Lesson 5/Learning Event 1
Two kinds of thermostats in common use are the bellows and pellet type. Both types operate on the same
principle; the difference is in the unit that opens and closes the valve. The bellows type consists of a flexible
metal bellows attached to a valve. The sealed bellows is filled with a liquid such as ether. When the liquid is
cold, the bellows is contracted so the valve is closed. When heated, the liquid turns to a vapor and expands the
bellows, opening the valve. This thermostat is sensitive to outside pressure, so it does not work very well if the
cooling system is pressurized.
137
Previous Page
