The flue of a chimney is the depression that originates in the base of the chimney, as a consequence of the difference of pressures created by the gases contained in it, that is to say, the fumes and the column of external air. The specific weight of the fumes, since they are always hotter than the ambient air is always lower, so: as the external pressure is higher than the internal pressure at the base of the chimney, the fumes experience a push that tends to evacuate them towards above; This push is the natural shot, created by the chimney.
6 Factors that will influence the natural shot
- The shot increases as the height of the chimney increases, and is related to the dimensions of the chimney. The higher the chimney height, the smaller the smoke outlet section is needed.
- The lower the specific weight of the fumes, the more shot. The specific weight of the smoke decreases as its temperature increases, therefore the draft will be higher the higher the smoke temperature.
- The material and the shape of the flue pipe affect transcendentally in the chimney flue. The circular sections are those that create less loss of load. The coefficient of friction will imply a different load loss for each material, and logically smooth interiors (metal), have an undeniable advantage over other types of materials (chimneys, prefabricated concrete etc.) Another important factor to have in Regarding materials, it is the temperature of the fumes, provided that a well-insulated duct with double-walled pipe can be installed, which guarantees a higher smoke temperature, and therefore a better draft.
- Trees, walls, roofs and in general any obstacle, in a radius of less than ten meters can negatively affect the chimney draft, this distance depending on the height of the obstacle in relation to our chimney.
- Also an inadequate cap can have a very negative impact on the chimney draft.
- Other factors that also have influence, although less, in the natural draft are the height above sea level, the location of the installation, the relative humidity of the air, etc.
Artificial shot
The draft is created by mechanical elements that are designed in each case in a manner appropriate to the needs, for this reason they do not depend so directly on the outside or smoke temperatures, nor on the circumstances of atmospheric pressure and humidity. They are generally used in stoves and pellet boilers, and are practically not used for firewood equipment.
Sources: www.chimecal.com y www.lacunza.net