Direct evaporative cooling is designed to cool the air by evaporating water which increases the moisture content of the air. Standard cooling systems use evaporative media of shredded aspen fibers, typically 1 to 2 inches thick. These systems have an effectiveness of 55 to 70 percent. (Effectiveness is a measure of how closely the supply air temperature leaving the evaporative cooler approaches the outdoor wet-bulb temperature.) Effectiveness is defined by:
TDB is the outdoor dry-bulb temperature while TWB is the outdoor wet-bulb temperature. SAT is the supply air temperature leaving the evaporative cooler.
The thickness of the media and air velocity contribute to the system effectiveness. More advanced evaporative cooling systems use a rigid medium 8 to 12 inches thick and have an effectiveness of 80% to 90%. Direct evaporative cooling systems are suitable for hot and dry climates where the design wet-bulb temperature is 68°F or lower. In other climates, outdoor humidity levels are too high to allow for sufficient cooling.
Indirect evaporative cooling uses an air to air heat exchanger to remove heat from the primary air stream without adding moisture. Hot and dry outside air is passed through a series of horizontal tubes that are wetted on the outside. A secondary air stream blows over the outside of the coils and exhausts the warm, moist air to the outdoors. The outside air is then cooled without adding moisture as it passes through the tubes. Indirect evaporative cooling typically has an effectiveness of 75%.
While the greater number of air passes increases the pressure drop and the required fan power, the high effectiveness extends the geographic range where the indirect evaporative cooler can fully meet the cooling demand.
Indirect cooling is often paired with a second direct evaporative cooling stage, to cool the supply air further while adding some moisture to the supply air. Two-stage systems (often referred to as indirect-direct or IDEC systems) can meet the entire cooling load for many industrial and commercial buildings in arid to semi-arid climates. IDEC systems provide cooler supply air at a lower relative humidity than direct evaporative cooling units. First, the indirect stage cools the supply air without increasing humidity. Since the air is cooled, it has a reduced capacity to hold moisture. Second, the supply air is then passed through a direct cooling stage, which cools the air further while adding additional moisture. IDEC systems typically have an effectiveness of 100% to 115%, cooling the air to a temperature slightly below the outdoor air wet-bulb temperature (see sidebar).
For commercial systems, indirect-direct systems can be coupled with a conventional DX cooling stage to meet cooling demand during hot and humid outdoor conditions (when outdoor air dry-bulb temperatures exceed 70°F). Since the systems use 100% outside air for cooling, they can also be paired with heat recovery to capture some of the energy that is lost in the exhaust air stream and reduce the ventilation cooling load.
IDEC systems used in arid climates (with a design wetbulb temperature of 66°F or lower) can have power consumption as low as 0.22 kW/ton, much lower than compressor-based cooling which can have power consumption on the order of 1 kW/ton. However, in more humid climates indirect-direct systems have less power reduction and energy savings than other cooling solutions.