Dalton’s Law of partial pressures considers a mixture of two or more gases, and states that the total pressure of the mixture is equal to the sum of the indi­vidual pressures, if each gas separately occupied the space.

Example 1.7

A cubic metre of air contains 0.906 kg of nitrogen of specific gas constant 297 J/(kg K),

0. 278 kg of oxygen of specific gas constant 260 J/(kg K) and 0.015 kg of argon of specific gas constant 208 J/(kg K). What will be the total pressure at 20°C?

PV = mRT

V = 1 m3 so p = mRT

For the nitrogen pn = 0.906 x 297 x 293.15 = 78 881 Pa

For the oxygen pO = 0.278 x 260 x 293.15 = 21 189 Pa

For the argon pA = 0.015 x 208 x 293.15 = 915 Pa

Total pressure = 100 985 Pa

(1.009 85 bar)

The properties of refrigerant fluids at the pressures and temperatures of inter­est to refrigeration engineers exhibit considerable deviation from the ideal gas laws. It is therefore necessary to use tabulated or computer-based information for thermodynamic calculations.

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