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An air door or air curtain is a mechanical device used for separating two spaces from each other, usually at the exterior entrance of a building, using a controlled stream of air aimed across the opening to create an air seal. This seal separates different environments, helping to protect Indoor Environmental Quality (IEQ), while allowing a smooth, unhindered flow of traffic and unobstructed vision through the opening. Because air curtains help to contain heated or air-conditioned air when the door is open, they provide size-able energy savings and occupant comfort when applied in retail, commercial, industrial, or warehouse settings. When considering energy savings only, a properly selected, installed and operated air curtain can pay for itself in one to two years by reducing the load on the building's heating or air conditioning system due to the door being opened. Air curtains are also used to stop the infiltration of flying insects and fumes.

Air curtains are utilized to protect the opening when the door is open while people, fork trucks, and/or other forms of conveyance pass through. Reasons for this are to prevent treated (air conditioned or heated) air from leaving the building, to keep insects out, to keep vehicle exhaust, tobacco smoke and other fumes out, with the dual purposes of saving energy and keeping building occupants comfortable, and in food service and similar situations, deterring contamination.

In 2012, the International Green Construction Code (IgCC) approved air curtains as a viable alternate to vestibules.

In 2013, an AMCA-sponsored research study confirmed that air curtains protect the doorway and save energy.

There are two major types of air curtains: recirculating and non-recirculating.

Air Curtain: Recirculating

Recirculating air curtains, which are mainly used in high foot traffic doorways such as supermarket and store entrances, emit air from a discharge grille located across the top of the door opening, collect it through a receiving grille on the opposite, floor side, and return it through duct work to the discharge grille.

They work like this:

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Air Curtain: Non-recirculating

Non-recirculating air curtains are mounted horizontally across the top, or vertically alongside, an opening, depending on the application, and blow a controlled air stream across the entire opening. They work like this:

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Non-recirculating air curtains are more common as they are much easier and significantly less expensive than re-circulating to install and are suited for both retrofits and new construction, whereas recirculating are installed as part of a major renovation or new construction. Non-recirculating air curtains are used across a wide range of applications than recirculating which are mainly used by supermarkets. Both have pros and cons depending on the individual application and usage intent of the space. Both styles have the option to provide supplemental heat (electric, steam, hot water, indirect gas-fired) to the space (popular option for both colder climates and for warm climates that have seasonal periods of cool weather).

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 threshold of the door opening. A properly-sized and AMCA-certified air curtain can contain approximately 70 to 80-percent of that air and return it to the space. The ideal air stream has a proper balance of velocity, volume and uniformity (see below).

For industrial conditions, high face velocities are acceptable. For commercial applications like store entrances, user comfort dictates low face velocities, which reduce effectiveness of separation of exterior air from interior air.

Non-heated air curtains are often used in conjunction with cold storage and refrigerated rooms. Airflow through a door depends on wind forces, temperature differences (convection), and pressure differences.

Air curtains can be used to save energy by reducing the heat transfer (via mass transfer when air mixes across the threshold) between two spaces, although a closed and well-sealed physical door is much more effective. A combination is often utilized; when the door is opened the air curtain turns on, minimizing air flow from inside to outside and vice versa. Air curtains are often used where doors are required to stay open for operational purposes, such as at loading docks and vehicle entrances.

Common applications are openings such as: main entrances, lobbies, vestibules, shopping cart doors, kitchen and cafeteria doors, loading docks, shipping and receiving doors, internal cold storage doors, walk-in cooler doors, and drive-thru (pass-thru) windows. Common building types: restaurants, hospitals, schools, grocery stores, convenience stores, office, hotels, warehouses, factories, food processing plants, guard & ticket kiosks, airplane hangers, sewage treatment plants, etc.

Controls: Usually, there is a mechanism, such as a door switch, to turn the unit on and off as the door opens and closes, so the air curtain only operates when the door is open. Applications that experience periods of high traffic (lots of people constantly entering and exiting) will often include a time delay along with the door switch. A time delay will keep the air curtain running for a certain amount of time (i.e., 60 seconds) after the door shuts, so that should the door open before then, the air curtain remains running so as to increase the effectiveness of the air curtain during this period. Heated air curtains are used when supplemental heat is needed for a space as well as to reduce the wind chill factor across the opening in colder climates. For heated air curtains, in addition to a door switch, a temperature sensor (thermostat) is installed inside the space so that the heat and the air curtain continue to operate until the thermostat has been satisfied, at which point, the air curtain turns off.

Proper air curtain performance relies on a balance of velocity, volume and uniformity. Velocity, volume and uniformity work together to create the ideal air curtain performance; 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. Weaker air streams are 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. NOTE: Few external doorways are not affected by wind loads.

Volume: Volume 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 sides.

Uniformity: Having the air stream flow in the same direction at the same velocity across the opening is the third piece to having a properly performing air curtain. Anything less than 75-percent will adversely affect the air stream effectiveness; on the other hand, an air curtain that focuses too much energy on generating a high uniformity loses velocity, therefore reducing its effective wind resistance.

Air curtains work best when the pressure differential between the inside and outside of the building is as close to neutral as possible. Negative pressures, extreme temperature differences, elevators in close proximity, stack effect, or extreme humidity can reduce the effectiveness of air curtains. Under ideal conditions of zero wind, the effectiveness of the air curtain is at its maximum. In windy locations, air curtains cannot create a perfect seal but are often used to reduce the amount of infiltration from the opening.

Building Codes and Standards:
The AMCA CRP (Certified Ratings Program) Seal assures that a product or product line has been tested and rated in conformance with AMCA’s test standards and rating requirements; a list of rated equipment can be found on the AMCA web site. The test standard for air performance testing for air curtain units is ANSI/AMCA 220-05 Laboratory Methods of Testing Air Curtains for Aerodynamic Performance Rating:

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Air Curtains and IgCC

The 2012 International Green Construction Code (IgCC) adopted language referencing AMCA 220-05 Laboratory Methods of Testing Air Curtains for Aerodynamic Performance Ratings, allowing air curtains to be used in place of vestibules.

Air Curtains for Food Service Industry (chemical-free insect control)

Insect Control Air Curtains, sometimes called “Fly Fans” are used in food processing plants, dairies, bakeries, bottling plants, restaurants, supermarkets, hospitals, school cafeterias, and other areas where the control of flying insects is required. The powerful stream of air that is produced by an Insect Control Air Curtain is an excellent deterrent to insects, which will usually avoid the air stream supply because of its unnatural high velocity.

The most effective air curtain design for insect control has a nozzle that can angle the air stream away from the area to be protected. An angle of about 20° from the vertical is usually optimal. The unit should meet or exceed U.S. Department of Agriculture and Food and Drug Administration standards, which call for an air stream 2 to 5 inches wide at the nozzle capable of producing a minimum velocity of 1600 fpm of air 3 feet above the floor and across the entire door opening. It is very important that the entire doorway be covered so that there are no gaps for the insects to enter.

The Insect Control Air Curtain can be mounted on the inside or outside of the doorway. When mounted on the inside, the unit can help control the building’s climate in the winter by stopping the influx of cold outside air. (For more on this means of energy savings, see the Industrial Climate Control or Commercial Entrance Sections.) When the building contains odors which are attractive to flying insects, it is recommended that the air curtain be mounted on the outside.

Numerous local health codes (state, county, municipal) around the country require the use of NSF-certified air curtains on kitchen back doors as part of preventing insects from entering the food preparation area. Acceptable certifications are: UL (Underwriters Laboratory) EPH (Environmental and Public Health) certified to NSF/ANSI standard 37 air curtains, and ETL Sanitation Listed for Standard NSF 37 air curtains.

FDA Recommends use of Air Doors

As the following excerpt from the “Food Establishment Plan Review Guide 1997,” illustrates, the US FDA specifically recommends the use of air curtains and controlled air currents for the exclusion of insects.

Food and Drug Administration and Conference for Food Protection FOOD ESTABLISHMENT PLAN REVIEW GUIDE 1997 SECTION III FOOD ESTABLISHMENT GUIDE FOR DESIGN, INSTALLATION, AND CONSTRUCTION RECOMMENDATIONS  PART 13 - INSECT AND RODENT CONTROL Openings to the outside shall be effectively protected against the entrance of rodents. Outside openings shall be protected against the entrance of insects by the installation of tight fitting, self-closing doors, closed windows, self-closing serving windows at drive-thrus, screening, controlled air currents, vestibules or other means approved by the regulatory health authority. Screen doors shall be self-closing and screens for windows, doors, skylights, transoms, intake air ducts and other openings to the outside shall be tight fitting and free of breaks. Screening material shall not be less than sixteen mesh to the inch. Seal openings around pipes, conduit or wiring entering the building from the outside. Loading docks and delivery doors must be provided with effective air curtains or vestibules with self-closing doors to preclude the entrance of insects. It is recommended that outside lighting around loading areas and entrances be sodium vapor rather than mercury vapor to decrease insect attraction.

USDA Recommends use of Air Doors

As the following excerpt from the Federal Register 1997 illustrates, the USDA also discusses the use of air curtains and make up air fans in their recommendations. Federal Register August 25, 1997 (Volume 62, Number 164)] [Rules and Regulations] . . . . This Guidebook is intended for use by meat and poultry establishments in considering decisions about design and construction of their facilities, as well as the selection of equipment to be used in their operations. . . . [Page 45032] . . . . * The air curtain or air door is a door that uses a layer of air generated by mechanical fans to separate two rooms or areas. If you select this door, consider the following: ** This door needs to be carefully selected, installed, and maintained to be effective. ** If an air imbalance (pressure imbalance) develops at the door opening, the separation effect may be diminished or eliminated. Air imbalance can occur from air flow changes from any other openings in the rooms especially other doors. . . . [Page 45033] . . . .* When exhaust fans are installed, provision should be made to provide enough outside make up air (emphasis added) to prevent air from being drawn into and through docks, coolers, and production areas to the area served by the exhaust fan. . . ..

EPCA: the Energy Policy and Conservation Act--Section 312: Mandates all walk-in coolers/freezers manufactured after Jan. 1, 2009 include either a strip curtain, spring-hinged swinging vinyl doors or other air infiltration reduction method such as an air curtain.

Standards:

NSF/ANSI 37-2005, “Air curtains for entranceways in food and food service establishments” Equipment covered by this Standard includes, but is not limited to, air curtains for entranceways in food and food service establishments (e.g., service and customer entries, service windows, cooler and cold storage entries). Housing, air moving equipment, air directional regulating devices, and other appurtenances to the air curtain are included. This Standard does not establish equipment installation requirements. While the requirements of this Standard are intended to ensure equipment may be installed in a sanitary manner, proper installation of equipment shall be governed by the applicable codes. Equipment components and materials covered under other NSF or NSF/ANSI Standards or Criteria shall also comply with the requirements therein. This Standard is not intended to restrict new unit design, provided such design meets the minimum specifications described herein. (22 pgs.) See techstreet.com for additional information. UL EPH A UL certification, the UL EPH (Environmental & Public Health) mark appears on products that have been evaluated to Environmental and Public Health Standards. EPH certified to NSF/ANSI standard 37. See ul.com for additional information.

Difference between an Air Curtain and an Overdoor Heater (a.k.a. space heater, cabinet unit heater) The UK based HEVAC Air Curtain Group [2] describes overdoor heaters as small electric or water heated fanned units with a low air volume flow rate. They are intended to be installed at doorways having low pedestrian footfall where the door is mainly closed. They are useful in providing warmth to a cold region adjacent to a doorway or opening which could become uncomfortable, similar to a radiator positioned under a window. They should not be seen as an alternative to an air curtain. The principal differences between air curtains and overdoor heaters are: Figure 1 Figure 2 1. Air curtains are designed to fully cover the width and height of a doorway plus an overlap – overdoor heaters are small, they could never cover the width of a doorway so would leave significant gaps where the outside air can enter. 2. The fans in an air curtain are powerful enough to provide an air stream to project across the whole doorway – overdoor heaters have only small fans that could never project an air stream more than approximately 1m. (ISO 27327-1:2009[3] describes a test method to measure air velocity projection.) 3. The discharge nozzle on an air curtain is optimized to provide a more uniform air stream across the whole width of the doorway – overdoor heaters could never provide uniformity comparable to an air curtain as they can only cover part of the doorway. (ISO 27327-1:2009[3] describes a test method to measure air velocity uniformity.) 4. Air curtains are fitted with sophisticated controls to regulate the strength and heating capacity of the airstream across the doorway – overdoor heaters have simple manual controls which are often left on the wrong settings, unnecessarily wasting energy. As way of illustration Figure 1 shows an air curtain and Figure 2 shows an overdoor heater, both installed over the same doorway. An authoritative engineering design procedure for calculating the supply air flow and thermal capacity of an air curtain for an HVACR application is explained in the BSRIA Application Guide 2/97[4]The procedure for a ‘Building with an Air Tightness Specification’ should be followed, i.e. a practical building with some air leakage. Within the BSRIA Application Guide, Section 4.2 explains the design procedure and Section 5.2 gives worked examples for buildings with a range of air tightness specifications. This allows the engineer to calculate the supply air flow rate and thermal capacity of the required air curtain for a particular application.

Definitions [edit]

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defines an Air Curtain as:

"In its simplest application, an air curtain is a continuous broad stream of air circulated across a doorway of a conditioned space. It reduces penetration of insects and unconditioned air into a conditioned space by forcing an air stream over the entire entrance. The air stream layer moves with a velocity and angle such that any air that tries to penetrate the curtain is entrained. Air curtain effectiveness in penetrating infiltration through an entrance generally ranges from 60 to 80%". – Taken from the ASHRAE Handbook 2004: HVAC Systems and Equipment, page 17.9

Air Movement and Control Association (AMCA) defines an Air Curtain as:

"A directionally-controlled airstream, moving across the entire height and width of an opening, which reduces the infiltration or transfer of air from one side of the opening to the other and/or inhibits flying insects, dust or debris from passing through".

AIR CURTAIN: A mechanical device that creates a wall of air to separate two areas without use of a structural wall or door. Also referred to as: air screen, fly fan, or air door.

AMCA	Air Movement and Control Association

BTU/HR	Amount of heat required to raise one pound of water one degree Fahrenheit. British Thermal Unit

CFM/VOLUME	Cubic feet of air per minute, the volume or quantity of air being moved.

DISCHARGE VELOCITY	Speed of air leaving the air door.

EPH	Environmental & Public Health. Has set standards for product certification.

EXTERIOR OPENING	Any opening exposed to the outdoors.

FPM/VELOCITY	Feet per minute, speed that air moves. 88 FPM equals 1 MPH (mile per hour).

INTERIOR OPENING	Any opening that is indoors, not exposed to outside elements.

NEGATIVE PRESSURE	Exhaust air not compensated by make-up air.

NOZZLE	The discharge opening on an air curtain, to which the discharge grill connects.

NSF	National Sanitation Foundation: Independent, not-for-profit organization dedicated to public health safety and protection of the environment.

ODP	Motor – Open Drip Proof

SPLIT LOCATION	The point where the air stream coming from the air curtain hits the ground and splits into two different directions, (to either side of the opening).

TEAO	Motor - Totally Enclosed Air Over

TEFC	Motor – Totally Enclosed Fan Cooled

References [edit] 1.	^ Interim Report on the Energy Appraisal of Retail Units: Assessing the effect of open doors on energy consumption and thermal comfort Technical Report Number: CUED/D-STRUCT/TR232, Murat Basarir & Dr. Mauro Overend, published 2010-10-18, accessed 2011-06-28 2.	^ HEVAC Air Curtain Industry Group (http://www.feta.co.uk). For technical and commercial information on air curtain technology click on the HEVAC icon and go to the Air Curtain Group area. The Federation of Environmental Trade Associations (FETA), Hare Hatch, Reading, UK 3.	^ ISO 27327-1:2009. Fans - Air Curtain Units - Part 1: Laboratory methods of testing for aerodynamic performance rating. International Organization for Standardization, Geneva, Switzerland. 4.	^ BSRIA Application Guide 2/97. Air Curtains – Commercial Applications. Building Services Research and Information Association, Bracknell, Berkshire, UK. 1997 5.	AMCA Standard 220-05, Laboratory Methods of Testing Air Curtains for Aerodynamic Performance Ratings 6.	AMCA Publication 222-08, Application Manual for Air Curtain Units 7.	Investigation of the Impact of Building Entrance Air Curtain on Whole Building Energy Use, AMCA International, Liangzhu (Leon) Wang, Ph.D. 8.	AMCA International Study Shows Air Curtains Save Energy over Vestibules 9.	Berner Air Curtain & Vestibule Performance Research 10.	Air curtains test 99% effective for insect control on airlines, Chris Woodyard, USA TODAY, March 9, 2004 11.	EPCA: Energy Conservation Standards for Walk-in Cooler and Freezer Standards

External Links:

Air Movement & Control Association International (AMCA) Energy Savings Calculator, from Berner Intl. International Green Construction Code

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