Important psychrometric properties

Dry bulb temperature (DBT) is the temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments.

Saturated vapour pressure (psat) is the saturated partial pressure of water vapour at the dry bulb temperature. This is readily available in thermodynamic tables and charts. ASHRAE suggests the following regression equation for saturated vapour pressure of water, which is valid for 0 to 100o C.

where psat = saturated vapor pressure of water in kiloPascals

T = temperature in K

The regression coefficients cto c6 are given by:

c1 = -5.80022006E+03, c2 = -5.516256E+00, c3 = -4.8640239E-02 c4 = 4.1764768E-05, c5 = -1.4452093E-08, c6 = 6.5459673E+00

Relative humidity (Φ) is defined as the ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour in saturated air at the same temperature and pressure. Using perfect gas equation we can show that:

Relative humidity is normally expressed as a percentage. When Φ is 100 percent, the air is saturated.

Humidity ratio (W): The humidity ratio (or specific humidity) W is the mass of water associated with each kilogram of dry air. Assuming both water vapour and dry air to be perfect gases, the humidity ratio is given by:

Substituting the values of gas constants of water vapour and air Rv and Ra in the above equation; the humidity ratio is given by:

For a given barometric pressure pt, given the DBT, we can find the saturated vapour pressure psat from the thermodynamic property tables on steam. Then using the above equation, we can find the humidity ratio at saturated conditions, Wsat.

It is to be noted that, W is a function of both total barometric pressure and vapor pressure of water.

Dew-point temperature: If unsaturated moist air is cooled at constant pressure, then the temperature at which the moisture in the air begins to condense is known as dew-point temperature (DPT) of air. An approximate equation for dew-point temperature is given by:

where Φ is the relative humidity (in fraction). DBT & DPT are in o C.

Of course, since from its definition, the dew point temperature is the saturation temperature corresponding to the vapour pressure of water vapour, it can be obtained from steam tables or by using below equation


Degree of saturation μ: The degree of saturation is the ratio of the humidity ratio W to the humidity ratio of a saturated mixture Ws at the same temperature and pressure, i.e.,

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