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Mollier Psychrometric Chart

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

mollierpsychrometricchartData Table

Overview

Engineering reference data for Mollier Psychrometric Chart 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

Mollier Diagram vs Psychrometric Chart

The Mollier diagram and psychrometric chart are two different graphical representations of the same psychrometric properties. They are both essential tools in HVAC engineering, drying process design, and air conditioning system calculations.

Transformation Between Representations

A Mollier diagram can be converted to a psychrometric chart (or vice versa) through a two-step geometric transformation:

  1. Reflection — Reflect the diagram across a vertical mirror (flip horizontally)
  2. Rotation — Rotate the reflected image 90 degrees

Practical Applications

PresentationTypical Use Cases
Mollier DiagramWidely used in European HVAC practice; preferred for visualizing enthalpy-based processes
Psychrometric ChartCommon in North American practice; useful for temperature-humidity relationships

When to Use Each Diagram

  • Psychrometric Chart: Best for analyzing sensible heating/cooling, humidification/dehumidification where dry-bulb temperature and relative humidity are primary variables
  • Mollier Diagram: Preferred when analyzing processes where enthalpy changes dominate, such as adiabatic mixing or evaporative cooling

Note: Regardless of which diagram you use, the underlying psychrometric relationships and calculations remain identical. The choice is primarily a matter of convention and personal preference for visualization.

References

Source: engineeringtoolbox.com