Arenas, convention centers, and other large, high-capacity, multiuse buildings present a number of challenges from the standpoint of air infiltration through open doors — some of which can be addressed using air curtains. An air curtain is a fan or a system of fans located inside a cabinet that blows a controlled stream of air across a door or window opening, creating an air seal. This air seal separates environments while still allowing a smooth, uninterrupted flow of pedestrian traffic. An air curtain saves energy by containing heated or conditioned air, increases comfort by preventing the infiltration of flying insects and other contaminants, improves safety by drying floors at entranceways during inclement weather, conserves space and costs by replacing vestibules where allowed by building energy codes, and more.
Air is drawn into an air curtain, where it is pressurized, shaped, and redirected, to form a high-velocity, uniform sheet of air across a door or window opening. Typically, the air drawn into an air curtain is conditioned air from inside of the building. Per the laws of entrainment, as air moves away from an air curtain’s discharge nozzle, it entrains air on both sides of the discharge. In the typical case of an air curtain that is mounted horizontally over an opening, discharge air splits when it reaches the floor, with some of the air redirected to one side of the opening and the rest to the other. If the building is properly balanced, the amount of air that goes to the inside of the building will be the same as that drawn into the air curtain plus that entrained on the inside; thus, the air splitting back into the building will be mostly conditioned air. Because of boundary mixing between the air curtain discharge and outside entrained air, a slight amount of unconditioned air may enter the building, but it will be much less than the amount that would enter in the absence of an air curtain. As additional air on either side of an opening moves toward an air curtain airstream, it is redirected downward and back to the side from which it came when it reaches the split at the floor (see Figure 1). A vertically mounted air curtain operates according to the same principles but blows air sideways across an opening instead of vertically. The discharge air splits when it strikes a surface on the other side of the opening or an opposing air curtain stream.
The primary purpose of an air curtain is to serve as an environmental separation barrier when a building’s doors/windows are open. As such, an air curtain keeps windborne dust, dirt, fumes, odors, and flying insects from entering a building or an area within a building; reduces the cross-migration of lighter warm air flowing through the upper part of an opening and heavier cold air flowing through the lower part; and minimizes the effect of wind blowing into a building opening. By separating environments, an air curtain can reduce energy and utility costs, provide comfortable and sanitary conditions, increase safety at busy doorways, and decrease equipment maintenance. Another potential benefit of air curtains is building cost and space savings from the elimination of vestibules. Codes and standards, such as the International Energy Conservation Code (IECC) and ANSI/ASHRAE/IES 90.1-2019, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” contain provisions that allow the use of an air curtain as an alternative to the requirement for a vestibule in certain instances.
Air curtain applications primarily fall under one of three designations: environmental separation, insect control, and processing. The first two, typically, are of particular interest to owners and managers of commercial establishments, such as arenas and convention centers. Environmental separation protects an exterior door from the unwanted infiltration of outdoor air and the escape of indoor air caused by the effects of wind and/or temperature differences. Research shows that in both warm and cold climates, an air curtain can reduce infiltration significantly (see references 1, 2, and 3). Between interior rooms connected by a common opening, air curtains can be very effective in preventing unwanted infiltration of unconditioned air or the loss of conditioned air from one room to the other. Specialized applications that fall under this category include coolers, freezers, and ovens. Flying insect control is a common requirement in facilities in which food products are produced or processed, such as kitchens, cafeterias, and counter services. For these facilities, air curtains provide the higher velocity airstream needed. It’s important to note that air curtains perform best in a balanced or a positive-pressure building. Applications where a negative pressure exists because of, for instance, mechanical ventilation or stack effect require special considerations. Under negative pressure, an air curtain unit will not adequately protect an opening from unwanted infiltration caused by wind and/or temperature differences.
Codes and Certifications
Energy codes, such as ANSI/ASHRAE/IES 90.1-2019 and the IECC, allow air curtains to be used as an alternative to vestibules as a path to compliance. High-performance energy codes, such as ANSI/ASHRAE/ICC/USGBC/IES 189.1, “Standard for the Design of High-Performance Green Buildings Except Low-Rise Residential Buildings,” and the International Green Construction Code (IgCC), allow air curtains to be used in conjunction with vestibules to provide an even more efficient way to reduce building envelope infiltration. For decades, air curtains have been included in state and local sanitation codes to help protect food service environments and, in some cases, are required to be in operation before an establishment can open. Safety codes applicable to air curtains are based on a wide variety of standards and remain focused on life, electrical, and fire safety. Several organizations, including Underwriters Laboratories (UL), Intertek’s ETL SEMKO division, CSA Group, and International Organization for Standardization (ISO), define standards and, in some cases, test air curtains for compliance. What these codes have in common is that they require validation through certification. While manufacturers are free to test products and publish their findings, certification requires a third party to test for compliance under its own programs. Aerodynamic performance testing is among the most influential certification and is required by all energy codes. Air Movement and Control Association (AMCA) International was among the first internationally recognized organizations to perform aerodynamic performance testing. ANSI/AMCA Standard 220, “Laboratory Methods of Testing Air Curtain Units for Aerodynamic Performance Rating,” and ISO 27327-1:2009, “Fans - Air Curtain Units - Part 1: Laboratory Methods of Testing for Aerodynamic Performance Rating," are used to determine aerodynamic performance in terms of airflow rate, outlet air velocity uniformity, power consumption, and air velocity projection for rating or guarantee purposes. The AMCA Certified Ratings Program (CRP) is used to certify that published performance data have been validated by a third party. Air curtains for food service applications may be required to be tested in accordance with NSF/ANSI Standard 37, “Air Curtains for Entranceways in Food and Food Service Establishments.” This standard has an established testing procedure to address public health safety in food service environments and has both performance and construction requirements. Lastly, there is sound performance testing. While there are many ways to measure sound, ANSI/AMCA Standard 300, “Reverberant Room Method for Sound Testing of Fans,” establishes procedures for determining the sound performance of air curtains. The data can be certified using the AMCA CRP.
Most air curtain manufacturers offer a wide range of accessories to assist users in the installation and operation of their products, including door limit switches, supplemental heat, controls, brackets, filters, and more. Custom or specialty accessories to address specific field conditions and/or applications are also available. In response to the global COVID-19 pandemic, more air curtain manufacturers are incorporating air-cleaning strategies and employing new and existing technologies to inhibit the spread of airborne pathogens and viruses in indoor spaces. These include:
Ultraviolet germicidal irradiation (UVGI) — The use of ultraviolet light at the 253.7-nm wavelength is used to kill pathogens. This technology has been studied extensively since the 1930s;
High-efficiency particulate air (HEPA) filtration — Airborne pathogens and viruses are collected and distributed over a large surface area using a fine-pleated mesh material; and
- Cold plasma ionization — A plasma field with highly positively charged ions that bond with airborne pathogens and viruses, disrupting their surface proteins, rendering them inactive and unable to replicate, and increasing their size and weight through agglomeration, making them easier to collect, is produced.
Most manufacturers offer a product selection guide or configurator to help users choose the right air curtain model and accessories. This selection is dependent on the type of application, the width and height of the opening, the building’s orientation, and field conditions. Beyond selecting basic installation and control accessories, choosing the proper air cleaning strategy can be challenging for users. The flow rate of the air curtain, the limitations of the air-cleaning device, the size of the opening, and the volume and exposure rates of the space all must be considered during the selection process.
An air curtain unit, at a minimum, must completely cover an opening, and, if possible, should slightly overlap. For wide openings, two vertically mounted, lower-velocity units can be used as an alternative to a single horizontally mounted, higher-velocity unit. The discharge path must be free and clear for the entire width and height of the opening and angled toward the higher pressure load. As products vary by maker, always follow the instructions and guidelines provided by the manufacturer when installing accessories. For units utilizing UVGI, HEPA filtration, and/or ionization, the traditional practice of installing only directly above or beside an opening may not apply. Depending on the size, configuration, occupancy rate, and HVAC system design of the space, additional units strategically placed in the room may be needed to reduce stagnation and maximize circulation through the air curtain. Although UVGI, HEPA filtration, and ionization are proven to reduce or even eradicate airborne pathogens and viruses, the data provided by manufacturers are based on standardized static test conditions using airtight spaces or chambers. Introduction of additional airborne pathogens and viruses by new hosts or carriers into a space may result in cleaning rates that are lower than those in published data. Additionally, the energy consumption and overall energy savings of an air curtain may be affected by the increased operational time of the “cleaning cycle” (typically, when the door is closed).
1. Wang, L., & Zhong, Z. (2013). Investigation of the impact of building entrance air curtain on whole building energy use. Montreal: Concordia University.
2. Wang, L. (2016). Energy saving impact of air curtain doors in commercial buildings. Montreal: Concordia University.
3. Wang, L., & Shu, C. (2018). Air curtain project - Evaluation and application of existing air curtain effectiveness methodology - Relating aerodynamics performance to air curtain effectiveness. Montreal: Concordia University.