Page 137 - Industrial Ventilation Design Guidebook

4.2 STATE VALUES OF HUMID AIR; MOLLIER DIAGRAMS AND THEIR APPLICATIONS 99

The quantity aA p is defined separately for each type of cooling tower, It
depends on many variables: jet pressure, jet division, airflow velocity, and oth-
ers. The total energy balance for a cooling tower is (see Fig. 4.19)

where Q vo is the temperature of the excess feed water. The need for excess
feed water represents the rate of vaporization 7%, Usually the term m hc pl,B l!0
has minor significance (vaporization rate 7% corresponds usually to just a
small percentage of the water flow n^), so on the basis of Eq. (4.152) we get
an equation for the cooling power <f> v:

We can dimension a cooling tower according to the equation above.

Example 13

A paper industry's cooling tower recovers heat from the outlet air. This
situation is represented by the following values:
• Inlet air enthalpy h kl = 293 kj/kg
• Outlet air enthalpy h k2 — 208 kj/kg (saturated 44.3 °C air)
• Outlet water temperature Q vl = 40 °C
• Inlet water temperature B l/2 — 5.0 °C
• Water flow m v = 44 kg/s
• Air flow riii = 44 • 4.186 • (40 - 5)7(293 -208) = 75.8 kg/s
2
• Cross-sectional area of the cooling tower A^ = 31 m and height
L = 3 m
It is discovered that in the cooling tower the water moving down-
ward from the jets changes its direction to upward after drop formation.
There is an effective heat transfer process when the drops move upward:
heat transfers from the outlet air to the drops through convection and
condensation.
Drops collide with the drop separator and drain down to the lower part of
the tower. These drops are large, so their total surface area is small and insig-
nificant. The effective heat transfer process takes place when the drops move
with the air flow, so this arrangement has to be treated as a parallel flow heat
transfer.
(a) Calculate aAp. According to the parallel flow principle, the situation is
as shown in Fig. 4.20.