A niche no more: engineers, interior designers, and architects now use fabric duct systems for air distribution in a variety of facilities. By outlining relevant applications, design parameters, sizing and installation tips and more, our monthly puzzle author spells out the fabric solution for moving air up, down, or across a space

Engineered Systems, Jan, 2005 by Steve Liescheidt

DESIGN INTENT

The 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 MATERIALS

With fabric duct, it is recommended to consider the use of performance specifications. 111 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 DUCTS

The 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 CONSIDERATIONS

One 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.