Open Recirculation

Open recirculation includes the standard cooling tower, spray pond, or evaporator condenser as shown in Fig. 4.39. An arrangement of this type pro­vides an efficient cooling system. Its main disadvantage is growth of microor­ganisms such as the Legionella species. To protect people from these bugs, the biological water treatment represents a very high cost in the operation of the plant. This arrangement is losing favor with many engineers and is being re­placed by the less efficient closed systems.

Typical applications include heat rejection from the refrigeration plant. The highest proportion of cooling takes place by evaporation.

Advantages

1. Can cool water down to 2 °C above the wet bulb temperature

Air discharge

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V

Heat exchanger

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 w’ater 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.

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