Sometimes the mouth of the extractor does not correspond to the size of the filter or tube of the same diameter and reductions are needed. For that we use the coupling of reduction in the appropriate measure.
It is also used for Y or T connections, to remove two tubes from one, since it is convenient to reduce the diameter of the tube.
When there are several bifurcations in a tube, only with step reductions can be controlled so that it absorbs the same from one site as another, for example, remove from a tube 200 two 150mm or 125mm, because if you continue with two of 200mm, the air can suck only 1 tube and leave the other unusable, thus affecting the ventilation of the crop.
If we put a reduction, but on the contrary, to make an extension, for example from 125mm to 150mm in the exhaust air output, we will get much less pressure when going out and make less noise.