Evaporative Cooling

Evaporative cooling is a process that uses the effect of evaporation as a natural heat sink. Sensible heat from the air is absorbed to be used as latent heat necessary to evaporate water. The amount of sensible heat absorbed depends on the amount of water that can be evaporated.

Evaporative cooling can be direct or indirect; passive or hybrid. In direct evaporative cooling, the water content of the cooled air increases because air is in contact with the evaporated water. In indirect evaporative cooling, evaporation occurs inside a heat exchanger and the water content of the cooled air remains unchanged. Since high evaporation rates might increase relative humidity and create discomfort, direct evaporative cooling can be applied only in places where relative humidity is very low.

Where evaporation occurs naturally it is called passive evaporation. A space can be cooled by passive evaporation where there are surfaces of still or flowing water, such as basins or fountains. Where evaporation has to be controlled by means of some mechanical device, the system is called a hybrid evaporative system.

indirect_evaporative_cooling_image3

Evaporative cooling is based on the thermodynamics of evaporation of water, i.e. the change of the liquid phase of water into water vapor. This phase change requires energy, which is called latent heat of evaporation- this is the energy required to change a substance from liquid phase to the gaseous one without temperature change. When non- saturated air (i.e. air that does not contain liquid water but only water vapor) comes in direct contact with water evaporation occurs. It is obvious that during this process the moisture content of air is increased. This process is represented on the psychometric chart by a displacement along a constant wet bulb line, AB. The air to be cooled is initially at point A. The air, as a result of the direct evaporative cooling process, reaches point B. This is a constant wet bulb temperature process and therefore line AB is parallel to the wet bulb temperature lines.

When evaporation occurs in the primary circuit of a heat exchanger, while the air to be cooled circulates in the secondary circuit, the air temperature decreases but its humidity ratio remains constant. It must be noted that since the air temperature drops, its relative humidity will increase, but less than during the direct evaporative cooling process. Since the humidity ratio of the air does not change, this process is represented on the psychometric chart by a displacement along a constant humidity ratio line CD. In this figure, the air to be cooled, initially at point C is sensibly cooled by the indirect evaporative cooler until it reaches point B.

direct_evaporative_cooling

direct evaporative cooling"

indirect_evaporative_cooling

indirect evaporative cooling"

Advantages and problems of direct and indirect evaporative cooling

Evaporative cooling uses large volumes or air. Forcing this volume of air through small ducts, around sharp corners, and out of small outlets, involves ducting costs. In some cases the best duct system is none. Just blow the air into a large daytime occupancy rooms.

If not properly designed direct type evaporative coolers may pose the following problems:

Indirect type evaporative coolers try to overcome these defects. Since the air in these types of coolers gets cooled without coming in direct contact with water, the problem of excessive humidity in the room air gets automatically solved. Simultaneously the required number of air changes also gets reduced.

The important advantages of the indirect type evaporative cooling are as follows: