Air curtain is a rectangular jet of air that has higher momentum and kinetic energy than its surrounding air. The higher momentum of the air curtain creates two independent air zones on its sides such that one side is insulated and protected by the air curtain against the other side. For this reason, air curtains are useful for splitting two side-by-side regions that are different in temperature, relative humidity, species concentrations, etc. In most applications, air curtains are forced vertically by one or a series of fan in downward direction; nevertheless, depending on the application they may be introduced in desired directions. In addition, they may also be of the same, lower or higher temperature and relative humidity than those of the ambient. In some applications, the air curtain impinges on the floor and creates a protected zone. However, in some applications there may be a return opening through which at least a portion of the discharged air is drawn back by the fan of the air curtain. The latter type is called a re-circulated air curtain and may cause some energy saving if the air is conditioned prior to being discharged. The performance of air curtains depends on, but not limited to the temperature and pressure gradients across it, the flow characteristics (such as velocity magnitude, velocity profile shape and also turbulence intensity) at the discharge, as well as the geometrical dimensions.
There is a wide range of application for air curtain from food industry to HVAC, traffic control and medical applications. Some of the common applications of air curtains are briefly explained in the following
Open Refrigerated Display Case
Open refrigerated display cases (as explained in Air Curtain Research Lab) are widely used in supermarkets to maintain the temperature of the inside food at a prescribed level. In such systems, an air curtain is mainly responsible to inhibit the penetration of the warmer air onto the cold shelves and food, by virtually creating a barrier between the two regions.
HVAC (Heating, Ventilation and Air Conditioning)
In HVAC industry the fan of the air curtains are usually installed over the entrance of buildings or the gate of storage rooms (Figure 1). The vertically generated air curtain effectively reduces the infiltration of outside cold or warm air into the conditioned spaces, or to prevent high outside humidity, dust and even insects.
Figure 1: Air curtain over the entrance of a building or storage room
In hospitals, clean rooms can be protected by air curtains. Incubators (Figure 2) may also benefit from air curtains to provide an insulated space. Chemical tests may also be performed within a control chamber equipped with a hood and an air curtain on the sides to prevent dispersion of the hazardous vapors into the room.
Figure 2: An infant incubator that benefits from air curtain (Courtesy of Dräger Co.)
In production line of the microelectronics like electronic chips (Figure 3), the presence of airborne particles may decay the quality of the devices. Air curtain provides a controlled zone during the production.
Figure 3: An electronic chip .
Fire Protection Systems in Tunnels
In case of fire in tunnels, not only spread of the fire, but also spread of the smoke can be hindered by air curtains. Usually re-circulated air curtains are used in tunnels.
Figure 4: Undersea tunnel in Persian Gulf (Source: www.khaleel.com)
Air curtains can be used to shield the antique statues, paintings, etc. against airborne particles that may cause damage in the long-term. Figure 5 shows the Statue of David in a museum in Italy and the CFD simulation of an air curtain shield around the statue to diverge the exhale of the visitors that contains noticeable amount of carbon dioxide, which is erosive. The air curtain is issued vertically from the bottom.
Figure 5: Statue of David shielded by an air curtain (Courtesy of FLUENT Corp.)
In an incineration device (Figure 6) the unburned particles can be trapped by using non-recirculated air curtains. This not only can reduce the environmental pollution, but also can enhance the burning by over-oxygenating the fire; this will result in cleaner burn and less emission. This type of air curtain is usually horizontal.
Figure 6: Incineration device with air curtain (Courtesy of Air Burners LLC)
Kettering Interdisciplinary Research Team