Page 102 - Industrial Ventilation Design Guidebook

4,2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 6 7

where c^(T) is the specific heat of dry air (J/kg K), c ph(T) is the specific heat
of water vapor, and l h(> is the heat of vaporization at 0 °C. Its numerical
value is

Specific heats c pi and c ph are somewhat dependent on temperature. In the
temperature range of -10 to +40 °C, their average values are

At the temperature of +50 °C, their values are c pl•= 1.008 kj/kg °C and
C M - 1.87 kj/kg °C.
Using numerical values mentioned above, the enthalpy of humid air b- K
(Eq. (4.87)) can be written as

where 9 is the temperature in Celsius. Equation (4.90a) can also be written as

We denoted the mass of dry air in a volume V as m^ that is, p i = m/ V {,
and the mass of water vapor in V as m h, that is, p h = m h/V h. In practical
calculations we usually handle volume flow g !;(nvVs) instead of volume V.
For instance, the value of volume flow is known in the suction inlet of a fan
when the operating point of the fan is defined. Volume flow q v, expressing
the total air flow or the combined volume flow of water vapor and dry air,
is not constant in various parts of the duct, because the pressure and tem-
perature can vary. Therefore in technical calculations dealing with humid
air, material flows are treated as mass flows. Also, while the humidity can
vary, the basic quantity is dry air mass flow w/(kg d.a./s). If, for instance,
we know the volume flow q v of a fan, the dry air mass flow through the
fan is

where p i is the partial pressure of dry air in the suction inlet of the fan, in the
same place where the total volume flow V is defined. Accordingly, the water
vapor flow m^ (kg H 2O/s) along the volume flow is

where p h is the partial pressure of water vapor.
Due to the definition of humidity (4.87), on the basis of the Eqs. (4.91)
and (4.92),