Fireplaces, stoves and stoves with fireplaces
Fireplaces, stoves, stoves with fireplaces
The article discusses some of the problematic issues of designing fireplaces, stoves, stoves with fireplaces, outdoor multifunctional complexes and their solution.
In the name of the fireplace, I put the concept that has developed in Russia for a long time. The fireplace is the most ancient, simplest, without a door, a heating device, contains a furnace, where the process of open burning occurs, does not have a convection system (gas ducts), joins the chimney in the upper part. The fireplace heats the radiant heat only visible area. Pleasant warmth of open fire, the ability to disconnect from earthly problems, the opportunity to fry kebabs. The fireplace decorates the room, ventilates it, it is an additional source of comfort in the room, these are the main advantages of an open fireplace. With seeming simplicity, an open fireplace is not a simple device and its good operation depends on many factors.
A significant drawback of the fireplace is a very low efficiency. The fireplace was constantly improved in order to increase efficiency. Additional convective heating of the air was carried out (heaters were inserted, hollow walls were made), external air supply systems (preheating) required for burning fuel in the fireplace were changed, and grate bars were inserted. However, these measures did not significantly improve their efficiency and did not solve the issue of efficient heating of the room. Due to the very low efficiency, the use of an open fireplace as a permanent heating device does not make sense.
Later in the fireplace began to install metal firebox (furnace insert) with sealed metal or glass doors. Such a device can no longer be called a fireplace. The fireplace has become a one-function heating device (heater) with a one-time laying of firewood and is used only for heating a single room. Some models can heat multiple rooms. Not every heater can be used as an open fireplace. Various designs of heaters are supplied to Russia with the name "fireplaces", which does not correctly reflect their purpose. The heaters use the method of glowing firewood burning (with the furnace door closed), which allows for a small hourly heat transfer, but for some time. It heats while the fuel is burning. The disadvantages include its rapid clogging with soot, especially when burning raw wood with a moisture content of more than 15% (wood with a moisture content of 15% can be obtained by storing them under a covered ventilated canopy for 2-3 years). In the summer they can serve only as a decoration. It should be noted that the operation of these heaters is ensured by the exact engineering calculation, since their effective work is ensured only under strict limitations of the vacuum limit that the pipe must create. They require a complete and expensive factory equipment parts (before the exit to the roof) and a qualified assembly. The release of flue gases into the brick smoke channel, without changing the system that creates the necessary vacuum in the pipe, can lead to its unsatisfactory work. Usually they are used as an auxiliary system (the second), which is heated only in the coldest period. It can also be used in places of temporary stay of people, for example, in garden houses, working premises, etc.
It is more expedient to use stoves with fireplaces in these cases, which are devoid of the indicated drawbacks and have high efficiency. They can be used as a fireplace or heating device at any time of the year. It is also possible to insert a metal firebox (heating insert) into the fireplace of the stove. It can be fired with both a closed and an open door (when connected to a pipe through a smoke chamber). In this case, the heat output of the stove with a fireplace increases. In the summer, when you want to sit by an open fire, the excess heat is removed through the oven to the street, and only radiant heat is used. In addition, such furnaces can have many additional properties that are useful to a person, such as: boil, roast, bake, smoke, dry, treat (stove), cook hot water, work on electricity, etc.
There is an opinion that in the stove with a fireplace the heat output of the stove decreases as the fireplace covers one wall of the stove. When designing stoves and devices for various purposes, including fireplaces, we had to solve a number of problems arising from their new design solutions.
There were questions: how to increase the craving, how to lower the temperature of the outgoing gases, how to improve and optimize the heat transfer of stoves with fireplaces? It took to solve problems:
1. Increased thrust, for example, in multifunctional outdoor complexes, in which it was required to reduce the exhaust gases from various devices into one pipe, with their large distance from it;
2. Reducing the temperature of exhaust gases from fireplaces, for example, in wall pipes, so that electrical wiring that crosses the chimney does not melt, or to prevent sparks from being ejected from a fireplace pipe, barbecue, etc .;
3. When blocking a stove with a traditional fireplace design, it closes the heat-release surface of the stove, reducing its heat transfer. This results from the fact that the fireplace of a traditional form has low located tooth and a smoke chamber. The firelighter has a thickening and inclined, full-bodied non-heating walls, which reduce the heat transfer of the furnace.
We do a lot of stoves with fireplaces. In this case, the walls of the fireplace are heat-giving walls of the furnace. In addition, we make multi-storey furnaces operating on one pipe. As part of these stoves may be open fireplaces.
When designing such furnaces, we had to solve the following tasks:
1. The walls of the stove with a fireplace should have the same thickness (for better heat transfer) and have the highest possible heat transfer surface;
2. The fireplace should work with a minimum pitch (the smallest section of the pipe);
The solution of these tasks became possible in the system of “free movement of gases” (SDG), where the heat generator's firebox is installed in a cap and is combined with it into a single space through a “dry joint” (slot). As an example of the possibility of this system, we can cite the 2-storey furnace shown above, built in Moscow in 2005. The furnace has one firebox and heats 10 rooms. And in 2 rooms there are warm stove benches of 77x182 cm in size. You can’t create anything like that in another system.
Combustion products are a simple mixture of several gases, including ballast gases, their molecules are completely independent, not linked to each other. Each particle of the gas flow has its own state: weight, heating, energy and takes place in the cap defined by this state for the entire time of free movement through the cap.Any interference with this movement, caused by a constructive change in the heat generator, leads to a change in the SDG system. Vertical cuts in caps, burnouts (bore-holes, departures, bypass) in the firebox do not ensure the free movement of each particle of the gas stream corresponding to its state.Heat transfer from the gas to the heat exchanger depends on the contact area of the heat exchange, on the temperature difference and on the contact time, the more they are, the greater the heat transfer. The hood can be of any shape and volume into which the heat exchanger can be inserted, that is, have a large heat exchange area. With such a construction of the heat generator, the area and time of contact of hot gases with the heat exchanger increases, that is, the heat exchange is improved. Passing through the system, the gas flows are separated in composition, degree of heating, weight and have a different speed of movement.The coldest jets have the highest speed, pass through the bottom of the bell and have little effect on the heat exchanger. The hot component of the flow under the action of Archimedean force directs up the cap, and is there all the time until the gases cool, that is, the heat of the flow is concentrated in the cap. By analogy, we can talk about the movement of water over a deep pool, in which the water temperature at the bottom practically does not change. In the system of forced gas movement (PDG), an undivided stream (with ballast gases) is passed through a convective system.As the gas flow rate increases, the contact time decreases. If the downward flow passes through a volume (channel) with a large cross section, the energy of the flow is dissipated. In either case, the heat transfer decreases, that is, the efficiency decreases. This is confirmed by tests conducted in Canada and France.
We had to look for new design solutions to the fireplace. Create your own design fireplace. Our fireplaces do not look normal, but they can work with pipe sections up to 20% smaller than classic fireplaces and do not impede the heat transfer from the furnace walls, which are common with the fireplace. In practical work on the laying of fireplaces, we often encounter poor-quality performance of smoke channels. They are partially overlapped with slabs, there are air leaks through the holes in the slabs, the seams are not overwritten. Therefore, life has forced us to carry out freestanding fireplaces according to our technology.
To simplify the understanding of the work of the fireplace (the process of movement of the gas stream), we will conditionally assume that the heat source will be an electric heater. In this case, hot air without combustion products will move through the fireplace.
The main elements of the fireplace fig1, defining his work: the firebox-1, cap-2 (with cornice), tooth-3, neck (with valve) -4, smoke chamber-5, pipe-6, rear inclined wall-7. There are also a portal (portal opening), side walls, under. In some cases, make an ashpit with a grate.
The movement of air through the fireplace occurs only when the temperature of the air in the pipe is higher than the temperature in the room and when the resulting thrust allows the air to overcome the resistance in the path of its movement.
Consider the movement of heat flow in the fireplace, shown in fig1.
If the valve-4 is closed, the hot air generated by the electric heater is constantly supplied to the cap-2. Part of the heat flow is perceived by the cap walls, and the rest is poured through the eaves into the room. At the same time in the cap there is increased pressure, increasing in height. If the valve is slightly opened, part of the heat will be supplied to the smoke chamber 5 and further to the pipe, some of the heat is perceived by the cap walls, and a part that is already smaller will also be poured into the room.At the same time pressure in a cap will decrease. You can choose a cross section of the valve, when all the heat generated by the electric heater will be perceived by the cap walls and go out into the smoke chamber, but it will not enter the room. From this we can conclude that the cross section of the neck with the valve should be sufficient to ensure the flow of hot gases with a permissible speed, or more. Usually the cross section of the neck with the valve is made equal to the cross section of the pipe, or more.
The same processes will occur if, instead of an electric heater, heat is given off by the warm walls of the fireplace, rear and side, heated by flue gases from the other side of the furnace.
The well-known interpretation of the mechanism for the movement of flue gases in the pipe-, fig2 does not explain many of the issues arising in the design of stoves and fireplaces. In accordance with this interpretation, the operation of the pipe depends on the average density of flue gases in the pipe (which in turn depends on the temperature of the gases) air density in the room and the height of the pipe. That is, the hydrostatic pressure in the pipe, acting per unit area- Δp, under the action of which the movement of flue gases is carried out can be calculated by the formula: Δp = H (ρx -ρd • cf.) g, where H is the height of the chimney, ρx and ρd • cf are the air density in the room and the average density of flue gases in the pipe. The pressure must be greater than the aerodynamic drag in the path of the gas flow through the fireplace (stove). It should be noted that in the SDG system the aerodynamic resistance to the driving flow is minimal.
In fact, everything is much more complicated. The density of a hot gas flow, even over a horizontal cross section of a pipe, has a different value, depending on the shape of its cross section and material. Fig3 shows sections of a square, rectangular, and round pipe.
The walls of the pipe are heated by passing gas, that is, heat is transferred from the gas to the pipe. For this reason, the temperature of the gas flow near the walls is less, and the density is greater and these indicators have different values at each point of the perimeter of a square and rectangular pipe. In a round pipe, the perimeter flux density can be considered the same. A rectangular pipe has the largest difference between the indicated parameters around the perimeter of the pipe. Due to the non-uniformity of the temperature field, aerodynamic phenomena are complicated by heat exchange processes.There are complex fields of speeds, concentrations of substances (with the flow of products of combustion), densities and temperatures. The description of such complex processes can be performed only on modern computers by specialists in computational fluid dynamics and heat transfer. All specified sides of the process are interconnected and influence each other. This can explain the difference in graphs, K. Mäkelä, the ratio of the cross-sectional area of the chimney of various shapes to the entrance opening of the open hearth (in%), depending on the height of the pipe. These graphs we use when calculating fireplaces. Graphs are shown in the book by K. Mäkelä, Stoves and Fireplaces, Moscow 1987. At the round section the ratio is the smallest, and at the rectangular section the largest. That is, a pipe with a circular cross section works better than a pipe with a square cross section and the pipe with a rectangular cross section is the worst working.
Fig.5 shows three identical volumes filled with hot air with a temperature t. All of these volumes take place when gas flows through a fireplace (stove) in the "free gas movement" system.
On fig.5a, open at the top and bottom; on fig.5v, open at the bottom, forming a cap; on fig.5c, open on top, forming a “glass”. The heated gas under the action of Archimedean forces tends to move upwards. In the volumes, forces F1, F2, F3 arise, directed upwards, this is the supporting (lifting) force of the gas.
The weight of the heated air in the pipe is less than the weight of the displaced cold air. The heated gas, in the case shown in fig.5a, under the action of the Archimedean force moves upwards relative to the less heated gas in the opposite direction to the force of gravity. In the case shown in fig.5b from the bottom up, the temperature in the cap increases. Due to the action of gravitational Archimedean forces, there arises an increased pressure equal to the difference in weights between the volume of hot and cold air in the room. The heated air displaces the cold air from the bell, with the cold air coming out of the bell through fields having the greatest density.
In the case shown in fig.5c hot air leaves the volume (cup), creating a vacuum there, and in its place, due to atmospheric pressure, cold air enters the lower part of the volume through places with a denser field (convection). If in the lower part of this volume there is hot carbon (coal), the source of heating the air, then it is oxidized, that is, slow (smoldering) burning. This phenomenon is based on the work of old Russians, sealed with a metal sheet, furnaces with hermetic doors, without grates and latches.Such stoves work according to the fig.5c scheme. In Europe, with a mild climate, at present, heating hearth furnaces (without grates) with thin walls, hermetic doors and without valves are being made. This ensures an increase in the time of exposure of hot gases to the heat exchanger (burning of fuel at a low temperature for 4-6 hours) and the possibility of prefabrication of furnaces. In a harsh climate, the device requires massive heat-consuming furnaces, in which combustion occurs at elevated temperatures for 1-1.5 hours, 1-2 times a day, and heat release during the day. Under these conditions, the use of few massive stoves as the main source of heating is not rational, since they will have to be frequently flushed.
This phenomenon is based on the method (mode) of glowing burning of solid fuel in boilers equipped with grate bars at night, which maintains the temperature in the boiler for a long time without human intervention. In this case, the boiler must be sealed and equipped with an airtight blower and furnace door. In this mode, when a large amount of red-hot coal accumulates in the firebox, the doors close tightly and the latch remains open.
As noted above, the hydrostatic head acting per unit area, Δp (kg • m / s2), can be calculated using the formula: Δp = H (ρx -ρd • cf) g where H is the chimney height (m), ρx and ρd • avg - the density of air in the room and the average density of flue gases in the pipe (kg / m3), g-acceleration (m / s2). According to this formula, the available hydrostatic head is calculated in the systems shown in fig.5a and fig.5b.
In public buildings, forced ventilation is usually performed, with forced air supply. This makes a balance calculation of incoming and outgoing air from the room, taking into account the work of the fireplace. In open fireplaces, the need for air for burning 1 kilogram of fuel, according to K. Mäkelä, can reach 150 m3. This amount of air must be supplied to the room, otherwise a vacuum occurs in the room, and the fireplace will smoke.
Where can air get into the room? Through a specially designed fireplace for ventilation. Through no density window and door openings. In the case of modern hermetic joinery and the absence of special ventilation for the fireplace, through the channel of natural exhaust ventilation of the room, which begins to work on the inflow. What troubles may arise in this case. Exhaust ventilation can be made of a garage, kitchen, toilet, pool, workshop, etc.When the doors to these rooms are open, all smells inherent in these rooms will enter the fireplace room. The same thing happens if the natural ventilation exhaust duct is located next to the fireplace tube and there is no special ventilation for the fireplace in the room. The exhaust channel begins to work on the influx, flue gases enter the room and after a while begins to pinch the eyes. It is not recommended to arrange exhaust ventilation with artificial impulse in the fireplace hall, and if the room is not compensated by inflow with artificial impulse, then it cannot be arranged.It follows from what has been said that it is necessary in each specific case to consider the need for special ventilation for a fireplace. The air can be brought outside, from the attic, the ventilated premises of the technical floor and basement, etc., the channel section is 160–340 square meters. see (total cross-section of supply channels). The air must be clean, without harmful impurities and dust. There are several ways to supply air for the fireplace. These are: directly to the upper area of the room (preferably next to the fireplace); into the fireplace through the holes in the bottom (possibly grate) or the walls of the fireplace; preliminary convective heating of the air supplied to the room with its distribution inside the room. You can do the same natural flow, opening the window, but in this case, the occurrence of drafts in the room.
There is a common mistake when in a residential building, with hermetic windows and doors, equipped with a fireplace, only natural exhaust ventilation is made, and artificial ventilation is not done. In a heated house, warm air through the exhaust ventilation comes out and a vacuum is created in the room. When you open the valve of the fireplace, counter-propulsion occurs (it blows from the fireplace) and the fireplace begins to smoke. In this case, it is necessary to make forced forced ventilation and air conditioning. This question is very difficult for an untrained person and should be solved by specialized design and installation organizations.
Consider the work of direct-flow fireplace fig.6. With a natural inflow, the movement of air from the outside to the inside of the room is provided by the vacuum in the room, which is created by the pipe. In any case, with the natural compensation of outgoing air, a vacuum occurs in the room, since the chimney must overcome the resistance of the incoming air in the path of its movement. Moreover, the greater the amount of outgoing air that is compensated for, the less vacuum occurs in the room and the smaller the compensation of the inflow, the greater the vacuum.In the absence of air supply or a large lack of incoming air, the working pipe sucks the air out of the room, a critical vacuum is created there, at which the external atmospheric pressure exceeds the pressure created by the supporting force of the gas, and atmospheric air polluted by flue gases from the pipe into the room according to the scheme shown in fig.5c. The supporting force of the gas is equal to the difference in the weight of air and gas in the volume of the pipe, minus the force expended to overcome the resistance in the path of gas movement and is directed upwards. Δp = H (ρx -ρd • cf.) g.In the pipe there is a counter movement of flue gases and air outside. This happens in a short period of time until the pressure inside and outside the room is balanced. Then the process repeats, - critical rarefaction, - air breakthrough, etc. The fireplace begins to smoke. Fig.6 shows how a breakthrough in the room takes place through a pipe. By analogy, we can talk about the breakthrough of air in a bottle, if you pour water out of it, dropping it down the neck.
Balancing pressure occurs due to two components:
1. Organized air intake from the outside (through specially designed ventilation);
2. The air entering the counter-flow from the pipe.
From the above, important conclusions follow to allow the correct design of stoves with fireplaces and other complex multifunctional devices for various purposes. Indoors for a longer period of time there will be no critical vacuum when the fireplace will smoke and the system’s operation becomes more stable in the following cases:
1.If the room has the largest volume;
2.With the greatest amount of air flow outside.
When building a fireplace built according to the scheme shown in (fig.6), and natural compensation of outgoing air, a critical depression and smoke of the fireplace arise in the room. For this reason, it is imperative to provide a device in the path of the gas flow that prevents the breakthrough of air into the room from the pipe. This device is the tooth-3, smoke chamber-5 and cap-2, shown in fig7. The cross section of the pipe must be large enough to ensure not only the passage of gases, but also the possibility of oncoming movement of flue gases and air entering from outside.If we consider the volume limited by the smoke chamber - 5 and pipe - 6, with the valve closed in the mouth - 4, then it can be seen that the movement of the gas flow in it occurs according to the diagram shown in fig.5c. In a smoke chamber, a critical vacuum is created, outside air enters the smoke chamber-5 (convection) by a counter flow, and the pressure is balanced in it. In the cap-2, the movement of the gas flow occurs according to the scheme shown in fig.5b and an increased pressure is created in it. At the level of the neck with a valve there is a change in the magnitude of the pressure, which has a positive effect on the work of the fireplace.
When the smoke chamber is built according to the scheme shown in fig7, the pressure is balanced in it, not in the room. It prevents the ingress of air polluted by flue gases from the pipe into the room. It takes the same process as in the system shown in fig.6. The conclusions drawn above are also applicable to it.
Balancing the pressure in the smoke chamber is due to two components:
1.Intake of air from the cap-2;
2.The air entering the counter-flow from the pipe.
In a smoke chamber, there will not be a critical vacuum for a longer time, during which the fireplace will smoke and the system becomes more stable in the following cases:
1.If the smoke chamber has the largest volume;
2. With the greatest amount of air intake outside, (in particular, from cap-2, this is due to the large difference in pressure drop in the smoke chamber-5, and cap-2.)
With increasing temperature, the force of gravitational pressure occurs in the cap, - D. The magnitude of this force depends on the cap height, the temperature of the gas stream and increases with their increase. At the same time, the temperature rises in the chimney. There is a force of thrust, - T in the pipe, its value is in the same dependence on the height of the pipe and the temperature of the gas in it, as in the cap, since the origin of these forces is the same.
On the movement of the gas flow in the system fig.7, there are two forces: in the cap, - the gravitational pressure (pressure) - D; in a pipe there is a draft (suction), - T. With an increase in the height of the cap, the pressure increases, - D and the draft decreases, - T, and with a decrease in height - vice versa. The total force driving the flow is made up of these forces and is directed upwards. The point of application of these forces is the outlet of the neck-4.
It should be noted that in the system shown in fig.7, the total value of the force driving the stream, with the same height of the pipe and the average density (temperature) of the gases in it, remains unchanged with increasing the height of the cap, 2. It can be said that in the place of change of positive pressure to negative, remains unchanged and equal to the sum of the pressure, - D and thrust, - T.As the height of the cap increases, a condition may arise when the pressure force is equal to or greater than the thrust force, but the total supporting force remains unchanged. This makes it possible to raise the tooth and the smoke chamber as much as possible in order to increase the area of the heat-giving surface of the furnace located behind the walls of the fireplace, that is, to increase the heat removal from the furnace. Balancing pressure occurs in the smoke chamber, where there is a pressure difference. The process of pressure equalization takes place constantly.
Usually we make a tooth in a fireplace high along or across the fireplace and a smoke chamber. At the same time, we strive to maximize the smoke chamber, 4-6 rows in height (28-42 cm), and raise it higher, and make the wall thickness between the fireplace and the stove the same. In stoves with fireplaces, the smoke chamber combines the outlets from the fireplace and stove, and the outlet from the fireplace should not be located under the chimney. The same solution, the smoke chamber, is used to combine gas streams from various devices into one pipe at their large distance from it in multifunctional complexes.The smoke chamber combines in plan all the emissions from various devices and the outlet of the pipe. This is usually a volume of 4-6 rows of masonry, the overlap of which rests on brick columns. The release of flue gases into the chimney is done in the upper part of the smoke chamber, placing it so that it is not located above the neck of the fireplace, as well as the paths of gas flows from the devices are evenly distributed over the cross section of the smoke chamber.If it is necessary to lower the temperature of the outgoing gases and eliminate the ejection of sparks from the pipe, we make the smoke chamber in the form of a cap. That is, the inlet of the pipe is lowered by 1-2 rows (7-14 cm) below the ceiling of the smoke chamber, while increasing the height of the chamber accordingly. The use of a metal mesh as a spark arrester with openings of no more than 5x5 mm is usually not operational, since soot is deposited on the mesh and clogs its openings. The picture shows an outdoor complex that has one pipe and combines a barbecue, a fireplace and a Russian heating furnace (RTIK). It is seen that they work simultaneously without smoking.
The ratio of the cross-sectional area of the chimney to the area of the entrance opening of the fireplace in%, depending on the height of the chimney, is taken according to the schedule given in K. Mäkelä’s book Stoves and Fireplaces, allowing for a decrease in its area to 20%. The depth of the fireplace, with a portal up to a meter wide, is 51 cm.
I am grateful to Kazachkov IV, Doctor of Technical Sciences, Professor of the National Technical University of Ukraine (KPI), for his constructive comments, which are taken into account in this article.
1. Edited by G.F. Knorre, “Introduction to the theory of furnace processes”, M 1968 2. Site www.stove.ru
2. Elementary textbook of physics. Edited by Academician G.S. Landsberg, Volume I, Nauka Publishing House, Moscow 1972 K.
3. Mäkelä, Stoves and fireplaces, Moscow, stroiizdat, 1987
4 .I.I. Kovalevsky, Furnace works, Moscow, High School, 1977
5. Yu.P. Sosnin, E.N. Bukharkin, Encyclopedia, Moscow, New Wave, 2001