What is the primary difference between direct and indirect evaporative cooling?

The primary difference between direct and indirect evaporative cooling lies in how they interact with air and humidity levels. In direct evaporative cooling, water is evaporated directly into the air stream, which increases the humidity of the air, allowing it to absorb more heat and lower the temperature effectively. This process relies on the principle that when water changes from a liquid to a vapor, it absorbs a significant amount of heat from the surroundings, resulting in cooler air.

While the other processes involve alternatives in energy efficiency and methods of cooling, option B specifically emphasizes the mechanism of mixing water vapor with the air, which is fundamental to how direct evaporative cooling operates. This method is straightforward and effective in suitable climates, particularly in hot and dry regions where humidity levels are lower, allowing for significant temperature reduction.

In contrast, indirect evaporative cooling systems do not mix water vapor directly with the air but use a heat exchanger to cool the air through the evaporation process, ensuring that the air supplied does not become humidified. This method is often employed in scenarios where maintaining lower humidity levels is crucial, providing a more controlled cooling effect without increasing moisture in the air.

Therefore, the statement that direct cooling mixes water vapor with the air accurately captures the essential difference, making it