ESC-Series Two-Stage Evaporative Cooling
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.