What is an Evaporative Cooler?

Basically, an evaporative cooler is a piece of equipment that cools the temperatures in your space. It does that through vaporization of water from a flat surface, typically a pad.
Usually, the evaporation of water from a body reduces its temperature. Unlike common belief, evaporating water from a body doesn’t necessarily make said body warmer.

An evaporative cooler is different from other types of air conditioning system in that it uses absorption refrigeration cycles. An evaporative cooler makes use of the fact that when water is absorbed, it absorbs a lot of heat in the process.

The temperature of the drying air is lowered significantly while liquid water is being converted into vapor. Accordingly, the air cools without using too much energy. In arid climates, an evaporative cooler not only cools the air, but also makes a little bit moist and much more bearable. The moisture makes the environment much more conductive.

The evaporative cooler has the potential to cool places without relying on a wet-bulb depression. In more arid climates, an evaporative cooler has the power to reduce the amount of energy it consumes and uses compressor-based cooling methods to condition the air.

In less arid climates, the evaporative cooler uses a cooling process that involves evaporation. Meanwhile, it doesn’t increase humidity. The process isn’t very different from that of the passive evaporative cooler, but comes with much less complex equipment and ductwork.

Brief History Behind the Schematic Design of a Typical Evaporative Cooler

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The usage of evaporative coolers dates back to the Ancient Egyptian and the Persian Empire. The windcatcher has been used for the longest time in history and can be dated back to thousands of years. They were used as a shaft for wind on rooftops of houses and buildings.

As its name suggests, the windcatcher catchers wind and passes it through a series of channels and expels the cooler air into the interior of your space. In modern Iran, the ancient invention has been widely re-adopted as the evaporative cooler.

In the United States, as well, different kinds of evaporative coolers have been in many states during the 20th century. Their usage started in 1906. One of their more distinct designs appeared as a patent in 1945. It had a water reservoir controlled on several levels by a floating valve. 

This design also has a pump that is used to transport the water through the pads, followed by an exposure to a centrifugal fan to pull the cooled air from the pads and into your space.

This distinct design and its material are still very prevalent in the evaporative cooler market. In the American Southwest climate, they seem to be more popular, as they can increase humidity. In the United States, they are commonly referred to as swamp coolers. This is because the earlier units produced algae and bad smell.

They can be used indoors but can also be mounted outside and used externally. They can even be used in vehicles to cool their interiors. Those are commonly referred to as car coolers. Externally mountable evaporative coolers are sold as outdoor accessories.

Alternatively, there’s a passive evaporating cooling technique. In this technique, the knowledge of William H. Goettl, who observed evaporative cooling technologies, is implemented. He invented the Astro air cooler in 1971, which combines refrigeration cooling and evaporative air cooling techniques.

Two researchers named W. Cunningham and T. Thompson developed a passive evaporation cooler in the University of Arizona in 1989. They invented an evaporation cooler tower in 1986, and the performance data is astounding. From then on, Arizona has become a foundational pillar for evaporative cooler design guidelines.

The Physics Behind the Evaporative Cooler

The air temperature is lowered with the help of an evaporative cooler by converting water into vapor using thermal energy that is already present in the air. It results in brisker breezes and cooler temperatures.

The required energy for the process is taken from the air. It’s known as sensible heat, as it uses wind energy to affect air temperature. It also converts energy to latent heat, which is the energy in the water vapor. The process of conversion from sensible heat to latent heat is named the Isenthalpic process. It typically takes place in a constant enthalpy value, in this case, the air.

Evaporative coolers can be simplified even more. If you a psychrometric chart, which moves the air through a process of constant enthalpy, your cooler will produce higher humidity.

Evaporative cooling happens naturally all the time. Sweat and perspiration from heat or excessive physical work as a form of evaporative cooling. It’s an autodfensive process that helps cool your body’s temperature. Evaporation rates vary from one body to the other.

Evaporation rates mainly depend on the outside temperature as well as on the humidity. This is exactly why people sweat more than usual when it’s humid. Looking closely at vapor-compression refrigerators, one notices that they use an evaporative cooling process. Because the vapor goes off into a closed space, it compresses to re-evaporate.

Water that evaporates isn’t recovered. Interior cooling units, however, keep the newly cooled air in an evaporation cooler and pass it through airflow exhausts.

Other Types of Cooling Phases

Another similar process is sublimation cooling. Sublimation cooling is different in that it transforms water from solid (ice) to vapor as opposed to from liquid to vapor. When observed closely, sublimation coolers can be anti-greenhouse.

Another use of phase change for cooling is the self-refrigerating process. This process can easily be applied in a soda can. There is a separate compartment in the can that contains the desiccant to be mixed with the liquid. 

When it dissolves, it absorbs latent heat. It should also be noted that the evaporative cooling work also in the phase change of liquid to vapour and the latent heats of vaporization, as the self-cooling can also make use of a change in solid to liquid with the latent heat of fusion to get the same results.

Design and Working Principles of an Evaporative Cooler

A lot of designs have been based on the fact that water has the highest vaporization enthalpies. As a result, evaporative coolers use very minimal energy for vapor compression. In arid climates, however, a single-step evaporative cooler would increase humidity to uncomfortable stages.

The Direct Evaporation Cooler

This system, which is also known as the open circuit, is used for lowering temperatures. The temperature increases humidity through its evaporation process and creates latent heat. During this conversion, the energy doesn’t change the air around it. Warm, dry air is converted to moist, cool air.

Outside heat is utilized for vaporizing water. As this increases relative humidity to about 70-90%, it causes a reduction of temperature by cooling down, the same way a human body does when it sweats. Moist air is released outside, making the space cooler and avoiding the saturation of the equipment.

The Mechanical Direct Evaporation Cooler

This is a unit that uses a fan to draw air onto a wet membrane and makes the water evaporate from the surface into your space. Then, from the top of the membrane, water is sprayed down onto its bottom to help keep it saturated. Excessive water would drip to the lower part of the pad, and get collected in pans to be re-circulated and re-used in cycles.

It is worth mentioning that direct evaporative coolers are smaller, more compact, and easier to carry around. They consist of the membranes, water, the pumps, and centrifugal fans. Mineral contents in tap water may cause scaling of the membrane, as well as clogging in the pipes. Regular cleaning and maintenance are crucial to keep your cooler alive and its performance at an optimal level.

Therefore, the air supplied from the single-stage evaporative coolers needs to be vented directly. That’s because of the high humidity levels. Very few designs have been able to use the present energy by manipulating the exhaust air in a two-sheet or double-glazed window. As a result, the amount of solar heat absorbed in the glazing process makes the air cooler. 

In comparison to the amount of energy need to get comparable cooling results from a compressor, the evaporative cooler proves to be much more efficient.

The Passive Direct Evaporative Cooler

This is a system that takes place in an evaporatively cooled space. Water can be used to cool different space without the assistance of fans. This is achieved by using fountains or downdraft cooling towers- commonly referred to as passive cooling towers. 

Passive cooling towers allow the passage of the outside air to flow down from the top of the tower. There is also a contact between the outside air and the water in the tower by means of a wet membrane.

As soon as the water begins to evaporate, the air from the outside becomes more refreshing as it allows a downward flow. Also, at the tower's bottom, there is an outlet that lets the cool air into the building. Just like the mechanical evaporative cooler, the towers can also be attractive for a low energy solution in hot and dry climates. That’s because they only need pumps to raise water.

For the passive evaporative cooling strategy to save energy, it also relies on the heat and climate.

An Indirect Evaporative Cooler

This cooler system, otherwise also known as a closed circuit, makes use of direct evaporative cooler methods with the help of a type of medium. It uses the medium to transfer the energy and them supplies fresher, cooler air. Furthermore, the cooled air doesn’t get in direct contact with the moist air as it does in the direct evaporative process. After that, the device releases the moist stream out of its vents to cool its exterior. It’s a really efficient way of keeping the device from overheating.

One evaporative cooling system that uses the indirect method is called The Maisotsenko Cycle. This cycle employs an iterative or multi-step heat exchanger that is capable of reducing the temperature.

Passive, Indirect Evaporative Cooler

This is cooler is quite a rare commodity. It uses architecture elements that act as heat exchangers. These elements can be cooled by the water’s evaporation from the surface of the elements. This passive, indirect evaporative method isn’t very common because of its high water consumption.

Different Applications of the Evaporative Cooler

Basically, residential or industrial evaporative coolers make use of direct evaporation. There is also a centrifugal-like fan that moves the air, or a blower that is usually powered by an electric motor with pulleys, or an axial fan that is directly driven by the power outlet. There is also a water pump that is used to wet the cooling pads

There are several places you can mount the cooling unit; it can be mounted on rooftops (downdraft, or down-flow), or it can be mounted on the walls, or , or around the exterior of the building.

For this device to work, the fan has to first draw ambient air into the vents at the unit’s sides onto the damp pad. The pad is constantly re-damped to continue the cooling process. Finally, cool moist air is delivered into the space through a vent in the roof or wall. Some of the typical applications of the evaporative cooling system are as follows:

Evaporative Cooling Tower

In the evaporative cooling tower, the towers are constructed to cool the water. Another transfer medium of heat close to the wet-bulb ambient controls the temperatures. The cooling tower works on the same principles as an evaporative cooling system, but they are optimized for cooling the water and not the air. The cooling tower can be found in most industrial buildings and other large spaces.

Misting Systems

The idea behind the misting system is that it forces water through a high-pressure pump as it goes through brass-stainless steel nozzles with an orifice close to 5 micrometers. As a result, they produce a very fine mist. Flash evaporation can be used to reduce the surrounding air temperature by as much as 35oF or 20oC in just a few seconds. This misting system can be used for an application in a lot of types of spaces.

Evaporative Coolers Pros and Cons

When you compare the design of an air conditioner to the design of an evaporative cooler, there is a significant difference between them; even though they both have one goal, which is to make the air cooler. In a lot of places, an evaporative cooler is an ideal choice for a perfect air conditioning; however, it has its pros and cons.

PROS

  • Evaporative coolers are very energy-efficient as they can use up to 75% less electricity than a typical air conditioner
  • They have also been designed to filter the air thereby making the air feel fresher to breathe
  • The evaporative coolers are eco-friendly as they do not make use of refrigerants such as hydrochlorofluorocarbons (HCFCs)
  • They require very minimal maintenance due to their simple design
  • The evaporative coolers are very easy to install

CONS

  • The evaporative coolers are not ideal for humid areas
  • The evaporative coolers also build up salts and minerals deposits which need to be removed from time to time
  • Although they are easy to maintain, the pad has to be kept moist at all times, and if at any time the pad gets dried, the evaporative cooler would not be efficient
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