What size of a Type B fire damper would be ordered for a lined duct measuring 24x12 ID?

To determine the appropriate fire damper size for a lined duct measuring 24x12 inches inside dimensions (ID), one must consider the need for the damper to fit properly within the duct system while accounting for the lining that typically adds to the overall dimensions.

In this scenario, a fire damper is designed to fit snugly within the duct; however, since the duct is lined, it requires a damper that accommodates the additional material. The lined duct dimensions will be greater than the internal measurements, necessitating a slightly larger damper size to ensure proper fit and function.

The correct size of 26 x 16 inches for the fire damper allows enough room for the internal lining and provides an adequate means of air flow as well as fire control. It is essential for the damper to properly seal against smoke and flames when closed, ensuring the safety features are upheld. This sizing reflects standard practices in the industry where the added dimensions of the insulation or lining are taken into account when selecting equipment like dampers.

In the context of the other sizes listed, the options measuring 24 x 12 would be too small, as they do not account for the lining. The other larger sizes, such as 28 x 18, are unnecessarily