The heart of a vacuum system is the vacuum pump. Powered vacuum pumps are mechanically
similar to air compressors, but work in reverse so that air is drawn from the closed system
and exhausted to the atmosphere. Vacuum pumps are designated by their vacuum
pressure potential or “Hg maximum” (Hg is the chemical symbol for mercury), their displacement
in cubic feet per minute (CFM) and the horsepower required to drive the pump.