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Evaporative Cooling of Air Conditioning Coils

Many parts of the country experienced extremely hot and dry weather this past summer. One effect of this was increased electricity demand for air conditioning in houses and places of business. An air conditioner works by compressing a refrigerant gas, which increases its temperature. The hot gas is contained in coils and outside air is forced over the coils, transferring thermal energy into the surrounding environment. After running through the exterior coils under pressure, the gas is allowed to expand in the internal chilling unit. The expansion of the gas causes its temperature to drop. This cooled gas is then put through another set of coils on the inside of the house and inside air is forced over these coils, cooling and drying the air inside the house. In dry parts of the world, people use a similar process to run evaporative coolers (sometimes called “swamp coolers”) that rely only on the evaporation of water to cool the air and not on the compression and expansion of a refrigerant gas.

Temperature and relative humidity changes as water is sprayed on AC coils

David Carter, Vernier’s STEM Training Director, wondered what effect using evaporative cooling of the outside AC coils would have on his central-air system. He used two Stainless Steel Temperature Probes to measure the ambient air temperature and the temperature of the air ejected from above the external fan of the AC unit. In addition, he placed a Relative Humidity Sensor above the unit. When the unit came on, he let it run for a few minutes and then sprayed the coils gently with water from a hose. The graph above shows the results of this experiment.

The green line represents the temperature of the ambient air. The red line is the temperature above the unit where the air drawn through the coils is exhausted. The blue line is the relative humidity of the air above the unit. Notice that the ambient air temperature remains fairly constant for the entire experiment, while the exhaust air temperature fluctuates widely as the coils were sprayed and then allowed to dry.

The first increase of the exhaust air temperature shows the temperature of the air exhausted by the outside unit as the unit turns on. Notice that it is about seven degrees warmer than the air entering the coils. The exhaust air temperature dropped by ten degrees, even dropping to a temperature below ambient, when water was sprayed on the external coils at the six-minute mark. This cooling effect was caused by the evaporation of the water on the coils. The relative humidity of the air above the unit increased by 50% when the coils were sprayed. It takes a tremendous amount of energy for water to undergo a phase change from liquid to gas, causing this drop in temperature. At ten minutes, water was no longer being sprayed on the coils and the air coming out of the AC unit returned to being warmer than ambient and the relative humidity dropped again.

The data show that spraying water on the coils causes the exhaust temperature to drop below ambient temperature. This may be a way, in the future, to reduce the energy consumption of air conditioning by making it easier for AC to transfer heat to the hot, outside environment.

An idea for further study would be to use an Extra Long Temperature Probe to measure the temperature of the air coming out of the inside vent pipe.

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