Definition - What does Vacuum Breaker mean?
A Vacuum Breaker, also known as atmospheric vacuum breaker is a backflow prevention device that is used to prevent the flow of non-potable liquids back into drinking water systems. Backflow is a fantastic mechanism that regulates the flow of liquid. These vacuum breakers are mainly used in water pipelines and tanks. They must be installed in a minimum height of 6 inches above the highest usage point in the system.
Petropedia explains Vacuum Breaker
A vacuum breaker is usually made up of brass and resembles a 90-degree elbow with a hood on the top to allow air to enter the water system if a siphon attempts to form. Inside this elbow is a poppet valve that is held by the water pressure in the system, closing the air entrance to the device. If the pressure in the "upstream side" is reduced to atmospheric pressure or below, the poppet valve drops and allows air to enter the system thereby breaking the siphon.
Since these devices work on atmospheric principles they cannot be installed in an enclosure containing air contaminants. Those contaminants could be drawn into the device, thus affecting the pipes.