Page 132 - Industrial Ventilation Design Guidebook

94 CHAPTER 4 PHYSICAL FUNDAMENTALS
When the airflow meets a surface whose temperature is lower than the
dewpoint, water vapor from the air condenses on the surface of the cooling
coil. If all air comes into contact with the cold surface, the state of the air after
the process will be at point 3. Some air always escapes the cold surface, and
therefore the state of air after contact with the coil is a mixture of saturated
air (3) and escaped air (1). The mixing point (2) lies on the line connecting
points 1 and 3, as shown in Example 8. The nearer point 2 is to point 3, the
more effective is the cooling coil.

Example 11
Adding steam to the air. From Eqs. (4.139) and (4.140) it follows that

where m h is the added steam flow and h k its enthalpy. From these equations
it follows that

On the other hand, differentiating Eq. (4.90b), h k = Cp iQ + x(c ph9 + // 30 ),
with respect to the variables 0 and x, we obtain

where h h(9) = l^ 0 + c ph9 is the steam enthalpy at air temperature Q.
From Eq. (4.149) we notice that if the temperature of the steam added to
air is below the temperature of the air, the air will cool down and
dO/dx < 0. If the steam temperature is higher than the air temperature, the
temperature of air will rise ((d0)/dx > 0).

Example 12
The room temperature is required to be 20 °C and the relative humidity
<p — 50%. The net heat load developing in the room is 2.45 kW and the net
steam flow 1.5-10" kg/s. What should the inlet air temperature and humid-
ity be when the inlet air is (a) rii; = 0.3 ke/s and (b) rii; = 0.6 ke/s