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6.12 2001 ASHRAE Fundamentals Handbook HUMIDITY PARAMETERS

Basic Parameters

Humidity ratio (alternatively, the moisture content or mixing

ratio) W of a given moist air sample is defined as the ratio of the

mass of water vapor to the mass of dry air contained in the sample: (7)

The humidity ratio W is equal to the mole fraction ratio x w/x da multiplied by the ratio of molecular masses, namely, 18.01528/28.9645

= 0.62198:

(8)

Specific humidity © is the ratio of the mass of water vapor to the

total mass of the moist air sample:

(9a)

In terms of the humidity ratio,

(9b)

Absolute humidity (alternatively, water vapor density) d v is the

ratio of the mass of water vapor to the total volume of the sample: (10)

The density 〉 of a moist air mixture is the ratio of the total mass

to the total volume:

(11)

where v is the moist air specific volume, ft3/lb da, as defined by Equation (27).

Humidity Parameters Involving Saturation

The following definitions of humidity parameters involve the

concept of moist air saturation:

Saturation humidity ratio W s (t, p) is the humidity ratio of

moist air saturated with respect to water (or ice) at the same temperature t and pressure p.

Degree of saturation ⎧ is the ratio of the air humidity ratio W to

the humidity ratio W s of saturated moist air at the same temperature

and pressure:

(12)

Relative humidity ⎫ is the ratio of the mole fraction of water

vapor x w in a given moist air sample to the mole fraction x ws in an air sample saturated at the same temperature and pressure:

(13)

Combining Equations (8), (12), and (13),

(14)

Dew-point temperature t d is the temperature of moist air saturated

at the same pressure p, with the same humidity ratio W as that

of the given sample of moist air. It is defined as the solution t d(p, W)

of the following equation:

(15)

Thermodynamic wet-bulb temperature t* is the temperature

at which water (liquid or solid), by evaporating into moist air at a

given dry-bulb temperature t and humidity ratio W, can bring air to saturation adiabatically at the same temperature t* while the total pressure p is maintained constant. This parameter is considered separately in the section on Thermodynamic Wet-Bulb Temperature

and Dew-Point Temperature.

PERFECT GAS RELATIONSHIPS

FOR DRY AND MOIST AIR

When moist air is considered a mixture of independent perfect

gases (i.e., dry air and water vapor), each is assumed to obey the perfect gas equation of state as follows:

(16)

(17)

where

p da = partial pressure of dry air