In each case, air provides the link between the controls and the mechanical equipment, and air properties influence the results of the processes. Psychrometrics refers to the properties of moist air. A psychrometric chart graphically illustrates the relationships between air temperature and relative humidity as well as other properties. A better understanding of air properties and the psychrometric chart can aid in the selection and management of a livestock building ventilation system, a grain drying system, or a home humidifier. This is the air we breathe and use for ventilation.
Dry air exists when all of the contaminants and water vapor have been removed from atmospheric air.
Dry air is used as the reference in psychrometrics. Moist air is a mixture of dry air and water vapor. Due to the variability of atmospheric air, the terms dry air and moist air are used in psychrometrics. For practical purposes, moist air and atmospheric air can be considered equal under the range of conditions normally encountered. The Psychrometric Chart Figure 1.