Pyschrometric chart

From Chemepedia
Psychrometric chart
Psychrometric chart.png

A psychrometric chart is a graphical representation of the psychrometric processes of air. Psychrometric processes include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, enthalpy, and air density.[1] For a specific location,information about wet and dry temperatures and humidity (relative and absolute) can be known from the chart. A psyhrometric process invovles no heat or loss during the mixing of two air streams.

Dry Bulb Temperature (DBT)

The dry-bulb temperature (DBT) is air temperature measured by a thermometer freely exposed to the air, but shielded from radiation and moisture and is the actual temperature a dry thermometer indicate of a mixture of an air-water vapor.It is the true thermodynamic temperature which is not affected by the moisture.It indicates the amount of heat in the air and is directly proportional to the mean kinetic energy of the air molecules. Temperature is usually measured in degrees Celsius (°C), kelvins (K), or degrees Fahrenheit (°F). In psychrometric chart, constant dry bulb temperature appears as vertical lines.

Adiabatic Saturation Temperature

The adiabaitc saturation temperature is the steady-state temperature when a large amount of water is contacted by the entering gas.[3]

Wet Bulb Temperature (WBT)

The wet bulb temperature is the steady-state nonequilibrium temperature reached when a small amount of water is contacted under adiabatic saturation condition by a continous stream of gas.[3]

The thermometer with the bulb kept in wet cloth or wick and immersed in a flowing stream of air-water vapor can measure wet bulb temperture. The flowing stream has dry bulb temperature and humidity.The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.[4] The latent heat of evaporation(heat loss) is balanced by the convective heat gain flowing from the stream at dry bulb temperature to the temperature of water around the wick.

The Wet Bulb temperature lies always between the Dry Bulb temperature (air temperature) and the dew point temperature(adiabatic saturation temperature).

Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric chart.[3]

Dew Point Temperature

Dew point temperature is the temperature at which the mixture of air and water vapor is saturated. If a mixture of air and water vapor is cooled enough to dew point temperature, the mixture is saturated. On further cooling, some water vapor condense as the partial pressure is not greater than saturation vapor pressure.



Humidity ratio lies on horizontal lines(right axis).

Psychrometric Ratio

The psychrometric ratio is the ration of the heat transfer cofficient to the product of mass transfer coefficient and humid heat at a wetted surface.

r= psychrometric ratio

hc=convective heat transfer coefficient

ky =convective mass transer coefficient

cs=humid heat

How to read Psychrometric chart

[1] [2] [3] [4]