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Psychrometric Terms

Reference data and engineering information about psychrometric terms for air psychrometrics applications.

psychrometricterms

Overview

Engineering reference data for Psychrometric Terms 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

Psychrometric Chart Overview

The psychrometric chart visualizes thermodynamic properties of moist air, allowing engineers to determine air conditioning processes. With at least two known properties, the state-point can be located, and all other properties can be read directly at that intersection.

Common Psychrometric Properties

  • Dry-bulb temperature (Tdb): Standard air temperature measured with a regular thermometer. Determines sensible heat content along the bottom axis.
  • Wet-bulb temperature (Twb): Associated with moisture content, measured with a moistened-wick thermometer. Slopes upward to the left on the chart, read at the saturation line.
  • Relative humidity (RH): Ratio of current water vapor mass to maximum possible at that temperature/pressure, expressed as a percentage. Lines curve upward to the right.
  • Humidity ratio (x): Mass of water vapor per unit mass of dry air (kg/kg or lb/lb). Indicated along the right-hand axis.
  • Specific volume (v): Volume per unit mass of dry air (m³/kg or ft³/lb). Lines slant upward to the left from the bottom axis.
  • Dew point temperature (Tdp): Temperature at which water vapor begins to condense (100% saturation). Found by following a horizontal line from the state-point to the saturation curve.
  • Enthalpy (h): Total thermal energy per unit mass of air (kJ/kg or Btu/lb). Read where the wet-bulb line crosses the diagonal scale above saturation.

Important Psychrometric Relationships

  1. At saturation (RH = 100%), the dry-bulb, wet-bulb, and dew-point temperatures are identical.
  2. The moisture-holding capacity of air increases dramatically with temperature, which is critical for drying processes.
  3. Air with the same enthalpy can be either hotter/drier (higher sensible heat) or cooler/moister (higher latent heat).

References

Source: engineeringtoolbox.com