23. March 2021

The h-x diagram and state changes

Zustädsänderungen der Luft im h-x-Diagramm

air warming

The simplest case of changing the state of air is heating. Water is neither added nor removed from the air (x = constant). However, as the temperature increases while the absolute humidity remains the same, the relative humidity decreases. In the diagram shown it can be seen that the line runs vertically upwards during heating. Heating the air to a desired temperature requires the amount of heat Δh.

In our example we increase the temperature from approx. 15°C to approx. 27°C. This requires a heat quantity of Δh = (h2 - h1) = (47 - 35) = 12 kJ/kg




Air Cooling

Air is cooled on surface coolers such as air coolers or evaporators. Two cases can occur when air is cooled, with the temperature of the surface being decisive. The dew point temperature of the air is above or below the cooler temperature.


Cool air without condensate separation

If the temperature of the cooler surface is above the dew point temperature, no water is separated from the air to be cooled. The absolute proportion of moisture remains constant (x = constant). Accordingly, the relative humidity of the air increases after cooling.

In the h,x diagram, the line runs vertically downwards, parallel to the line with constant water vapor content x.





Air cooling with condensate separation

If the temperature of the cooler surface is below the dew point temperature, water is separated from the air to be cooled

This process is shown in simplified form in the h,x diagram. The line runs from point Θ 1 along the imaginary line to the radiator surface temperature. Depending on the air flow, structure and surface area of ​​the surface cooler, the temperature Θ 2 is set . As a result, the line runs slightly obliquely. This creates a difference of Δx. Through further calculations, it is possible to specify an absolute amount of water with the Δx for a specific period of time for the cooling process. This value is relevant for the design of defrost water pipes and defrost water pumps or lifting systems. The relative humidity increases during the cooling process, but not as much as in the previous example "Cooling the air without condensation occurring".





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