A Niche No More
Applications that require noncondensing duct surfaces, or those requiring lightweight material installations in facilities with lightweight roof construction or nondenting materials of construction are just a few of the applications that have found solutions with fabric duct systems.
Advantages To Using Fabric DuctSome of the advantages to using fabric duct systems are that they:
- May be less expensive to install than other air distribution systems;
- May be installed in less time than other air distribution systems;
- Are flexible and not subject to denting;
- May provide more even air distribution than traditional grilles and diffusers;
- Are colorful and aesthetically pleasing;
- Are easily cleaned with minimal system or process downtime;
- Are essentially self-balancing;
- Are easily shipped in boxes;
- Provide lower shipping and handling cost due to less weight;
- Provide quiet air delivery;
- Have multiple fabric material options to fit essentially any application;
- Come in a variety of shapes and sizes with optional fittings;
- Provide draft-free air distribution;
- Are not subject to condensation on the "duct" surface;
- Have some manufacturers' certification as "green" product for LEED™ certified facility designs;
- Are a soft fabric that provides an additional acoustical absorptive surface in the space;
- May be used for spot cooling;
- Can handle a wide range of pressures;
- May be used in VAV as well as constant volume systems.
Installations/ApplicationsThe use of fabric duct systems is generally limited to exposed applications and not allowed by code for concealed applications. However, there may be some areas such as crawl spaces and other nonoccupied concealed spaces where fabric air distribution systems may be allowed. Consult the local building code AHJ in these applications.
Food Processing and ManufacturingSeveral types of food processing industries could be considered good candidates for fabric duct installations. Some examples of successful installations include such industries as meat packing, dairies, canneries, candy factories, beverage producers, etc. The fabric ducts can be used in both the production and storage areas. One of the key benefits of using fabric ducts in food processing facilities is the ease of cleaning the fabric duct as compared to traditional sheet metal duct. When properly designed and installed, the fabric duct can be cleaned faster and allow for shorter production downtimes during the cleaning periods.
Any large open facility where ventilation is needed is a good candidate for fabric air distribution. Providing forced supply air as opposed to using exhaust fans may often be a more effective way to provide proper ventilation in these types of large spaces.
Natatoriums/GymnasiumsFabric ducts have found a major market in buildings with enclosed pools and natatoriums. These spaces have high humidity and chlorine in the air that can cause corrosion in some steel duct installations. Fabric ducts can offer a cost-effective, noncondensing, corrosion-proof solution to these air distribution challenges.
Gymnasiums have also become very popular places for fabric ducts. Fabric ducts don't dent when a ball is kicked into the duct. Fabric ducts provide a soft material to help absorb noise from the yelling and screaming in a gymnasium during a sporting event. Fabric ducts can be used to provide custom colors, logos, or advertising in a fun yet functional way. Fabric ducts are lightweight compared to steel ducts and thereby don't impose as much weight on the structure. Fabric ducts can be installed quickly in existing facilities where ventilation needs to be added or improved.
Auditoriums/Sports Arenas/Convention CentersPublic facilities like major auditoriums, arenas, indoor stadiums, and convention centers generally need low initial first cost construction methods when tax dollars are on the line. Fabric duct can answer this challenge.
Large Public SpacesLarge public spaces like casinos, concert/music halls, museums, and convention center meeting rooms typically require air distribution from high ceilings. Fabric air distribution systems can accomplish this in a manner that is not only effective for air distribution but also adds another dimension to the interior design of the space. Signage and advertising can be provided on the fabric ducts to help inform the public of areas of interest while in these facilities. Likewise, facilities like hospitals and places of worship have spaces that architects attempt to make aesthetically pleasing. The use of fabric ducts provides a softer look, provides a quiet method of air distribution, and provides good air coverage in the space.
Office SpacesComfort and IAQ hinge on good air distribution. Fabric ducts can get air distributed in an office space where it is not generally cost effective or possible to do so with traditional ceiling diffusers. With fabric air distribution, the air can be distributed more evenly by using vents/nozzles evenly spaced across the entire length of the fabric duct. This provides a great way to cover a large length of floor-to-ceiling glass.
Retail/Grocery StoresPublic spaces designed to get shoppers to buy products need to have architectural and interior designs that create a mood for shopping. These are design challenges that require creative solutions. With fabric ducts, the designers can use colors and shapes and air distribution creativity that is not generally economically feasible otherwise. In areas where open case refrigeration displays are used, the fabric air distribution systems can provide draft-free supply air so that the "air curtain" of the display case is not affected. This becomes a major energy saver for the store and allows the refrigerated product to be unaffected by the supply airstream that might be blowing warm air onto it when using traditional metal supply air devices.
Electronics RoomsRooms that have electronic equipment generally require materials of construction that minimize or eliminate static electricity charges. Some manufacturers of fabric ducts have found fabrics that do not cause static electricity charges. These fabrics can also provide draft-free air distribution in rooms where this is needed.
Temporary Structures/TentsThe use of temporary tents and other structures for various events like weddings, parties, holiday celebrations, family reunions, or more tragic events like tornado and hurricane disaster relief efforts and military operations have become very common. Since these structures and tents need to be mobile, it only makes sense that the air distribution systems are mobile as well. Fabric duct systems provide the flexibility, durability, and ease of installation that meet these design requirements. In addition, fabric ducts are lightweight and do not impose an unnecessary structural load on the framework of these structures and tents.
Exhaust EnvironmentsLaboratories generally require large amounts of supply air to compensate for the research hood exhausts. It is important to get this supply air into the space without disturbing the airflow pattern into the hood. It is also generally desirable to provide a comfortable environment with minimal drafts in the space for the occupants. Properly designed fabric air distribution can provide solutions to these design challenges.
Along the same lines, fabric ducts can provide a draft-free solution for providing replacement air in kitchens required because of the kitchen hood exhaust. Draft-free air distribution is important so that the effectiveness of the hood is not compromised, worker comfort is maintained, and the food is not cooled by air blowing on it before it is served. Consult the AHJ for this application to ensure compliance with local codes and ordinances. Special fabrics with USDA approval for food processing applications may be considered for kitchen applications as well.
Design IntentThe design intent of using fabric duct air distribution systems needs to be established from the beginning of the design. As with the selection and installation of traditional air distribution duct and diffusers, the functional requirements need to be clearly outlined in order to ensure that they are met by the end result. Terminal velocity, supply air temperature, available pressure, and throw distance are all parameters that need to be taken into account just like in a traditional metal duct and diffuser system.
Selection Of Fabric Duct MaterialsWith fabric duct, it is recommended to consider the use of performance specifications. In the sheet metal industry, there are some long-established standards for the construction materials and gauges used to fabricate the duct (ASTM, SMACNA, etc.). In the fabric air distribution industry, the manufacturers essentially research and select the materials that they use with no industry guidelines on fabric types, porosity, etc. In this regard, the basis-of-design manufacturer may be able to provide a material type and size that meets the design functional requirement; however, another manufacturer may have to use a different fabric and size to meet the same functional performance requirements. The project specifications and submittal review need to allow for this type of product uniqueness.
Sizing Of Fabric DuctsThe sizing of the fabric duct system is manufacturer specific due to the uniqueness of the fabrics used by each manufacturer. Manufacturers publish sizing information and charts; however, care should be used when comparing "or equal" products when such things as duct diameters, orifice sizes, and material porosity are compared.
The design criteria needed for sizing of fabric duct is essentially the same as for sizing metal ducts and diffusers. However, for sizing the fabric duct, it is necessary to decide what inlet pressure is available at the inlet to the fabric duct, and then the sizing is done based on this available pressure (generally 0.5 in. wc minimum) and velocity in the duct. For the air distribution nozzles/vents, it is necessary to determine the throw distance, terminal velocity, and supply air temperatures for cooling and heating.
Manufacturers of fabric duct do not all provide NC levels for all of their fabric options; however, sound data is available in some cases. If an ADPI analysis is desired, it needs to be done with some engineering discretion since there is some subjective judgment as to which type of air diffuser a fabric nozzle/vent might most represent depending on the installation location of the nozzle/vent on the fabric duct. Fabric air distribution would most likely fall under the classification of a "high sidewall grille" when using the ADPI selection procedure shown in the ASHRAE handbooks. However, if placed on the ceiling by using a semi-circle shape, it could possibly function as a "ceiling slot diffuser."
Installation ConsiderationsOne of the primary reasons for using fabric duct is the ease of installation. It is essentially a three-step process. First is installation of cable and turnbuckles. Second is installation of fastening devices such as clips, hangar clamps, or corded fabric. The fabric duct is ready to be hung.
Fabric Duct and Accessory SpecificationsFabric. The fabric can range from the inexpensive polyethylene used in greenhouses to the high-tech specialty anti-microbial and anti-static fabrics that are used in food processing or cleanroom areas.
Things to consider in the specifications for fabrics are the material type, fabric weight, porosity, weave, fire resistance, electrostatic characteristics, resistance to aging and discoloring, UV protection, chemical resistance, etc. In some cases, the fabric is porous or has vents/nozzles for air distribution, whereas in others it is non-porous where air transport is desired without diffusion.
Duct construction and fittings. Just like sheet metal ducts, fabric ducts are usually constructed in round shapes. However, some manufacturers offer half- and quarter-round fabric duct profiles to fit flat to ceilings and in corners. Also, some fabric duct manufacturers can offer radius elbows, transitions, saddle "T's", and other fittings similar to those used in sheet metal duct systems.
The fittings used to connect sections of fabric ducts are generally made with zippers with a fabric flap that can be folded over to conceal this joint. The zippers are generally made of heavy-duty, commercial quality, nonmetallic materials. The initial start fitting of a fan, air handler, or metal duct system is made by connecting the fabric duct with a belt or drawband fastened around the round start collar. A bead is sometimes rolled onto the metal start collar to help provide an additional point of security and fasten the drawband around to prevent the fabric air duct from working itself loose from the start collar.
Fabric colors. One of the major benefits of fabric duct systems is the variety of colors that are available. Manufacturers of fabric ducts generally have a stock of standard colors and can also provide custom color fabrics for those installations that need the colors to conform with the client's color schemes.
Some manufacturers also offer silk screening of letters, wording, motivational messages, corporate logos, or other advertising or signage. By using a Velcro-type fastening system, the signage can be changed out as needed and sold as a revenue-generating technique at sporting events or in other public spaces.
Air distribution vents/nozzles. Some manufacturers have perfected techniques to punch or cut their fabrics to provide a hole or series of holes directly in the fabric without fraying of the fabric. Other manufacturers use plastic orifices inserted in the fabric. Some of the vents are continuous series of smaller holes, while others are slotted openings covered with a mesh material.
Cleanability. Fabric ducts are designed to be easy to clean. The whole concept of fabric ducts is centered around the ability to take the duct down easily and wash it. Generally, the cleaning involves commercial laundering of the fabric air distribution material. Polyethylene materials can generally be washed or hosed down in place if the application requires this type of cleaning. Drain ports may be desired in washdown type applications to allow the water to be drained from the system; however, the polyethylene can also be dried out when the air system is re-energized if excessive water has not accumulated in the system.
Mounting hardware and accessories. The mounting of fabric ducts is generally done with galvanized cabling. However, other mounting materials and systems are available, such as stainless steel cables, plastic coated cables, and aluminum tracks.
Performance criteria. As with any product selection, the end result is what needs to be considered before selecting the product. This is the performance criterion that needs to be understood from the beginning of the job. These include such things as requirements for noncondensing duct surfaces, space air velocity, terminal velocity, throw distance of the air being distributed, temperature of the air during cooling and heating, ceiling heights, etc.
Design parameters/units of measure. It is essential to have a performance-type specification that can be used for comparison of alternate products. The design parameters listed below are some of the typical parameters that are used by many fabric air distribution product manufacturers in their literature and typical specifications. Metric equivalent units can also be used if desired or required.
- Weight (oz/sq yd);
- Thickness (mil);
- Tensile strength (lb/in.);
- Resistance to tearing (lbf [pound-force]);
- Elongation (%);
- Permeability (in. wg);
- Shrinkage in wash (%);
- Melting point (°F);
- Spontaneous ignition temperature (°F);
- Water absorption (%);
- Electrostatic resistance (Ohm/in.); and
- Operating temperatures (max/min, °F).
Warranty. Manufacturers generally provide a standard one-year parts warranty with extended warranties available if specified. Current manufacturer standard warranties range up to 10 years on some fabrics.
Submittals. Manufacturers of fabric ducts can provide product data sheets showing the performance and construction features of their products. Installation and O&M instructions can be provided; certified construction drawings and the fabric material safety data sheets are also available upon request.
Demonstration/commissioning. As with other HVAC installations, the end result is the most important aspect of the design. The design intent should be specified in a manner that provides measurable results at the actual installation. It may be found that some fabric duct products cannot provide the required level of performance when air performance is specified in a manner that requires proof of performance.