Honeywell AUTOMATIC CONTROL, Engineering Manual

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PSYCHROMETRIC CHART FUNDAMENTALS
38
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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
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.
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.
British thermal unit (Btu): The amount of heat required to
raise one pound of water one degree Fahrenheit.
Density: The mass of air per unit volume. Density can be
expressed in pounds per cubic foot 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.
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.
Moisture content (humidity ratio): The amount of water
contained in a unit mass of dry air. Most humidifiers
are rated in grains of moisture per pound of dry air
rather than pounds of moisture. To convert pounds to
grains, multiply pounds by 7000 (7000 grains equals
one pound).
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.