Evpaorative Cooling

What is evaporative cooling? is it phase-change cooling

How evaporative cooling works:

When warm air moves over water surface some water will start evaporation this action of transforming water to vapor will observe energy (the heat from the warm air) , this heat which taken from remaining amount of water in the liquid state, resulting in a cooler water.

The common mistake people think that the air get cooler, but this is not true, the remaining water after evaporation effect getting cooler,The cooler air is the result of the warm air when its interact with the cold water while passing throw that cold water (simple heat exchange action)

Evaporation of 1 liter of water observes about 2300 BTU  or 674 watt of thermal energy (lower temperature)

So basically the amount of water can be evaporated per hour can be converted to BTUs of cooling

Evaporative cooling system capacity:

The cooling capacity of evaporative cooling can’t be calculated as fixed formula, it really all about wet-bulb depression, as we know wet-bulb temperature related current ambient temperature and humidity level which keep changing during day and night hours

Wet-bulb Depression: the difference in temperature between the ambient air temperature (Ta) and the wet-bulb temperature (Tw)

Supply Air temperature = Intake air temperature – (Wet-bulb Depression x evaporative cooling system Efficiency(%)

Generally the cooling capacity can be calculated with following formula:

Cooling capacity [kW] = Air mass rate [kg/s] x  air specific heat capacity [kJ/kg K] x air temperature change [K]

 Evaporative cooling Efficiency:

The evaporative cooling is the most efficient cooling system consumes about 15-25% power compared to traditional compressor based cooling system to provide the same amount of cooling

Evaporative cooling capacity usually calculated as the percentage of wet bulb temperature can be achieved (wet-bulb efficiency), for example if the system able to supply the temperature of the wet-bulb that means the system efficiency is 100%

wet-bulb efficiency =( ( intake air temperature – Supply air temperature ) / ( intake air temperature – wet-bulb air temperature ) ) x 100

EER and COP rating also can be applied into the evaporative cooling system, but can be calculated only according to known design condition the ambient temperature, humidity level, and location elevation

EER= Cooling capacity (Btu/h)/ Total unit electrical power (Watt)

COP= heat removed  / electricity consumed

Evaporative cooling in history:

Civilizations throughout the ages have found ingenious ways to combat the heat in their region.

The Arabs hung wet blankets over doors and windows to benefit from evaporative cooling

also the windcatcher (Malakef) was used in ancient Egypt , and Persian Gulf thousands of years ago in the form of wind shafts on the roof, which caught the wind, passed it over subterranean water in a tunnel and discharged the cooled air into the building.

Examples of evaporative cooling in nature:

When you sweat, this is a very clear example for evaporative cooling. When you do any effort you sweat, your skin will drop in temperature.

A vulture can urinate on itself, thereby increasing the moisture on its legs. The vulture is cooled as the urine evaporates. Birds can flutter their neck region to bring air quickly in and out of the mouth, which increases evaporative cooling. Animals such as elephants can also increase evaporative cooling by getting their bodies wet.

click here for : Evaporative Cooling  Types