Sealing open doorways and saving energy with air barriers
by Elaina Adams | December 1, 2011 11:26 am
By Michael Launer
One of the greatest sources of energy loss in a building is through an open door. For years, facilities have been losing substantial amounts of energy and money, while occupants suffer the discomfort of cold drafts blasting in when the doors are open. What some people do not know is there is a technology on the market that can create a significant energy-efficient seal on open doors by simply re-circulating facility air across the doorway.
These ‘air barriers’––sharing a name but differing in function from the building assembly discussed on page 50––have been saving facilities energy and money as well as enhancing employee/client occupant comfort for years. The air velocity from the air barrier must be great enough that the resulting air velocity is directed downward. It should be set so a small part of the airstream is directed out of the opening, while the rest re-circulates into the premises––resulting in cold outside air being kept out, and warm indoor air being kept inside.
The principle of using an air current as an invisible ‘barrier’ between hot and cold zones in entrance areas originally comes from Europe where heated duct systems are constructed using powerful blower fans in the ceiling and extraction grids in the floor. These systems are often energy-inefficient and expensive to install and operate. Air barriers have evolved this concept to provide maximum efficiency, minimal costs, and simple installation.
It is also very important for one to know the difference between an air barrier and an air curtain. The former should not be confused with a heated warm air curtain, whose primary objective is to provide warmth at an open doorway. Without a proper seal over an open doorway, the expensively heated or air-conditioned air, along with the warm blast from a typically heated air curtain, is immediately conveyed out the door. A typical 8-kW air curtain, operating eight hours per day, five days per week, will consume in excess of $1300 of electricity per annum. Under the same conditions, an air barrier unit will consume $90 of electricity.
Various applications
Air barriers can be manufactured for any size opening––from foot traffic doors to aircraft hangars and everything in between. The application for the technology extends far beyond sealing the cold winter air out. Some of the different applications in which air barrier technology has been successfully applied are:
- stopping bugs, dust, and warm summer air from entering restaurants and retail/food processing facilities;
- keeping warm air from entering the coolers/freezers in cold storage facilities; and
- preventing outside air from affecting processes such as precision-machining, manufacturing, or welding operations.
The benefits of fitting a doorway with an air barrier are just as endless as the applications. The most significant advantage of installing one is the potential for energy savings. Both door and heating and air-conditioning system maintenance are quickly becoming a costly expense that comes off the bottom line. By keeping the door open while preventing the cold air from entering, door cycles are reduced, along with the constant running of a heating system trying to heat the cold air coming in through the open door. The ability to stop cold drafts from entering the facility will not only increase employee and customer comfort, but it is also a large factor in using the air barrier technology.
In the food storage industry, stopping warm air and humidity from infiltrating the tightly controlled cooler or freezer prevents temperature fluctuations, reduces product spoilage, and lowers maintenance requirements on the cooling system. When it comes to the food processing industry, the ability to keep dust and insects out of consumer product processing is a huge benefit to any company wanting to keep quality control as stringent as possible. Car washes have also seen the benefit of reducing the ice fog generated when the cold air from outside hits the warm, humid air of the car wash when their doors open to either enter or exit the car wash.
Selecting the right manufacturer
With numerous types of air barriers on the market, it is crucial for one to choose the right product for the doorway. There are four essential critical design criteria when it comes to selecting the right manufacturer.
Uniform air flow
Consistent fan placement every 0.6 m (2 ft) allows for uniformity across the air barrier, eliminating low-pressure areas. Each fan wheel needs to be independently driven by its own motor. Twin-shafted motors that drive multiple fans are not recommended as they create additional gaps in the airstream.
Velocity and volume
It is important to ensure a sufficient amount of air at the correct speed reaches the floor to obtain a seal––a minimum 4 m/s at 0.3 m (800 fpm at 1 ft) from the floor is recommended.
Laminar air flow
A specially designed direct discharge nozzle promotes a non-turbulent airstream. The smoother the air, the greater resistance to outside air infiltration.
Solid articulating nozzle
A nozzle creates the ability to direct the entire airstream outward to counter for incoming air. The discharge angle is application-specific based on varying regional and facility conditions. The nozzle should be manufactured to directly receive blower air discharge to maintain laminar air flow. Directing, deflecting, or ducting discharge air off a backpan or with a plenum should not be used due to air turbulence caused. All vanes within the nozzle are designed to move in unison, and independently adjustable vanes or vanes that do not cover the entire nozzle area should be avoided as this disrupts the laminar air flow required.
Several engineering research studies done in various parts of the world have demonstrated the capabilities of air barriers to stop drafts. (See the article, “Effectiveness and Optimum Jet Velocity for a Plane Jet Air Curtain Used to Restrict Cold Room Infiltration,” by A.M. Foster, M.J. Swain, R. Barrett, P. D’Agaro, and S.J. James from the International Journal of Refrigeration, pages 692-699 [2006]). They operate on a very simple law of physics––one force changes direction if subjected to greater force. The ‘force’ is air mass and the velocity at which it travels. The outside wind has a certain mass and velocity, as does the air barrier airstream. If the air barrier force is equal and opposite to the force from the draft through the door, then the draft is prevented from entering. If the draft is much stronger than the air barrier, it will break through and air will enter. However, even in this circumstance, the air barrier prevents outside wind from entering most times. If the air barrier is much stronger than the draft, some of the air will go outside. Its strength follows the formula, ‘mass times the velocity squared.’ Therefore, an air barrier that blows at twice the speed has four times the wind-stopping power at the same air volume.
Air barriers can be mounted above a door, on one side, or, depending on its width, on both sides. The most common is to mount the unit above the door as this helps to de-stratify the waste heat from the ceiling. To achieve the best possible result, the air barrier should be positioned as close to the opening as possible––no more than 25.4 mm (1 in.) above the header––and should extend over the entire width of the entrance. The unit must be installed to allow air discharge to be angled toward pressure sides up to 20 degrees. This is done to counter incoming air flow and allow the unit to run at its optimal efficiency. About 610 mm (24 in.) of headroom above the door header is required to allow for proper clearance for unit installation.
Energy-efficient thinking
As fuel costs continue to rise to record levels, pressure is mounting for all companies to become more energy-efficient in their operations and reduce their carbon footprint. People are scrambling to find technologies that can truly help save energy and money. Air barriers are not only an energy-saving solution, but they also help maintain a more comfortable indoor climate and help reduce wear and tear on heating/cooling systems and doors.
A proprietary engineered calculator has been developed to determine the potential energy savings by using air barrier technology. Based on American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) industry guidelines, this calculator defines the energy amount being lost out the door opening as well as determines how much energy is saved. Typically, any doors within Canada that see 1 to 1.5 hours of use daily have a two- to three-year payback.
In the past, when it was cold in a building, the natural solution was to add heat. Mechanical system layouts usually call for heaters to be aimed at doorways to temper cold air coming in and make up for the loss of heated air. However, a change is being seen as people are starting to heat the building envelope, rather than the outdoors. Typically, buildings are designed with adequately sized heating and cooling systems; therefore, doors are not allowed to be left open for extended periods. It is far more efficient to seal off the door openings than to add components that are going to increase the heating or cooling load.
Making the decision to seal off an open doorway with air barrier technology is just the first step to thinking about energy efficiency. Properly selecting the manufacturer that can create an efficient and effective seal is as important as the decision to install the technology.
Michael Launer is the president of Enershield Industries. Ticketed in 2nd Class Gas, Industrial Steam, he has 13 years of experience in the air barrier industry, and 19 years of experience in heating and ventilation. Launer is a member of the Canada Green Building Council (CaGBC), Better Business Bureau (BBB), and the Edmonton Chamber of Commerce. He can be reached via e-mail at mlauner@enershield.ca.
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