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Air Curtains and Air Screens

Air curtains or air screens in open doorways are used to keep acceptable indoor comfort in buildings

Actual Applications

Door-less shop fronts
Workshop entrances
Doors of public buildings which are frequently opened

Actual Designs

Vertically mounted - on one or both sides of the opening
Horizontally mounted - on top, bottom or both top and bottom of the opening

air curtains screens

Forces acting on an opening in the wall

The forces acting on an opening like a door into a warehouse, mall or similar can be summarized to

wind
natural draught

The natural draught depends on the height of the building (open inside height) and the temperature difference between outside and inside air. The differential pressure due to natural draught in a building doorway with inside temperature 20^oC, outside temperature 0^oC and building height 10 m is approximately 10 Pa. With -20^oC and a building height of 20 m the natural draught pressure is close to 40 Pa.

Calculate the natural draught force!

The wind force acting on an opening in the wall is proportional with mass times the velocity squared. The pressure at 10 m/s (36 km/h, 22 mph, Beaufort Fresh Breeze ) is 60 Pa.

Air Velocities

By direction the airflow with an angel out of the door it is possible to resist the natural draught and wind forces. The strength of an air curtain follows the formula

mass times the velocity squared

An Air Curtain that blows at twice the speed has four times the resisting power at the same air volume.

In general the discharge velocities should not exceed

discharge from above > 5 - 15 m/s (15 - 50 ft/s)
discharge from below > 2 - 5 m/s (5 - 15 ft/s)
discharge from side > 10 - 15 m/s (30 - 50 ft/s)

Note that maximum discharge velocities depends on location of the discharge nozzles. The discharge velocity must be increased if the air curtain unit is mounted higher if the air stream shall hit the floor with the required velocity that shall stabilize the air stream.

For ware houses, shopping malls and similar buildings with openings up to 2.5 m the velocity should not exceed 5 - 9 m/s. For industrial buildings the velocity can be exceeded to 35 - 40 m/s.

Air quantities

The quantities required depends on many variables and an exact calculation may often be hard to perform. Values of 2,000 - 5,000 m³/h air per m² door opening are common.

Note! Exposed systems with

lot of wind
low out door temperatures
high buildings

may even double the values.

Air Flow and Potential Differential Pressure

The strength of an air curtain is the maximum potential differential pressure it can resist. The potential pressure resistance generated by an airflow through an inlet opening can be expressed as

Δp = 2.2 q² sin(α) / b H^3/4 (1)

where

Δp = potential differential pressure over the opening in the wall (Pa, N/m²)

q = air flow through discharge nozzle (m³/s per meter opening width in wall)

α = airflow angle (normally between 20 - 30^o)

b = depth of discharge nozzle (m)

H = height of door opening (m)

The average air velocity through the discharge nozzle can be expressed as

v = q / b (2)

where

v = average velocity (m/s)

Note! The average air velocity is per meter opening width in wall. The velocity should not exceed the values mentioned above.

Air Curtains Calculator

The calculator below can be used to estimate the strength of an air curtain by calculating the pressure difference and velocity in the air flow. Replace the default values with the actual values.

The differential pressure shall compensate the differential pressure caused by natural draught and wind velocity.

Example - Air Curtain

The height of an entrance opening in to a mall is 2.5 m. The depth of the inlet is 1 m. The air flow angle through the inlet is 25 degrees and the air flow per meter width of the opening is 8 m³/s.

The force against natural draught and wind forces can be calculated with (1) as

Δp = 2.2 (8 m³/s)² sin(25) / 1 (2.5 m)^3/4

= 29.9 Pa

The velocity through the inlet can be calculated with (2) as

v = (8 m³/s) / (1 m)

= 8 m/s

Modulating Air Curtains

Forces compensated with air curtains constantly varies with outside temperature and wind velocity and some control devices modulating the air flow angles and volumes are often required.

with outside temperature close to inside temperature and low wind - the curtain air flow is minimized and the air flow is directed straight through the doorway
with outside temperatures far from inside temperature - and a lot of wind - the curtain air flow is maximized and the air flow is directed out of the doorway

A modulated control can be achieved with one or more temperature transmitters located as indicated in the figure above.