What type of duct connection uses angle iron flanges at each section with matching bolt holes?

The type of duct connection that utilizes angle iron flanges at each section with matching bolt holes is accurately identified as companion flanges. Companion flanges are designed to provide a secure and robust method for connecting duct pieces, ensuring that the sections are held firmly together while allowing for some flexibility in alignment.

This type of connection is particularly beneficial in applications where the ducts may need to be disassembled for maintenance or modifications, allowing for easy access and reassembly. The matching bolt holes ensure a precise fit and secure fastening, which helps maintain the integrity of the ductwork system and reduces the chances of air leaks.

Other options, such as TDF (Tack Drip Flange) or TDC (Tack Drip Corner), refer to a specific type of flange system that does not necessarily involve angle iron or bolt holes in the same manner as companion flanges. These are typically used for simplicity and efficiency in manufacturing and require different installation methods. Therefore, companion flanges are the most fitting choice for the described connection method.