Homemade boilers for long burning on wood

Recommendation points

• How it works: why does not all firewood burn at once?
• Disadvantages of DIY
• Mine boiler – we disassemble the scheme
• Integration into the design of a water heat exchanger
• Optional equipment

From seventy to one hundred thousand rubles – this is the average price of solid fuel boilers with a daily burning duration. But, having the skills of arc welding, you can make a long-burning boiler with your own hands. It will be much cheaper, and there are a sufficient number of schemes and designs. Let’s talk about them.

How it works: why does not all firewood burn at once?

Many designers have fought over this task for a long time: how to make solid fuel boilers more convenient, increase their autonomy and reduce the time spent on maintenance. One of the first solutions was the development of devices for batch and automatic fuel supply. However, this approach justified itself only in units that burn size-formatted fuel in the form of granules and briquettes..

After that, other features of the wood combustion process were noted. We know that the hearth of the flame is supported by a stream of fresh air. The incoming air, burning, forms a high-temperature gas, which quickly heats the logs lying above the flame. And the higher the stack, the faster it flares up.

Obviously, by counteracting these two phenomena, longer work can be achieved. For example, if you burn the firewood placed in the boiler shaft not from the bottom up, but only from the top. Another innovation was implemented in generator-type boilers. The wood in them burns, but heats up strongly, forming a pyrolysis gas. He, at its high temperature, only needs a small amount of oxygen to flare up and inflame even more..

The generation of pyrolysis gas is possible only with a limited supply of oxygen, sufficient to maintain smoldering in the furnace, and not too large so that the gas does not burn out prematurely. Both of the principles described can be relatively easily implemented in homemade devices..

Disadvantages of DIY

Please note, however, that calculating the final power and combustion parameters is not so easy. In particular, the generation efficiency depends on both the traction force and the firewood used (moisture, breed). In addition, there are a number of purely technological nuances that are difficult to implement in a home workshop..

First of all, it is the material of manufacture. The combustion temperature of generator gas is slightly higher than wood and can reach 800 ° C. Under these conditions, structural steel can only work with a significant intrinsic heat capacity, that is, the thickness of the walls of the combustion chambers should be about 12 mm or even more. Heat-resistant steels may also be used, but only if suitable processing and bonding means are available. Unfortunately, there are no other ways: all other products have a short service life until they burn out (no more than 10 years), and undergo a change in shape due to internal stresses.

Another problem is that a special shape of the chambers and their separators is required for stable operation. In the conditions of factory stamping and automatic welding, it is possible to create arbitrarily complex shapes, but in home production, a fancy channel system cannot be performed. This leads to increased aerodynamic resistance of the boiler and does not allow for normal draft. The inverse flow of the upper blast can be compensated for either by a very high (about 10–12 m) exhaust duct pipe, or by forced draft due to an electric smoke exhauster.

The installation of the latter is strictly recommended for all long-burning boilers without exception. During the day, the natural draft force can change, which leads to incomplete combustion of generator gas and its accumulation in an explosive concentration even inside the boiler room.

Mine boiler – we disassemble the scheme

The boiler with a shaft vertical loading has a body in the form of a high (up to 2 meters) column of a square or round profile with an internal section of about 0.3 m². This column has a blank bottom and, possibly, an outer casing that limits heat radiation from the hot walls..

A structure that is very similar to the piston of a disposable syringe is lowered into the firebox. The wide slab at the base exactly follows the internal profile of the shaft, creating gaps at the walls of 4–6 mm. The gap acts as a nozzle and restricts the flow of air through the combustion chamber, so an accurate fit is important here. Piston rod – 100 mm steel pipe with a length not less than the total height of the shaft, welded through the center of the plate.

The flue duct is located on the side of the shaft with an offset of 200 mm from the upper edge. This is an ordinary steel sleeve welded through one of the walls of the shaft. Another 250 mm lower inside the shaft along its walls a hoop of wire 4–5 mm thick is welded on. The clearance of the shaft is thus limited, but it is still enough for the piston to pass.

A steel plate rests on the resulting side, exactly like on a piston. A 250–300 mm hole was made in its center, along the edge of which a steel strip bent into a ring was welded to form a 70–80 mm high side. There is the same side along the outer edge..

Now we have the bottom of the chamber for mixing and burning pyrolysis gases. Its function is to localize secondary combustion, because otherwise the volume of the hearth will grow with the subsidence of the piston as the fuel burns out with a consequent drop in the boiler efficiency. Another detail is the boiler lid. This is the third slab, but with dimensions along the outer contours of the shaft. At the edges, it has a small (30–40 mm) side for a tight vestibule, and a 120 mm hole was made in the center and a sleeve was inserted, protruding up and down by 250–300 mm.

Integration into the design of a water heat exchanger

The resulting chamber has a volume of about 0.15-0.2 m³, any heat exchanger can be relatively freely placed in it, but heat-resistant cold-drawn tubular coils are mainly used. Its ends pass through the walls of the boiler and are supplied with 5-6 turns of pipe thread – everything is as usual.

It is much more important to solve another problem: after installing the heat exchanger, the piston can no longer be pulled out, but then how to lay firewood? For these purposes, two holes are cut in the front wall of the boiler and two rectangular steel sleeves are welded for mounting hinged doors..

One of them is small, about 200 mm high, and it is installed in the upper part of the boiler, exactly opposite the pyrolysis combustion chamber above the support ring. With its help, you can control the combustion process and adjust the draft, so it is better to choose options with tempered glass..

The second door should be almost as wide as the boiler shaft. It is intended for loading and igniting fuel, as well as removing ash after it has burned out. The opening must be placed below the support ring.

There are also alternatives. The boiler, for example, can have a circumferential tank around the body, or it can be wrapped with a conventional seamless steel pipe for 3-4 turns. It is necessary to place the heat exchanger in the upper part of the boiler, although the rest of the outer surface (including the chimney) is suitable for taking heat of all kinds of economizers.

Optional equipment

It remains only to make some improvements. To begin with, let’s make the distribution of primary air over the generator’s hearth more efficient. To do this, weld 6–8 parabolic petals perpendicularly onto the piston plate to form a kind of impeller.

The second useful addition to such a boiler is its correct piping. A recirculation unit must be installed between the supply and return of the internal heat exchanger, which will not allow the water temperature in the boiler to drop below 50–60 ° С. Otherwise, the formation of condensation from residual moisture and rapid wall corrosion is inevitable..

Weld a small stud to the top of the piston pipe and put a damper with a hole on it to regulate the amount of air supplied to the generator. Also, you should not replace the smoke exhauster with a blower fan, which is much more difficult to adjust. The exhaust pump must be installed at a point in the flue, 2 meters away from the boiler. It is optimal if radiator fins are installed in this section of the pipe..