An eccentric reducer is a fitting used in piping systems between two pipes of different diameters. They are used where the diameter of the pipe on the upstream side of the fitting (i.e. where flow is coming from) is larger than the downstream side.

In a centrifugal pump there is a reducer in suction and expander in discharge. … Horizontal liquid reducers are always eccentric, top flat, which prevents the build up of air bubbles in the system. Eccentric reducers are used at the suction side ofpumps to ensure air does not accumulate in the pipe.

A concentric reducer is used to join pipe sections or tube sections on the sameaxis. The concentric reducer is cone-shaped, and is used when there is a shift in diameter between pipes. For example, when a 1″ pipe transitions into a 3/4″ pipe and the top or bottom of the pipe doesn’t need to remain level.