Psychrometrics deals with thermodynamic properties of moist air and uses these properties to analyze conditions and process involving moist air. Atmospheric air contains many gases components as well as water vapor and miscellaneous contaminants (e.g., smoke, pollen and gaseous pollutants). Dry air exists when all water vapor and contaminants have been removed from atmospheric air. The composition of dry air is relatively constant, but small variations in the amounts of individual components occur with time, geographic location, and altitude. The apparent molecular mass or weighted average molecular weight of all components, for dry air is 28.9645, based on the carbon-12 scale. The gas constant for dry air, based on the carbon-12 scale is 1545.32/28.9645 =53.352 ft lbf / lbm ^oR.

Moist air is a binary mixture of dry air and water vapor. The amount of water vapor in moist air varies from zero (dry air) to a maximum that depends on temperature and pressure. The latter condition refers to saturation, a state of neutral equilibrium between moist air and the condensed water phase. Unless otherwise stated, saturation refers to a flat interface surface between the moist air and the condensed phase. The molecular weight of water is 18.01528 on the carbon-12 scale. The gas constant for water vapor is 1545.32/18.01528 = 85.778 ft lbf / lbm ^oR

The temperature and barometric pressure of atmospheric air vary considerably with altitude as well as with local geographic and weather conditions. The standard atmospheric gives a standard of reference for estimating properties at various altitudes. At sea level, standard temperature is 59^oF; standard barometric pressure is 29.921 inch Hg. The temperature is assumed to decrease linearly with increasing altitude throughout the troposphere (lower atmosphere), and to be constant in the lower reaches of the stratosphere. The lower atmosphere is assumed to constant of dry air that behaves as a perfect gas. Gravity is also assumed constant at the standard value, 32.1740 ft/s².

Humidity ratio (alternatively, the moisture content or mixing ratio) is defined as the ratio of the mass of water vapor to the mass of dry air. Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air. Absolute humidity (alternatively, water vapor density) is the ratio of the mass of water vapor to the total volume of the moist air. Saturation humidity ratio is the humidity ratio of moist air saturated with respect to water at the same temperature and pressure. Degree of saturation is the ratio of the air humidity ratio to humidity ratio of saturated air at the same temperature and pressure. Relative humidity is the ratio of the mole fraction of water vapor in a given moist air to the mole fraction in an air saturated at the same temperature and pressure.

The enthalpy of a mixture air is the sum of the individual partial enthalpies for dry air and for saturated water vapor at the temperature of the mixture.

Example 1-1. Calculate the air density, specific volume, and enthalpy in US units at the ambient conditions of DBT 87.8^oF, RH 80% and sea level.

• Air Density: 0.0714 Lb/ft³
• Air Specific Volume: 14.3309 ft³/Lb dry air
• Air Enthalpy: 46.3774 Btu/Lb dry air

Download the example file (exe1_1.zip)
This file covers the examples of 1-1 through 1-4.

Example 1-2. Calculate the air density, specific volume, and enthalpy in US at the ambient conditions of DBT 87.8^oF, RH 0% (Dry Air), and sea level.

• Air Density: 0.0723 Lb/ft³
• Air Specific Volume: 13.8224 ft³/Lb dry air
• Air Enthalpy: 21.1196 Btu/Lb dry air

Example 1-3. Calculate the air density, specific volume, and enthalpy in US at the ambient conditions of DBT 87.8^oF, RH 100%, and sea level.

• Air Density: 0.0711 Lb/ft³
• Air Specific Volume: 14.4639 ft³/Lb dry air
• Air Enthalpy: 52.9849 Btu/Lb dry air

Example 1-4. Calculate the air density, specific volume, and enthalpy in US at the ambient conditions of DBT 87.8^oF, RH 80%, and 1,000 feet in altitude.

• Air Density: 0.0688 Lb/ft³
• Air Specific Volume: 14.8824 ft³/Lb dry air
• Air Enthalpy: 47.3494 Btu/Lb dry air

Example 1-5. Find a relative humidity which the relationship of 1/ air density = specific volume is established at an ambient condition of DBT 87.8^oF and sea level.

• Air Density: 0.0723 Lb/ft³
• 1 / Air Density: 1 / 0.0723 = 13.8224 ft³/Lb dry air
• Air Specific Volume: 13.8224 ft³/Lb dry air

The relationship of 1/air density = specific volume is only valid at the point that the relative humidity is zero. That is, only valid for the dry air condition.

Download the example file (exe1_5.zip)