Evaporative cooling is responsible for the chill you feel when a breeze strikes your skin. The air evaporates the water on your skin, with your body heat providing the energy. Traditionally, since ancient times, people used to hang wet mats in their doors and windows, and wind blowing through the mats cooled the air, it was one of the earliest attempts at air cooling. This basic idea was refined through the centuries, mechanical fans to provide air movement in the 16th century, cooling towers with fans that blew water cooled the air inside factories in the early 19th century, desert or swamp coolers in the 20th century.
Direct Evaporative Cooling
The simple examples given above illustrate direct evaporative cooling. Here, outside air is drawn (by means of a fan) through water saturated medium or pad and cooled by evaporation. A re-circulating water pump is used to keep the pad wet.
Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation. The dry bulb temperature is reduced, while the wet bulb temperature stays the same. The total heat content, or enthalpy, of the air remains unchanged since the latent heat required to vaporize some of the water is drawn from the air which gets cooled in the process.
Type of Pads
The most common wetted pads used in evaporative cooling are made of shredded wood fibers, which have a naturally wetable surface, packed in the plastic net. These pads are 20 to 25 mm thick. The thicker pads are more effective since they can hold a larger quantity of water. Fiber pads must operate at low air velocities (less than 1 m/sec) to prevent water from being pulled off the pad by the air stream. They are therefore used on coolers that have air inlets on many sides. The pads are simply discarded every year or two and replaced with new ones. Wood fiber pad coolers usually cost the least and require the most maintenance.
More advanced evaporative coolers use a rigid sheet pad, comprising a stack of bonded corrugated cellulose sheets. The pads have a corrugation or fluted pattern that forces water to flood the pad’s air inlet side where most of the evaporation of water occurs. The pads may also be chemically impregnated to stiffen the material, improve wetability and prevent decay and decomposition. These pads are usually 100 to 300 mm thick.
Rigid pads allow air to move at higher velocities (of up to 3 m/sec) than is possible with wood fiber pads. Coolers using rigid sheet pads usually have a single air inlet (and are often referred to as “single inlet coolers”). These pads are substantially more expensive than wood fiber pads, but they can last for many years if water quality is properly maintained with a bleed off or sump dump system. Therefore the life cycle cost for three pads can equal the cost of fiber pads.
Indirect Evaporative Cooling
With indirect evaporative cooling, a secondary (scavenger) air stream is evaporatively pre-cooled by water by direct contact. The cooled secondary air stream goes through a heat exchanger, where it indirectly cools the primary air stream (without coming into contact with it).
Indirect evaporative cooling units are used as pre coolers with some very distinct advantages :-
To provide sensible cooling at a high energy efficiency ratio. Compared to conventional refrigerated units the cooling output, per watt of input, can be increased ten or more times, significantly decreasing operation costs.
To pre-cool the (fresh) air entering cooling coils in a conventional air-conditioning system. This will reduce the sensible cooling load resulting in a reduction in the size of coil, refrigeration unit and heat rejection unit. The connected electrical loads, as well as operating costs, are reduced.
To sensibly pre-cool the air to direct evaporative units to increase their effectiveness. This is the two stage system concept.
Double Stage Air Washers
We are also providing two stage cooling Air Washers. In two stage cooling (also referred to as indirect / direct evaporative cooling), the primary air stream is first pre-cooled sensibly by indirect evaporative cooling. Since this pre-cooling adds no humidity to the air, it can still be subsequently direct evaporatively cooled, which is carried out in a direct evaporative section with a rigid pad. However, since the pre-cooled air can hold less moisture, the final relative humidity is lower (about 60% to 65%) than that reached with direct evaporative cooling.
Advantages of Two Stage Evaporative Cooling
Two stage cooling allows the use of energy saving evaporative cooling technology for design conditions where direct evaporative cooling is inadequate.
The energy consumption is around half than that of air conditioning, while the capital cost is also significantly lower.
In many cases two stage systems can provide better comfort than a compressor based system because they maintain a more favorable indoor humidity range. Hence, they can replace mechanical refrigeration in many applications.
The conditioned space is cooled by 100% fresh air, hence there are no IAQ related problems. Typically, two stage cooling systems are designed for 25 to 40 air changes per hour, similar to ventilation.
Since no refrigerant is used, the system is environmentally friendly.
Disadvantages of Two Stage Evaporative Cooling
It is less effective in coastal areas which have high relative humidity.
There is a significant temperature variation in the cooled space round the year, depending on the prevailing ambient dry and wet bulb temperatures.
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