The air curtain (or air door, as it also is known) has been around since the early 20th century, with the first North American patent granted to, coincidentally, the inventor of the revolving door in 1904. Air Movement and Control Association (AMCA) International defines an air curtain as “a directionally controlled air stream 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.” So, the air curtain is both a HVAC device and a non-chemical solution for flying-insect (and small-bird) control … birds, bees, and Btus.
Air curtains often are used on frequently opened doors in health-care and food-preparation facilities to keep out bugs and birds. NSF International has a standard—NSF 37-2012, Air Curtains for Entranceways in Food and Food Service Establishments—governing air curtains used for this purpose. Although this application is not intended to save energy—it’s mainly for health and nuisance abatement—there nevertheless will be some energy-use reduction if a protected space is heated or cooled.
Here in South Florida, air curtains most commonly are used on exterior doors to reduce the infiltration of unconditioned air into air-conditioned spaces or, conversely, the exfiltration of conditioned air to the outside. Of course, the installation of air curtains on exterior doors is not limited to controlling cooled air in warmer climates; air curtains are very effective in controlling heated spaces in cold climates. Air curtains used for this type of environmental separation can be equipped with airflow-rate control (two-speed or variable-speed motors or dampers) to allow air volume to be varied based on outside wind speed and direction and outside-inside temperature differential. Based on reduced energy spending only (that is, without accounting for lower maintenance costs attributed to reduced runtimes of HVAC equipment), simple payback periods on air-curtain projects on which I have been involved typically have been less than two years. In one case, with a high level of personnel traffic in and out of a service office, it was approximately eight months.
Another common application for air curtains—usually an indoor one—concerns the control of refrigerated air from and between coolers and freezers in cold-storage applications. This reduces energy loss and helps to prevent product spoilage, which, in the case of food or vaccines, could represent a significant safety hazard. Air curtains also can be used as defrosters to prevent ice buildup on the doors of freezers. Similarly, air curtains can be used to prevent the loss of heated air from commercial and industrial ovens. As with cold-storage applications, this usually is an indoor application, so wind is not an issue.
In addition to these applications, there are some that are less obvious. For example, air curtains are used in paint booths to reduce overspray and in wastewater-treatment facilities for odor control. There also are applications that intuitively seem good, but aren’t practical. For example, several years ago, we suggested air curtains on the exterior doors of a university student center. There is a large lobby and one or more exterior doors that almost constantly are being opened. Unfortunately, the administration felt freshly coiffed donors attending functions would object, and the project died.
Regardless of the primary intended purpose of an air curtain, if it’s being used to control a space that is mechanically air-conditioned or heated, there almost certainly will be resultant energy savings.