How Evaporative Coolers Function

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A maintenance worker is performing upkeep on a commercial rooftop evaporative cooler. David Spates/Shutterstock

If you’ve ever gauged the wind’s strength by testing it with a damp finger, you’ve utilized evaporative cooling. This same principle cools you down after a swim and also powers one of the earliest and most straightforward types of air conditioning. Known as evaporative coolers in the United States, they have roots that trace back to ancient Egypt. They’re inexpensive, energy-efficient, and eco-friendly, but they do have some limitations. Therefore, don’t throw your standard air conditioner out the window just yet.

The ancient Egyptians required air conditioning, and they achieved it by hanging wet blankets across the doorways of their homes or having their servants fan them from large jugs of water if they were royalty. When hot, dry air passes over water, or preferably through it, the air cools off. Nowadays, we use electric fans instead of servants, but the principle of cooling the air by evaporation remains the same.

Unfortunately, evaporative coolers are not practical everywhere, and swamps are not suitable places for swamp coolers. It’s unclear where they got their name, but it probably refers to the humidity they add to the air or the swampy odor that can develop if not cleaned frequently enough.

Swamp coolers are utilized all over the world, but they require a hot, dry climate to function. In the United States, they work well in the arid western and southwestern regions of the country. The global market for evaporative coolers was valued at $7.6 billion in 2019 and is expected to reach $19.8 billion by the end of 2026.

Swamp coolers are based on a simple, energy-efficient technology that has been around for a long time. The principles of evaporative cooling worked for the pharaohs, and they can still work for you.

Evaporative Cooling Basics: From Ben Franklin’s Undergarments to Modern A/C

July 1758 was a sweltering month in Philadelphia, Pennsylvania, with temperatures reaching 100 degrees Fahrenheit (37 Celsius). As he later wrote in a letter, Benjamin Franklin was in his room, reading and writing with “no other cloaths on than a shirt, and a pair of long linen drawers, the windows all open, and a brisk wind blowing through the house.” Even the founding fathers perspired, and as he changed into a dry shirt, he noticed something: it felt warm, as if it had been near a fire, compared to the damp shirt he had just removed. Franklin theorized that he wasn’t being cooled by the hot air blowing through his room, but by the perspiration evaporating off his skin. Later, he conducted some experiments, wetting the bulb of a thermometer, and watched as the temperature decreased. He discovered that liquids’ evaporation causes heat loss. What he described in his letter is what we now know as evaporative cooling.

Liquids evaporate by releasing molecules into the atmosphere, transforming from a liquid state to a gas. As the molecules become suspended in the air, they draw some of the heat from the hotter air, cooling it down as the water and air find equilibrium. The process also cools the remaining liquid, as the hotter, faster-moving molecules are more likely to escape into the atmosphere.

Swamp coolers function by utilizing this cooling reaction. The only thing necessary is a means to circulate the now-cooler, more humid air throughout the house.

When you think of an air conditioner, you may picture a metal box on the outside of a window. In contrast, a swamp cooler is much simpler, with a blower or fan that brings air in from outside and pushes it into the house after passing through damp pads for evaporation. To keep the pads moist, a small pump is used. This method goes back centuries, with the Egyptians and Benjamin Franklin using woolen blankets and sweaty shirts to cool themselves. Evaporative cooling is still used today in various forms, including to keep fruits and vegetables from spoiling in hot climates. Traditional air conditioners cool air using coils and re-circulate it through a closed system, while swamp coolers rely on the flow of air through the building. Both systems can use central or window units, but swamp coolers need a way for the air to escape. Windows and doors can be used to control the air flow, while air conditioner ducts direct the flow of air.

Standard air conditioners remove moisture from the air, condensing water vapor from the cooled room as it passes over the cold coils. This results in a drier room, which can be beneficial in humid climates where too much humidity can prevent perspiration. In contrast, swamp coolers add water to the dry air, making them act as humidifiers. This is ideal in drier climates where the humidity can be too low for comfort. However, because of their different operating methods, it is not possible to use a swamp cooler and a standard air conditioner in the same house as they would cancel each other out.

When comparing the two systems, swamp coolers have several benefits. They are affordable to build and install and require minimal materials. They also have lower operating costs than central air conditioning units and use 60-80% less electricity. This results in savings on energy bills and benefits the environment. Additionally, swamp coolers do not use refrigerants, which have negative environmental impacts.

Despite their advantages, swamp coolers only work effectively in certain climates. They are not suitable for areas with high humidity, such as Philadelphia. Ultimately, the choice between a swamp cooler and an air conditioner depends on the climate and personal preference.

Swamp coolers are effective in hot and dry conditions, as the cooling effect of evaporating water decreases when the dry bulb temperature approaches the wet bulb temperature. If the wet bulb temperature exceeds 70 degrees Fahrenheit (21 degrees Celsius), the swamp cooler may not be able to sufficiently adjust the temperature to maintain comfort levels. Excess humidity can also limit the cooling effect and cause problems like mold, mildew, and rust. Regular maintenance is necessary to avoid issues with air quality and mineral buildup. To determine the cooling effect, it is important to know both the dry and wet bulb temperature. The dry bulb temperature is the regular air temperature, while the wet bulb temperature is the temperature of adiabatic saturation measured with a thermometer covered in a wet cloth sock and exposed to airflow.

The wet bulb temperature is always lower than the dry bulb temperature. The difference between them is referred to as the wet bulb depression. The efficiency of your swamp cooler determines how much it can reduce the temperature, up to 95% of the wet bulb depression. For example, if you are in Las Vegas with an outside temperature of 108 degrees Fahrenheit (42 degrees Celsius) and a wet bulb temperature of 66 degrees Fahrenheit (18 degrees Celsius), an evaporative cooler operating at 85% efficiency can bring the temperature down to a comfortable 72.3 degrees Fahrenheit (22.3 degrees Celsius).

If you want to learn more about this topic, there is lots of information available. Check out the related articles, such as “How Air Conditioners Work,” “How Solar Air Conditioners Work,” “How Humidifiers Work,” “How Home Thermostats Work,” and “How Refrigerators Work.” You can also find more great links, including the Air Conditioning, Heating and Refrigeration Institute and the Department of Energy’s page on evaporative coolers.

FAQ

1. What is a swamp cooler?

A swamp cooler is a type of air conditioner that uses water to cool the air. It is also known as an evaporative cooler.

2. How does a swamp cooler work?

A swamp cooler works by drawing hot, dry air into the unit through a series of moistened pads. The air passes through the pads, causing the water to evaporate. The process of evaporation cools the air, which is then circulated back into the room.

3. What are the benefits of using a swamp cooler?

Swamp coolers are more energy-efficient than traditional air conditioners, as they use less electricity and do not rely on harmful refrigerants. They also add moisture to the air, which can be beneficial in dry climates.

4. Are swamp coolers suitable for all climates?

No, swamp coolers are most effective in dry climates with low humidity. In humid climates, they may not be as effective and can even increase the humidity levels in a room.

5. How often do I need to refill the water in a swamp cooler?

The frequency of refilling the water in a swamp cooler depends on the size of the unit and the climate. In hot, dry climates, it may need to be refilled every few hours. In cooler, more humid climates, it may only need to be refilled once a day.

6. Can I use tap water in a swamp cooler?

Yes, tap water can be used in a swamp cooler. However, it is important to regularly clean the unit and replace the water to prevent the buildup of minerals and bacteria.

7. How do I maintain a swamp cooler?

To maintain a swamp cooler, it is important to regularly clean the unit and replace the water. The pads should also be replaced annually. Additionally, the motor and fan should be checked and lubricated regularly.

8. Can a swamp cooler be used in conjunction with a traditional air conditioner?

Yes, a swamp cooler can be used in conjunction with a traditional air conditioner. This can help to reduce energy costs while still maintaining a comfortable indoor temperature.

9. How much does a swamp cooler cost?

The cost of a swamp cooler varies depending on the size and features of the unit. On average, a swamp cooler can cost anywhere from $300 to $1,500.

10. Can a swamp cooler be used outdoors?

Yes, a swamp cooler can be used outdoors. However, it is important to ensure that the unit is designed for outdoor use and is protected from the elements.

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