What type of pipe fitting allows for two pipes to be connected, allowing flow in opposite directions?

The type of pipe fitting that allows for two pipes to be connected while permitting flow in opposite directions is the return bend. This particular fitting is specifically designed to redirect the flow of fluid and connect two lengths of pipe at a 180-degree angle, thereby allowing fluid to travel back in the reverse direction seamlessly.

On the other hand, a reducer is used to connect pipes of different diameters, adjusting the flow area but not allowing for flow in opposite directions simultaneously. A 45° elbow changes the direction of the piping system but does not facilitate two-way flow; it redirects fluid in a singular direction. A coupling serves the purpose of joining two pipes end-to-end, which also does not support flow in opposite directions. Hence, the return bend is the only fitting explicitly designed to enable the desired flow characteristics in this scenario.