Single  impeller:  used to transport medium with a little large grains because this impeller is not easy to damage grains. The shortage is high impulse, low pressure, small capacity, so seldom using.

Double  impeller:  used to transport medium with  large grains, with  low damage  rate  and  outlet  flow  rate pulsation is smaller than single impeller, low pressure.
Butterfly impeller: used to transport medium with middle or small grains, with low damage rate and low impulse, high pressure. 
Tri-impeller: this is commonly type, the capacity is higher than all other rotors, but it has certain breakage to the grain medium.

Multi-impeller: The capacity will decrease with impeller increase, low impulse, but easy to damage grain medium.