Chimney effect

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Pressure conditions in a chimney

The chimney effect is the physical effect that leads to the intensification of a fire and that arises when hot smoke gases are guided through a chimney or similar vertical shaft. It is in contrast to the burning of an open hearth fire (such as a smoke kitchen , where the smoke gases passed through the house) or an open campfire .

If a fire is lit in a firebox below a chimney, a large volume of hot flue gases is quickly created. As a result of the static buoyancy , the hot gases rise up into the chimney . First, the buoyancy occurs in the middle of the chimney ( thermal convection ), while the colder air stored in the chimney and later flue gas condensate flow downwards on the inner wall of the chimney . The rapidly rising and turbulent flow of the exhaust gas stream mixes with the countercurrent and also carries it away from this "air bubble" of the colder temperature, just as quickly flowing water carries air bubbles with it. In addition, the turbulent mixing (as well as its wall friction ) heats the "cold air", which is then even more easily carried away by the subsequent hot exhaust gases. Finally, a uniformly homogeneous single-phase upward flow is formed. The Bernoulli effect of the flow causes a negative pressure in the combustion chamber (the so-called delivery pressure ) compared to the air pressure. The pressure difference is compensated by sucking in fresh air (or, depending on the point of view, the now overpressure of the air pressure allows fresh air to flow in), creating a chimney draft or chimney effect . A Venturi effect also occurs at narrow air inlet openings . Both effects lead to the fact that more atmospheric oxygen reaches the combustion material per unit of time, whereby the fire is fanned more (this increases the fire temperature and the exhaust gas volume, which leads to a further increase in the chimney draft).

A single match will not start the chimney draft; a decoy with fast-burning ignition aids (ball of paper, fidibus , sprinkler ) is required, which quickly create a large, hot volume of exhaust gas (low density). To properly light a fire in the stove, see Fireplace stove # Correct lighting

The chimney effect is based on natural convection . It is used technically to discharge exhaust gases from furnaces through chimneys ( natural draft firing ). An admixture of light water vapor in cooling towers or in water pants increases the buoyancy enormously.

A chimney effect can also occur in facade fires if the smoke gases rise in a narrow atrium or behind a thermal insulation system or in the cavity of a ventilated facade . The same effect also occurs in fires whose smoke gases escape into a stairwell or an elevator shaft and from there further up through an opening to the outside.

Even without a chimney pipe, similar phenomena occur in nature (see firestorm and thermals ).

The rising air cools down until it reaches layers of air at the same temperature, where the lift ends.

Waterspout : Moist air has a lower density than dry air.

In interaction with the hydrostatic pressure exerted by the colder fresh air, thermal convection occurs .

Flue gases containing steam rise almost vertically above the chimney head because they (also) flow out at a higher speed due to the chimney effect.

Fire of (moist) leaves in slow open combustion (without chimney effect) results in cooler smoke gases that are heavier due to soot and show little buoyancy.

Physical background

The difference in density is maintained by heating on one side and normal temperature on the other side of the circuit. The resulting differential pressure is also called “driving pressure” or “effective pressure” .

The differential pressure depends on the density difference and the effective height according to the formula: ${\displaystyle \Delta p}$ ${\displaystyle \Delta \rho }$ ${\displaystyle h}$

${\displaystyle \Delta p=h\cdot g\cdot ({\rho _{2}}-{\rho _{1}})}$

• ${\displaystyle h}$: effective height in m
• ${\displaystyle g}$: Acceleration due to gravity in m / s²
• ${\displaystyle \rho _{1}}$: Density at temperature  in kg / m³ ${\displaystyle T_{1}}$
• ${\displaystyle \rho _{2}}$: Density at temperature  in kg / m³ ${\displaystyle T_{2}}$

The pressure difference makes the air flow calculable with the speed : ${\displaystyle v}$

${\displaystyle v={\sqrt {0{,}5\cdot g\cdot h\cdot {\frac {T_{1}-T_{2}}{T_{2}}}}}}$

Because the chimney draft / delivery pressure depends on the chimney height and the cross-sectional area of ​​the chimney, the output of a new stove should be adjusted accordingly. If the delivery pressure is too high, the emissions from the fireplace and the load on the fireplace increase. Chimney sweeps do such calculations or approvals . A draft limiter would, for example, be a remedy against excessive delivery pressure . A wind blowing over the chimney head can increase or decrease the delivery pressure.

hazards

The currents and pressures created by chimney effects and the positive feedback can be dangerous and must be taken into account when planning and constructing buildings. Tunnels and stairwells are particularly at risk here. In the fire disaster on the Kaprun 2 glacier lift , the chimney effect in the ascending tunnel intensified the fire and also caused a large number of deaths above the source of the fire due to the smoke gases. Those who fled down survived.

Use

The chimney effect is made use of in addition to the useful firing and chimneys, also in updraft power plants , in which air heated under one roof is blown up in a tower by the prevailing air pressure and thereby drives turbines , as well as in natural draft cooling towers in which the air heated by the cooling water unites without further aids Experiences buoyancy. In addition, this process takes place on all heated surfaces such as radiators, cooling fins (of motors or electrical components), house facades, etc., if the geometric shape and the proportions resemble a chimney. In a case of wind power station air is cooled at the top of the chimney pipe, after which the air moves downward and the air flow drives a turbine wheel.

Chimneys use the chimney effect to fire up charcoal faster.

The effect has been used in house building for a long time; traditional Arab houses, for example, are cooled in this way. The Persian wind tower Bādgir has used the chimney effect to ventilate and cool buildings for centuries.

literature

• Frieder Kircher, Rainer Sonntag: Die Roten Hefte, Issue 25 - Preventive Fire Protection . 1st edition. Kohlhammer, Stuttgart 2014, ISBN 978-3-17-016996-8 .

Individual evidence

1. ^ Herbert Sigloch: Technical fluid mechanics. ISBN 3642030890 p. 48 ( limited preview in Google Book Search).
2. ↑ FVLR, Issue 10 [1] , accessed January 31, 2017
3. ↑ Thomas Lechner: "Traditional Arab houses, for example, the chimney effect has been cooling from sixty degrees Celsius ambient temperature to forty degrees for thousands of years.", In: Roland Wengenmayer: Cool head on the 41st floor , in: Frankfurter Allgemeine Sonntagszeitung July 11, 2010. ( online) (PDF; 495 kB).