Types of Evaporative Cooling Systems | Evaporative Cooling ~ Biography of Stars, Shayari, Latest News

1.5 Types of Evaporative Cooling Systems

Two principle methods of evaporative cooling are

1) Direct cooling: In direct cooling water evaporates directly into the airstream, thus reducing the air’s dry-bulb temperature while humidifying the air.

2) Indirect cooling: In indirect cooling, one stream of air called primary air is cooled sensibly (without addition of moisture) with a heat exchanger, while the secondary air carries away the heat energy from the primary air. Direct and indirect processes can also be combined (indirect/direct). The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply air stream.

1.5.1 Direct Evaporative Cooling (open circuit)

Direct evaporative cooling introduces water directly into the supply airstream (usually with a spray or some sort of wetted media). As the outside air is blown through a water-saturated medium (usually cellulose) and cooled by the evaporation as the water absorbs heat from the air, it evaporates and cools the air. In direct evaporative cooling the dry bulb temperature is lowered but the wet bulb temperature remains unchanged. In operation, a blower pulls air through a permeable, water-soaked pad. As the air passes through the pad, it is filtered, cooled, and humidified. The cooled air is circulated by a blower. A recirculation pump keeps the media (pad of woven fibers or corrugated paper) wet, while air flows through the pad. To ensure that the entire media is wet, more water is usually pumped than can be evaporated and excess water drains from the bottom into a sump. An automatic refill system replaces the evaporated water.

Direct evaporative cooling adds moisture to the air stream until the air stream is closed to the saturation.

Direct Evaporative cooling

1.5.2 Indirect Evaporative Cooling (closed circuit)

Indirect evaporative cooling lowers the temperature of air via some type of heat exchanger arrangement, in which a secondary (scavenger) airstream is cooled by water and the cooled secondary air stream passes through a heat exchanger where it cools the primary airstream. The cooled primary airstream is circulated by a blower. The cooled air never comes in direct contact with water or environment. In indirect evaporative cooling system both the dry bulb and wet bulb temperatures are reduced. Indirect evaporative coolers do not add humidity to the air, but cost more than direct coolers and operate at a lower efficiency.

Indirect Evaporative cooling

1.5.3 Two-stage Indirect/direct Evaporative Cooling

Two stage evaporative coolers combine indirect with direct evaporative cooling. This is accomplished by passing air inside a heat exchanger that is cooled by evaporation on the outside. In the second stage, the pre-cooled air passes through a water-soaked pad and picks up humidity as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air, whose affinity for moisture is directly related to temperature. The two-stage evaporative cooling provides air that is cooler than either a direct or indirect single-stage system can provide individually. In many cases, these two-stage systems provide better comfort than a compressor-based system, because they maintain a more favorable indoor humidity range. An advanced two-stage evaporative cooler uses 100 percent outdoor air and a variable speed blower to circulate cool air. Two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems, according to the American Society of Heating and Engineers (ASHRAE). Yet this relative improvement depends on location and application.

Two stage Evaporative cooling

In first stage, the primary air stream is cooled by indirect evaporative cooling. In second stage, water used in first stage cooling passes through the wet side of a coil. Additional sensible heat is removed from the primary air stream, and no moisture is added to the primary air.

To read more more about Evaporative Cooling - See our series on evaporative cooling click here