Page 127 - Industrial Ventilation Design Guidebook

4,2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 89
Substituting Eqs. (4.135) and (4.136) in Eq. (4.134), we get

which is equivalent to Eq. (4.133).
Equation (4.137) is almost exactly the same as the approximation equa-
tion (4.123) derived for wet bulb temperature. When the partial pressure of
water vapor is low compared with the total pressure—in other words when
the humidity x is low—the specific heat of humid air per kilogram of humid
air, Cp, and the specific heat of humid air per kilogram of dry air, c^, are al-
most the same: c p~c pk. Therefore, in a situation where the humidity is low
and Le ~ 1, the thermodynamic wet bulb temperature is very nearly the same
as the technical wet bulb temperature Q M.

Example 7
(Table 4.7)Draw in the Mollier diagram at the 14 °C point of the satura-
tion curve (a) the state change line of the adiabatic humidification and (b) an
auxiliary line, associated with the wet bulb temperature measurement, by
means of which the state can be defined. The pressure of air is p = 1 bar.