What is the series of pumps? What is the difference between a unipolar and a multilevel? What is the meaning of each "first class"?
You're talking about a centrifugal pump. His series says the number of slices of the impeller (excluding the wheel, that is, the wheel)
A single-stage pump is a pump with only one impeller. 2, a multistage pump is a pump with two or more impellers. The multistage pump can increase the pressure and lift the head higher. Level 1 means a booster impeller.
Each impeller is of the first stage. (two impellers share only one level, and exit is only level one)
