Fundamentals of stove construction

In any hearth the hot gases coming out of the burner and moving inside it return their heat to the walls and after they are cooled they are discharged into the chimney. What is meant by the notion hearth? The hearth includes a firebox in which the process of combustion of fuel takes place, and convection system to use the fuel heat. This may be the stove of any functional purpose (heating stove, heating and cooking stove, Russian stove, bath stove, etc.) stove with a fireplace, heating boiler.

While heating or burning of wood part of its substance is fumed, the other part remains on the hearth bottom. The quantity of matter which vaporizes from the wood, e.g. birch, reaches up to 88 %, and the quantity of solid matter amounts to 12 % only. The majority of the substance that vaporizes are combustible fumes, therefore in order to use the energy content efficiently combustion of these gases is also foreseen. The most common error while designing a hearth lies in the fact that one forgets about the existence of the gaseous fuel part of wood. Energy or heat content of wood is a maximum amount of chemical energy incorporated in it that theoretically can be separated from the fuel during its combustion.

Being guided by this, we shall specify the following concept. In order to have a good hearth it is necessary to provide the following:

1. ensure the best conditions for the complete carrying out of combustion reaction, i.e. obtain maximum useful quantity of heat inherent in the fuel;
2. organize the natural movement of cold and hot gases, separate the gas flow coming through the hearth, direct the hot gases for heat accumulation and vent the cold ones;
3. fulfill the structural part of the convective system of the hearth in accordance with the necessary functional requirements in order to reduce the heat losses from the smoke fumes, ensure optimum heating of all parts of the hearth , turn the greatest part of wood energy content into useful heat, that is to achieve the highest efficiency factor. The possibility to use electric energy as reserve fuel.

Simplifying the gist of the concept: we have to obtain maximum heat from fuel combustion and accumulate the heat that was obtained in the hearth. The hearth design must meet the functional requirements and ensure optimum heat transfer.

These requirements are best of all met by the theory of stove construction worked out by I.S. Podgorodnikov. The most suitable conditions for carrying out the combustion reaction as well as organization of movement of the gases being cooled inside the hearth is to be solved on the basis of natural (free) movement of gases. The temperature of the exhaust hood fuel gases fed into the chimney is not big (something about 120 °C) that conditions high thermo-technical capacities of the hood constructions. The efficiency factor of channel-free convection systems is 93.7 % (do not mix up with the stove efficiency factor). The fundamentals of the right construction of household stoves have been worked out by I.S. Podgorodnikov on the basis of hydraulic theory of stoves, by the Russian scientist and metallurgist Prof. V. E. Grum-Grzhimailo. In order to better understand the question we shall view these fundamentals.

Provisions of hydraulic theory of stoves in regard to household stoves have fully preserved their meanings. The main idea underlying this theory is very simple. A hot gas plume surrounded by the cold gas comes up, as it is lighter, and a small stream-line of cold gas surrounded by the hot one goes down because it is heavier. V. E. Grum-Grzhimailo while designing the stove recommends to set such a direction of movement of the gases in its every part that would correspond to their natural strive: a stream-line of hot gas surrounded by cold gas should be sent upwards, while the stream-line of cooling gas downwards. The hot gases coming from the firebox into the stove space are accumulated under the arch as they are light, forming, as Grum-Grzhimailo puts it, «a sack of hot gases». Cold gases being heavier, go down to the lower part of the stove, and in case they are not extracted they form «a sack of cold gases». Therefore fume extraction should always be done at the bottom where the most cool gases are accumulated. The stoves built on this principle do not need «zigzag movement». The movement of gases in them is done by means of natural forces.

Such stoves are built using the principle of natural (« free», as M. V. Lomonosov puts it,) gas movement.

Refusal from zigzag movement made it possible to reduce considerably the resistance to gas movement inside the stove which is most important in the Russian stoves with quite a small height of the chimney. Only through the use of «free» (licentious) gas movement it became possible to solve the task of creating a heating furnace in the lower part of «teplushka» which chimney creates a reduced pressure of only 1.5-2 mm of water column. To solve this task using «zigzag movement» is practically impossible.

Flame flow coming out of fuel layer represents a mixture of burning gases stream-lines, air and products of combustion. In order the burning reaction in the flame ended, it is necessary to mix up its separate stream-lines; in the volume where the burning ends high temperature has to be maintained.

Under roof volume (at the top of hood) - «a sack of hot gases» is an ideal combustion chamber.