In its simplest application, an air curtain is a continuous wide stream of air circulating over a door in a conditioned space. It reduces the penetration of insects and unconditioned air into a conditioned space by forcing an air flow over the entire entrance. The air flow layer moves at a speed and angle so that all air that tries to penetrate the curtain is captured. Air curtain efficiency to prevent infiltration through an inlet usually ranges from 60 to 80%.
The Air Movement and Control Association (AMCA) defines an air curtain as: “A directional airflow that moves over the entire height and width of the opening, which reduces the infiltration or transfer of air from one side of the opening to the other and / or inhibits flying insects, dust or debris from passing through ”.
Users of Air Curtains
Air doors are often used where doors must be kept open for operational purposes, for example at loading docks and vehicle entrances. They can be used to help keep flying insects out by creating powerful turbulence or to keep the outdoor air, which reduces infiltration through the opening. Cold drafts can be avoided by mixing in hot air heated by the air door. Heated air doors are usually used when extra heat is needed for a space and to reduce the wind cooling factor inside the opening, in colder climates.
Additional applications include customer entrances, aircraft hangars, cargo doors, restaurant doors or cargo reception doors. Unheated air curtains are often used in connection with cold storage and cold rooms.
Air doors can be equipped with or without heaters to heat the air. The fan must be powerful enough to generate an air jet that can reach the floor. There are some studies in the scientific literature that present analytical methods for predicting the sealing efficiency obtained with an air curtain.
The air flow through a door depends on wind forces, temperature differences (convection) and pressure differences. Air doors work best when the pressure difference between the inside and outside of the building is as close to neutral as possible. Negative pressures, extreme temperature differences, elevators nearby or extreme humidity can reduce the efficiency of air doors.
The most efficient air door to contain conditioned air inside a building with an open door will have a high surface velocity at the opening, generated by top-down flow, and air recovery through a recirculating air plenum and duct return to the source fans. This configuration is possible for new construction, but difficult to implement in existing buildings. The air door is most efficient with low external wind speed; at higher wind speeds the speed of air mixing increases and the outer air portion of the total surface flow increases. Under ideal conditions with zero wind, the efficiency of the air door is maximum, but in windy places, air doors cannot create a perfect seal, but are often used to reduce the amount of infiltration from an opening.
For industrial conditions, high speeds are acceptable. For commercial applications such as store entrances, user comfort dictates low speeds, which reduces the efficiency of separating the outside air from the inner air.