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Vapor Pressure Moist Air Wet Dry Bulb Temperature

Reference data and engineering information about vapor pressure moist air wet dry bulb temperature for air psychrometrics applications.

vaporpressuremoistair

Overview

Engineering reference data for Vapor Pressure Moist Air Wet Dry Bulb Temperature in air psychrometrics.

Key Formulas

Humidity Ratio

ω=0.622PvPa\omega = 0.622 \frac{P_v}{P_a}

Mass of water vapor per mass of dry air.

Relative Humidity

ϕ=PvPvs×100%\phi = \frac{P_v}{P_{vs}} \times 100\%

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

Twb=Tdb−Pvs−PvγT_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

h=cpT+ωhgh = c_p T + \omega h_g

Sensible + latent heat per unit mass of dry air.

Variables

SymbolDescriptionUnit
ω\omegaHumidity ratiokg/kg
ϕ\phiRelative humidity%
PvP_vVapor pressurePa
PvsP_{vs}Saturation vapor pressurePa
TdbT_{db}Dry bulb temperature°C
TwbT_{wb}Wet bulb temperature°C

Chart Interpretation

The provided charts illustrate the relationship between dry bulb temperature, wet bulb temperature, and the resulting vapor pressure of water in moist air. This relationship is fundamental to psychrometry and is used for humidity calculations.

How to Read the Charts:

  1. Locate your measured Dry Bulb Temperature on the horizontal axis.
  2. Locate your measured Wet Bulb Temperature on the diagonal lines or secondary axis.
  3. The intersection point projects onto the vertical axis to give the Vapor Pressure in the air.

Key Psychrometric Relationship: The wet bulb temperature is always less than or equal to the dry bulb temperature. The difference (T_dry - T_wet) is known as the wet-bulb depression. A larger depression indicates drier air. When the air is fully saturated, the wet bulb, dry bulb, and dew point temperatures are all equal.

Practical Application

This data is essential for:

  • HVAC System Design: Calculating latent cooling loads and sizing dehumidifiers.
  • Process Engineering: Determining conditions for drying, evaporation, and moisture control.
  • Weather Analysis: Interpreting raw meteorological data to find humidity levels.

Example: For a dry bulb temperature of 30°C and a wet bulb temperature of 25°C, you can use the chart (in kPa or inches Hg) to find the vapor pressure, which can then be used to calculate the relative humidity or absolute humidity using additional psychrometric formulas.

References

Source: engineeringtoolbox.com