Saturation Curve

Saturation curve (φ = 100 %) is the boundary of the Mollier h–x diagram. Every point on the curve represents air saturated with water vapor at that temperature — the air cannot hold any more moisture at that temperature.

Physical basis

Air is saturated when the partial pressure of water vapor $p_w$ reaches the saturation pressure $p_{ws}$ at that temperature:

$$\varphi = \frac{p_w}{p_{ws}} = 1 \quad (100\ \%)$$

The saturation pressure rises steeply with temperature (approximately exponentially, per the Clausius–Clapeyron relation), which gives the saturation curve its characteristic shape.

Regions of the diagram

Region	State
Above the saturation curve	Real unsaturated air states (φ < 100 %)
Saturation curve	φ = 100 %, air saturated
Below the curve (fog region)	Unstable state — excess moisture condenses

Dew point and the saturation curve

The dew point of any air state always lies on the saturation curve — it is the intersection of the vertical line (constant $x$) with the φ = 100 % curve.

Processes that end on the saturation curve

• Adiabatic humidification (evaporative cooling) — follows a constant-enthalpy line and can reach the saturation curve
• Cooling with dehumidification — the coil cools the air below its dew point; the state reaches the saturation curve and then moves along it (toward the coil surface temperature, the ADP), the excess moisture draining off as condensate
• Steam humidifier to 100 % — the air can reach the saturation curve during isothermal humidification (adding dry steam)