To get good interaction between particles and droplets the air linear velocity should be high. This also has an additional advantage in that it breaks up droplets.

LINEAR VELOCITY = Q/A.

A narrow throat venturi saves on fan power for a given collection efficiency. The pressure drop at the converging section of the venturi obeys the following equation:

In the diverging section, the equation above is almost satisfied except for slight frictional loss.

Since in this instance the air is slowing down we get back almost all the pressure we lost. Looking at it another way, energy loss depends on friction, which increases with velocity. If air is moving fast, over only a small length of the system, the frictional loss is only small yet we still maintain the interaction we need.