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CMGT 235
Chapter 9 Psychrometrics
Discussion #12
PROPERTITIES OF AIR
Atmospheric Air is the mixture of dry air, moisture, and particles.
The properties of air are the characteristics of air:
Temperature
Humidity
Enthalpy
Volume
Temperature
Temperature is the measurement of the intensity of heat.
Measured with a dry bulb thermometer
Dry bulb temperature (db) is the measurement of sensible heat in °F or °C
Humidity
Humidity is the moisture (water vapor) in the air.
Relative Humidity (rh) is the amount of moisture in the air compared to the amount it would hold if the air were
saturated. It is always expressed as a percentage.
Humidity Ratio (W) is the ratio of the mass (weight) of the moisture in a quantity of air to the mass of the air
and moisture together. It is expressed in grains (gr) of moisture per pound of dry air (gr/lb) or in pounds of
moisture per pound of dry air (lb/lb).
Humidity represents latent heat.
Wet Bulb Temperature
Wet Bulb Temperature (wb) is a measurement of the amount of
moisture in the air.
Dew Point (dp) is the temperature below which the moisture in
the air begins to condense.
At dew point, the air is saturated with moisture and the dry and
wet bulb temperatures are the same.
Humidity Measurement Equipment
A psychrometer is an instrument used for measuring humidity
that consists of a dry bulb thermometer and a wet bulb
thermometer mounted on a common base.
A psychrometer measures humidity by comparing the
temperature readings on the dry bulb and wet bulb
thermometers.
A sling psychrometer is an instrument used for measuring
humidity that consists of a wet bulb thermometer and a dry bulb
thermometer mounted on a common base. The base is mounted
on a handle so it can be rotated rapidly.
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Enthalpy
Enthalpy (h) is the total heat contained in a substance measured from a baseline of 32°F and is the sum of
sensible heat and latent heat.
Enthalpy of air is expressed in Btu per pound of moist air. The enthalpy of air at different conditions is found on
a psychrometric chart.
Volume
Air, like most substances, expands when heated and contracts when cooled.
Volume (V) is the amount of space occupied by a three-dimensional figure. It is expressed in cubic units such as
cubic inches or cubic feet.
Specific volume (v) is the volume of a substance per unit of the substance.
The specific volume of air is expressed in cubic feet per pound at a given temperature.
Standard Conditions
The properties of air are related so that a change in one of the properties causes a change in the other three.
Standard conditions are values used as a reference for comparing properties of air at different elevations and
pressure.
One pound of dry air and its associated moisture at standard conditions has a pressure of 29.92" Hg (14.7 psia).
Temperature of 68°F, volume of 13.33 cu ft/lb, and density of 0.0753 lb/cu ft.
Relationship between Properties
The two properties used most often for identifying specific condition of the air are temperature and humidity.
A change in wet bulb temperature changes the humidity ratio and the relative humidity of the air. A change in
wet bulb temperature indicates that moisture has been added to or removed from the air. Wet bulb
temperature, humidity ratio, and relative humidity are all directly related to the amount of moisture in the air. A
change in one property changes the others.
A change in both dry bulb and wet bulb temperatures affects enthalpy, which is total heat content. A change in
dry bulb temperature indicates a change in sensible heat. A change in wet bulb temperature indicates a change
in latent heat.
A change in dry bulb temperature affects the specific volume of the air. If the dry bulb temperature increases,
the specific volume increases. If the dry bulb temperature decreases, the specific volume will decrease.
When considering the effect of changes in humidity on other properties of air, the humidity ratio must be
considered. As the humidity ratio increases, the latent heat content of the air also increases. As the humidity
ratio decreases, the latent heat content of the air also decreases.
PSYCHROMETRIC CHART
A psychrometric chart is a chart that defines the condition of the air for any given property.
The properties of air for any condition can be determined by using a psychrometric chart.
Using the Psychrometric Chart
The properties of air found on a psychrometric chart are dry bulb temperature, relative humidity, humidity ratio,
wet bulb temperature, dew point, enthalpy, and specific volume. The properties of the air define the condition
of the air.
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