A liquid band vacuum pump comes with an 
impeller with blades mounted on a Screw Air Compressor center hub positioned in a cylindrical body but off-set from the guts.
In Figure 1 to the right, you can observe the blades close to the the surface of the pump are nearer to the outside wall than at the sides and bottom of the pump. You can observe this more obviously in Figure 2.
The impeller sits among two end plates (port plates) that have shaped holes cut into them called ports.
The pump requires a liquid (also called the sealant) to create vacuum the following. Before you start the pump, it must be partially filled up with the liquid sealant. The liquid can be water (rendering it a water ring pump), oil or a solvent, depending upon the application.