Evaporative air cooling systems draw air over wetted pads and surfaces. This results in the evaporation of the water into the air which reduces the air temperature and increases its moisture content. The cooled humidified air is distributed inside to the space, or process to be cooled, before leaving via natural relief or mechanical ventilation openings.
Evaporative Air coolers use the water to cool the air. Some system like cooling tower use the air to cool the water
How evaporative air cooling systems work
Evaporation is the conversion of a liquid substance into the gaseous state.
Have noticed a temperature drop after a light shower of rain? Its evaporative cooling effect. Perspiration & sweating is also the example of very effective evaporative cooling process adopted by human body.
Heat energy from air is supplied to change the water from liquid to gaseous state which brings down the temperature of air.
In the case of sweat, the energy is supplied from the body in the form of a heat flow resulting in a temperature drop of the skin.
When air moves over a surface of water it causes some of the water to evaporate. This evaporation results in a reduced temperature and an increased vapor content in the air. The bigger the area of contact between the air and water the more evaporation occurs, resulting in more cooling and the addition of moisture.
Evaporative air coolers rely on the difference between the wet bulb and the dry bulb temperatures for their cooling effect. The wet bulb temperature is different from the dry bulb temperature that we are used to considering. The wet bulb temperature is a function of the dry bulb temperature and the moisture content of the air. It is easily measured using an instrument such as a sling psychrometer; it can be calculated from air dry bulb temperature and relative humidity data or read directly off a psychrometric chart.
Applications:
This System are effective in hot, dry, low humidity climates and where increased humidity levels do not adversely affect comfort or process.
Typical applications are:
- Data centers
- Schools
- Factories
- Green Houses
- OEM’s etc
There are two types of Evaporative Cooling system
- Direct Evaporative cooling
- Indirect evaporative cooling
Direct Evaporative Cooling
Direct Evaporative coolers are commonly used for residential cooling. Developments in the evaporative cooling industry have reliably increased the efficiency or effectiveness of the cooling media. All direct EAC’s use 100% outside air. Electricity is used by a supply fan motor and a small sump pump.
Indirect Evaporative Cooling
The other type of EAC is called “indirect” because the evaporative cooling is delivered across a heat exchanger, which keeps the cool moist air separated from the room air. These indirect evaporative air coolers (IEAC’s) can be used in conjunction with direct EAC’s and/or with refrigerated air coolers. IEAC’s use electricity for the supply fan motor, a sump pump, and a smaller secondary fan motor used for the heat exchanger’s airflow.
IEAC’s use a heat exchanger, and do not add moisture to the room air stream (known as sensible cooling). There are four different types of IEAC’s, and all of them use the same evaporative cooling process as direct evaporative cooling, known as adiabatic cooling. The main kinds of IEAC’s predominately in use are:
- Air-to-air heat exchangers;
- Combination IEAC and refrigerated systems;
- Cooling tower “free cooling”;
- Refrigerant migration.
IEAC’s can be used in combination with direct evaporative cooling, in combination with refrigerated air systems or as a stand-alone system. When combined with direct EAC’s the effectiveness is additive. “Typical indirect/direct evaporative air coolers have a rated effectiveness of 120 to 130 percent”
- Air supplied to the conditioned room by main supply fan is called the primary air stream
- Air-to-air heat exchanger consisting of alternating dry and wet channels.
- All of the primary air passes along the dry channels and gains no additional moisture.
- A portion of the conditioned air is returned through the wet channels.
- Through evaporation and conduction, it gains both moisture and heat. The channels are continuously soaked with water. This moist, warm air is then exhausted outside of the building.
- No moisture is transferred across the membranes between the dry and wet channels; only temperature (heat) is transferred.
- The heat passes out of the air in the dry channels through the membrane and into the air passing through the wet channels.
- In this way, the air in the dry channels becomes progressively colder but gains no moisture.
- This fresh, cool air passes into the building.
- Further it can also be passed through Direct evaporative cooling system where it gets further cooled with little addition of moisture(in case of Indirect+direct evaporating arrangement).
Evaporative cooling effectiveness
The difference between the wet bulb and the dry bulb temperatures of the outdoor air is called the Wet Bulb Depression (WBD).
A properly designed and installed evaporative air cooling system can reduce the dry bulb temperature of the outdoor air by a factor equivalent to the rated evaporation efficiency of the evaporative air cooler, multiplied by the wet bulb depression of the outdoor air. For example, for a direct evaporative air cooler rated at 80% evaporative efficiency and with a wet bulb depression of 14°C, the evaporative air cooler should reduce a 35°C outdoor dry bulb temperature to:
so, 35-(0.8×14)= 23.8 deg C
That is 11.2 deg C temperature reduction for the system operating in this condition.
Psychrometric Representation
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