Evaporative Cooling

Khaled Eyad Almusa

013 2 95 2

The concept of evaporative cooling came from the sweating of our bodies!

The rate of evaporative cooling is a function of both humidity and air movement.

When sweat evaporates from the skin, a large amount of heat is required. This heat of vaporization is drawn from the skin, which is cooled in the process. The sensible heat in the skin is turned into the latent heat of the water vapor.

As water evaporates, the air next to the skin becomes humid and eventually even saturated. The moisture in the air will then inhibit further evaporation. Thus, either air motion to remove this moist air or very dry air is required to make evaporative cooling efficient.

Evaporative cooling is something that we have all experienced. Wearing a damp tee shirt on a warm but windy day gives us a chill. The phenomenon that causes this is the latent heat of vaporization.

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What does all this mean? ss

At critical temperature points in the diagram (0 degrees Celsius and 100 degrees Celsius) water needs to draw in heat energy from the environment to change phase.

In order to melt or evaporate the water requires energy from the environment: this is the latent heat of vaporization.

So Evaporation is rapid when the humidity is low and air movement is high. Evaporation is slow when the humidity is high and air movement is low.

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The reason we care about this in terms of evaporative cooling is that the more energy that water draws the more we can cool the contents of our Evaptainer. By the numbers this shakes out to 1g of evaporated water reducing the temperature of 1kg of water by half a degree

However, this assumes 100% efficiency. Celsius.

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The energy required to free an atom from the liquid is equivalent to the energy needed to overcome the surface resistance of the liquid. You may remember from school that water has relatively high surface tension from its hydrogen bonds, thus water needs to absorb a large amount of energy to go through a phase change.

What impacts this rate of efficiency is ambient conditions or "Wet Bulb to Dry Bulb" :

What does that

mean ? 3

The potential for evaporative cooling depends on the difference in wet bulb and dry bulb temperatures of the air. Humid air has a high relative humidity, and not as much capability to evaporate moisture.

As the relative humidity of the air increases, the performance of the system will decrease, limiting its application in moist climates.

Evaporative cooling is most effective in climates where average relative humidity is less than 30%. As humidity increases, and the cooling capability declines, the temperature difference between the outside and inside of the chamber decreases.

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To test if evaporative cooling wet bulbwill be effective, the

temperature can be measured by on the end of moist clothplacing a

and waving it thermometera through the air. The temperature read by the thermometer is the theoretical minimum temperature that can be achieved through evaporative cooling.

For a visual representation of this phenomenon we can use useful are a charts Psychometric .psychometric charts

bulb temperature given the -tool for predicting a particular wetoutside ambient conditions: pressure, temperature, and

humidity.

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Above and beyond Psychometric charts (which only have 3 variables) anything that increases the rate of evaporation of a system will make evaporative cooling more effective.

This includes: 1 Lowering ambient humidity 2 Decreasing atmospheric pressure 3 Increasing ambient temperature 4 Increasing surface area of evaporation 5 Choosing different evaporative media 6 Adding air movement/wind

Using all of these variables we are able to optimize the cooling effect of our system across the widest range of applications.

Evaporative cooling is provided by the evaporation of water:

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This system works in dry climates and uses less energy than conventional cooling but consumes water

A variety of thermal energy storage technologies are available. Ice storage is commonly used to shift peak electric loads from daytime to the night. Rather than saving energy or reducing carbon emissions, the focus of this technology is to reduce peak summertime electric demand and thereby reduce peak demand charges.

Depending on the site, ice storage can in some cases reduce energy and carbon emissions, but in other cases, it will increase energy use and carbon emissions. The substantial embodied energy of the storage vessel, which is often concrete, should be accounted for in the analysis.

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Evaporative cooling VS. Traditional cooling

While evaporative cooling is one of the oldest forms of climate control still found today, evaporative coolers remain a highly sought-after alternative to traditional air conditioning. While very effective and efficient when used properly, these cooling units are not for use everywhere. Due to the evaporative process used by these swamp coolers, these cooling units are best used in hot, dry climates, In these climates, however, an evaporative cooler can be your most effective and efficient means of climate control. There are many different benefits of evaporative cooling, but to truly understand them, you have to compare evaporative coolers to more traditional air conditioners.

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There are two types of Evaporative cooling:

Direct Indirect

With direct evaporative cooling, outside air is blown through a water-saturated medium (usually cellulose) and cooled by evaporation. The cooled air is circulated by a blower

Direct evaporative cooling adds moisture to the air stream until the air stream is close to saturation. The dry bulb temperature is reduced, while the wet bulb temperature stays the same.

With indirect evaporative cooling, a secondary (scavenger) air stream is cooled by water. The cooled secondary air stream goes through a heat exchanger, where it cools the primary air stream. The cooled primary air stream is circulated by a blower.

Indirect evaporative cooling to the does not add moisture

primary air stream. Both the dry bulb and wet bulb temperatures are reduced.

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Direct Evaporative Cooling

When water evaporates in the indoor air, the temperature drops but the humidity goes up. In hot and dry climates, the increase in humidity actually improves comfort. However, direct evaporative cooling is not appropriate in humid climates because the cooling effect is low and the humidity is already too high.

See the Figure below for the kind of climate that supports direct evaporative cooling:

When water evaporates in the indoor air, the temperature drops but the humidity goes up.

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The most popular form of direct evaporative with commercially cooling is accomplished

(swamp coolers). evaporative coolersavailable Although they look like active mechanical devices from the outside, they are actually quite simple and use little energy.

A fan is used to bring

outdoor air into the building by way of a wet screen.

A modest amount of water is required to keep the screen wet. To maintain comfort, a high rate of ventilation is required during the day.

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Misting the air has become a popular direct evaporative-cooling strategy in all hot climates but works best in dry climates. Water under high pressure is atomized into tiny droplets, which then readily evaporate to cool the air.

Misting is mainly used to cool outdoor spaces. Unfortunately, if the area is too sunny or too windy, the benefit of misting will be minimal. However the cooling effect can be significant in sheltered outdoor spaces and greenhouses.

Misting is often used more for the atmosphere it creates than for its cooling benefits.

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Indirect Evaporative Cooling

A critical aspect of evaporative cooling is that the heat of vaporization must come from what is to be cooled. Thus, spraying a sunlit roof is not especially good because the heat of the sun will evaporate most of the water. On the other hand, the heat to evaporate water at night or from a shaded roof pond comes mainly from the building itself.

The cooling effect from evaporation can also be used to cool the roof of a building, which then becomes a heat sink to cool the interior. This technique is an example of indirect evaporative cooling, and its main advantage is that the indoor air is cooled without increasing its humidity.

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This figure illustrates the basic features of roof-pond cooling. An insulated roof shades the pond from the sun. Openings in the roof enable air currents to pass over the pond during the summer. As water evaporates.

The pond will become cooler together with the ceiling structure, will act as a heat sink for the interior of the building during the winter, the pond is drained and the roof openings are closed. The main disadvantage of this system is the cost of the concrete or metal ceiling and waterproofing.

A clever alternative to the above roof pond is the roof pond with floating insulation.

At night a pump sprays the water over the top of the insulation, and it cools by both evaporation and radiation.

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Meanwhile, the water together with the roof structure acts as a heat sink for the interior. Although the cooling occurs only at night, it is very effective because of the combined action of evaporation and radiation.

When the sun rises the pump stops and the water remains under the insulation, where it is protected from the heat of the day.

A more conventional version of the water spray design is to store the cooled water in a tank and to precool the floor slab at night.

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Cases for Direct and Indirect Evaporative coolers

Evaporative coolers are widely used in hot and dry regions.

- Direct evaporative cooler on the roof a house :

- An evaporative cooler, Colorado, used in the drier parts of the American West provide economical cooling:

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- Indirect Evaporative cooling unit:

Indirect evaporative coolers reduce the indoor air temperature without increasing its humidity.

- Figure for Indirect evaporative cooler:

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- Muscatese Evaporative cooling window system:

- Sample of Malqaf using fountain (Hassan Fathy 1986)

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Conclusion:

The process of evaporation happens all the time. Our bodies, for example, perspire in hot weather; through evaporation the sweat dries and drops our body temperature.

Whenever dry air passes over water, some of the water will be absorbed by the air. That's why evaporative cooling naturally occurs near waterfalls, at rivers, lakes and oceans. The hotter and drier the air, the more water that can be absorbed. This happens because the temperature and the vapor pressure of the water and the air attempt to equalize. Liquid water molecules become gas in the dry air, a process that uses energy to change the physical state. Heat moves from the higher temperature of the air to the lower temperature of the water. As a result, the air is cooler. Eventually the air becomes saturated, unable to hold more water, and evaporation ceases.

We can use evaporative cooling in our design, to raise the energy efficiency of the building, and reducing the need of mechanical equipment.

Evaporative cooling can be made by the design wise of the building or by using evaporative cooling units, even if these unit are actually mechanical equipment, their embodied energy is very low in comparison with other equipment.

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References ……………………………………………………………………

[1] HEATING, COOLING, LIGHTING, Sustainable Design Methods for Architects -Norbert Lechner

[2] Green Building Illustrated - Ching, Francis D. K.

[3] “Review Article: Passive Design for Thermal Comfort in Hot Humid Climates,” Journal of Architectural/Planning Research and Studies Volume 5

http://www.ap.tu.ac.th/jars/download/jars/v5-1/01 Review

[4] Application of passive cooling systems in the hot and humid climate: The case study of solar chimney and wetted roof in Thailand, Building and Environment

http://gse.cat.org.uk/downloads/passive_cooling.pdf

[5] “Experimental Studies on the Roof Pond House under Tropical Climatic,” 2006.

http://www.en.kku.ac.th/enjournal/th/images/stories/files/published/33No-5.pdf

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Evaporative cooling Report done by Khaled Eyad Nihad Almusa