• Same principle as perspiration on the human body.
• As air passes over the skin evaporation takes place and a cooling effect is felt.
• Evaporative air coolers use filter pads as the cooling outer skin.
• A fan draws air through the water-soaked pads, evaporating the water and cooling the air that is then blown into the house.
• A temperature reduction of as much as 20 degrees can be achieved.
• Evaporative cooling is most efficient in areas of low humidity.
• Achieved by natural evaporation of water.
• Evaporation of water requires 540 kcal of heat per kg. Conversely, the same quantum of cooling is generated when water evaporates.
• Dry bulb temperature of air drops as a consequence of the evaporative effect.
• On the psychometric graph air moves towards water saturation and the circulating water cools down till it reaches its saturation temperature or wet-bulb temperature.

Direct Evaporative Cooling

• The evaporative cooling addresses sensible cooling requirement of ambient air Latent cooling requirement remains unaffected
• No reduction in ‘total heat content’ or enthalpy of air. Hence cooling effectiveness represented by the reduction in dry-bulb temperature and not ‘TR’
• Requires low relative humidity (i.e. the ability for air to absorb significant additional moisture) to yield effective cooling

Indirect-Direct Evaporative Cooling

• Cools air without adding humidity, Dry-Bulb and Wet-Bulb temperature of air drops as a consequence of the evaporative and heat-exchange effect
• On the psychometric graph air moves horizontally along the constant humidity line (but increasing RH % – since at the cooler DBT it can hold less additional moisture) towards saturation and the circulating water cools down till it reaches it’s saturation temperature or wet-bulb temperature
• ‘Cooling Effect’ generated in one path-way is transferred to a physically separated stream of air (which will eventually be supplied to the occupied space) through a heat-exchange process across ‘thin walls’
• Indirect Evaporative Cooling addresses the sensible cooling requirement of ambient air
• The latent cooling requirement remains unaffected
• Leads to the reduction in ‘total heat content’ or enthalpy of air
• Can operate in moderately humid air conditions as well (i.e. more robust in its climatic range relative to Direct Evaporative Coolers)
• Combined Direct and Indirect Evaporative Cooling in sequence (2 stage Evaporation) provides cooler air than is possible through each process separately.

Direct Evaporative Cooler

• A fan moves the supply air past a wetted media, adding moisture to the supply air stream to accomplish the evaporative cooling effect
• Cellulose Pads impregnated and treated to resist biological and mechanical degradation
• Cross fluted design enhances surface area to hold water on which air flows and causes air to change directions to ensure all air touches wetted media before leaving the area
• Combined Direct and Indirect Evaporative Cooling in sequence (2 stage Evaporation) provides cooler air than is possible through each process separately.

Indirect/Direct Evaporative Cooler

Indirect Evaporative cooling

• Secondary air stream passes through the wetted media and is cooled down by the water
• Primary air stream loses its heat while passing through a heat exchanger containing chilled secondary air stream. However, there is no moisture added to the primary air stream.
• This cooled air is now circulated in the room by blower

Indirect-Direct Evaporative Cooling

• The first stage is indirect evaporative cooling where primary air loses its heat to cool and humid secondary air stream through a heat exchanger.
• In the second stage, additional cooling happens when this cooled primary air directly passes through a wetted media. The evaporation effect further reduces the air temperature leaving the system
• The amount of moisture gained by the primary air in this system is much lower than direct evaporative cooling.

Direct Evaporative Cooler

• Outside air is pulled inside via fan
• Water is pumped up to pass through wetted media
• Air when passes through wet media, loses heat to water and absorbs the moisture. At this stage, Dry Bulb Temperature of the air reduces but the Wet Bulb Temperature remains constant (as cooling happens through the phase change of water)
• Cold humid air is circulated inside the room by a blower

Indirect Direct Evaporative Cooler

• A fan pulls a secondary air stream which passes through the sensible heat exchanger, thereby absorbs moisture and loses its heat
• Another fan pulls primary air stream, which passes through a heat exchanger with secondary air stream inside. Heat is transferred from primary to secondary air without the addition of moisture.
• At this stage, both the Dry Bulb and Wet Bulb temperature of the primary air reduces
• In the next stage, the primary air stream passes directly through the wetted media, where it absorbs moisture and further loses its heat. Since the temperature of this air stream is lower than the secondary air stream, the amount of moisture absorbed is also lower.
• With the help of a blower, cooled primary air is circulated inside the room

Direct Evaporative Cooling

• Industries – Provides comfort to staff from hot indoor climate, thus leading to a reduction in errors and increased productivity
• Commercial – Customized sizes to fit in any air handling unit, thus improving the working environment of the businesses.
• Residential – Customized sizes to fit specific residential requirements
• Poultry Industry – Lower air temperatures, reduces heat-stress related problems
• Horticulture Industry – Ensures the balance of temperature in greenhouses

Indirect Direct Evaporative Cooling

• Factory sheds
• Warehouse
• Laboratories
• Office complex
• Entrance lobbies
• Cafeterias
• Malls
• Lecture / training rooms
• Function halls
• Kitchen/Laundry

• Extremely low power consumption relative to conventional ACs: EER of EAC ~ 20 in Hot & Dry Regions vs. EER of 2.8 for 3-Star AC
• Use of natural refrigerant for cooling: Water
• Much lower capital cost and life-cycle operational cost, energy consumption, and GHG emissions
• Better building occupants’ health condition due to more fresh air intake and circulation

• Lobby Atrium of 18,000 ft2 is cooled only with Indirect-Direct Evaporative Cooling; no use of air conditioning
• The atrium door needs to be open at all times to enable free movement of people
• Space temp. < 300 ℃
• Water addition max 500 liters/hr
• Peak ventilation air required – 136,000 m3/ hr
• Peak power requirement to cool the space is 48 kW/hr
• Installed with 4 X 34,000 CMH capacity cooling system

Summertime temperature less than 300 C with an ambient temperature of 400 C

View Evaporative Cooling resources for Architects and for Engineers.