PRESSURE in REFRIGERATION

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PRESSURE in REFRIGERATION

Pressure is defined as force per unit of area. This is normallyÂ expressed in pounds per square inch (psi). Simply stated,Â when a 1-lb weight rests on an area of 1 square inch (1 in2),Â the pressure exerted downward is 1 psi. Similarly, when aÂ 100-lb weight rests on a 1-in2 area, 100 psi of pressure isÂ exerted, Figure 1. If the 100-lb weight rested on a surfaceÂ that was 100 in2, the pressure exerted would be 1 psi.

When you swim under the surface of the water, you feelÂ a pressure pushing inward on your body. This pressure isÂ the result of the weight of the water and is very real. YouÂ would feel a different sensation when flying in an airplaneÂ without a pressurized cabin. Your body would be subjectedÂ to less pressure instead of more, yet you would still feel uncomfortable.Â It is easy to understand why the discomfortÂ under water exists, the weight of the water pushes in. In theÂ airplane, the situation is just the reverse. There is less pressureÂ high in the air than down on the ground. The pressureÂ is greater inside your body and is pushing out.Â Water weighs 62.4 pounds per cubic foot (lb/ft3). A cubicÂ foot (7.48 gal) exerts a downward pressure of 62.4 lb/ft2Â when in actual cube shape, Figure 2. How much weightÂ is then resting on 1 in2? The answer is simply calculated.Â The bottom of the cube has an area of 144 in2 (12 in. *Â 12 in.) sharing the weight. Each square inch has a total pressureÂ of 0.433 lb (62.4 / 144) resting on it. Thus, the pressureÂ at the bottom of the cube is 0.433 psi, Figure 3.

A volume of 1 ft3 is equivalent to 1728 in3 (12 inches * 12 inches * 12 inches = 1728 in3), so the dimensions ofÂ the container can be changed as long as the volume remainsÂ the same. If the same 1 ft3 of water had the shape shown in
Figure 1.19 (24 inches * 24 inches * 3 inches = 1728 in3),Â the pressure at the bottom would be different. The area atÂ the bottom would be 576 in2, since the base is 24 in. byÂ 24 in. (24 in. * 24 in. = 576 in2). The resulting pressureÂ would then be only 0.108 psi (62.4 / 576). As the baseÂ gets larger, the pressure it exerts in the downward directionÂ gets lower. Have you ever seen someone walking across theÂ snow wearing snowshoes? The purpose of the snowshoes isÂ to increase the size of the base to lower the pressure pushingÂ down. This reduces the chance of sinking into the snow.

Copied fromÂ REFRIGERATION &Â AIR CONDITIONINGÂ TECHNOLOGY byÂ WILLIAM C. WHITMAN,Â WILLIAM M. JOHNSON,Â JOHN A. TOMCZYK andÂ EUGENE SILBERSTEIN