Pressure

Since the operation of a refrigerator depends mainly on pressure differences in the system, a basic understanding of pressure and the laws of pressure are very important. Pressure is defined as the weight or force per unit area, and it is usually expressed in pounds per square inch or pounds per square foot. The normal pressure of the atmosphere at sea level averages about 14.7 pounds per square inch.

(Note: There are 144 square inches 1 square foot; therefore to get the pressure per square foot, 14.7 must be multiplied by 144.)

Substances always exert pressure on the surfaces supporting them. A refrigerator (a solid) exerts pressure on its legs because if they were removed the box would fall; a liquid always exerts pressure on the sides and bottom of its container, such as a bottle; and gas always exerts pressure on all the surfaces of its container, such as a balloon. If a solid weight of one pound were made with its bottom the surface area on an inch square it would exert a pressure of one pound per square inch upon a flat surface.

A liquid in a container maintains an increasing pressure on the sides and bottom as the liquid depth increases. Gases, however, do not always exert constant pressure on the container because the pressure is determined by the temperature and the quantity of the gas in the container.