The operation of a chimney is based on the principle of communicating vessels, that provides that a fluid contained in two or more vessels, communication between them, tend to assume and maintain the same level, should that have the same density, in each vase. E ', in effect, an application of the principle of gravity.

 

  Even the air is a fluid, which has a mass, and therefore a weight. As for all gases, the weight of the air is a function of its density; that is, the amount of matter per unit volume. In other words: the more gas is "sparse", the lower its quantity per unit volume (say: one cubic meter), the lower its density, the lower the weight of that cubic meter. Finally, we consider that the density of a fluid is also a function of its temperature: the higher the temperature, the lower the density.

 

  Observing the drawing on the left, is Imagine two chimneys, "A" and "B", of identical height, set in communication between them, at the base, by a horizontal portion, fitted in the center of a gate valve that separates them. At room temperature, the two columns of air contained in the chimneys, will have the same density, and thus the same weight: will therefore be in static equilibrium. If, however, we provide, at the base of column A, to provide heat, the temperature of the column of air contained in A will begin to rise. The temperature increase, accelerating the motion of the gas molecules, cause its expansion; a part of the gas will be ejected from the top of the chimney, while the remaining part inside will decrease its density, and consequently will become lighter.

  At this point, by opening the gate valve and putting in communication the two columns, we break the static equilibrium between them. The air contained in the column A, less dense, and therefore lighter, will tend to be ejected from the top of the chimney from the air contained in column B, which, being colder, and therefore denser and heavier, gravity will tend to take his place, to restore balance. In contact with the heat source, however, the cold air coming from column B will increase in temperature, and also become lighter, it will be pushed upward by always new air that comes: the union of the two columns continuous heat, determine the continuous operation of the system.

 

   

 

 

The fireplace in practice

  We adapt now this theoretical model to reality. The column "A" is our fireplace; the stove is the source of heat, provided with a sluice which is the air intake, while the "B" column is formed simply from the outside air: the principle of communicating vessels applies whatever their section; we can therefore consider a "vase" having also the basis for the entire earth's surface, and a height equal to that of our fireplace. The layers of atmospheric air located beyond the outlet of the chimney can be ignored, since exercising an identical pressure on both vessels.

  When we turn on the heater, the hot fumes produced by the combustion are expanded by virtue of their high temperature, the volume of gas contained in the fireplace decreases in density, and the outside air heavier tends to take its place, passing through the heater. In this way oxygen it is also supplied to the combustion, which can continue until it runs out of fuel. Continuing to provide fuel, it is the process can be continued indefinitely.

 

 

 One thing you must keep in mind in this scheme is that the pressure differences in the game are minimal: a fireplace is considered in good "draw" when the difference between the external atmospheric pressure and the lower internal pressure (or, as is commonly said , depression) is between 10 and 20 Pascal, that is between 0.1 and 0.2 millibars, that is, still, between 1 and 2 ten-thousandths of normal atmospheric pressure! The balance of operating a fireplace is therefore delicate, and is influenced by many factors that make reason of the "strange" behavior of the chimneys.

 

  The hot air contained in the chimney moves slowly, at a speed of a few meters per second (mostly 1, 5-2 ms), driven from the outside air, whose pressure must win the inertia. Rough walls that cause friction, bottlenecks that create turbulence, sudden changes of direction, such as sharp bends, constitute serious obstacles to the movement of the fumes.

 

  The chimneys seem to suffer in the weather because weather changes are always accompanied by changes in atmospheric pressure: in good weather, the high pressure favors the operation of the chimney; in the rainy days, however, the low atmospheric pressure, it makes the task more difficult.

 

  The cold days, increasing the temperature difference between the flue gas inside the chimney and the outside air, favoring the draw; hot days make it more difficult.

 

  Increase the height of the chimney makes it easier to "draft", because in this way it causes an equal increase of the external air column, so that it becomes heavier total: if the initial pressure difference between the two columns is too low, for each fraction of added height, is added a fraction of a weight difference in more, until the sum of these fractions creates a difference in total pressure sufficient to put in motion the column of air lighter.

 

  A flue with many curves and horizontal or inclined, increases its length without increasing its height: it follows that the amount of air contained in it, even if more light outside air, can have a total weight in excess of the capacity Push the corresponding column of outside air. For this curves and horizontal sections, if necessary, they must be compensated for with an extension of the vertical section of the chimney.

 

  A plant placed at sea level will generally need a chimney shorter of a plant placed in the high mountains, because with the altitude, the atmospheric pressure decreases.

 

GAS Formula

Chemical Density

Kg / m3 Relative density to air

Air - 1,293 1,000

Hydrogen H2 0,090 0,070

Carbon monoxide CO 1.250 0.967

Carbon dioxide CO2 1,977 1,529

Oxygen O2 1,429 1,105

Sulphur dioxide SO2 2,926 2,264

Nitrogen N2 1.251 0.967

 

 

Further work

 

Determine the natural draft (head pressure of the FP)

    From what has been described above, it should be clear that the 'draw' natural function is above the height of the chimney and the temperature difference between the flue gas within the flue and the ambient air. Better still, the difference in density between the combustion fumes inside the chimney and the outside air. In fact, more fumes are hot, are less dense, more are lighter, more easily outside air will tend to expel them from the chimney.

    The density of the smoke is normally evaluated as mass, in kilograms, per unit volume, expressed in cubic meters, that is kg / m3.

    The first thing to do, therefore, is to try to quantify this difference in density.

    A formula is normally used to determine the density of the fumes it may be expressed as:

 

 

 

where dfumi is the density of the fumes searched, d0 is the density of the fumes to 0 ° C, 273 is the transposition in degrees Kelvin (absolute) of the value 0 ° C, T is the average temperature of the combustion fumes.

    Given the huge excess of air necessary for completely burning the wood, we can consider, with reasonable approximation, the density of the fumes as equal to that of air, that is, about 1.3 kg / m3 (at 0 ° C and to the sea ​​level) and set the temperature of the flue gas, say, at 300 ° C. The formula, then, becomes:

 

 

 

ie 0,62 kg / m3, which is the average density of the fumes of combustion of the wood at 300 ° C of temperature.

 

    At this point, to calculate the static pressure of the chimney, or the natural draft, we can apply the following formula:

 

 

   

 

where Ph is the static pressure sought, expressed in Pascal, H is the height of the chimney in meters, and g is the gravity acceleration, 9.8 m / s2, (in effect, is the force of gravity the engine of the system), by the density of the ambient air (equal to about 1.2 Kg / m3 at 20 ° C and at sea level) and dt is the gas density at the given temperature (300 ° C), both expressed in kg / m3.

    Let us assume that our fireplace has a height of 10 m. Substituting in the formula, you have:

 

 

 

namely

 

 

 

that is again: Ph = 56,84 pascal.

 

    If, for a given height of the chimney, the temperature of the flue gas were, say, 200 ° C, the natural draft would correspond to 44.1 pascals, while for a flue gas temperature of 400 ° C, there would be a theoretical depression of about 66 pascal.

    On the other hand, considering the temperature of 300 ° C, if our fireplace was high only 5 m, instead of a vacuum of 52.92 pascals, it would obtain a of 28,42 pascal, that is half, while with a chimney 15 m high, we would have a theoretical depression, always at 300 ° C, of ​​well-85,26 pascal.

    Consider again that, if our chimney was placed at 800 m altitude, the pressure of the outside air, always at 0 ° C, would be about 1,124 Kg / m3, and therefore the same chimney of 10 meters in height, the the same temperature of 300 ° C, would produce a vacuum of 49.39 pascals.

    Just to finish to complicate things, we recall that the density of air varies, as well as a function of altitude, also in function of the temperature (colder temperature means denser air and vice versa) and atmospheric conditions: weather variations can lead to variations in atmospheric pressure also of 50/60 pascal.

 

    E 'therefore evident how the flue gas temperature and the height of the chimney are important to have a good draft. However it must be remembered that the higher the temperature of the flue gas, the lower is actually the efficiency of the stove, or the heat generator in general. In other words, the warmer the fumes, less heat is available for heating of the environments. With a wood stove, you should not exceed 180/200 ° C temperature of the smoke, to have an efficient heating regionevolmente. With an average temperature of the fumes of 200 ° C, and a chimney 5 m high, we would have a vacuum of about 20 pascal that, considering that the majority of applications require a minimum vacuum of 8/10 pascal to function properly, do not They seem few. But to depression theoretical be subtracted the inevitable losses of load of the system, ie the sum of the resistances which the heat generator and the chimney oppose the free sliding of the fumes.

    This talk about in the next section.

 

Determine the load losses.

    A fluid flowing inside a tube encounters a series of resistances that tend to slow down stroke, absorbing, so to speak, along the route up to the chimney, energy imprinted to the fumes from the natural draft to the base of the chimney.

    The calculation of the load losses of a fluid flowing inside a duct is one of the more complex calculations and difficult technical physics (the standard UNI 9615, which regulates the sizing of flues, requires the detection of 23 different parameters combining in about 80 mathematical passages) and its complete explanation here it is obviously impossible. We confine ourselves therefore to provide some general guidance, useful to identify the main problems of the "functioning" of the chimneys.

    Among the main causes of the pressure drop of the fumes, we can list the following:

    

Load losses for thermal dispersion

    Even the flue better insulated disperses a certain amount of heat through its walls. The thermal dispersion cools the fumes, making them more dense and therefore heavier.

    Such heat transfer depends mainly on:

- By the thermal resistance of the material constituting the walls. Obviously, the greater the thermal insulation of the walls of the chimney, there is less heat loss. For this flues must always be insulated, especially if carried out.

- The diameter and the length of the chimney. The greater the diameter and the length of the chimney, the greater the heat exchange surface.

- The temperature difference between the inner wall and the outer. Much colder the outdoor temperature, the greater is the thermal exchange between the inside and the outside of the chimney. For this flues should run, as far as possible, within the housing, where the 'outdoor temperature' is still higher.

- The speed of the fumes. The greater the speed of the fumes, so much less time remain inside the flue, so less heat can disperse.

 

    Hereinafter we provide the value of thermal conductivity of some materials used for the construction of flues. As shown, the flues prefabbricaterealizzate with two thin layers of steel around a thick mineral fiber are the best compromise between heat resistance and light weight.

 

Values ​​of thermal conductivity and density of some materials

 

Material

 

Temperature

 ° C Thermal Conductivity

(W / mk)

Density

 Kg / m3

 

Steel

20

 

15 7850

Refractory 1 2000

Masonry with concrete sprayed from 0.4 to 0.6 1290/1930

Masonry bricks from 0.35 to 0.52 in 1000

Mineral fiber 0.035 100

 

   The power losses for thermal dispersion can be evaluated with a certain precision, knowing the 'height, the section and the materials of which it is composed of the chimney, in addition to the value of the mass of the fumes, which (together with the section of the chimney) determines the speed of the fumes themselves. The calculation is rather complex, but it can be said in general that, if the fireplace is well insulated, the load loss for thermal dispersion is quite negligible, at least for domestic installations working with wood or coal, in which the height of the chimney rarely exceeds 10-12 meters, at most.

 

  Loss of head by friction

    A fluid flowing within a tube is slowed down by the friction produced by contact with the walls of the tube. The friction is directly proportional to the roughness of the walls and the speed of the fumes, as well as dependent on the shape of the tube and on its length.

    More specifically:

- The greater the roughness of the inner walls of the conduit, the greater the friction and therefore the resistance that is opposed to the flow of the fumes. This is fairly intuitive. Slide a hand on a glass plate is not the same thing that slide on a sheet of sandpaper.

- The higher the speed of the fumes, the greater the friction. Although this should be obvious. You can slowly slide a hand on a sheet of sandpaper, perhaps to test its roughness, without too much damage; slide it very quickly, however, can cause serious damage: the roughness more speed provoke a strong friction.

- More irregular is the section of the conduit, the greater the pressure loss. This is a bit 'less intuitive, but suffice it to say that the more irregular is the section, the more easily the motion of the fluid becomes turbulent, and that the increase of turbulence greatly increases the friction.

- The longer the pipe, the greater the loss of head by friction. Here the same observation made for pressure losses due to thermal dispersion: the longer the conduit, the greater will be the friction surface.

    Below we provide the value of roughness of some materials used for the construction of flues. Also in this case, as can be seen, the steel is the material far more convenient. It also takes into account the fact that the roughness values ​​reported for the materials do not apply in the case of steel pipe made in a workmanlike manner. But if, as almost always happens, the excess grout used to seal the various elements of the pipes in the form of deposits within the masonry chimney, the roughness values ​​rise catastrophically.

Average roughness "r" of the inner wall for some materials / td>

Type of material Roughness

"R" in meters

Steel tube 0.0005

Concrete 0.001

Refractory 0.001

Conducted masonry 00.003

 

   The loss of head by friction depend on the nature of the duct and by the speed of the fumes, are therefore generally calls distributed load losses, given that their total value basically depends on the length of the tube, and distinguished from localized pressure losses, of which speak below.

    

    Localized head loss

   The pressure drops localized (or concentrated) are pressure drops due to obstacles such as bends, elbows, valves, branches, narrowing (or widening) of the section, etc. The values ​​of the friction factor for the most common barriers can be derived from tables, of which we give below a partial example.

 

Systems design Wood

 

 

 

 

 Make wood heating alternative to traditional heating it requires careful design. As with every major domestic installation, the installation of a wood stove can be a significant financial investment, so it is important to choose the right unit and position in view of better efficiency.

 

  The first step is to decide how much of the house is expected to warm by the stove. A stove too large will produce too much heat, and groped to reduce the amount to a minimum by reducing the pace can cause the formation of creosote deposits and a decrease in yield. A stove too small, on the other hand, will be almost useless, and force long the gait of combustion to obtain a heat nonetheless insufficient will usurarla ahead of time.

 

Also important is the position of the stove inside the house. The correct position will allow you to fully exploit the heat of the stove, while a wrong position will disseminate only in a small area, leaving the rest of the cold environments.

 

   Decided on the location, you will have to choose the right stove for your situation. As we have seen, there are a quantity of stoves commercially available. With a little 'patience, you can find one that meets the needs of heating, is suitable furnishings and is easy to use and maintain.

 

 

 

  

  Define needs

 

  A fireplace can be the only source of heat in the house, or be a supplement for a room very cold. If there were no previous experience with wood-burning plants, you can start with a small heater easy to install. This way you can tell if the wood heating is compatible with their lifestyle.

 

  If you plan to largely replace the traditional heating, the best location for the stove is the main area "lived" in the house. The living room is usually the largest, and is often centrally located, making it easier to transfer heat to adjacent rooms.

 

  A very large living space, separated by walls and intermediate rooms, requires a system leading the heat away.

 

  In the design phase, we need a rough map of the living space concerned, indicating the size of rooms and location of the openings (doors and windows), and partition walls (you can use a cadastral map or the original architectural design). A house with several floors to be heated requires a section in elevation, indicating the rooms and the placement of the stairs. Check the height and thickness of the floors and the maximum height to the roof ridge, in order to establish the extent of the flue required

 

 The volume to be heated. To find the volume of each room, calculate the floor area (length and width), and multiply it by the height to the ceiling. If it is irregular in shape, it divides the surface into squares or rectangles, are the surfaces of each, and then add up. If the roof is inclined (attics), we proceed on the contrary, by first finding the surface of a wall that contains the two heights, and then multiplying by the depth of the perpendicular walls.

 

  Insulation of the house. If you do not know, you try to find out the quality of the insulation of the house. In houses with their attics not finished, the isolation of the slab may be in view. To check the insulation of the outer walls, you can disconnect the electricity and remove from its housing a switch with its box; so you should be able to check if there is the insulator in the interspace between the internal wall and the external finish.

 

 

 

 

 Also check the number and size of windows, and if the windows are sealed with tinted windows. If the windows are not well insulated, consider the possibility to replace them before installing a stove. A well-insulated home requires less heat, and therefore a smaller stove.

 

  Place the stove

 

  Once you have determined the space to be heated, you have to decide which room to place the stove, and where in the room. This is a delicate decision, because the positioning of the stove will largely determine its efficiency in heating, and once installed, the stove, the flue, the chimney and any ducts for warm air distribution can not easily be moved .

 

  

Location in the house

In general, the stove should be located as centrally as possible in the house, in order to distribute the heat evenly throughout the living space. In most homes this position corresponds to the lounge and the dining room, the room where he usually spends more time, and where they mostly place family activities.

 

  Of course, the need for heating may not coincide with this area of ​​the house; you might need more warmth in a bedroom that's never hot enough, or in a studio or workshop is not heated by the current system and where a small heater can solve the problem.

 

 

 

 

 

In a two-story house, place the stove near the stairs will help to warm up the local high school. The same result can be obtained by opening of the slits in the floors to circulate the hot air. You can also place small fans wall to circulate air better in different environments. For this type of installation are building regulations that must be respected. It 'good use adjustable louvers, to modulate the amount of heat required and that they can be closed when not in use, so as to limit the spread of noise.

 

  The hot air tends to stagnate upwards, close to the ceilings. In houses with high ceilings, it can be a good idea to put the paddle fans, who will circulate the hot air more evenly.

  For the same reason, a room with ceiling lower than the other tends to be cold, since warm air is sucked from the higher ceilings of other rooms. A stove in this position it will compensate for the cooling and help to circulate more efficiently the heat in other rooms.

 

 

 

 

 

  If the thermostat that regulates the current heating system is located in the room where the stove will be placed, it will be the case to move it, or calibrated again to take into account the heat produced by the stove.

 

  If the house is very wide and distributed, it may be convenient to place two smaller stoves in place of a single very powerful, to better distribute the heat, or you can opt for a real system of ducts for warm air distribution , which, however, it requires a minimum of masonry to be installed. The first solution can be convenient for houses that are only partially inhabited for most of the time and where the members of the family group stable conduct their usual life in a few rooms: rooms not used can remain cold for the period in which are uninhabited . Place more stoves still it means having to use multiple flues.