What is the h-x diagram?

Components and parameters:

The h,x diagram shows all the essential parameters required to describe the state of the air:

Temperature = t in °C

Absolute humidity = x in g/kg

Relative humidity = RH in %

Specific enthalpy = h in kJ (1+x)kg

Density = p in (kg/m3)

Structure

The coordinate system increases the reading accuracy for the unsaturated region of humid air. To construct the oblique diagram proposed by Mollier, the x-axis is rotated clockwise until the isotherm t = 0 °C runs horizontally in the unsaturated region of the moist air. The lines of constant specific enthalpy h run from top left to bottom right. The lines of constant water content x run vertically.

For practical reasons, the horizontal axis on which the water content x is plotted does not pass through the origin. The partial pressure of water vapour can be specified as a second x-axis, as this depends only on the water content x and the air pressure p. The specific enthalpy h is plotted on the diagonal lines. Curves for relative humidity are shown in the diagram.

Using the scale at the edge, changes of state can be easily represented graphically, e.g. the change of state during steam-air humidification. The index 1/x indicates that the enthalpy of the moist air is composed of the enthalpy of the dry air and the enthalpy of the water.

The lines of equal temperature (isotherms) rise slightly in the region of unsaturated air, namely by the sensible enthalpy component of the water vapour. At the saturation point (relative humidity = 1), the lines bend downwards because, beyond the maximum vapour content, water can then only be present in the air in liquid form as small water droplets (mist). In the fog region, the isotherm deviates from the isenthalp passing through the saturation point only by the small sensible enthalpy of the additional water content.

In the region of unsaturated air, there are now curves of equal relative humidity, which arise from an equal division of the respective isotherm sections between = 0 and = 1. The relative humidity therefore decreases as the air becomes warmer, provided the amount of water x remains constant.

Calculations using the h,x diagram

Heating at constant absolute humidity

The fifth diagram shown here illustrates the heating process of an air mass without any change in its water vapour content. What can be observed from this process?

Heating begins at 11°C (point 1) and ends at 25°C (point 2). The absolute humidity x remains constant at 4 g/kg during this process. In contrast, the relative humidity changes from 50% at 11°C to 20% at 25°C. Furthermore, the enthalpy h (1/x) changes from 21.4 kJ/kg to 35 kJ/kg, and finally, the density also changes from 1.24 kg/m³ to 1.17 kg/m³.

Atomisation or evaporation of water (adiabatic humidification)

When water is atomised or evaporated without the simultaneous addition of heat, the energy required for evaporation is drawn from the ambient air. The air therefore cools down. As the cooling process follows the adiabatic lines, it is referred to as adiabatic cooling. In the h,x diagram, the exact direction of the cooling curve during humidification can be determined from the marginal scale ∆h/∆x.

Calculation of ∆h/∆x:

∆h=kJ/kg

∆x kg H2O / kg dry air

Humidification using steam (isothermal air humidification)

When humidifying air with steam, the temperature remains essentially constant, as the water vapour is at the same energy level as the air.

Calculation of ∆h/∆x:

∆h = kJ/kg

∆x kg of dry air