Air flow

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Underlying technologyHeat dissipationAir flow


In a closed room without heat sources or ventilation, the air will settle in stable layers, with the warm air at the top and the cool air below.


When you introduce a heat source to this room, the heated air will rise up, since warm air is lighter than cold air. At the same time, cool air takes the place of the warm air, is also heated up and also rises. The result is a stable air flow around the heat source for as long as the cool air taking the place of the heated air is cooler than the surface temperature of the heat source. The air flow also speeds up the heat transfer (convection).


The force of the air flow is determined by the difference in density ∆ρ of the air, the effective height h and the gravity of earth g, provided no obstacles restrict these. The warmer the air, the lower its density.


p = 2 - ρ1) * h * g


How much the air heats up (and thus its density ρ1) depends in turn on the temperature difference of the cool air taking the place of the warm air and the heat-emitting surface of the heat source. If the difference in temperature is great, more heat can be transmitted to the air in the same amount of time than when the difference in temperature is small.


The air flow from a heat source can be obstructed by the following:

Solid obstacles in the way of air ciculation.

Insufficient space around the heat source (and thus insufficient heat exchange)

Air circulation in the opposite direction generated by other heat sources.



If the natural air flow is not sufficient to maintain the ambient temperature at the heat source (device) within the permitted range, it must be backed up by an additional air flow generated by a ventilator.


Click to expand or collaps object.Example of air circulation inside a heated room

The air flow shown here represents an idealised flow and are not meant to be an exact representation of air circulation. Actual air flows are not as clearly outlined and may blend into one another at the edges. Also, an air flow may be generated above the heated up control cabinet which then has an impact on the other flows of air.


Two heat sources exist in the control room: a heater and an HMI in a closed control cabinet (without any ventilation slits). Both generate air flows which are analysed below:


A.The heater generates air circulation in the area behind the control cabinet, which mainly effects this area. Due to its low position the air flow covers the entire height of the room. Via the heated up air spreading along the ceiling this air is also transported in front of the control cabinet.

B.The HMI inside the control cabinet generates an air flow whose strength is determined by the effective height and the difference in air density determined by the difference in temperature. A key factor is the amount of heat that can be transported to the outside via the control cabinet walls.

C.There are two factors supporting air circulation here: the heat emitted by the HMI's front display and some of the air heated up by the heater. Both flows of aircool off at the colder wall and sink down again. How low the air flow sinks at the wall is determined by the difference in temperature between wall and air flow. The upwards pull at the display is determined by its temperature.

D.This air flow is generated by the air heated up at the control cabinet, and it needs the temperature of the ambient air in the room to be lower than that of the air heated up by the control cabinet's surface. The air flow can therefore collapse completely if the room temperature is equal to or higher than that of the control cabinet's surface.



When warm air cools off due to contact with a cooler object (e.g. cabinet wall), heat is transported to the cooler object, warming it up.



Please note that air flows will change with every new source of heat and / or obstacles and forced ventilation systems.


See also:

Kapitelseite Thermal resistance

Kapitelseite Thermal transfer resistance



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