Commercial Systems
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 difference 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.
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. 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.