Page 129 - Industrial Ventilation Design Guidebook

4,2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 9 I

accuracy. The intersection of the constant enthalpy line with the isotherm re-
sponding to the temperature of air gives the humidity of air. For more accurate
calculations Eq. (4.116) should be used, or its approximation (4.122) when the
steam pressures (/?/, and pi] are low compared to the total pressure of air (p),
As an example of using a Mollier diagram in defining the state of air, we can
take a typical measurement from the local exhaust hood of a paper machine. The
temperature of the exhaust air is 82 °C and its wet bulb temperature 60 °C. In Fig.
4.Wd we move from the saturation curve at the point 60 °C straight up along the
constant enthalpy line ( h^ = 460 kj/kg d.a.) until we reach the isorherm
0 = 82 °C. The intersection represents the state of air, and from Fig. 4.10d we
see that to the accuracy of the diagram x - 0.14 and the corresponding humidity
relation /= x/x'(S2 °C) = 0.20. Based on the values x - 0.14 and p = 1.()
bar the relative vapor pressure <p can be calculated. From Eq. (4.84} we have
p h = 0.183 bar and from Table 4.7 p' h (82 °C) = 0.5133 bar; then on the ba-
sis of definition (4.110) <p = (ph/p'h ) = 0.356 = 35.6%. We see that the values
of /and (p clearly differ from each other.
According to Eq. (4.122) when Le~ I, 1{0 M) = 2450 kj/kg and
c / ,sl.OkJ/kg°C:

and by means of this the state of air can be approximately calculated. Often
we call the temperature of air the dry bulb temperature to distinguish it from
the wet bulb temperature.
It is important to emphasize that, especially in process measurements,
radiation can have an essential influence on the wet bulb temperature,
and therefore generally the wet bulb temperature is dependent on the
measurement device and the method of measurement. If the airflow is
very low, the radiation can have a remarkable contribution in addition to
the convective heat transfer. Basically, an equation analogous to Eq.
(4.138) can be empirically determined for each wet bulb temperature and
method of measurement.

4.2.7 State Changes of Humid Air

Now we will consider a balance borderline of the system presented in Fig.
4.14. The system can be any part of the air surrounding the process device. If
an air-handling application is considered, the balance can be calculated over
the inner air of a room, an office, or an industrial hall, for example.
The energy balance of the system, consisting of the area inside the balance
borderline, is in a stationary situation:

where <f> is the net heat power received by the system, W m is the net work
power to the environment preferred by the system, m v is the water flow
(1 = inflow, 2 = outflow), thj is the ice flow, and ra^is the separate clean
steam flow not included in the air flows. The steam flows included in the