Units in this category use an external low pressure hot water supply to raise the temperature of circulating air. To be fully effective as thermal barriers they should cover the full width of the door, otherwise cold air may penetrate at the edges. If a door opening is double width, then a suitably sized (double width) air curtain will be required. If the unit is to be mounted significantly above a doorway, or if the door opening is unusually tall, then a more powerful unit is likely to be required to maintain the airflow. Larger units may require a three phase electricity supply.
Warm air in a room is normally retained using a physical barrier -- such as a door. However there are many situations where this in not practicable, such as in a busy shop or restaurant where doorways will be open for substantial periods of time. An open doorway leads to loss of heated air due to natural convection and wind forces, and can result in reduced customer/staff comfort levels and higher energy costs. Circulation of heated and cold air current is quickly established though an open door. This circulation is driven by differences in air buoyancy, with lighter warm air flowing through the top of the door entrance and colder dense air entering through the bottom as indicated below:
The second problem is wind, which is often more troublesome than convection. Cold gusts can penetrate an open doorway and displace heated air, which is lost to the outside environment. The actual airflow patterns at a door opening can be quite complex, but are illustrated simply below:
Air curtains are designed to counteract the heat losses at an open door entrance, and consist of a fan and a nozzle typically located inside the doorway. They work by drawing heated air from the interior of the room and directing it downwards as a thin jet across the full width of the door opening. This moving barrier of air needs to be sufficiently strong to overcome convection and wind flows.
Although air curtains can greatly reduce energy losses, some mixing of the cold external air with the curtain airstream will inevitably occur. For this reason air curtains usually incorporate heating elements to raise the temperature of the recirculating air. A heated air curtain may also be used to supplement existing space heating. Close
Selection of Air Curtains
Firstly is it worth bearing in mind that depending on the specific requirements, an air curtain may not necessarily be the best solution. If, for example, a door is only occasionally opened it may be more cost effective to provide additional space heating close to the door to counter any cold spots rather than to install an air curtain. Another option that may be worth considering is to build a vestibule to prevent cold air from entering a temperature controlled area, so that there is a double door barrier between the heated area and the external (cold) environment. This may be more satisfactory in a restaurant , for example, where customers may feel that the air flow generated by an air curtain is a distraction.
Assuming that some kind of air curtain is the best option, the next consideration is 'what type?' If the situation is undemanding, an overdoor heater mounted above the door may be a practical solution. These units provide provide heating close to the point of heat loss, thereby eliminating cold spots. However, they generally have only limited effectiveness as thermal barriers (especially against wind) as they are unable to produce the aerodynamic flow across the full door width required of a 'true' air curtain. However they do have a number of benefits, such as small size, low capital cost and ease of installation (being typically rated at 3kW, they can be wired to a fused spur from a standard ring main).
If the situation is more demanding, then a full door width air curtain is likely to be required. Whilst these units come in a variety of sizes and capabilities (with specific details varying depending on manufacturer and model) the following principles generally hold true:
Please note that products must be installed and operated in accordance with the manufacturer's specific requirements and installation must be carried out by competent personnel. Close