The evaporative condenser is a highly efficient method of heat rejection. The heat is rejected at a lower temperature than with simple air-cooling and less fan power is required.
The cooling effect of the evaporation of water is applied directly to the condenser refrigerant pipes in the evaporative condenser (Figure 6.7) . The mass flow of water over the condenser tubes must be enough to ensure wetting of the tube surface, and will be of the order of 80-160 times the quantity evaporated.
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The mass flow of air must be sufficient to carry away the water vapour formed, and a compromise must be reached with expected variations in ambient conditions. An average figure is 0.06 kg/(s kW).
An evaporative condenser rated at 880 kW and the water-circulating pump takes another 15 kW. What will be the evaporation rate, the approximate circulation rate, and the air mass flow?
Total tower duty = 880 + 15 = 895 kW
Evaporation rate = 895 x 0.41 x 10~3 = 0.37 kg/s Circulation rate, 80 times = 30kg/s (AT = 7.1K)
160 times = 60kg/s (AT = 3.6K)
Air flow = 895 x 0.06 = 54kg/s
It will be seen that the water and air mass flow rates over an evaporative condenser are roughly equal.
Evaporative condensers have a higher resistance to air flow than cooling towers and centrifugal fans are often used, ganged together to obtain the required mass flow without undue size. This arrangement is also quieter in operation than axial flow fans. Most types use forced draught fans (Figures 6.7 and 6.8).
Figure 6.8 Evaporative condenser installation (Baltimore Aircoil)
Cooling towers and evaporative condensers may freeze in winter if left operating on a light load. A common arrangement is to switch off the fan(s) with a thermostat, to prevent the formation of ice. The water-collection tank will have an immersion heater to reduce the risk of freezing when the equipment is not in use or the tank may be located inside the building under the tower structure, if such space is conveniently available.
Materials of construction must be corrosion resistant. Steel should be hot galvanized, although some resin coatings may suffice. GRP casings are used by some manufacturers. The water-dispersal packing of a cooling tower is made of treated timber or corrugated plastic sheet.
The atmospheric condenser is a simplified form of evaporative condenser, having plain tubes over a collecting tank and relying only on natural air draught. This will be located on an open roof or large open space to ensure a good flow of air. The space required is of the order of 0.2 m2/kW, and such condensers are not much used because of this large space requirement. Atmospheric condensers can still be seen on the roofs of old breweries. They are in current use where space is plentiful.
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