Evaporative cooling

Pro May 5, 2020 Winter

Evaporative cooling is the cooling effect provided by the adiabatic evaporation of water, guaranteeing lower energy consumption than compressor cooling.

Evaporative cooling and evaporative air conditioning are the most underutilized technologies of air conditioning in Spain, perhaps due to ignorance of the available technologies in our country. In dry regions, evaporative air conditioners provide cooling, whereas in the humid regions they are used for more specialized applications. Evaporative air conditioners can be used effectively in houses and in all types of commercial establishments. This type of air conditioning consumes less energy than compressor cooling by varying the energy consumption according to the humidity and temperature levels. Savings can reach up to 60 to 80% of energy consumption in regions with low humidity.

One of the benefits of evaporative air conditioning is that it does not use CFC refrigerant gases and only requires water returning to the atmosphere as steam. This allows for the elimination of CFCs and the gases responsible for the greenhouse effect and the destruction of the ozone layer.

Evaporative air conditioning is competitive both in capital and acquisition costs and in operating and maintenance costs, especially when compared to the equivalent installation of compressor air conditioning. Evaporative cooling systems create an overpressurized environment by generating a constant flow of air by injecting large quantities of fresh and clean air, moistened after passing through a few running water filters. Overpressure is generated by creating a balance between the incoming and outgoing air. Any moisture problems are caused by an imbalance between the incoming air and the outgoing air. This problem is solved by increasing the size of the air vents to generate more air flow.

Generally, each air conditioning unit requires 4m² of air outlet, usually created by opening the doors or windows. In case the environment has to be hermetic, for production reasons, it is sufficient to install air extractors that allow the proportion between the incoming and outgoing air to be maintained.

The amount of water consumed depends directly on the temperature, the relative outdoor humidity and on the performance of the air conditioner. For example, during 8 hours of operation, with an outside temperature of 36ºC and an average relative humidity outside of 40%, an evaporative air conditioner consumes 180 liters of water. This amount of water is similar to what is needed to fill a domestic bathtub and almost the same amount of water that is used in a 10 minute shower.

With evaporative air conditioners, the drier the climate is, the greater the cooling capacity will be, although they are also effective in humid climates. What really needs to be considered is the difference in temperature outside and inside. Evaporative air conditioners are able to reduce the internal temperature to 14ºC in dry climates (20 to 30% of average relative humidity), and to 5ºC in very humid climates (70 to 80% of average relative humidity) with respect to the outside temperature.

For example, in industrial environments where, for production reasons, the heat inside the building is usually higher than outside, even with relatively high humidity, the evaporative air conditioners are still the best option.

There are two models of evaporative air conditioners, direct and indirect. In direct air conditioners, the water evaporates in the air stream, reducing the dry bulb temperature, while increasing the humidity in the air. They use a humid panel through which they circulate a large flow of outside air. This hot, outdoor air passes through moist, hot air pads, evaporates some of the water and cools down. The cold air obtained, which has increased its relative humidity, is driven into the space to be heated. The large incoming volume fills the entire space, by overpressure, renewing every few minutes. The hot air, dirty with fumes or odors, is expelled outside, and in its place the new filtered air remains, humidified and cooled, and that will be renewed every few minutes.

Indirect evaporators use the base of the direct evaporative system to cool a network of small ducts, which in turn cool the air that circulates through other small ducts. The first air (hot and humid) is expelled to the outside, and the air that is introduced into the room is the air that circulates through the other channels of the cold exchanger, but unlike direct evaporative, this air does not acquire humidity.

Neither system uses refrigerant gases to cool, but need only water and evaporation. The results are surprising when consuming 1/10 of the electricity of conventional cold systems and reducing the temperature by 15ºC compared to the outside temperature. Proof of its capacity is that a standard evaporative equipment of 12,000 m3 of flow, having a maximum consumption of 1 kw/h, provides air conditioning to 200 m² of surface at a comfortable temperature in the moments of maximum heat and an average consumption of 400w/h.

This consumption represents savings in CO2 emissions. Each electric Kw not consumed is 0,40 kg of CO2 not emitted into the atmosphere, so the standard equipment of 12,000 m3/h saves 4.320 kg of CO2 per year (calculated on 9 kw/h X 0,40 Kg CO2 X 8 hours / day x 150 days / year). If the savings is multiplied for the existing air conditioning equipment, the results are impressive.