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| Fig. 1-18. The temperature-pressure curve for water. At atmospheric pressure, water boils at 212 F. At point (A)which is at a pressure of 24" Hg water boils at 142 F. Increasing the pressure above atmospheric increases the boiling temperature. At point B, which is at a pressure of 30 psi, the boiling temperature is 272 F. |
The boiling temperature for any liquid depends upon the pressure under which the liquid is placed. Water normally boils at 212 F., but if the pressure on the surface of the water is increased to 100 pounds per square inch gauge, the boiling point will be raised to 338 F. If instead of increasing the pressure it is decreased to an absolute pressure of 3 pounds per square inch, (24 in. Hg. vacuum) the water will boil at 142 F. as shown in Fig. 1-18.
It is the effect of reduced pressure on the boiling temperature of certain liquids that makes the operation of the mechanical and absorption refrigerator possible. As an illustration of this, the refrigerant R-12 boils at -21.7 F. under atmospheric pressure while at 10 inches of vacuum, the boiling temperature is -38.5 F. See Fig. 9-6 for a graph of the temperature characteristics of R-12.
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Basics of Refrigeration