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PSYCHROMETRIC CHART FUNDAMENTALS
INTRODUCTION
This section provides information on use of the psychrometric
chart as applied to air conditioning processes. The chart provides
a graphic representation of the properties of moist air including
wet- and dry-bulb temperature, relative humidity, dew point,
moisture content, enthalpy, and air density. The chart is used to
plot the changes that occur in the air as it passes through an air
handling system and is particularly useful in understanding these
DEFINITIONS
To use these charts effectively, terms describing the
thermodynamic properties of moist air must be understood.
Definition of these terms follow as they relate to the
psychrometric chart. Additional terms are included for devices
commonly used to measure the properties of air.
Adiabatic process: A process in which there is neither loss
nor gain of total heat. The heat merely changes from
sensible to latent or latent to sensible.
Density: The mass of air per unit volume. Density can be
expressed in kilograms per cubic meter of dry air.
This is the reciprocal of specific volume.
Dew point temperature: The temperature at which water
vapor from the air begins to form droplets and settles
or condenses on surfaces that are colder than the dew
point of the air. The more moisture the air contains,
the higher its dew point temperature. When dry-bulb
and wet-bulb temperatures of the air are known, the
dew point temperature can be plotted on the
psychrometric chart (Fig. 4).
Dry-bulb temperature: The temperature read directly on an
ordinary thermometer.
Isothermal process: A process in which there is no change of
dry-bulb temperature.
Joule (J): The unit of measure for energy, work, and heat. This
section uses joule as a unit of heat where 4.2 joules
will raise the temperature of 1 gram of water 1 kelvin.
Latent heat: Heat that changes liquid to vapor or vapor to
liquid without a change in temperature or pressure of
the moisture. Latent heat is also called the heat of
vaporization or condensation. When water is
vaporized, it absorbs heat which becomes latent heat.
When the vapor condenses, latent heat is released,
usually becoming sensible heat.
changes in relation to the performance of automatic HVAC control
systems. The chart is also useful in troubleshooting a system.
For additional information about control of the basic
processes in air handling systems, refer to the Air Handling
System Control Applications section.
Moisture content (humidity ratio): The amount of water
contained in a unit mass of dry air.
Relative humidity: The ratio of the measured amount of
moisture in the air to the maximum amount of moisture
the air can hold at the same temperature and pressure.
Relative humidity is expressed in percent of saturation.
Air with a relative humidity of 35, for example, is
holding 35 percent of the moisture that it is capable of
holding at that temperature and pressure.
Saturation: A condition at which the air is unable to hold any
more moisture at a given temperature.
Sensible heat: Heat that changes the temperature of the air
without changing its moisture content. Heat added to
air by a heating coil is an example of sensible heat.
Sling psychrometer: A device (Fig. 1) commonly used to
measure the wet-bulb temperature. It consists of two
identical thermometers mounted on a common base.
The base is pivoted on a handle so it can be whirled
through the air. One thermometer measures dry-bulb
temperature. The bulb of the other thermometer is
encased in a water-soaked wick. This thermometer
measures wet-bulb temperature. Some models provide
slide rule construction which allows converting the
dry-bulb and wet-bulb readings to relative humidity.
Although commonly used, sling psychrometers can
cause inaccurate readings, especially at low relative
humidities, because of factors such as inadequate air
flow past the wet-bulb wick, too much wick wetting
from a continuous water feed, thermometer calibration
error, and human error. To take more accurate readings,
especially in low relative humidity conditions,
motorized psychrometers or hand held electronic
humidity sensors are recommended.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
38
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