How does Evaporative Cooling work?

evaporative cooling

Evaporative Air Conditioning uses the natural process of evaporation to provide cool air throughout your home.  The process is identical to a refreshing ‘sea breeze’ – as air is blown across the water it gives off its heat and we enjoy the cool air.

The temperature that we feel as a direct result of air movement across our body on a hot day is known as the effective temperature.  An effective temperature should not be confused with the actual temperature that you would read from a thermometer.

When sensing a breeze, we feel colder even though the air stream produced by the breeze is the same temperature as the air.  The cooling effect of air in motion is due to increased skin evaporation.  Heat is removed from the body so you feel cooler.

By moving a large volume of cool air through the house, usually changing all the air 35 times per hour, the cool air removes the heat build-up in the walls and structure of the house itself.  This provides a nice, comfortable home filled with fresh, filtered, cool air.

THE TECHNICAL SIDE OF EVAPORATIVE COOLING

Most of Australia is blessed with a climate that is ideal for evaporative air conditioning.  It is the natural and healthy way to cool your home.  Unlike refrigerated air conditioning systems where the air is dried, chilled and re-circulated, evaporative systems use the natural element of water to cool the air.  With evaporative cooling the air is never re-circulated as stale air is constantly expelled through open doors and windows.  Fresh, filtered air is always used to cool your home.

REDUCTION OF RADIATED HEAT TO OCCUPANTS

  • Normally heat radiates from the body out to surrounding walls and ceilings
  • As the temperature reaches 34o – 35o, heat is radiated back to the human body
  • Evaporative air conditioning prevents this by absorbing heat from walls/ceilings with a constant flow of cool air
  • The human body can then radiate heat and so feels cooler

SO HOW COLD DOES AN EVAPORATIVE AIR CONDITIONER GET?

  • An easy answer is to just “estimate” around 12 – 14 deg C temperature differential between inside your house to the outside temperature.

But there is a bit more science to it!

It does involve two important readings –

  • Dry Bulb Temperature (Tdb) – The ambient temperature taken with a standard thermometer
  • Wet Bulb Temperature (Twb) – The theoretical lowest temperature to which a given body of air can be cooled by evaporating water in it.
  • Wet bulb temperature is affected by humidity, height above sea level, dry bulb temp and a number of varying factors

Calculating an estimated supply air temperature (from the unit) –

  1. Take the difference between the Dry Bulb and Wet Bulb temperatures (eg: 38 – 21 = 17o)
  2. Multiply this number (17o) x the cooling efficiency of the filter pads (usually about 80-85%)
  3. 17 x 80% = 13.6o
  4. Subtract this number from the Dry Bulb temperature … 38o – 13.6o = 24.4o … and that’s the expected cooling ability.

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