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Air Curtains: The New Advantage in the Energy Battle
A recent research study has confirmed what some energy-savvy retailers and commercial building owners have known for some time: Air curtains (air doors) provide a better environmental separation and save significantly more energy than vestibules.
Air curtains, which are typically mounted above doorways, separate indoor and outdoor temperatures with a stream of air strategically engineered to strike the floor with a particular velocity and position. The air prevents outdoor air infiltration while also permitting an unobstructed pedestrian entryway.
Prior to the study, architects and engineers never knew for certain if air curtains were a viable energy-saving substitute for vestibules, because there hadn’t been any organized research comparing the two entryway methodologies. The fact that air curtains aren’t listed on some codes as viable vestibule alternatives also limited them as commercial building specification choices by consulting engineers and architects.
However, the skepticism has been answered with the recent release of “Air Curtains: A Proven Alternative to Vestibule Design,” a three-month-long research study funded by air curtain manufacturer, Berner International, New Castle, Pa., with certified results using computational fluid dynamics (CFD) analysis from second-party research/validation consultant, Blue Ridge Numerics, Charlottesville, Va.
“If you ask most engineers or architects if air curtains are more energy efficient or less expensive than vestibules, they probably don’t know for sure, but this study reveals what we’ve known for some time,” said Timothy Londeree, R.A., architect, David A. Levy & Associates, Akron, Ohio, a retail design and development firm that has specified air curtains on dozens of commercial projects to save clients construction costs.
The study proved that air curtains clearly outperformed vestibules in energy savings by 10 percent. The study’s researchers used vestibule construction dimensions and statistics from three leading pharmacy chains as a model. Using a typical pharmacy chain entrance, the study CFD-modeled three scenarios: 1) air curtain with automatic door; 2) vestibule; and 3) air curtain with vestibule. Each scenario was subjected to wind loads of up to four mph and different frequencies of traffic.
Eliminating vestibules saves construction costs. However, the energy angle is only the tip of the advantages iceberg. An estimated vestibule cost is anywhere from $20,000 to $60,000 while an air curtain for a single six-foot-wide entrance/exit opening plus installation labor costs less than $6,000. Add the fact a vestibule consumes anywhere from 50 square feet at a small retail store on up to 2,000 square feet at a large home center retailer and the advantages of air curtains appear even more impressive. A conservative estimate of today’s construction costs per square feet is $125. Multiply that by 50 and 2,000 square feet and the cost savings of eliminating a vestibule can amount to $6,250 for a small retail store to $250,000 per project at a large home center.
While environmentally-minded grocers such as Whole Foods, clothing retailers such as Urban Outfitters, and pharmacies such as Walgreens, have smartly used air curtains for energy savings and other benefits, the challenge has been changing building codes to reflect the new study’s results.
Currently there’s no mention of air curtains in the International Building Code’s (IBC) building energy efficiency section; the American Society of Heating, Air-Conditioning, & Refrigerating Engineers’ (ASHRAE) Section 90.1--energy standards for buildings; and the International Energy Conservation Code (IECC).
These codes, which establish minimum commercial building requirements for energy efficiency that architects and engineers must comply with, don’t disallow air curtains; they just don’t list them as vestibule substitutes. Additionally, architects and engineers have inaccurately believed vestibules were always the best entryway energy-saving choice until rising energy costs prompted closer scrutiny.
The Air Movement & Control Association (AMCA), Arlington Heights, Ill., a nonprofit organization that creates standards for, and tests/rates everything from air curtains to dampers, louvers, outside air grilles, fans and other “air-side” HVAC equipment, hopes to change codes to address air curtains as a vestibule substitute. Armed with the research study, the association is currently lobbying code committees.
Architects, engineers and specifiers can still comply with codes and use air curtains as vestibule substitutes however, but it’s usually done at the local inspection level. Info on the research study that can be passed on to local inspectors is now available at www.berner.com.
Advantages Go Past Construction & Energy Savings
Another sometimes overlooked advantage of air curtains is employee comfort. Because point-of-purchase (POP) stations typically are located near entryways for security reasons, POP employees don’t enjoy the same air comfort of their interior counterparts. Cashiers tend to get the brunt of unbridled wind in the absence of vestibules or air curtains.
In the case of vestibules, cold outdoor air can penetrate the entryway especially when both sliding door entrances open simultaneously during heavy traffic periods. Supplemental space heaters, which are both costly to install and operate, temper the vestibule space air that infiltrates the POP area. Conversely, air curtains can be specified with built-in electric heaters or heating coils that are supplied by the building’s central heating supply loop to temper the air for employee comfort.
The same happens in summertime; only hot humid air infiltrates the space and makes work areas near doors uncomfortable for employees.
Besides employee comfort, some retailers such as convenience stores are discovering air curtains also dry the interior entryway floor during inclement weather. This reduces maintenance costs as well as liability issues from patron slippage.
Air Curtain Applications Beyond Vestibules
While air curtains appear to be the perfect substitute for vestibules, they also have a variety of other applications as well. For example, the extension of the TD Banknorth Garden’s commuter foot traffic area 100 feet into the adjacent train shed of the Massachusetts Bay Transit Authority’s (MBTA) North Station was an energy-saving coup. Architect Sasaki & Associates, Boston, and HVAC project engineer, Jeff Fleishman, Cosentini Associates, Cambridge, Mass., were concerned with energy efficiency as much as aesthetics. Consequently, Sasaki’s idea of a 12-foot-high lobby/soffit design now presents an environment that’s comfortably separated from the outdoor air of the mammoth train shed via 12 air curtains at the mouth of 12 automatic sliding doors. During rush hour when the automatic doors are constantly open with passing commuters, the air curtains keep cold air from infiltrating the new train station lobby area at a great savings to building owner, Delaware North Companies, Inc. Summertime operation prevents hot humid air from infiltrating the lobby space. Except for the gentle air stream at the entrance, commuters don’t notice the aesthetic air curtains because they are flush-mounted above into the new lobby’s perimeter soffit.
Cosentini custom ordered factory-installed coil/filter combination options that all have hot water coils that tap into the soffit’s heating supply loop. The 95,600-Btu/h coils supplement the new entrance area’s main HVAC system by providing thermostatically controlled heating near the door areas, when needed.
Each air curtain also includes a control package, which includes a thermostat, three-speed fan with easy maintenance access from an on-board control panel, a timer delay function, and a low voltage relay for tapping compatibly into any direct digital control (DDC) building automation system. Essentially, the control package offers TD Banknorth Garden two units for the price of one; an air curtain performance/wind stopping unit when the door is open, plus a cabinet heater or low velocity and low noise with high heat output when the door is closed.
Productivity & Energy Efficiency: McCormick Place
Another innovative use of air curtains is Chicago’s new McCormick Place West convention center. When officials of the Metropolitan Pier and Exposition Authority (MPEA), which owns and operates the facility, were presented with energy conservation estimates of $70,000 annually and equipment cost payback of less than two years, choosing air curtains was a sure thing. In fact, air curtains were one of several energy conserving technologies that helped the project achieve two of its Leadership in Energy and Environmental Design (LEED) points from the U.S. Green Building Council (USGBC), Washington, D.C. in the “Optimized Energy Performance” category.
The huge $850 million exposition facility has more than 375 linear feet of freight and pedestrian doors typically open 10 to 14 hours daily during the required five to seven days of setup prior to major winter events such as February’s annual Auto Show. Many air curtain manufacturers can perform an energy savings estimate for customers by gathering and calculating dozens of site conditions data. Some of McCormick Place West’s data calculations considered a 26-week heating season with each door open 10 hours weekly for a total of 260 hours annually. Besides the annual savings and payback, there was also a calculated hourly savings of $269.79 when all the doors are open simultaneously.
Aside from the energy savings, air curtains also have an impact on indoor air comfort in the facility’s 470,000 square feet of open exhibition space. Without the air curtains on all large roll-up doors and man doors, the temperature would drop dramatically during set-up/tear-down and it would require a minimum of 24 to 48 hours to regain the target temperature of 73°F to 75°F for an event, according to Susan Van Klompenburg, senior mechanical design engineer, A. Epstein & Sons International, Inc., which performed consulting engineering for the McCormick Place West expansion as a member of the joint-venture, McCormick Place Design Team, Take II LLC, Chicago.
Furthermore, employee air comfort can affect productivity significantly. Shows are set up more efficiently in cold wintertime months at McCormick Place West because employees are more comfortable and the fact that forklifts can pass freely through freight doors without waiting for them to cycle.
Energy prices aren’t expected to decrease much any time soon, therefore facility managers should be considering every possible energy advantage in retrofits and new construction. Air curtains may not be as well known an entity as vestibules, but they certainly win out in operational savings comparisons and quick paybacks in the battle against rising energy costs.
