Air Curtains and Air Screens

With air curtains or air screens in the doorways, it is possible to keep acceptable indoor comfort in the building - even with open doors

Air curtains and air screens blows heated air (or cooled air in summertime) across door openings and reduces the ingress of cold air (or hot air in summertime) from the outside due to wind forces and natural draught through the building.

• Air screens acts with heated (or cooled) air
• Air curtains acts with unheated (or not cooled) air

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

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 velocity depends on the location of the discharge nozzle. The discharge velocity must increase as the air curtin unit is mounted higher so the air stream will hit the floor with the required velocity that stabilizes 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 applications with

• a lot of wind
• lower 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.

q - volume capacity per meter port - (m³/s)

α - airflow angle  (degrees)

b - depth of inlet opening (m)

H - height of port (m)

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

• Calculate the natural draught force 
• Calculate dynamic pressure due to 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 8² sin(25) / 1  2.5^3/4

    = 29.9 Pa

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

v = 8 / 1 = 8 m/s

Modulating Air Curtains

Since the forces compensated with air curtains constantly varies with outside temperatures and wind velocities, it may be necessary with some control devices modulating air flow angles and air flow volumes.

• With outside temperatures close to inside temperatures and low wind - the curtains air flows are minimized and the air flows are straight directed through the doorways
• With outside temperatures far from inside temperatures and a lot of wind - the curtains air flows are maximized and the air flows are directed out of the doorways

A control of the modulation can be achieved with one or more temperature transmitters located as shown in the figure above. 

Discharge temperatures (winter conditions)

The air screen discharge temperature should be kept within certain limits. For winter conditions

• small installations temperature range 35 - 50^oC (95 - 125^oF)
• large installations temperature range 25 - 35^oC (80 - 95^oF)
• suction temperature temperature range 5 - 15^oC (40 - 60^oF)

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