Psychrometry

1. Talking Points: Psychrometry

2. Definition and Key Elements
• Psychrometry is the study of the physical
and thermal properties of air and water
vapor mixtures.
• The seven key physical and thermal
characteristics used to describe
psychrometry are: dry bulb temperature,
humidity ratio, relative humidity, enthalpy,
dew-point temperature, wet bulb
temperature, and specific volume.

3. Common Examples
• When the body sweats to cool itself, it is
using psychrometry.

4. Definitions
• Dry Bulb Thermometer-regular
thermometer.
• Humidity ratio. Saturated air is air holding
100% of the water possible.
– For every 20 degrees Fahrenheit temperature
increase, the holding capacity of the air
doubles. (The inside of a hot car is often 20
degrees more than the outside air.)

5. Definitions
• Relative humidity is a percentage of the
saturated air content.
• Enthalpy: Heat energy content of the
air/water vapor mixture.
– The air’s energy content changes if either or
both the dry bulb and humidity ratio changes.
• Dew Point: The temperature at which
moisture starts to condense from the air.

6. Definitions
• Wet-bulb temperature: measured using a
thermometer whose bulb or sensor is covered
with a moistened wick in a moving air stream.
– The difference between wet and dry bulb
temperatures is important in evaporative cooling.
• Specific volume: the volume in cubic feet
occupied by a pound of dry ear at a specific dry
bulb temperature and pressure.

7. Keys to defined variables
• Knowing only two of the seven outlined
definitions will lead to knowing all seven
variables using the psychrometric chart.
– A typical local weather forecast usually
includes the dry bulb temperature in degrees
Fahrenheit, dew point in degrees Fahrenheit,
and the humidity measured in inches of
mercury.

8. Common Sense Answers
• Evaporative cooling works even in humid areas,
particularly in the afternoon.
– As the temperature rises, so too does the amount of
water that can be carried.
• Water temperature has little effect on
evaporative cooling.
– Placing a gallon of 50 degree water on 90 degree
cement produces 9000 Btu’s of cooling
– Placing a gallon of 90 degree water on 90 degree
cement produces 8700 Btu’s of cooling
• A difference of only three percent.

9. Common Sense Answers
• The key to getting the most out of any
evaporative cooling system is to maximize
the amount of air that comes into contact
with the moisture added.

10. Psychrometry
• Psychrometry is the study of the physical
and thermal properties of air and water
vapor mixtures.
– “The key to getting the most out of any
evaporative cooling system is to maximize the
amount of air that comes into contact with the
moisture added.”
– “Evaporative cooling works even in humid
areas, particularly in the afternoon.”
• As the temperature rises, so too does the amount of water that
can be carried.
Source: University of Kentucky