Specifying air curtains for savings and performance

04/01/2009


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Air curtains provide a controlled stream of air across a building entrance or doorway to separate interior and exterior environments. When the conventional door is opened, the air curtain's airstream prevents cold, hot, or humid outdoor air, as well as insects and dust, from infiltrating the indoor environment.

Although air curtains require power to run the blowers that create the airstream, they save much more energy than they expend, making them an attractive option for building owners and operators. Air curtains have long been used in industrial settings; however, they are increasingly finding their way into new markets such as hotels, hospitals, stores, public facilities, and other commercial buildings.

Air curtain manufacturers' catalogs can be misleading, however, because some manufacturers do not have their performance data certified. Such misleading information might not surface until after an installation, when an air curtain is found to be underperforming. For example, some manufacturers use the term maximum velocity, a specification that carries no industry certification. Determining the location of the maximum velocity, whether it's at the discharge nozzle, the floor, or somewhere in between, can lead to confusion. A more accurate term is velocity projection, which is the actual measured velocity at specific distances along the air curtain's airstream.

A better design parameter is the combination of velocity, volume, and uniformity and the proper balance between each.

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 (i.e., one that barely splits) is viable only for applications involving temperature differential without wind, such as internal doorways. These types of installations can stop 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 4 to 5 mph. Obviously, an air curtain for a fast-food restaurant's drive-through window doesn't need as strong a volume as a 16-ft-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. Uniformity impacts the airstream effectiveness only when it drops below 75%. An air curtain that focuses too much energy on generating a high uniformity loses velocity, therefore reducing its effective wind resistance.

Velocity, volume, and uniformity work together to create the ideal air curtain performance—relying on only one or two could skew performance results.

Design considerations

There are several considerations when specifying air curtains:

  • Size of the doorway. Openings can range anywhere in size from drive-through windows to 16-ft-wide industrial building openings and even customized 140-ft-wide doorways in airplane hangers. Multiple air curtains can be tandem-mounted to work together for larger openings.

  • Size of the building. Stable internal pressures typically exist with larger buildings that have multiple doors. Contrarily, smaller buildings with multiple doors have unstable pressurizations because each time an opening is breached, other areas such as doorways with air curtains are affected.

  • Expected wind load. The building's geographical location and surrounding environment impact the expected wind load and exposure. The National Oceanic and Atmospheric Administration (NOAA) publishes geographical wind load data for all major cities.

  • Exposure of the door. Outdoor temperature based on geographical location is vital. What direction the doors face, their relationship to prevailing winds, and nearby obstructions such as retaining walls, other buildings, and topography can all figure into air curtain sizing. Here again, NOAA provides data.

  • Building pressurization. Most buildings that are designed with proper HVAC have a positive pressurization, which is ideal for air curtain performance. Standard air curtains don't perform well in negative pressure situations unless makeup air is involved or filtered bypass air is allowed.

Typically, an engineer starts with doorway size and wind load to specify the correct air curtain. There has to be proper design balance between the air curtain's fan motor hp, plenum size, discharge velocity, air thickness, etc., which all can be increased or decreased by selecting the correct model to reach ultimate performance.

Therefore, specifying the correct air curtain for an application is difficult when its performance specifications aren't accurate, which is frequently the case in the air curtain industry. One source engineers can rely on is AMCA International, which certifies many manufacturers' air curtain specifications through its Certified Ratings Program (CRP). AMCA International has just introduced a new publication developed by a committee of air curtain experts, AMCA Publication 222-08, Application Manual for Air Curtain Units.

Air curtains as vestibule substitutes

Some engineers are using a recent study, “Air Curtains: A Proven Alternative to Vestibule Design,” to lobby local code officials to allow the substitution of vestibules with air curtains combined with automatic doors on new building projects. The three-month-long research study was funded by Berner International and is available at www.berner.com . The study compiled certified results using computational fluid dynamics (CFD) analysis from second-party research/validation consultant Blue Ridge Numerics, of Charlottesville, Va. (see Figures 1a, 1b, and 1c).

The study showed that air curtain and automatic door combinations are 10% more efficient than vestibules, and significantly less expensive in construction costs. 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: air curtain with automatic door, vestibule, and air curtain with vestibule. Each scenario was subjected to wind loads of up to 4 mph and different frequencies of traffic. It is hoped that the study will help air curtains become an optional alternative to vestibules in the International Energy Conservation Code (IECC), which is published by the International Code Council (ICC). Currently, the IECC doesn't disallow air curtains; there just isn't a provision for them as a vestibule substitute.

In an age when energy is one of the most important political issues on the planet, air curtains can be an invaluable energy conservation tool for consulting engineers. The bottom line, however, is that not all air curtains are the same, and they aren't just rectangular metal boxes with blowers. Extensive engineering is involved in creating a smooth, nonturbulent, projecting airstream with the properly designed air discharge that accomplishes the engineer's goal of effectively separating interior and exterior environments.


<table BORDER = '0' CELLSPACING = '0' CELLPADDING = '0' WIDTH = '100%' ID = 'id1395136-47-table'><tbody ID = 'id1395146-47-tbody'><tr ID = 'id1395148-47-tr'><td ID = 'id1395150-47-td' CLASS = 'table' STYLE = 'background-color: #eeeeee'> Engineering resources </td></tr><tr ID = 'id1395160-50-tr'><td ID = 'id1395162-50-td' CLASS = 'table'>AMCA Publication 222-08, Application Manual for Air Curtain Units. www.amca.org .</td></tr><tr ID = 'id1395174-53-tr'><td ID = 'id1395176-53-td' CLASS = 'table'>“Air Curtains: A Proven Alternative to Vestibule Design.” www.berner.com .</td></tr><tr ID = 'id1395188-56-tr'><td ID = 'id1395191-56-td' CLASS = 'table'>The National Oceanic and Atmospheric Administration (NOAA). Geographical wind load data for all major cities. www.noaa.gov .</td></tr></tbody></table>

Famed train station selects air curtains for retrofit

Architects are increasingly using air curtains for building designs such as the retrofit of the TD Banknorth commuter train station in Boston.

To eliminate pedestrian congestion, the entrance lobby of the renowned North Station terminal was moved 100 ft out into the open-ended train shed (see Figure 2). Air curtains keep the new lobby area and outdoor train shed environments separated and safeguard against the infiltration of train emissions (see Figure 3). Jerry Fleishman, HVAC project engineer, Cosentini Associates, Cambridge, Mass., specified high-efficiency, 3,624 cfm air curtains for 12 sliding commuter doors that are continually open during rush hours.

Aesthetics was a key factor in the project. Architect Sasaki Associates Inc. built a curtain wall with a perimeter soffit. Fleishman accommodated the quest for aesthetics by specifying in-ceiling mounted air curtains, which are hidden in the soffit and appear only as aluminum ceiling grills inside each doorway.

For added comfort during winter operation, the air curtains supplement the space's general heating with 95,600 Btuh coils supplied by the building's hot water loop. The air curtains also include a control package consisting of a thermostat, a three-speed fan, a timer delay function, and a low-voltage relay for tapping compatibly into any direct digital control (DDC) building automation system.



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