Psychrometric Chart Mollier

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

psychrometricchartmollierData Table

Overview

The Mollier diagram (also called the Mollier i-x chart) is a graphical representation of the thermodynamic properties of moist air. It plots enthalpy against specific humidity (moisture content), with overlay lines for dry bulb temperature, wet bulb temperature, and relative humidity. The Mollier diagram is a rotated variant of the standard psychrometric chart and is widely used in HVAC system design for analyzing heating, cooling, humidification, dehumidification, and air mixing processes.

Key Formulas

Humidity Ratio (Specific Humidity)

$\omega = 0.622 \frac{P_v}{P_a}$

The mass of water vapor per unit mass of dry air, where $P_v$ is the partial pressure of water vapor and $P_a$ is the partial pressure of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

The ratio of actual vapor pressure to saturation vapor pressure at the same dry bulb temperature.

Enthalpy of Moist Air

$h = c_p \cdot T_{db} + \omega \cdot h_g$

Sensible heat plus latent heat per unit mass of dry air. The constant $c_p \approx 1.006$ kJ/(kg·°C) for dry air and $h_g$ is the enthalpy of saturated water vapor at the given temperature.

Wet Bulb Temperature

$T_{wb} \approx T_{db} - \frac{P_{vs} - P_v}{\gamma}$

An approximation relating wet bulb depression to vapor pressure deficit, where $\gamma$ is the psychrometric constant (approximately 6.66 Pa/°C at 1 atm).

Variables

9 rows

Key psychrometric variables used on the Mollier diagram
Symbol	Description	Unit
ω (omega)	Humidity ratio (specific humidity)	kg/kg dry air
φ (phi)	Relative humidity	%
Pv	Partial pressure of water vapor	Pa
Pvs	Saturation vapor pressure at Tdb	Pa
Tdb	Dry bulb temperature	°C
Twb	Wet bulb temperature	°C
h	Specific enthalpy of moist air	kJ/kg dry air
cp	Specific heat of dry air at constant pressure	kJ/(kg·°C)
hg	Enthalpy of saturated water vapor	kJ/kg

Source: engineeringtoolbox.com

Example: Reading the Mollier Diagram

Case 1 — Known: Dry Bulb Temperature and Relative Humidity

Given Tdb = 21 °C and φ = 50%, read the following from the Mollier chart:

3 rows

Mollier chart readings for 21 °C dry bulb and 50% RH
Property	Value
Enthalpy (h)	~40 kJ/kg dry air
Wet Bulb Temp (Twb)	~14.5 °C
Humidity Ratio (ω)	~0.0076 kg/kg dry air

Source: engineeringtoolbox.com

Case 2 — Known: Dry Bulb and Wet Bulb Temperatures

Given Tdb = 19 °C and Twb = 12 °C, read:

2 rows

Mollier chart readings for 19 °C dry bulb and 12 °C wet bulb
Property	Value
Enthalpy (h)	~33 kJ/kg dry air
Humidity Ratio (ω)	~0.0056 kg/kg dry air

Source: engineeringtoolbox.com

Saturation Vapor Pressure Reference

The saturation vapor pressure $P_{vs}$ increases sharply with temperature. Representative values from the Antoine equation or steam tables:

9 rows

Saturation vapor pressure and latent heat of vaporization for water at standard atmospheric pressure
Temperature(°C)	Saturation Vapor Pressure(Pa)	Enthalpy of Saturated Vapor(kJ/kg)
0	611	2501
5	872	2510
10	1228	2520
15	1705	2529
20	2339	2538
25	3169	2547
30	4246	2556
35	5628	2565
40	7384	2574

Source: engineeringtoolbox.com

Saturation Vapor Pressure vs. Dry Bulb Temperature

Mollier Chart Example Points

The original images show example readings from the Mollier chart. The extracted readings are represented below as data so they can be inspected without relying only on the bitmap chart.

2 rows

Readings extracted from the original Mollier chart example images.
Chart case	Dry bulb(°C)	Relative humidity(%)	Wet bulb(°C)	Humidity ratio(kg/kg dry air)	Enthalpy(kJ/kg dry air)
Dry bulb 21 °C and 50% RH	21	50	14.5	0.0076	40
Dry bulb 19 °C and wet bulb 12 °C	19	40	12	0.0056	33

Source: engineeringtoolbox.com

Extracted Mollier Example States

Mollier Diagram Reproduction

The original Mollier diagram is redrawn below as an equation-generated psychrometric chart for moist air at standard atmospheric pressure. The chart uses humidity ratio on the horizontal axis and dry-bulb temperature on the vertical axis, with relative-humidity curves, constant-enthalpy lines, humidity-ratio grid lines, and the saturation boundary.

Redrawn psychrometric Mollier reference chart for moist air at 101.325 kPa. Curves are generated from standard equations for saturation vapor pressure, humidity ratio, dry-bulb temperature, and moist-air enthalpy.

3 rows

Validation points for the redrawn Mollier chart.
State	Calculated from equations	Original chart reading
21 °C dry bulb, 50% RH	ω = 7.71 g/kg, h = 40.71 kJ/kg	ω ≈ 7.6 g/kg, h ≈ 40 kJ/kg
19 °C dry bulb, ω = 5.6 g/kg	h = 33.32 kJ/kg, RH ≈ 41%	h ≈ 33 kJ/kg, RH ≈ 40%
0 °C saturation	ω = 3.77 g/kg, h = 9.44 kJ/kg	saturation boundary near 0 °C

Source: engineeringtoolbox.com

Mollier vs. Psychrometric Chart

The Mollier diagram and the standard ASHRAE psychrometric chart display the same thermodynamic data but with different axis orientations:

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Comparison of the Mollier diagram and the standard psychrometric chart
Feature	Mollier (i-x) Chart	Psychrometric Chart
Horizontal axis	Humidity ratio (ω)	Dry bulb temperature
Vertical axis	Enthalpy (h)	Humidity ratio (ω)
Temperature lines	Nearly horizontal, slight slope	Vertical
Enthalpy lines	Nearly vertical	Diagonal
RH curves	Upward concave	Upward concave
Common region	Europe, industrial HVAC	North America, ASHRAE standard

Source: engineeringtoolbox.com

Calculator: Humidity Ratio from Vapor Pressure

Humidity Ratio (ω)

Vapor Pressure(Pa)

Atmospheric Pressure(Pa)

Moist Air State from Temperature and Relative Humidity

Dry bulb temperature(°C)

Relative humidity(%)

Atmospheric pressure(Pa)

Unit Converter

Psychrometric Unit Converter

Temperature(°C)

Humidity ratio(kg/kg dry air)

Enthalpy(kJ/kg dry air)

Pressure(Pa)

Note: the direct kJ/kg to Btu/lb conversion preserves the metric enthalpy reference. The IP psychrometric enthalpy value uses the common inch-pound psychrometric reference and should be used when comparing with IP psychrometric charts.

Mollier Chart Reading Examples

The annotated example charts below are redrawn from the same psychrometric equations used by the main chart.

Example reading: 21 degC dry-bulb temperature and 50% relative humidity gives about 7.7 g/kg humidity ratio and 40.7 kJ/kg enthalpy.

Example reading: 19 degC dry-bulb temperature and 12 degC wet-bulb temperature gives about 5.6 g/kg humidity ratio and 33 kJ/kg enthalpy.

Engineering Notes

Atmospheric pressure matters. The Mollier diagram is typically drawn for standard atmospheric pressure (101.325 kPa). At significantly different altitudes or pressures, corrections are needed or a pressure-specific chart should be used.
Axis orientation can confuse users. In the Mollier chart enthalpy runs nearly vertically and humidity ratio nearly horizontally, which is the reverse of what many ASHRAE-trained engineers expect. Verify which axis convention your chart uses before reading values.
Chart accuracy. Reading values from a printed chart introduces interpolation error. For precise calculations use the governing equations or digital psychrometric tools rather than visual interpolation.
Wet bulb temperature approximations. The formula $T_{wb} \approx T_{db} - \frac{P_{vs} - P_v}{\gamma}$ is a linearization. For high accuracy, use iterative thermodynamic models (e.g., ASHRAE's formulation) or ASHRAE Psychrometric Chart No. 1.
Process visualization. The Mollier diagram excels at visualizing air-handling processes: sensible heating moves horizontally to the right; cooling and dehumidification move down and left; mixing of two airstreams follows a straight line between their state points.
Saturation line. The curved boundary on the Mollier chart represents 100% relative humidity. Air states cannot exist above (to the right of) this line—condensation occurs beyond saturation.

References

Original Source — Engineering ToolBox: Psychrometric Chart / Mollier Diagram
ASHRAE Handbook — Fundamentals, Psychrometrics chapter

Source: engineeringtoolbox.com