Evaporative Cooling

When moisture is added to air, the relative humidity increases and we perceive the temperature to have decreased. This works where the air is very dry and not too hot, and requires a large quantity of water and outdoor air. For centuries, fountains have cooled courtyards in hot arid climates. Passive evaporative cooling systems can be as simple as a sprinkler on the roof of a conventional building, or as complex as a roof pond with adjustable louvers.

Outdoor conditions in about half of the United States are suitable for mechanical evaporative cooling. Known as swamp coolers or desert coolers, evaporative coolers (Fig. 25-1) are also used in high-heat applications such as restaurant kitchens. Dry fresh air from outdoors is circulated through a wet pad, where it absorbs moisture as water vapor. After use, the air exits the building through grilles or open windows. Evaporative coolers are often located on the roof. Through-wall coolers are also common.

When the outdoor air is at 41°C (105°F) and the relative humidity is a low 10 percent, evaporative cooling can produce indoor air at 26°C (78°F) and 50 percent relative humidity with only the power necessary to operate a fan. However, the fans that drive evaporative coolers are noisy, and the aroma of the wetted cooler may be unpleasant.

AA lU-"" Absorptive pad

Figure 25-1 Evaporative cooler:

Misting or fogging systems make people feel cooler with no total change in the heat content of the treated air. Roof sprays have been used in the past to keep poorly insulated roofs cool. Misting can be used for small outdoor areas, such as team benches at football stadiums or refreshment pavilions. It has also been used in very large spaces in hot dry climates, including a railroad station in Atocha, Spain, and a conservatory in Michigan. Night roof spray thermal storage systems cool water on the roof at night via radiation and evaporation. The water is stored on or below the roof for use the next day in building cooling, or in a tank to precool entering air.

Indirect evaporative cooling actually uses both direct and indirect evaporative techniques, and may be combined with direct refrigerant cooling systems. Warm, dry outdoor air that has been cooled by evaporation is passed through a rock bed under a building at night. The next day, very hot dry air is cooled by passing through the rock bed, and then through an evaporative cooler.

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