Page 191 - Industrial Ventilation Design Guidebook

4.4 WATER PROPERTIES AND TREATMENT I 5 3

2. Average temperature drop through tower in 10-18 °C range depends on
wet bulb temperature

Disadvantages
1. Corrosion due to absorption from the atmosphere of pollutants as the
water droplets pass through the tower
2. Fouling of surfaces, resulting in decreased heat transfer efficiency
3. Scale buildup, resulting in a reduction of fluid flow through the heat
exchanger and loss of effectiveness
4. Microbiology problems (such as 2 and above 3) together with
corrosion of materials and health hazards
5. Decay problems in wooden cooling towers
6. Spray water loss, resulting in costly additional water treatment for the
makeup water
7. Spray drift may cause annoyance to people in its path, as well as
corrosion of adjacent metals and concrete breakdown; improved design
of drift eliminators available (in PVC) for critical control of drift
The temperature difference between the recooled water temperature and
the inlet air wet bulb temperature is called the approach. The lower the ap-
proach, the more complex the tower's design becomes. The normally used
minimum approach temperature is 2 °C.
4.4.3.2 Closed Recirculation
The arrangement for closed recirculation is shown in Fig. 4.40, It over-
comes many of the problems encountered with the open system. Typical appli-
cations include engine cooling and heat rejection from refrigeration plant.

Advantages
1. Compared with the open system, the average cooling water
temperature drop is small, only in the 6-8 °C range.
2. Provided no water losses occur due to pipeline leaks, the cost of water
treatment is at a minimum.

Disadvantages
I . Corrosion and fouling will occur, though normally considerably less
than that experienced in the open system.
4.4.3.3 Once-Through System

This is the simplest of the three systems (see Fig. 4.41). Typical applica-
tions include

• Process water
• Potable water
• General service supplies

Advantages
1. Low initial cost
2. Space savings, small footprints