Facilities-HVAC Systems - Energy Conversion and Management - page 218

thoroughly wetted with water. When the wet bulb is placed in an airstream, water
evaporates and an equilibrium temperature, called the wet-bulb temperature, is
reached. This temperature can be assumed as the adiabatic-saturation temperature.
For practical purposes, the enthalpy of the mixture is calculated by summing the
contributions of dry air and water vapor. This value is usually referred to the mass
of dry air (Ma) and the humidity ratio is then introduced:

H mix ¼ M a h a þ M v h v

ð H mix = M a Þ ¼ h mix ¼ h a þ ð M v = M a Þ h v ¼ h a þ ω h v

The enthalpy of dry air can be obtained from ideal gas tables or calculated by
using specific heat (1.005 kJ/kg K which varies slightly over the interval 10-30 C
(50-86 F). The enthalpy of water vapor, which is superheated at operating points
such as point 1 in Fig. 13.1 , is assumed to be equal to the saturated vapor value
corresponding to the given temperature (as in the steam table or the Mollier
diagram; see Figs. 6.1 and 6.2 ). It can also be expressed as the sum of the enthalpies
of the saturated vapor at standard temperature 273.15 K (0 C, 32 F) and of the
superheated vapor (the latter's enthalpy is equal to vapor-specific heat, when the
air-water mixture is in atmospheric conditions assumed equal to 1.8 kJ/kg K,
multiplied by the temperature).

Approximately, the enthalpy of the mixture of dry air and water vapor is

h mix ¼ 1 t þω ð 2500 þ 1 : 8 t Þ ð kJ = kg Þ

where
1 kJ/kg K is the specific heat of the dry air.
t is the difference between the mixture temperature in K and the
standard temperature (273.15 K), that is, the temperature in C.
2,500 kJ/kg is the enthalpy of the saturated steam and 1.8 kJ/kg K is
the specific heat of the superheated steam when the air-water mixture is
in atmospheric conditions (p v < p a ; see Table 2.5 ).

The humidity ratio of a moist air mixture varies with the temperature; if the
mixture temperature T* is reduced at constant mixture pressure, the water vapor
state moves from point 1 to the dew-point temperature (point 3 in Fig. 13.1 )at
constant partial pressure p v and at constant humidity ratio. Then, if cooling
continues condensation occurs and the final temperature is less than T*. The system
will then consist of a saturated mixture of dry air and water vapor at a pressure less
than p v , because the mole fraction of water vapor at the final state is less than the
value at the initial temperature.
The dew-point temperature of a given air sample is the temperature of saturated
moist air at the same pressure and with the same humidity ratio as the air sample.