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Without AMCA Certification, Air Curtain Specification Is No Sure Thing
by David Johnson, Engineering Manager, Berner International Corp.
An engineer would never specify major unitary equipment, such as a building's HVAC air handlers without first assuring the fans and blowers were certified to deliver the manufacturer's promised air performance. Air curtain specifications should receive equal discretion, but many engineers simply rely on manufacturer data sheets.
Because an air handler has a significant effect on the entire building, it gets most of the attention when it comes to requiring certifications to verify performance. But air curtains have fans too and many engineers and architects don't link the interrelationship of certification and performance. In today's drag-and-drop world of specifying equipment, sometimes smaller equipment such as an air curtain isn't scrutinized enough for performance.
Air curtains provide a controlled stream of air across a building doorway to separate interior and exterior environments. When the doorway is open the air curtain’s airstream prevents cold or hot outdoor air, as well as insects or dust from infiltrating the indoor environment. Therefore performance specifications are critical for energy efficiency, sanitation and other air curtain benefits.
In a perfect world, all air curtains would perform to their manufacturer's specifications, but this isn’t always the case. An uncertified air curtain can result in a product that might not operate up to expectations. Consequently, this tarnishes the reputation of air curtain technology, not to mention the engineer that specified them. Air curtain technology does work, but proper sizing is critical.
In 1978 the not-for-profit organization Air Movement and Control Association International (AMCA), (www.amca.org) Arlington Heights, Ill., formed a committee of its air curtain manufacturer members to develop a test standard to assess air curtains and certify their performances. By 1982 AMCA Standard 220, "Laboratory Methods of Testing Air Curtain Units for Aerodynamic Performance Rating" was introduced. The test standard became ANSI/AMCA 220, an American National Standards Institute (ANSI), standard in 2005 and an ISO standard as well in 2009.
An AMCA rating label should rank as highly with engineers as Underwriters Laboratories, (UL), Northbrook, Ill., or National Sanitation Foundation (NSF), Ann Arbor, Mich., certifications.
How Specifications Get Skewed
Some manufacturers' specifications might be written around a specific product feature, therefore they might not have best interest of the end-user in mind. For example, a manufacturer might specify a particular motor housing because its their exclusive design and it will eliminate other manufacturers without that motor housing. This makes it difficult for engineers to separate what is truly an end-user benefit, such as motors with ball bearings, galvanized surfaces or powder-coating, for example, from what is an attempt to exclude the competition.
Other marketing strategies by manufacturers include listing an unproven five-percent or more air volume increase on their data sheets for uncertified models versus the data of AMCA certified models with the same horsepower motor. Some air curtain manufacturers use this marketing ploy because bigger is presumed as better. Consequently, engineers will specify this inflated performance over an AMCA certified model and sadly discover it's not possible because the uncertified model's data is skewed from unfounded data. In some cases air volume performance will be based on raw fan data that does not account for the cabinet or plenum that it is mounted in. It's simply against the law of physics to have a significant increase in air volume using the same or lesser horsepower on a fan at the same rpm and static pressure.
Inflated data can also ruin an application's effectiveness. For example, any door with an air curtain that has inflated air volume and velocity data won't have a strong enough airstream to reach the floor. Instead, outdoor air, vehicle fumes and flying insects will be able to penetrate the doorway near the floor, unbeknownst to the specifying engineer, because the air curtain is undersized. In this instance, the air curtain is ineffective, a waste of capital investment and an additional drain on energy when operating.
Some undersized air curtains might work under perfectly calm conditions, but fluctuations in wind load or building pressurization can create a situation where the air curtain will become ineffective, resulting in energy losses.
AMCA certification is also invaluable because air curtain manufacturers must biannually check test each certified model, which assures any modifications or redesigns in the last two years are also updated.
Engineers will also find many of the industry's AMCA-certified models may not be certified when an option such as a heating coil is added, which can reduce volume and velocity. The fact it's not certified with an option doesn't necessarily mean the performance specifications are skewed. More air curtain options are being certified each year, however adding steam, hot water, electric and gas-fired heating options can quadruple certification expenses because each model must be tested with each individual option.
How AMCA Tests Air Curtains
AMCA tests air curtains for air volume, power (consumption) rating, air velocity and uniformity.
Air volume and power rating requires a test chamber that measures the air curtain's volumetric flow rate by plotting a curve. The test subjects the air curtain to a predetermined resistance down to free air delivery and records the airflow data. The test chamber, which is calibrated, certified and operated by trained laboratory technicians, uses a variety of measuring instruments such as volumetric flow nozzles, monometers, pressure taps and watt meters.
The air curtain is tested for power consumption during the air volume test, which is important when designing green buildings. This allows engineers to compare the power consumption of similar air curtains. For example, two different units that have a flow rate listing of 1,200-cfms but operate with a power rating of 1.2 and 1.6-kW respectively, demonstrate an energy usage difference.
For velocity projection and uniformity, the air curtain is placed on its side on the floor. Peak velocities are measured with an anemometer across the width of its air discharge nozzle at different planes. The plane locations vary in distance ranging from a nozzle width to one meter increments up to the distance the air curtain is designed for Velocity projection is determined for each plane by taking an average of the measured peak velocities along the discharge nozzle. The uniformity is determined by using a standard of deviation to establish the consistency of the velocities using the plane that is one nozzle width away from the discharge.
Specifying an Effective Air Curtain Application
Once an engineer arrives at two or more AMCA-certified manufactured models that fit the application, then each brand's attributes, such as features, serviceability, installation ease, reputation and other considerations should be considered.
Engineers and building owners looking for the great advantages that air curtains can provide in saving energy, indoor air comfort and eliminating sanitation concerns of flying insects, should be as particular about selecting an air curtain as they are with an air handler. In other words, look for the AMCA ratings seal on an air curtain for absolute assurance and guaranteed performance.
BIO: David Johnson is engineering manager of Berner International Corp., New Castle, Pa., (www.berner.com), which has 75 percent of its air curtain line certified by the Air Movement & Control Association International (AMCA). Johnson has over 21 years experience in the engineering field. He has several patents including those for an air curtain digital controller, a specialty application air curtain, and air curtain nozzle design. Johnson is also a member of the Air Conditioning, Heating and Refrigerating Engineers Society (ASHRAE) and serves as chairs on AMCA’s Air Movement Division, Air Movement Engineering Standards Committee, and the Air Curtain Engineering Standards Committee. Johnson can be reached at email@example.com or (724) 658-3551.
SIDEBAR: Specify Air Curtains for Velocity, Volume and Uniformity
Velocity, volume and uniformity work together to create the ideal air curtain performance, therefore relying on only one or two of three could skew performance results.
• Velocity: To properly design an air curtain installation, the airstream must hit the floor with enough velocity to create a split. The split, which creates stability, strength and direction for the air entrained on each side of the airstream, should occur right at the doorway’s threshold. An installation with a weak airstream, one that barely splits for example, is only viable for applications involving temperature differential without wind, such as internal doorways. They’re capable of stopping infiltration or cross contamination of environments due to airflow caused by the temperature differential, but they become ineffective once wind is introduced. Few external doorways are not affected by wind loads.
• Volume: Volume, on the other hand, is the building block that allows a properly designed and pressurized discharge plenum to generate a high velocity laminar jet stream. The taller the opening, the more volume that is required to generate a thicker, higher velocity airstream to resist wind loads of four to five mph. Obviously an air curtain for a fast food restaurant’s drive-through window doesn’t need as strong a volume as a 16-foot high door in a shipping area. Once an air curtain activates and creates a split, it creates a “skin” over the building’s volume of indoor air and uses this internal pressure to resist wind. The split then rolls the entrained conditioned and unconditioned air back to their respective areas.
• Uniformity: Another important factor to consider in air curtain performance and selection is uniformity, which only impacts the airstream effectiveness when it drops below 75-percent. An air curtain that focuses too much energy on generating a high uniformity loses velocity, therefore reducing its effective wind resistance.