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 .: Use of heat of effluent furnace gases :.

Use of heat of effluent furnace gases.

The results of operation of four boilers in the Monastery in Ganina Yama pit during heating period of 2003-2004/2005 showed that the boilers are capable to heat water up to 90 C even in case of using recently cut wood, which is not dry enough. The boilers were designed and built on the basis of theory laid out in the article " Basics for the design of wood-burning..", therefore, in this article you can find references to some provisions and pictures of the above-mentioned article. Fig.5 features combustion of wood, which is not dry enough in one of the boilers. The boilers are designed to burn wood only during the day. At night a smoldering mode is used when the wood is being punky when the ash-pit door (or regenerator heater, for one of the boilers is built with this device) is completely closed. During operation in this mode the temperature of effluent gases proved to be very high, and an additional device is needed for utilization of this heat.

The gases in the boiler cannot cool down up to water temperature, and water boils at the heating surface or is close to boiling temperature. Therefore the gases when they leave the boiler are heated to high temperature, approximately to 250-300or even higher. If the gases are let at once into the pipe they will take out a lot of heat. In order to make use of this heat, a special device called economizer is usually installed through which effluent gases flow. It is used for heating of recycling water that is returned into the boiler. The economizer can have various designs. All types have one thing in common, namely water movement in economizer shall be in the opposite direction as compared to the heat flow. In other words, cold water contacts with cold gases. Further, as the water flows in the economizer it gets warmer and contacts with hot gases. This condition is met if the economizer is made in the form of bell, in which in every cross-section located above the temperature is getting higher. At such oncoming flow of water and gases (heat flows) there remains the temperature difference necessary for heat transfer. At the same time the probability of condensation, which is the cause of pipe corrosion, is not so critical. This is particularly important if we burn wood with high moisture content. Such movement of gas and water is called counter flow. It shall be noted that in the system of free gas movement (when the coils of the boiler or economizer is installed in the bell) the coils of the boiler are designed and connected correctly, this condition is met due to natural forces of nature; it doesnt require an external impact, and therefore is natural and optimal. The design and the connection of coils in the boiler and that of economizer has to be made in such a way that the water flow is in the opposite direction as compared with heat flow. Piping of economizer shall ensure the possibility of water drain from it into hot water heating system from time to time. This is necessary for burning out of soot from the pipes surface through a special sealed door made in the bell where the economizer is installed. As the temperature of gases coming into economizer is significantly less as the temperature in the convective system of the boiler, the coils of the economizer shall have a larger heat absorbing surface. In order to use unified design components, it is necessary to use the same coils in the boiler and in economizer.

The economizer shall be installed in the second bell as viewed in the direction of gas movement, in two horizontal consecutive bells, as shown in Fig. 4. From the left side of E axis a convective system of the boiler of continuous operation is shown, Fig.2, with coils marked with letter C. (" Basics for the design of wood-burning.."). From the right side you can see the bell with the coils of economizer. The following designations are used: 1- outlet for exhaust gases, 2- economizer in the bell, 3- channel of direct stroke, 4- direct stroke shut-off damper , 5- temperature sensor, 6- sensor of the content of gases coming out from the pipe, 7- pipe, 8- by-passing channel.

The principle of operation is as follows: Sensor 6 (Fig.4), depending on the content of exhaust gases acts upon the actuating mechanism of a heater of regeneration system 6 (Fig.1 and 2) and optimizes air supply necessary for combustion. Sensor 5 depending on the temperature of gases coming out from the pipe actuates shut-off damper 4, opening or closing it. At increased temperature of exhaust gases shut-off damper 4 closes and hot gases from the boiler come to economizer through opening 1 and after cooling come to pipe 7 through channel 8. When the temperature goes down below the permissible limit shut-off damper 4 opens, and a certain amount of hot gases comes into the pipe through channel 3. Of course, this system can also operate without automation.

Water flow along any pipe shall be equal. Such a design ensures a minimum force exerted on the boil structures due to temperature elongation of coil pipes, low resistance to water flow, a good head and easy replacement of the pipes. When the system is filled with water no air-locks arise in the coils if they are installed with a slope for water drain in the return pipe. The piping diagram of the boiler and economizer (with natural circulation) and direction of water flow is shown by dotted line and arrows.

I receive letters from all over the world, e.g. from the U.S.A., Canada, Australia, Spain, Latvia, Lithuania, Estonia, Ukraine, Byelorussia, Uganda, Croatia, Slovakia, Czech Republic, South Africa, Poland, Sweden, Finland and other countries asking for boiler drawings and other things. I started to publish on site drawings of some wood-burning boilers using brick and furnace fixtures that are available in Russia. Different types of brick and stove fixtures having different sizes are used in other countries as compared with Russia. They can also be used for building boilers using my drawings that are published. Below you can find some explanations.

Brickwork for boilers is usually made of standard brick manufactured in Russia. We normally use red solid brick having the dimensions of 25012065 mm of grade 150 and fire-brick 8 (8) having the dimensions of 250123 65 mm. It is also possible to use fire-brick having other dimensions, in small amount. Due to that the external dimensions of boilers as well as the internal dimensions of fire-boxes and the bell (bells) in plan view are multiples of 13 cm. The width of a bell (bells) can be 13 cm if one coil is installed inside or 26 cm if two coils are installed inside. It is possible to have another width, multiple of 13 cm. The minimum dimension of the bell in plan view is 1313 cm. One vertical pipe can be installed in it (in minimum cell) of a coil having a diameter of 60 mm, that is 16,7% of the cell square. The number of vertical pipes shall be equal with the number of minimum cells. The height of one story of brick with joint is 7 cm. The boiler height is 30 stories, i.e.210 cm. It is possible to build boiler with a different height.

If bricks having other dimensions are used, the dimensions of the boilers shall be multiples of one half of the brick length with a joint. The diameter of coil pipes shall not exceed 16,7 % of the square of minimum cell. The boiler equipped with economizer can operate in the same way as the boiler without it, in the mode of heat supply adjustment in hot-water heating system, due to redistribution of way of gas flow without reduction of efficiency.

The excess of heat can be accumulated in accordance with the diagram shown in Fig.3. From the left side of E axis there is a boiler of continuous operation with economizer, Fig.2, marked with letter C. From the right side there is a system shown in accordance with the scheme single-two-story bell D making use of excess of heat. The core accumulating the excess of heat is marked by figure 2.

In accordance with this scheme it is possible to solve the question of hot water supply in solid fuel-burning boilers of continuous operation in closed circuit. In this case the system D can be represented by a stove with boiler (water heater) of hot water supply, in which water heater for hot water supply is indicated by figure 2. This stove as well as the boiler have a common pipe and can also be laid out above the hot water heating boiler. Hot water of one and the same temperature is needed all the year round, which is not the case for hot-water heating while the boiler is not used during the summer. When hot water heating boiler is in operation the excess heat of exhaust gases is used for hot water supply. If the boiler does not function during the summer or the heat of effluent gases is insufficient for heating water to the required temperature, in this case we may fire the stove for hot water supply separately.

I. V. Kuznetsov.

16/01/2005 Igor Kuznetsov "Kuznetsov's stoves"