Stephen Benes 2015-07-27 13:04:36
The basic air curtain concept of protecting an open doorway from the exterior environment with an “air seal” hasn’t changed much in the last 50 years, but what has changed is the variety of businesses using them today to save energy. Air curtains have been decades-long, energy-saving devices for industrial facility shipping doors. However, recent technological advancements have made air curtains more applicable to a variety of business types ranging from health care, office buildings, hotels, and retail stores, to supermarkets, restaurants, and even compressed natural gas bus maintenance garages. Two things haven’t changed, however. Energy prices continue to skyrocket, and heated and cooled air continues to escape through open, unprotected doorways. Therefore, air curtains should be considered as a doorway energy loss solution for new buildings as well as doorway retrofits. An air curtain, sometimes referred to as an “air door,” is typically fashioned of highly engineered blowers, motors, and nozzles in a metal cabinet mounted above a doorway. Air curtain technology draws interior air from the facility and discharges it through field-adjustable (+/-20 degree) linear nozzles that seal the doorway with a non-turbulent air stream that meets the floor approximately at the door opening’s threshold. Because the air curtain discharges air at velocities generally in the 1,000 to 3,000 feet-per-minute range at 3 feet above the floor, it effectively prevents outdoor air infiltration, as well as flying insects. Air curtains are typically activated by a limit switch that’s triggered when the door opens, and deactivated when the door closes. Many facility managers and engineers have the misconception that air curtains are useful only for minimizing winter energy losses. Conversely, energy losses through open doorways in summer air-conditioning months are typically more costly than winter operation, thus air curtains are useful year-round. How Efficient Are Air Curtains? Air curtains can be expected to return 70 to 80% of the indoor energy at an open doorway back into the space. That type of return is not guaranteed by all air curtain models, however. Therefore, air curtains should be properly sized and certified under criteria in accordance with Standard 220 established by the Air Movement & Control Association—International, Arlington Heights, IL, (AMCA), a trade association dedicated to certifying manufacturers’ air performance statistics on all types of air movement devices including blowers, fans, motors, and air curtains. Volume, velocity, and uniformity (VVU) of the air stream are critical performance factors in an air curtain’s effectiveness. Without certification, air curtain manufacturers can inflate statistics and mislead specifiers, resulting in poor VVU and underperforming doorway energy conservation. Many businesses prefer vestibules with automatic doors on both ends for doorway energy savings. However, a three-month-long 2008 research study, “Air Curtains: A Proven Alternative to Vestibule Design,” used computational fluid dynamics (CFD) analysis from second-party research/validation consultant Blue Ridge Numerics, Charlottesville, VA, (since acquired by AutoDesk, Inc.) to prove air curtains save more energy. The study’s results overwhelmingly confirmed that an air curtain and a single automatic door entrance combination is up to 10% more energy efficient in environmental separation performance, versus a conventional vestibule with two automatic doors—on the exterior and interior doorways. An air curtain/automatic door combination is also considerably cheaper in new construction costs and can enable anywhere from 20 to 250 square feet of vestibule space for other uses, such as merchandising in retail applications. Vestibule costs range from a minimum of $20,000 to $100,000 or more, depending on size, extra heating equipment requirements, extra lighting, and additional doors with accompanying hardware. Conversely, an air curtain(s) would cost only several thousand dollars and is now considered as an acceptable and inexpensive alternative to vestibules by many building codes, such as the International Green Construction Code (IgCC) overlay of the International Code Council’s (ICC) International Energy Construction Code (IECC). New Energy-Saving Applications for Air Curtains Air curtain manufacturers offer architectural products, such as flush-mounted units in ceilings, to meet aesthetic requirements for commercial uses in places like health care facilities and hotels. An upscale health care lobby, for example, is more likely to use an air curtain for energy conservation if it’s concealed in the ceiling rather than appearing as an industrial metal cabinet suspended above the doorway. The University of Pittsburgh Medical Center’s (UPMC) new $252 million Monroeville, PA, facility, the most sustainable of its 30-hospital network, uses 10 in-ceiling air curtain models that appear as flush ceiling grilles. The air curtains are an important part of the 302,000-square-foot, 155-bed facility’s overall sustainable design that was granted Leadership in Energy and Environmental Design (LEED) Silver certification. The facility was also awarded “Project of the Year 2013” honors in the Commercial Category from the Engineers’ Society of Western Pennsylvania, Pittsburgh, and is racking up an estimated $350,000 to $500,000 in annual energy savings versus the network’s older buildings, according to Joseph T. Badalich, corporate construction project director, UPMC. Besides energy savings, the air curtains all have electric heaters that are critical for lobby indoor air comfort and make the building’s larger heating systems work less. The heaters can be thermostatically controlled with a timer/delay that continues spot heating at a lower air velocity until the doorway area’s indoor set point temperatures are reached. An additional 11th air curtain is a conventional model mounted above the emergency room (ER) doorway that saves energy, but also prevents idling ER vehicle fume infiltration, which is a common hospital industry indoor air quality (IAQ) problem. All the air curtains use three-speed fans that can be adjusted for proper doorway sealing and minimal operational decibel levels. Saving Energy in Hazardous Locations A totally new use of air curtains is appearing in facilities tapping into the trend of compressed natural gas (CNG). The Central Ohio Transit Authority’s (COTA) ongoing $75 million conversion to CNG-fueled buses includes the HVAC retrofit of its 400,000-square-foot CNG bus maintenance and garage areas into an energy-saving showcase that's aimed at LEED silver certification. One product complementing the energy-savings specifications for project HVAC consulting engineer, Dynamix Engineering Ltd., Columbus, OH, was air curtains specified on seven large 14 (h) x 16-foot (l) bus doorways. Three additional air curtains separate environments at one large interior common doorway shared by the maintenance and garage areas. “Using air curtains on those large doorways has proven to be a huge benefit in energy savings and indoor air comfort,” says Jon Hancock, a former COTA transportation facility manager, who oversaw much of the design. Because of a potential natural gas leak from a bus CNG storage tank, the air curtains, as well as all other mechanical equipment, include life/safety measures. Spark-resistant fans, explosion-proof motors, aluminum blower wheels and other components were mandated for compliance under National Fire Protection Association (NFPA) governing standards for CNG and vehicle fuels such as NFPA-30A, 52, 70, 88A, plus to satisfy state and local fire marshal compliances. Each 1,600-cfm air curtain is activated by the opening of its accompanying high-speed roll-up door. The 16-foot-long air curtains are the longest single construction in the industry and don't create inefficient airstream disrupting obstructions as do models with multiple smaller lengths bolted together. TD Banknorth Garden Train Terminal Retrofit For the Massachusetts Bay Transit Authority’s commuter train station terminal retrofit at the famous TD Banknorth Garden (formerly called Boston Garden) terminal, air curtains were the only alternative. The retrofit called for extending the arrival/departure area into the open-air train shed to alleviate previous foot traffic congestion. When architect Sasaki Associates, Boston, designed an ingenious, suspended utility soffit that houses 12 doorways dozens of feet under the towering train shed roof, consulting engineer firm, Cosentini Associates, Cambridge, MA, recommended 12 accompanying air curtains. This was viewed as the only viable method of creating a barrier that would lessen energy losses to the exterior train shed area, but still allow pedestrians unrestricted passage. The only other alternative was cabinet heaters, which would have used more energy and simply dumped heat into the space without energy loss protection. When compared to other energy reduction methods, an air curtain is an inexpensive solution that delivers a payback typically in 1.5 years or less. While the concept of air sealing a doorway to prevent energy losses remains viable decades after its invention, technological advancements have opened the door to many other applications that can use air curtains to minimize energy losses at doorways. Stephen Benes is a regional sales manager at Berner International Corp., in New Castle, PA.
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