How residentia | commercial | industrial air curtain works?
What is an Air Curtain?
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defines an air door as follows: "In its simplest application, an air curtain is a continuous broad stream of air circulated across a doorway of a conditioned space. It reduces penetration of insects and unconditioned air into a conditioned space by forcing an air stream over the entire entrance. The air stream layer moves with a velocity and angle such that any air that tries to penetrate the curtain is entrained. Air curtain effectiveness in preventing infiltration through an entrance generally ranges from 60 to 80%".
The Air Movement and Control Association defines an air curtain as: "A directionally-controlled airstream, moving across the entire height and width of an 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".
How Does an Air Curtain Work?
Once the power is on, air is brought into the unit through the intake, enters the fan housing and is accelerated by the fan. This fast moving air goes into a plenum, which allows for an even distribution of air along the full length of the discharge nozzle. Aerofoil-shaped vanes in the nozzle create a uniform air stream with minimal turbulence.
For efficient air curtain performance, the angle of the discharge nozzle must be set correctly. An air curtain nozzle should be adjustable to aim inward or outward up to 20 degrees from the opening.The air discharged through the nozzle creates a jet stream to the floor. Approximately 80% of the air returns to the intake side of the air curtain, and 20% goes in the opposite direction.
Non-recirculating air curtains can be mounted horizontally above the door or vertically on one or both sides of the door, depending on the space available and the height, width and physical characteristics of the opening. Horizontal mounting above the door minimizes the chances of damaging the air curtain. Protective measures should be taken if the air curtain is mounted vertically.
The proper size, power rating and features needed for a particular air curtain application should be selected according to the following considerations:
- The physical dimensions of the opening, including height, width and space available for installation (clearance above the mantle).
- The type of door being used.
- The type of opening — customer entry, service entry, dock door, etc.
- The climate. Would supplemental heat at the door be appreciated by the building occupants?
- The prevailing winds and exterior temperatures on outside openings.
- The existence of any drafts due to pressure differences at the opening.
What's the most effectiveness?
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
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.
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. 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. 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.