Air door

An air door or air curtain is a device used for separating two spaces from each other, usually at the exterior entrance. The most common configuration for air curtains is a downward-facing blower fan mounted over an opening. Normally this opening is an entrance to a building, and the air curtain is intended to help keep flying insects out by creating forceful turbulence. It also helps keep outside air out, avoiding cold drafts by mixing-in warm air from the air curtain. The fan must be powerful enough to generate a jet of air that can reach the floor.

The most effective air curtain for containing heated or cooled air inside a building with an open door will have a high face velocity at the opening, generated by top-down flow, recovered by a recirculating air plenum and duct return to the source fans. This configuration is available for new construction, but difficult to provide in existing buildings. The air curtain is most effective with low exterior wind velocity. At higher wind velocities, the rate of air mixsing increases and the outside air portion of the total face flow increases. Under ideal conditions of zero wind, the effectiveness of the air curtain is at its maximum.

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.

Non-heated air curtains are often used in conjunction with cold storage and refrigerated rooms. Airflow through a door depends on wind forces, temperature differences (convection), and pressure differences.

Air curtains can be used to save energy by reducing the heat transfer between two spaces, although a closed and well-sealed physical door is much more effective. A combination is usually utilized; when the door is opened air curtain turns on, minimizing air flow from inside to outside and vice versa.