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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, blowing air across the surface of the opening. Air Curtains can come with, or without heaters to heat the air. Normally the opening is an entrance to a building, or opening between two space conditioned at different temperatures. Air curtains can serve many purposes. They can be intended to help keep flying insects out by creating forceful turbulence. It also helps keep out outside air, reducing infiltration though the opening. They can also be used to avoid cold drafts by mixing in warm air heated by the air curtain. The fan must be powerful enough to generate a jet of air that can reach the floor.
Air curtains are utilized when a barrier is needed across an opening, and when reducing energy costs is desired. Some applications include customer entryways, airplane hangars, cargo doors, drive through windows, restaurant doors, or shipping receiving doors. The air stream the air curtain places across the opening creates an invisible barrier to contain an air conditioned space and reduce infiltration. Typically, an air curtain can pay for itself in one to two 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 when the door is open. Heated Air curtains are commonly used when supplemental heat is needed for a space, and to reduce the wind chill factor across the opening in colder climates. Air Curtains 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 Curtains.
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 mixing 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. In windy locations air curtains 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.
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 (via mass transfer when air mixes across the threshold) between two spaces, although a closed and well-sealed physical door is much more effective. A combination is often utilized; when the door is opened the air curtain turns on, minimizing air flow from inside to outside and vice versa. Air curtains are often used where doors are required to stay open for operational purposes, such as at loading docks and vehicle entrances.
Research by the University of Cambridge for the Close the Door campaign found that air curtains failed to provide adequate thermal comfort if store doors were left open, and that they could be eliminated if the doors were shut.
It should be noted that the air movement equipment positioned over the shop doorways in the two case studies in the University of Cambridge interim report are not air curtains but overdoor heaters. There is a clear distinction between an overdoor heater and an air curtain and this was not acknowledged by the authors of the interim report who referred to these units as air curtains. 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 footfall where the door is mainly closed. They are useful in providing warmth to a cold region adjacent to a doorway or opening which could become uncomfortable, similar to a radiator positioned under a window. They should not be seen as an alternative to an air curtain. It is quite understandable that the overdoor heaters in the University of Cambridge study did not perform well at the doorways examined as they were being used as a cheap alternative to air curtains. Unfortunately this happens all too often as people do not understand the principal differences between air curtains and overdoor heaters, these are:
1. Air curtains are designed to fully cover the width of a doorway plus an overlap – overdoor heaters are small, they could never cover the width of a doorway so would leave significant gaps where the outside air can enter.
2. The fans in an air curtain are powerful enough to provide an air stream to project across the whole doorway – overdoor heaters have only small fans that could never project an air stream more than approximately 1m. ISO 27327-1:2009 describes a test method to measure air velocity projection.
3. The discharge nozzle on an air curtain is optimized to provide a more uniform air stream across the whole width of the doorway – overdoor heaters could never provide uniformity comparable to an air curtain as they can only cover part of the doorway. ISO 27327-1:2009 describes a test method to measure air velocity uniformity.
4. Air curtains are fitted with sophisticated controls to regulate the strength and heating capacity of the airstream across the doorway – overdoor heaters have simple manual controls which are often left on the wrong settings, unnecessarily wasting energy.
As way of illustration Figure 1 shows an air curtain and Figure 2 shows an overdoor heater, both installed over the same doorway.
An authoritative engineering design procedure for calculating the supply air flow and thermal capacity of an air curtain for an HVACR application is explained in the BSRIA Application Guide 2/97The procedure for a ‘Building with an Air Tightness Specification’ should be followed, i.e. a practical building with some air leakage. Within the BSRIA Application Guide, Section 4.2 explains the design procedure and Section 5.2 gives worked examples for buildings with a range of air tightness specifications. This allows the engineer to calculate the supply air flow rate and thermal capacity of the required air curtain for a particular application.
- American Society of Heating, Refrigerating and Air-Conditioning Engineers defines an Air Curtain as:
"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 penetrating infiltration through an entrance generally ranges from 60 to 80%". – Taken from the ASHRAE Handbook 2004: HVAC Systems and Equipment, page 17.9
- 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".
- Interim Report on the Energy Appraisal of Retail Units: Assessing the effect of open doors on energy consumption and thermal comfort Technical Report Number: CUED/D-STRUCT/TR232, Murat Basarir & Dr. Mauro Overend, published 2010-10-18, accessed 2011-06-28
- HEVAC Air Curtain Industry Group (http://www.feta.co.uk). For technical and commercial information on air curtain technology click on the HEVAC icon and go to the Air Curtain Group area. The Federation of Environmental Trade Associations (FETA), Hare Hatch, Reading, UK
- ISO 27327-1:2009. Fans - Air Curtain Units - Part 1: Laboratory methods of testing for aerodynamic performance rating. International Organization for Standardization, Geneva, Switzerland.
- BSRIA Application Guide 2/97. Air Curtains – Commercial Applications. Building Services Research and Information Association, Bracknell, Berkshire, UK. 1997