welded to the opposite ends. Some form of insulation may be fitted, as it may need to be removed during operation, and will be hot!

The Concentrator: This is a thin sheet disk with a central hole of approx 50 mm diam and 3 locating lugs on the perimeter to slide over the inner drum. The concentrator is needed if any cooking is required on the hotplate, as it has been found that it concentrates the flame for greater intensity – else the hotplate will never boil! For space heating alone, it is not needed.

Lighting:

Dry sawdust must be packed tightly into the inner drum, using a pounding stick. (The tighter the pack – the longer it burns!) However, first, a smooth 30-40 mm diam tube must be found to act as an air tube former. An old shock absorber is very good for this. When the sawdust has been packed, remove the airtube former carefully, using a twisting motion and position the concentator plate onto the inner drum. This should leave a neat hole down the centre of the sawdust directly above the air intake tube (which should be seen!). An oil soaked rag may be lit and stuffed into the airtube from outside, so that the flames sweep up the vertical airtube in the sawdust. It may be an idea to position the (removable) chimney on top of the concentrator plate, not only to encourage the initial combustion, but to warm the chimney and so encourage a good draft when the chimney and the hotplate are returned to their proper stations. After some 15 mins of burning, the restrictor plate may be inserted in place to slow combustion for the night. If more heat is neede, the plate may be left out, but the burning time will be shortened and the fumes may escape from the stove top. (It may be better to make a larger air hole in the plate).

Final notes:

1. Make sure that the chimney is designed to rid the hut of all fumes – they are most obnoxious. Properly set up, the stove should provide a fume-free environment inside the hut. To prevent rain entering the chimney, it is suggested that a short piece of larger-diam tube be welded onto the top of the chimney, such that the lower join is open, allowing rain to run out.

2. Make sure that the outer drum is removable. It will need cleaning and some ash/sawdust will collect in the outer drum ad well as the inner one (although most ash will be removable with the inner drum). The chimney also will need de-coking periodically.

3. A poker/hook (from 6mm def bar) is advisable for positioning the burning rag and occasional cleaning out of fallen ash from the airtube.

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Introduction: "The Watchman Stove" - Sawdust-burning stove for cooking and space heating

The next blog post is about the Watchman Stove. This is a term I've coined for a small sawdust-burning stove for cooking and space heating. I will include pictures and describe the process of sawdust burning, how to make the stove, and how to use it. Six years on, I wish to show photos of the original stove, the prototype, and give some comments on its performance.

I have written a 750-word document on this topic and will draw the majority of the content for this blog from this document.

A new adventure

This is my first blog (or website!), and I hope that it will be of an interest and help to those involved in developing technologies using underused energy resources. Here, that means sawdust and the African sun!