BOILING POINT
The temperature at which a liquid boils is not constant, but varies with the pressure. Thus, while the boiling point of water is commonly taken as 100°C, this is only true at a pressure of one standard atmosphere (1.013 bar) and, by varying the pressure, the boiling point can be changed (Table 1.1) . This pressure-temperature property can be shown graphically (see Figure 1.4).
|
Table 1.1 |
|
|
Pressure (bar) |
Boiling point (°C) |
|
0.006 |
0 |
|
0.04 |
29 |
|
0.08 |
41.5 |
|
0.2 |
60.1 |
|
0.5 |
81.4 |
|
1.013 |
100.0 |
|
Figure 1.4 Change of state with pressure and temperature |
The boiling point of a substance is limited by the critical temperature at the upper end, beyond which it cannot exist as a liquid, and by the triple point at the lower end, which is at the freezing temperature. Between these two limits, if the liquid is at a pressure higher than its boiling pressure, it will remain a liquid and will be subcooled below the saturation condition, while if the temperature is higher than saturation, it will be a gas and superheated. If both liquid and vapour are at rest in the same enclosure, and no other volatile substance is present, the condition must lie on the saturation line.
At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas.
The liquid zone to the left of the boiling point line is subcooled liquid. In refrigeration the term saturation is used to describe the liquid/vapour boundary, saturated vapour being represented by a condition on the line and superheated vapour below the line. More information on saturated properties for commonly used refrigerants is given in Chapter 3.
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