Air Cond: Psychrometric

Moist air can be treated as binary mixture, composed of:

Dry air – essentially mixture of nitrogen, oxygen and traces of argon, carbon dioxide, neon etc.

Water vapour – which exists at low partial pressure. At low low atmospheric pressure and temperature, water vapour can be treated as ideal gas.

Assumptions.

(1) vapour contains no dissolved gases

(2) gaseous phases can be treated as ideal gas

(3) no interaction between components and all components are at the mixture temperature

Psychrometric properties All properties are based on the “dry-air part”. Consider a volume of moist air

Barometric pressure = 101.325 kPa Gibbs phase rule, F = 2 + 2 – 1 = 3 (ie need 3 intensive properties to define the state). Since pressure is fixed, need 2 other properties:

(a) dry bulb temperature, TDB …………… easily measured

(b) moisture content, w

(c) dew point temperature, TDP

(d) percentage saturation,

(e) relative humidity,

(f) specific enthalpy, h

(g) specific volume, v

(h) wet bulb temperature, TWB ……….…….usually measured

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