How Evaporative Cooling Works
Cooling is provided by Evaporative heat exchange which takes advantage of the principles of the latent heat of evaporation where tremendous heat is exchanged when water evaporates. It makes use of the free latent energy in the atmosphere. Compared to air-conditioning which uses mechanical refrigeration, the operating cost of heat evaporative exchanging can be up to 80% less than air conditioning, however these systems do use water, so although they are cheaper to run in electrical terms they also use water were the standard air conditioning systems do not use any water.
Mechanics of Unit
Water is brought into the cooler from the mains water supply and is pumped up to the top of the unit using a circulation pump. The water is then dispersed over the Celdek pads using a water distribution system which allows the water to flow continually over the pad. The pads become saturated, air is drawn through the pads and the water evaporates causing the air to cool. The cool air is then ducted round the building or kitchen.
The hotter the outside temperature and the lower the humidity, the greater will be the cooling effect. On the hottest dry days in the Australia up to 15°C of cooling can be achieved through this process, however as the relative humidity increases the temperature difference decreases. This is why in Newcastle these type of systems are mainly used as fresh air make up for commercial kitchens as a cheap means of getting some tempered air into the hot kitchen. When we start to get further into the hunter valley and into the West were they experience dry heat, these type of systems start to get good temperature differences from outside ambient air to the tempered air drawn through the evaporative cooler and supplied into the building.
During a typical hot period the ambient temperature approaches 30°C. This coincides with a Relative Humidity of under 50%. As the air passes over the pads it will typically cool down to about 22°C. There is less cooling in the night as the temperatures reduce and the humidity rises.
Types of Evaporative Coolers
Most designs take advantage of the fact that water has one of the highest known enthalpy of vaporization (latent heat of vaporization) values of any common substance. Because of this evaporative coolers use only a fraction of the energy of vapour-compression or absorption air conditioning systems. Unfortunately, except in very dry climates, the single-stage (direct) cooler can increase relative humidity to a level that makes occupants uncomfortable. Indirect and Two-stage evaporative coolers keep the RH lower.
Direct Evaporative Cooling
Direct evaporative cooling (open circuit) is used to lower the temperature of air by using latent heat of evaporation, changing liquid water to water vapour In this process, the energy in the air does not change. Warm dry air is changed to cool moist air. The heat of the outside air is used to evaporate water. The Relative Humidity increases to 70 to 90% which reduces the cooling effect of human perspiration. The moist air has to be continually released to outside or else the air becomes saturated and evaporation stops.
Indirect Evaporative Cooling
Indirect evaporative cooling (closed circuit) is similar to direct evaporative cooling but uses some type of heat exchanger. The cooled moist air never comes in direct contact with the conditioned air. The moist air stream is released outside or used to cool other external devices such as solar cells which are more efficient if kept cool.
One indirect cooler manufacturer uses the so-called Maisotsenko cycle which employs an iterative (multi-step) heat exchanger that can reduce the temperature to below the wet-bulb temperature. While no moisture is added to the incoming air the RH does rise a little according to the Temperature-RH formula.
Conditioned air without added moisture increases the evaporation of perspiration improving the cooling effect of Indirect compared to Direct.
Two-Stage Evaporative Cooling
In the first stage of a two-stage cooler (or indirect-direct cooling), warm air is pre-cooled indirectly without adding humidity (by passing inside a heat exchanger that is cooled by evaporation on the outside). In the direct stage, the pre-cooled air passes through a water-soaked pad and picks up humidity as it cools. Since the air supply is pre-cooled in the first stage, less humidity is transferred in the direct stage, to reach the desired cooling temperatures. The result, according to manufacturers, is cooler air with a relative humidity between 50-70%, depending on the climate, compared to a traditional system that produces about 70–80% relative humidity in the conditioned air.
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