User Manual

19
11.2 h,x-diagram
Simplifying calculation of air state changes graphically was attempted a long time
ago. Diagrams for psychrometric calculations (e.g. enthalpy, dewpoint etc.) exist in
various forms. The Mollier diagram normally is used in Europe, and Carrier in the
U.S. Both diagrams use the same principal approach, only the orientation of the
axes varies. The temperature axis in the Mollier diagram is vertical, and horizontal
in Carrier. Water content in Mollier is horizontal, and vertical in Carrier.
Figure 11-1
h,x diagram by Mollier
Name Unit Description
A Temperature T
[°C]
Designates the heating condition of air. Indicated in degrees °C or
absolute in Kelvin K.
B Absolute
humidity
x
[g/kg]
Absolute humidity x is the amount of water per kg in dry air,
expressed in g.
C Relative
humidity
%
[r.h.]
Dry air can only absorb a specific maximum amount of steam at a
particular temperature. The higher the temperature, the greater the
possible amount of water in the air.
D Heat unit or
enthalpy
h
[kJ/kg]
One of the most important calculations in terms of ventilation is
determining the amount of heat needed to achieve an air state
predefined by temperature and humidity. In this case, air, whose state
is unknown, must either be mixed, heated, cooled, humidified, or
dehumidified to achieve the required air state. The heat contents h (in
kJ/kg) plays a significant role.
E Steam
pressure
Pw
[mbar]
Overheated steam mixed in air has a particular steam pressure Pw.
This steam pressure is part of the entire air pressure and, for this
reason, is also referred to as partial pressure for steam. This partial
pressure depends on the mixing ratio of steam to dry air. The higher
the steam content of air, the greater the partial pressure of steam Pw.
As a result, steam partial pressure Pw can be displayed in the mbar
parallel, horizontal to water content x, thus allowing for determining
the partial pressure Pw corresponding to a particular water content x
[g/kg].