Abstract
An experimental study was conducted to evaluate the pressure loss per unit length of nonmetallic flexible duct under fully stretched conditions when a controlled bend was present and to determine a pressure corrective K factor to compare to a straight section of the same length. All tests were conducted in accordance with ANSI/ASHRAE Standard 120-2008. Ducts of 203-, 305-, and 356-mm (8-, 12-, and 14-in.) nominal diameters were tested. The bend was created by wrapping the duct around a mandrel to generate a 45° and 90° bend. In every instance, the duct was subjected to only one bend with a dimensionless bend radius of either R/D = 1 or R/D = 1.5. The pressure loss per unit length versus flow rate was described by a power law relationship. Positive K factors were obtained when a bend was present, and it was found that the K factor typically increased as volume flow rate increased. Generally the K factor for a 90° bend approximately equaled 2.0, whereas the K factor for a 45° bend tended to have a value roughly equal to 1.25. For a given duct diameter with either a 45° bend or a 90° bend, the K factor for R/D = 1 exceeded that for R/D = 1.5. Suggestions for additional testing of flexible duct configurations more representative of typical field installations are provided.
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