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El Efecto chimenea es el movimiento del aire hacia dentro y hacia afuera de edificios, chimeneas, conductos de gases u otros sistemas, como resultado de la flotabilidad del aire. La flotabilidad tiene lugar debido a la diferencia entre la densidad del aire entre el interior y el exterior, como resultado de las diferencias de temperatura y humedad. El resultado es una fuerza de flotabilidad positiva o negativa. Cuanto más grande es la diferencia térmica y la elevación de la estructura, mayor será la fuerza de flotabilidad y, en consecuencia, el efecto chimenea. El efecto chimenea ayuda a impulsar la ventilación natural, la infiltración y los fuegos (véase Desastre de Kaprun y el incendio de Kings Cross).

Efecto chimenea en edificios

Dado que los edificios no son totalmente herméticos (aunque mínimo, siempre existe un ingreso a nivel del suelo), el efecto chimenea causará la infiltración de aire. Durante la temporada de calefacción, el aire interior más cálido se eleva a través del edificio y escapa por la parte superior o a través de las ventanas abiertas, ductos de ventilación, o huecos no intencionales en el techo, tales como ventiluces o ventiladores de techo. El aire cálido ascendente reduce la presión en la base del edificio, aspirando aire fresco a través de puertas, ventanas u otras aberturas y fisuras. Durante la estación de ventilación, el efecto chimenea es inverso, pero es generalmente más débil debido a la menor diferencia entre las temperaturas.

En un edificio moderno de altura, con una envolvente bien sellada, el efecto chimenea pueda crear diferencias de presión significativas que deben recibir consideraciones de diseño y pueden necesitar resolución con ventilación mecánica. Las escaleras, los conductos, los ascensores y similares tienden a contribuir al efecto chimenea, mientras que las divisiones interiores, los pisos, y los parallamas pueden mitigarlo. Especialmente en el caso de fuego, se debe controlar el efecto chimenea para prevenir la dispersión del humo y del fuego, a fin de mantener condiciones adecuadas para los ocupantes y para los bomberos.​

Efecto chimenea en conductos verticales de gases y chimeneas

El efecto chimenea en chimeneas: los medidores representan la presión absoluta del aire y el flujo del aire es indicado con flechas de color gris claro. El dial del indicador se mueve en el sentido de las agujas del reloj con el incremento de la presión

The stack effect in industrial flue gas stacks is similar to that in buildings, except that it involves hot flue gases having large temperature differences with the ambient outside air. Furthermore, an industrial flue gas stack typically provides little obstruction for the flue gas along its length and is, in fact, normally optimized to enhance the stack effect to reduce fan energy requirements.

Large temperature differences between the outside air and the flue gases can create a strong stack effect in chimneys for buildings using a fireplace for heating. Fireplace chimneys can sometimes draw in more cold outside air than can be heated by the fireplace, resulting in a net heat loss.

Before the development of large volume fans mines were ventilated using the stack effect. A downcast shaft allowed air into the mine. At the foot of the upcast shaft a furnace was kept continuously burning. The shaft (commonly several hundred yards deep) behaved like a chimney and air rose through it drawing fresh air down the downcast stack and around the mine.

Cause for the stack effect

There is a pressure difference between the outside air and the air inside the building caused by the difference in temperature between the outside air and the inside air. That pressure difference ( ΔP ) is the driving force for the stack effect and it can be calculated with the equations presented below.​​ The equations apply only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building. For multi-floor, high-rise buildings, h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference​ explains how the NPL affects the stack effect in high-rise buildings.

For flue gas stacks and chimneys, where air is on the outside and combustion flue gases are on the inside, the equations will only provide an approximation and h is the height of the flue gas stack or chimney.

$\Delta P=Cah{\bigg (}{\frac {1}{T_{o}}}-{\frac {1}{T_{i}}}{\bigg )}$

SI units:

where:
ΔP	= available pressure difference, in Pa
C	= 0.0342, in K/m
a	= atmospheric pressure, in Pa
h	= height or distance, in m
To	= absolute outside temperature, in K
Ti	= absolute inside temperature, in K

U.S. customary units:

where:
ΔP	= available pressure difference, in psi
C	= 0.0188, in °R/ft
a	= atmospheric pressure, in psi
h	= height or distance, in ft
To	= absolute outside temperature, in °R
Ti	= absolute inside temperature, in °R

Induced flow

The draft (draught in British English) flow rate induced by the stack effect can be calculated with the equation presented below.​​​ The equation applies only to buildings where air is both inside and outside the buildings. For buildings with one or two floors, h is the height of the building and A is the flow area of the openings. For multi-floor, high-rise buildings, A is the flow area of the openings and h is the distance from the openings at the neutral pressure level (NPL) of the building to either the topmost openings or the lowest openings. Reference​ explains how the NPL affects the stack effect in high-rise buildings.

For flue gas stacks or chimneys, where air is on the outside and combustion flue gases are on the inside, the equation will only provide an approximation. Also, A is the cross-sectional flow area and h is the height of the flue gas stack or chimney.

$Q=CA{\sqrt {2gh{\frac {T_{i}-T_{o}}{T_{i}}}}}$

SI units:

where:
Q	= stack effect draft (draught in British English) flow rate, m3/s
A	= flow area, m2
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)
g	= gravitational acceleration, 9.81 m/s2
h	= height or distance, m
Ti	= average inside temperature, K
To	= outside air temperature, K

U.S. customary units:

where:
Q	= stack effect draft flow rate, ft3/s
A	= area, ft2
C	= discharge coefficient (usually taken to be from 0.65 to 0.70)[cita requerida]
g	= gravitational acceleration, 32.17 ft/s2
h	= height or distance, ft
Ti	= average inside temperature, °R
To	= outside air temperature, °R

This equation assumes that the resistance to the draft flow is similar to the resistance of flow through an orifice characterized by a discharge coefficient C.

See also

• Chimney
• Flue gas stack
• HVAC
• Solar chimney
• Solar updraft tower
• Draft (boiler)

External links

• Stack Effect: When Buildings Act Like Chimneys --Green Building Advisor
• National Research Council Canada - CBD-104 Stack Effects in Buildings