PSYCHROMETRICS
(Much of this material was taken from the Course
Manual and Lecture Notes AGRICULTURAL STRUCTURES & ENVIRONMENT
for AgEngr 351, Spring Semester 1994, by Dave Bohnhoff, University of
Wisconsin.)
Definition:
Psychrometrics is the study of the physical and thermodynamic
properties of air-water vapor mixtures.
Two governing laws which are important in understanding the
relationshipships between psychrometric properties are
Dalton's Law and the Ideal
Gas Law. The most commonly used terms to describe air-water vapor
properties are temperature and humidity. There are actually three
measures of air temperature that should be considered; these are
dry-bulb temperature,
wet-bulb temperature and
dewpoint temperature . The moisture content in
air is often described by relative humidity.
Other terms that relate to moisture content are
humidity ratio, degree of
saturation, vapor pressure and
wet-bulb depression. The
heat energy content or
enthalpy of air includes
sensible and latent
heat. Specific heat relates the change in
temperature and the change in heat energy content. A final air
property that is important in the study of environmental control is
specific volume. The relationships of these
properties may be described by
equations or depicted by a
psychrometric chart.
Changes in air-water vapor properties are brought about by
psychrometric processes. The processes that are used to that are used
in environmental control, or that occur naturally, or are used in
crop drying are: heating, cooling,
humidifying, dehumidifying ,
evaporative cooling, adiabatic
drying, and mixing.
Frequently these individual processes do not occur alone, but rather
in combination. Livestock in a confined space add heat and moisture
or cause the combined heating and humidifying
processes. A typical home air-conditioner brings about both
cooling and dehumidifying.
