Chapter 3: PSYCHROMETRY
Eng. Miral Al-Shraideh
Department of Renewable Energy
Al al-Bayt University
What is Psychrometry
Psychrometry is the study of the properties of mixtures of dry air and
water vapor.
Why study Psychrometry?
To be able to analyze various processes involving moist air.
Example: if the air is warm and wet, what will be the air-conditioning
design required
Basic Definitions
Dry air
The dry air is a complex mixture of several gases such as nitrogen,
oxygen, carbon dioxide and other gases such as argon , carbon
monoxide and neon. It does not contain water vaper
 Moist air
The moist air is mechanical mixture of dry air and water vaper. Thus,
when the air is cooled, it looses moisture due to condensation of the
water vapor in the air
Dry Bulb Temperature
Dry bulb temperature is the temperature of the moist air as measured
by a standard thermometer or other temperature measuring
instruments.
Wet Bulb Temperature
Air temperature measured using a wetted thermometer bulb is known
as wet bulb temperate.
Relative Humidity
The amount of moisture that air holds relative to the maximum
amount of moisture the air can hold at the same temperature.
Relative humidity is normally expressed as a percentage. When Φ is
100 percent, the air is saturated
Humidity Ratio (w)
Is the mass of water vapor associated with each kilogram of dry air.
the humidity ratio is given by:
mv – Mass of vapor
ma – Mass of dry air
Units – g / kg
Dew-Point Temperature
he temperature of saturated moist air at the same pressure and
humidity ratio of a given air mixture.
This is the temperature at which condensation begins when the air is
cooled at constant pressure
Specific Volume
The specific volume is defined as the number of cubic meters of moist
air per kilogram of dry air.
Unit – m³ /kg of dry air
Enthalpy
Is equal to the sum of the total enthalpy of dry air and that of the water
vapor.
Psychrometry Chart
A
Psychrometric
chart
is
graphical
representation
thermodynamic properties of moist air.
Need two quantities for a state point
– Can get all other quantities from a state point
of
the
Example 1
Using the Carrier’s psychrometric chart, Find the wet bulb temperature,
humidity ratio, dew point temperature and specific volume of dry air at
24˚C dry bulb temperature and 50% relative humidity.
Example 2
The inside design condition of a room is 24 C dry bulb temperature and
60 % relative humidity. Find the following by using Carrier’s Chart
1. Humidity ratio
2. Specific enthalpy
3. Dew point temperature
4. Specific volume of the dry air