IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report, November 2009
1. cooling circuit
2. inlet flow
3. cooling element (heat exchanger)
4. return flow
5. heat source
6. circulating pump
7. cooling air
8. fan drive
9. fan
4.1.2 Wet Cooling Towers
Fig. 8: Sketch of a dry cooler (SWKI, 2005)
Cooling towers are characterized by the primary cooling being evaporation of water. The
lowest achievable temperature is the wet bulb temperature for the ambient air. The wet bulb
temperature is depending on both the dry bulb temperature and the moisture content of the
air. At rising dry bulb temperature and constant moisture content the wet bulb temperature
will rise. As a consequence the capacity of a cooling tower will go down as the ambient
temperature raise doing the day.
Cooling towers can be
• open type, having direct contact between cooling water and the air stream in the
tower
• closed type, not having direct contact between cooling water and the air stream in the
tower
The open wet cooling tower (open loop evaporative cooling tower) consists of a shell
containing packing/fill material with a large surface area. Nozzles arranged above the
packing, spray and distribute the cooling water onto the packing. The water trickles through
the packing into a basin from which it is pumped back to the chiller. The water is cooled by
air and drawn or blown through the packing by means of a fan. The air flow, which is either
in counter or cross flow to the water flow, causes some of the water to evaporate, thus latent
heat, is exchanged from the water to the air.
The evaporated water is continuously replenished by make-up water. However, evaporation
also increases the concentration of the dissolved solids in the cooling water and blow down
of the cooling water is therefore necessary. In wet cooling towers the wet-bulb temperature
determines the degree of cooling and thus cooling below the ambient dry bulb temperature is
possible. The characteristic approach temperature, which is the difference between the
water outlet temperature and the ambient wet-bulb temperature, of open wet cooling towers
lies between 4 to 8 K (SWKI 2005).
Compared to dry coolers wet cooling towers are able to cool the cooling water to a lower
temperature level, require less space and have much lower investment costs. The main
disadvantages of wet cooling towers are hygienic problems, water consumption and high
maintenance effort.
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