Wet/dry vacuums work without a paper bag or filter. They pick up wet and dry debris through a small tube and pull them into a large tub, where the air movement slows and gravity causes the items to fall to the bottom of the tub while the machine expels air back into the room. They use an electric motor and fan to pull air through the system, according to HowStuffWorks.
The wet/dry vacuum works much like a soda straw, where the individual creates suction with the lungs in the mouth and through the straw. This pulls the soda through the straw, but once the soda enters the larger opening of the mouth, gravity pulls it to the bottom of the mouth rather than directly into the lungs.
Wet/dry vacuums include a float ball in the debris chamber. If the unit fills with liquid, the ball floats to close the entrance into the fan and stop the vacuum. As a result, the unit stops pulling water into the tub.
Central vacuums work in a similar manner, although most designs are only for dry materials. A fan creates a vacuum in a large canister. It connects to ports in various rooms of the home through a system of pipes in the walls. The user plugs a hose into a port for cleaning the room. Removal of the hose allows the lid to the port to close, which prevents vacuum leaks.