Concentric Reducer
Buttweld concentric reducer is also called welded concentric coupling. It allows from connecting a large pipe to a smaller pipe by means of welding. A concentric reducer allows welded connection between two pipes with the same center line.In concentric reducers the area reduction is concentric and the center line of the pipe on a bigger end and smaller end remains the same.  These styles are normally used for vertical lines. So in concentric reducers, the pipe axis remains the same. Reducers are reversible and can be used in any direction.

Eccentric Reducer
Eccentric reducer is a fitting used in piping systems between two pipes of different diameters. The same fitting can be used in reverse as an eccentric increaser or expander. 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, and where there is a danger that vapor may accumulate. In Eccentric Reducer the reduction of the pipe size is achieved by decreasing the diameter of the fitting at a constant rate over a specified length, maintaining one side of the fitting horizontally.