3822 (RP-690) -- Equivalent Round Diameter of Spiral Flat Oval Ducts PDF

Friction factors for spiral flat oval ducts were measured over the Reynolds number range 6 x 10 to the power 4 to 6 x 10 to the power 5, based on hydraulic diameter. Aspect ratios ranged from 1.4 to 5.5. When plotted on a Moody diagram, the data closely followed a single relative roughness curve. It was shown that the flat oval equivalent round equation in the ASHRAE Handbook (ASHRAE 1993) is within 6.2% of a rigorous solution. Since this accuracy is within the accuracy of duct pressure loss calculations, it is recommended that the subject equation be retained for future editions of the Handbook. The absolute roughness of the ducts ranged from 0.055mm (0.00018 ft) to 0.381mm (0.00125ft), averaging 0.158mm (0.00052 ft) for the eight tests. It is therefore recommended that spiral flat oval duct be categorised as "average" in the "Duct Design" chapter of the Handbook.

KEYWORDS: diameter, ducts, spiral wound ducts, friction factor, measuring, Reynolds numbers, accuracy, pressure drop, calculating, testing, speed