Air Curtains
An air curtain is a fan-powered device that creates an invisible air barrier over the doorway to separate efficiently two different environments, without limiting the access of the people or vehicles.
The energy saving air screen reduces heating and cooling costs by up to 80% while protecting the internal climate and increasing people comfort.
It keeps the atmosphere clean from pest and insects, dust, airborne, pollution, smells, odours and stops draughts and cold or hot air entrance.
The functioning of an air curtain is based on a high velocity jet of air that covers all openings. Heated air curtains creates comfort by the jet of air on people as they cross the air screen and helps to maintain the temperature on the entrance.
Are air curtains effective?
Absolutely!
All studies and tests have proven that air curtains are effective. When an efficient air curtain is well installed it save lots of energy and at the same time the entrance areas remain climate protected, comfortable and free of draughts, insects, odours, etc.
The following advanced Computational Fluid Dynamics CFD diagrams made by UPC (Polytechnic University of Catalunya) show clearly the efficiency of air curtains.
The surface of next diagram is divided in two parts (at X=2): left side considered inside has a heater on the wall and refrigerated right side simulates outside.
Situation without air curtain:
Following the laws of physics, the light heated air on the left of the image, goes up by natural convection and escapes to the cooler side while the heavier unheated cold air penetrate from the outside flowing through the floor. As the air tends to equalize temperatures and pressures and the warm inside are not thermally protected from the outside, the air mixes and we can't see two separate thermal areas. In this situation we waste lots of energy and the heater is not able to keep comfortable the internal area.
Situation with air curtain:
The air jet of the air curtain placed on top, clearly divide the warm and the cold area in two thermal zones. The air curtain keeps internal environment on the left side of the image at comfortable warm temperature and avoids cold air penetration from the outside. In this second situation, the air curtain saves up to 80% of the energy given by the heater placed on the left wall.
Important Note: Poorly designed air curtains, wrong selected units (less powerful) or inadequate installation will loose part or even all the advantages and became worse than unprotected door.
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.
There are 3 possible directions of blow-out air streams:
Most air curtains are installed horizontally, across the top of the opening, and air is expelled from top to bottom of the door. Vertical air curtains can be installed in one or both sides of the door. Horizontal installation integrated on the floor is the most effective in thermodynamic terms and at the same time the most expensive one to install and problematic where public pedestrian traffic crosses the doorway for obvious reasons. So it's only used in some special industrial applications.
Types of air curtains: non-recirculating or recirculating. A non-recirculating unit discharges the air to the environment. Otherwise recirculating collect and return the air from the discharged air. Recirculating air curtains are more efficient but the majority of installations are non-recirculating due to lower costs.
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 equalise 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.
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