Aircon Air Curtains

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Definitions

A typical Commercial 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 preventing infiltration through an entrance generally ranges from 60 to 80%.

The Air Movement and Control Association (AMCA) 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". In North America, the more commonly used term for an air door is "air curtain".

USES

Architectural Air Curtains

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 curtains.

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 curtain image

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. 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. 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 operates only while the door is open.

How air curtains work?

  • Generally the air entering through the inlet grille, sometimes with filter functions, is compressed by internal fans and forced thought an air outlet, which is directed at the open doorway
  • The filter protects the interior components (heat exchanger, fans, electronics, etc.) from dust and particles.
  • The fans of the air curtain can be direct or belt driven. The most frequently used types are centrifugal, axial and cross-flow.
  • Some air outlets and/or lamellas can be adjustable to increase the performance of the air curtain according to each situation.
  • Heated air curtains have a coil (electric, hot/chilled water, steam, indirect or direct gas, direct expansion, etc.) to heat or cool the jet. Heating is used to avoid people feeling a cold jet of air when crossing the doorway and also to heat the volume of air coming in at the entrance.
Air curtain image

Horizontal discharge from top to bottom

Air curtain image

Non Circulating Air Curtain

Air curtains: Technology

It is known that on open doorways with two adjacent areas in different conditions the air is interchanged between them because the laws of physics tend to equalize the temperature and pressure difference between both sides.

Basically air transfer on doorways happens due to these 3 factors:

  • Temperature difference: is the natural convection effect that creates an air transfer between two areas with temperature difference. Warm air will escape through the top of the doorway and being replaced by cold air coming in at the bottom. As bigger temperature difference bigger air infiltrations and energy losses
  • Pressure difference: it is recommended to equilibrate the pressure difference as much as possible because it affects the air curtain performance. But in some installations like clean zones a little pressure difference helps to prevent the particles entering from outside
  • Wind, stack effect and draughts: by modifying the air jet strength and the outlet discharge angle, the air curtain can work against forced air movements like wind or draughts. But if the velocity of the incoming air is excessive the air curtain will become less efficient

Schematic representation of the main parameters involved in the performance of an air curtain produced by the UPC (Polytechnic University of Catalonia).

The efficiency of an air curtain depends on the optimization of performance factors.

Air curtain image
  • h = effective width of the jet
  • α = discharge angle
  • U0 = discharge velocity
  • θ = negative impact angle
  • H = opening height
  • P1 = outside pressure
  • P2 = inside pressure
The most important ones are:
  • Turbulence of the jet: low turbulence jet will be much more efficient and save energy
  • Air speed: air velocity should be enough across the doorway
  • Air volume: a wider jet makes the air curtain stronger against air transfer on doorways
  • Angle discharge: according to a situation if jets are well oriented it will increase the energy saving
  • Fan type: axial, tangential wheel, centrifugal, etc. Higher pressure fans create a higher pressure jet that reaches farther. For instance, if we compare an air curtain with tangential fan against an air curtain with centrifugal fan (with same air volume), the jet made of centrifugal fans will be stronger and larger.

UPC University air curtain studies have proven that air turbulences are one of the most important parameters that will affect the distance of the air jet.

UPC University schema shows the air turbulences behaviour:
Air curtain image

Optimized shape of the outlet plenum, the position and fans type, the shape of lamellas, etc. substantially affects the air jet performance.

Air curtains angle discharge

Tests and University studies have proven that the angle discharge helps substantially helps the air curtain to be more efficient.

Tests and University studies have proven that the angle discharge helps substantially helps the air curtain to be more efficient. When factors like wind, temperature or pressure difference causes air transfer from outside to inside, we can aim the jet towards outside some degree. Then the jet direction against the air entrance will help to keep the air outside. The trajectory of the jet will be parabolic but at the end will reach the floor approximate by the doorway. If we can’t adjust the angle discharge, the jet will be pushed in by the external air forces.

The forces parallelogram theory explains how the forces behave on a doorway. The following diagrams show the difference between air curtains with fixed lamellas against adjustable ones.

Air curtain image

Air curtain with fixed lamellas less efficient (1)

Air curtain image

Air curtain with oriented lamellas more efficient (2)

(1) The first one, fixed vanes, where the entrance air velocity pushes the jet of the air curtain, the resultant of the parallelogram of forces is diverted inwards. This allows outside air to enter inside.

(2) The second one, adjustable lamellas, when the air jet is oriented against the entrance, the resultant of the parallelogram forces is directed perpendicularly to the floor. This means that outside air does not enter and inside air does not escape. Besides internal temperature level is maintained.

Advantages and benefits of air curtains

We classify the air curtains advantages in 4 groups:

Commercial profitability

  • Sales increase due to the “open door effect”. It is known that when doors are open, people affluence increases substantially
  • Doors open is an invitation for customers
  • Doorway acts as a shop window and allow people to see the inside of the shop
  • Free access for disabled people (wheelchair) or people using baby strollers, trolleys, umbrellas, etc.
  • Help to control uncomfortable sensations of draughts
  • Increases available usable space on entrances
  • Increases customers and staff comfort
Hygienic and healthy atmosphere
  • Helps maintain adequate environment
  • Reduces employee absenteeism
  • Pest and insect control
  • Barrier against airborne dust, pollution, fumes and bad odors
Energy saving
  • Reduces the energy losses from the conditioned space
  • Reduce central plant capacity (heating/cooling)
  • Reduces the building running cost
  • Reduces the CO2 emissions
Increased safety
  • Increase visibility and avoid collisions because there isn’t a physical obstruction
  • In cold rooms reduces misting, dry out the doorway and prevents ice forming on floors
  • Easy evacuation through the exit doorway in case of fire or emergency
  • Act as a barrier against fire smoke (special application)

To achieve those advantages it is absolutely necessary to select the right air curtain.

Air curtain image

Inefficient air curtain (losing money)

Air curtain image

Efficient air curtain (saving money + advantages)

If we select a wrong unit the air jet won’t reach the floor and the separation of two adjacent areas will disappear. Then all heated/cooled air will cross the doorway and we will loose energy and all other advantages.

Some people only care about the air curtain price and select a less performance or inadequate air curtain, following sometimes other seller recommendations. Being confident you should always compare the characteristics of the air curtain, especially the kind of fans used and the air volume.

At the end if we calculate the amount of money of the air curtain and the energy savings we will achieve that selecting the wrong unit can be more expensive and moreover the end client won’t be satisfied.

In conclusion, on open doorways it is highly recommended to use an air curtain because of the many benefits.

Door climate protection

First and foremost air curtains are designed to prevent a climate area (heated or cooled) from the influx of outside air through an open doorway.

The air curtains reduce energy costs by keeping heated or cooled air in the internal building atmosphere.

Efficient air curtains will save up to 80% energy looses across a doorway compared with a door without air curtain.

During the winter an air curtain creates a barrier that keeps out the cold air while in the summer the air stream keeps out the hot air from outside.

Bearing in mind the energy saved, the average payback time for an air curtain is between 2 and 5 years depending on usage and climate conditions.

Pest and flying insect control

Insect air curtains are commonly used, when necessary, to stop insects from entering a buildings like food processing plants, bakeries, restaurants, hospitals, etc. Bugs in flight approaching will be stopped by a barrier of air too powerful to penetrate.

Air curtain image

Air curtain stops flying insects entrance from the outside

You need to select the correct air curtain for insect repellent applications. A very powerful air jet along the all doorway is needed (stronger than standard applications).

Cold storage

Due to big temperature differences, it is highly recommended to install air curtains to reduce the energy losses when the door is open. The higher the temperature differences between inside/outside, then higher are the energy losses when a door is unprotected.

So the payback period of the air curtain is very short and then the energy/money saving is very high.

Not only we will save money avoiding air transfer through the door but also will keep the frozen goods in perfect condition and help to reduce defrost costs of the cold room.

Dust, pollution and airborne control. Clean rooms.

Always installed in the clean side of the doorway, air curtains act as a barrier against dust, particles and humidity.

Other applications

Drive-thru windows, separate smoking from non-smoking areas, industrial ovens, refrigerated display cabinets, etc.

Installation examples


Common places where air curtains are installed:
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Buildings

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Airports

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Train stations

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Hospitals

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Universities

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Museums

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Theaters

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Stores (general)

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Chain clothes

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Chain shoes

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Chain supermarkets

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Chain food

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Chain cars

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Banks

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Factories

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Shopping centers

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Hotels

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Restaurants

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Theme parks, sports

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Others