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Effectiveness Of Air Door
May 22, 2017


Airflow through a door depends on wind forces, temperature differences (convection), and pressure differences. Air doors work best when the pressure differential between the inside and outside of the building is as close to neutral as possible. Negative pressures, extreme temperature differences, elevators in close proximity, or extreme humidity can reduce the effectiveness of air doors.

The most effective air door for containing conditioned air inside a building with an open door will have a high face velocity at the opening, generated by top-down flow, and air recovery by a recirculating air plenum and duct return to the source fans. This configuration is feasible for new construction, but difficult to implement in existing buildings. The air door is most effective with low exterior wind velocity; at higher wind velocities, the rate of air mixing increases and the outside air portion of the total face flow increases. Under ideal conditions of zero wind, the effectiveness of the air door is at its maximum, but in windy locations air doors cannot create a perfect seal, but are often used to reduce the amount of infiltration from an opening.

For industrial conditions, high face velocities are acceptable. For commercial applications like store entrances, user comfort dictates low face velocities, which reduce effectiveness of separation of exterior air from interior air.

Comparison to overdoor heaters[edit]

Air flow of an air door (top-down configuration)

Air flow of an overdoor heater

The UK based HEVAC Air Curtain Group describes overdoor heaters as small electric or water heated fanned units with a low air volume flow rate. They are intended to be installed at doorways having low pedestrian traffic where the door is mainly closed, and are useful in providing warmth. However, they should not be seen as an alternative to an air curtain, which also serves to separate the indoor and outdoor air spaces.

The main differences are:

  • Air doors are designed to fully cover the width of a doorway, whereas overdoor heaters may be too narrow.

  • The fans in an air door are powerful enough to provide an air stream to project across the whole doorway opening. Overdoor heaters may have less powerful fans.

  • The discharge nozzle on an air door is optimized to provide a uniform air stream across the whole width of the doorway, which may not be the case with overdoor heaters.

Energy savings[edit]

Air curtains consume electrical energy during their operation, but can be used for net energy savings by reducing the heat transfer (via mass transfer when air mixes across the threshold) between two spaces. However, a closed and well-sealed physical door is much more effective in reducing energy loss.[3] Both technologies are often utilized in tandem; when the solid door is opened the air curtain turns on, minimizing air exchange between inside and outside.

An air curtain may pay for itself in a few years by reducing the load on the building's heating or air conditioning system.[citation needed] Usually, there is a mechanism, such as a door switch, to turn the unit on and off as the door opens and closes, so the air curtain only operates while the door is open.

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