Refrigeration & Air Conditioning Technology, 8e - 25

Unit 1

25

Heat, Temperature, and Pressure

1.11 PRESSURE
Pressure is defined as force per unit of area. This is normally expressed in pounds per square inch (psi). Simply
stated, when a 1-lb weight rests on an area of 1  square
inch (1 in2), the pressure exerted downward is 1 psi. Similarly, when a 100-lb weight rests on a 1-in2 area, 100 psi
of pressure is exerted, figure  1.16. If the 100-lb weight
rested on a surface that was 100 in2, the pressure exerted
would be 1 psi.
When you swim under the surface of the water, you feel
a pressure pushing inward on your body. This pressure is the
result of the weight of the water and is very real. You would
feel a different sensation when flying in an airplane without
a pressurized cabin. Your body would be subjected to less
pressure instead of more, yet you would still feel uncomfortable. It is easy to understand why the discomfort under water
exists, the weight of the water pushes in. In the airplane, the

situation is just the reverse. There is less pressure high in the
air than down on the ground. The pressure is greater inside
your body and is pushing out.
Water weighs 62.4 pounds per cubic foot (lb/ft3). A cubic
foot (7.48  gal) exerts a downward pressure of 62.4  lb/ft2
when in actual cube shape, figure 1.17. How much weight
is then resting on 1 in2? The answer is simply calculated. The
bottom of the cube has an area of 144 in2 (12 in. 3 12 in.)
sharing the weight. Each square inch has a total pressure of
0.433 lb (62.4 4 144) resting on it. Thus, the pressure at the
bottom of the cube is 0.433 psi, figure 1.18.
A volume of 1 ft3 is equivalent to 1728 in3 (12 inches 3
12  inches 3 12  inches 5 1728  in3), so the dimensions of
the container can be changed as long as the volume remains
the same. If the same 1 ft3 of water had the shape shown in
figure 1.19 (24 inches 3 24 inches 3 3 inches 5 1728 in3),
the pressure at the bottom would be different. The area at the
bottom would be 576 in2, since the base is 24 in. by 24 in.

Figure 1.16 Both weights are resting on a 1-square-inch (1-in2) surface.
One weight exerts a pressure of 1 psi; the other a pressure of 100 psi.

Figure 1.18 One cubic foot (1 ft3) of water exerts a downward pressure
of 62.4 lb/ft2 on the bottom surface area of a cube.

100 LB

1 LB

1 IN.

12 IN.

1 IN.

12 IN.

Figure  1.17 One cubic foot (1 ft3) of water (7.48 gal) exerts pressure
outward and downward. 1 ft3 of water weighs 62.4 lb spread over 1 ft2.
Figure 1.19 If the bottom surface is 4 ft2, or 576 in2, the downward pressure is 0.108 psi.

1 CUBIC FOOT OF WATER
3 IN.

24 IN.

WATER IN CUBIC
FOOT CONTAINER

24 IN.
1 FT

1 FT



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
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