Hi,
That is called a liquid ring pump. They are similar to a centrifugal pump but because it has the closed ring impeller it will re-prime itself and draw a very powerful suction even with air in the line. They are used for CIP return pumps because they will suck the tank dry and not lose their prime like a standard centrifugal pump.
As a centrifugal pump goes they are not very efficient and you wouldn't want to use one to pump mash but they are expensive and untouchable for CIP return.
Let me know if you need more information on it.