Ventilation

Psychrometric chart

Water

7,4g/kg

Enthalpy

40,9kJ/kg

Outlet

32,0°C

The psychrometric chart finds the state of moist air from temperature, humidity and pressure using the Magnus formula. It shows humidity ratio, enthalpy, dew point, density and specific volume. Heating, cooling and mixing can be read as air processes.

x

7,37

g/kg

h

51,1

kJ/kg

Heat

1,67

kW

hx g/kg01020304050600123456789100°5°10°15°20°25°30°35°40°121

1Heating

StateInlet and outlet
Temperature 1
22,0°C
Humidity 1
45%
Dew point
9,5°C
Wet bulb
14,8°C
Density
1,191kg/m³
Partial pressure
1.187Pa
Temperature 2
32,0°C
Humidity 2
25%
x₂
7,37g/kg
h₂
51,1kJ/kg

Process

1013 hPa

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How it's calculated

The psychrometric chart finds moist air with the Magnus formula for saturation vapour pressure. Temperature, relative humidity and pressure give humidity ratio, enthalpy, dew point, wet-bulb temperature, density and specific volume. Heating and cooling keep the humidity ratio until the saturation line is reached. Mixing is the mass balance of both streams. The limits are about −40 to 95 °C and 700 to 1,200 hPa.

Worked example

22 °C and 45% at 101,325 Pa: humidity ratio 7.37 g/kg, enthalpy 40.9 kJ/kg, dew point 9.5 °C.

FAQ

Why does this differ from a printed chart?
It uses the Magnus formula, not the full IFC formulation. That is enough to plan an air state, not to stand alone as a commissioning record.
What happens when cooling passes the dew point?
The humidity ratio falls and condensate is reported. The result is a change of state, not a coil selection.
Can pressure come from altitude?
Yes. Zero altitude is the standard pressure of 101,325 Pa. The standard atmosphere sets the pressure inside the allowed range.
Is volume flow needed for the state?
Not for the air state. It is needed for capacity and condensate, because both follow the mass flow.