The equivalent length of an 8-inch radius smooth 90-degree elbow used as a domestic dryer exhaust duct fitting is ______ ______.

To determine the equivalent length of an 8-inch radius smooth 90-degree elbow for a dryer exhaust duct, it's essential to understand how to calculate equivalent lengths for fittings. Equivalent length is essentially a way to convert the effects of bends in ductwork into a straight length of duct for the purpose of calculating airflow resistance.

In the case of a smooth-radius elbow, the bend is less restrictive compared to a sharp or crimped elbow. For an 8-inch radius 90-degree elbow, industry standards typically provide a reference for equivalent lengths. The formula often used involves looking at tables that provide specific equivalent lengths for various duct sizes and fittings.

For an 8-inch radius, a smooth 90-degree elbow generally equates to approximately 1 foot and 7 inches of straight duct. This accounts for the reduced turbulence and friction loss due to the smoother flow path provided by the larger radius. When evaluating the other options, they fall short of this specific equivalent length, either being shorter or longer than what is recognized as standard for this type of fitting. Thus, the equivalent length being 1 foot, 7 inches accurately reflects the reduced resistance of the airflow through the smooth elbow compared to a straight duct measurement.