Basic Characteristics

1/ Cross section


Note: We also make transitions to take the duct from one of the shapes above to a different shape. Some shapes deform slightly despite being properly tensioned, this is due to positive or negative pressure and the material flexibility (applies to cross sections S and T).


2/ Dimension

We manufacture fabric ducting and diffusers ranging from 4” to 80” diameters (custom diameters upon request), depending on the project requirements. The inlet adapter dimensions are always approximately 1/2” larger than the metal connection for ease of installation.

In general, fabric diffusers are designed for similar flow speeds as traditional ducting and the volume of air determines the inlet size. Maximum velocities range from 1400-1800fpm for round ducts and 700-1400fpm for half round ducts (the higher the inlet velocity, the more static pressure required). Factors to consider are potential flow turbulence from fittings, available pressure, and sound generated by fast moving air. Specific conditions of flow, static pressure and weight of the fabric used must be taken into account to avoid vibrations/duct movement. Do not hesitate to contact us for design advice, we would be glad to help.


Fabric Duct Sizing Calculator

3/ Length

Determining the length of fabric ducting and diffusers typically depends on the space. Generally the same air flow may be supplied into an area using a 10’ - 650’ long diffuser depending on the material used, its modification, and the supply fan’s delivery pressure. The length of diffuser will usually correspond with the space dimensions depending on throw, velocity and installation height. We can help to determine the length and flow model to specify for the most economical and effective solution.



A - beginning (Inlet/Collar) – length 4”-8”

B - continuous part (Straight section) – length 16.4’-33’, may be repeated several times in sequence

C - end part – length from 3’ to 36’ (6” Zippered end caps also available upon request, otherwise the endcap        is sewn directly to the last section.)

- Individual parts are connected with zippers; the number of zippers may be modified per customer request.

- Only the overall length in feet (thus A + B + C) is provided in the material list; ducting and diffusers are separated into segments during production.


4/ Pressure

Fabric ducting is typically designed with the static regain method. Pressure losses of fabric diffusers & ducting are very similar to those of traditional metal ducting, but also slightly different. The biggest difference is that for most fabric duct applications the air is continuously distributed along the entire length so the velocity of the air continuously decreases. This equates to a lower velocity or friction pressure loss per running foot for fabric ducting than traditional metal ducting. For very long duct runs, typically over 100’, we will also reduce the duct diameter similar to metal ducting to ensure even air delivery from inlet to the endcap.

Minimum static pressure necessary to keep a fabric diffuser or duct fully inflated depends on the weight of the fabric used and if reinforcement options are used. Sufficient pressure for light weight materials is .10 INWG and 0.20 INWG for medium and heavy weight fabrics (if no reinforcement options used).

Generally Fabric ducts are designed on average for .50” wg inlet external static pressure loss. Do not hesitate to contact us for design assistance of the distribution system or ducting, as our software can easily provide accurate pressure loss calculations.



5/ Types of Ending