A
typical air curtain enclosure
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 penetrating 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".
Uses
Air doors are often
used where doors are required to stay open
for operational purposes, such as at loading
docks and vehicle entrances. They can be
used to help keep flying insects out by
creating forceful turbulence, or help keep
out outside air, thus reducing infiltration
through the opening. Cold drafts can be
avoided by mixing in warm air heated by
the air door. Heated air doors are commonly
used when supplemental heat is needed for
a space, and to reduce the wind chill factor
inside the opening, in colder climates.
Further applications
include customer entryways, airplane hangars,
cargo doors, drive through windows, restaurant
doors, or shipping receiving doors. Non-heated
air curtains are often used in conjunction
with cold storage and refrigerated rooms.
Air doors can be equipped
with or without heaters to heat the air.
The fan must be powerful enough to generate
a jet of air that can reach the floor. There
are some studies in the scientific literature
that present analytical methods to predict
the sealing efficiency obtained with an
air curtain.[2]
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.
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.
Energy
savings
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.[3]
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.[citation needed] 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.
Design
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 2/97[4] The 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.
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