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How it Works?
An evaporative cooler works on the principle of pulling hot air from the surroundings through moist pads, cooling it, and circulating it.
The principle you read above is very simple, and one of the oldest forms of cooling systems. I'll give you an example. After you exercise, you perspire, and I think almost everybody knows the reason behind that. It is the body's natural phenomenon of cooling down after a bout of physical activity that increases the body's internal temperature. It's the same simple physics principle that is applied in the working of an evaporative cooler.

Sometimes, it is also referred to as a 'Swamp Cooler'. It got this name because many people believed that its use makes the house moist and humid, as it uses water to cool the air. These coolers are really simple machines, because neither their working consists of any rocket science nor is it a pain to own them. While these coolers can be quite cost-effective, energy-efficient, low-maintenance, and have the least environmental impact, there are some instances that make them lousy appliances. Therefore, you will have to consider some important things before bringing one home.
Choosing the Right Swamp Cooler
Is the Climate Conducive?
Depending on the area you live in and the climatic conditions, you will have to decide the type of cooling that you can use. Evaporative coolers can be used successfully if you stay in arid areas, like the Southwestern United States. The wind has to be hot and dry. Humidity levels should be less than 50 percent for you to enjoy the benefits of an evaporative cooler.
Swamp cooler
Swamp Cooler
You may get confused by the name 'swamp cooler', and think that it is meant to be used in mushy marshlands, but the reality is just the opposite. They are of negligible use in damp areas, like the Southeastern United States, and also in the monsoon season. The reason this happens is, when hot and dry air passes through water, due to the difference in vapor pressure, the water heats up and evaporates, thus, leaving the area cooler. The same does not happen with humid air, as the difference in vapor pressure between water and humid air is very less.

Therefore, the type of climate you experience is the key factor in determining whether or not you should opt for an evaporative cooler.
Determine the Correct Size of the Cooler
Once you know that your climate supports the use of an evaporative cooler, the next step to consider is the appropriate size of the cooler that would meet your needs. If you select a very small size, then it will have to work extra hard to keep the area cool, and a larger than required size would mean bringing the North Pole to your home.
So, how do you determine the appropriate size of cooler, that can give you the best results? You need to calculate a simple figure called CFM (cubic feet per minute). For that, you will require the square feet area of the room, and height at which the ceiling is. You have to multiply the square footage by the height of the ceiling, and then divide by 2.

For Example: 500 × 8 ÷ 2 = 2,000 CFM
This figure gives you the amount of fresh air that needs to be circulated throughout the room to effectively cool it. If we follow the above example, a swamp cooler that offers at least 2,000 CFM or higher, will be required for your room.
Determine the Correct Type of Cooler
Residential evaporative coolers are of two types: portable and mounted. If you want to cool the air just where you are, then maybe a portable cooler is the best option for you. The moving type of coolers come with wheels, and are usually lightweight. If you are opting for a portable cooler, you could go in for the one with a higher CFM. These usually cool up to 300 to 500 square feet of area.
Portable and mounted cooler
Mounted coolers are of two types. The first is a window-mounted cooler. If you think that you only require cooling in one room or a particular area, then you need to probably install a window-mounted cooler. Nowadays, these coolers come with all the necessary installation hardware. They can be easily installed on any type of window. The second option is the down-discharge type of cooler, where the cooler is installed on the roof, and the cooled air is discharged downwards. This can cool the entire house. Sometimes, an elbow is attached to the roof-installed cooler, which is directed inside the house through an opening at the side of the building.
The type of padding is also essential. There are two types of padding: fiber and rigid media. The fiber pads are made of many different material, like aspen wood shavings, cellulose fiber, or synthetic fiber. They are usually cheap, and are 1 to 2" thick, but require regular maintenance. The pads made with rigid media are usually stacked corrugated material, and have a thickness of around 8 to 12". They are low in maintenance, and have a very long life, if maintained properly.
Consider the Accessories Required
If you stay in an area where the water is hard and contains a lot of mineral salts, then probably, you will need a purge pump. It will automatically keep removing the water, and eventually help prevent mineral buildup.

If keeping the windows open is a security issue for you, then a ceiling vent is the answer. It will pull out warm air, and let it out into the attic from where it would leave through the roof vents. You may have to install additional ventilation in the roof and attic.
Ceiling vent and air filter
Some filters known as pre-filters can also be installed, if you want the incoming air to be dust-free. There are a few antibacterial filters available too, that can be installed to free the air of microbes. These filters are optional, and can be done without.
Comparison With a Traditional Air Conditioner
The main aim of both, an air conditioner and an evaporative cooler, is cooling. But both of them should never be used together, a because a cooler will humidify the air, while an air conditioner will dehumidify it. As compared to a traditional air conditioner, evaporative coolers are easy to install, and require less maintenance. If we speak of energy efficiency, operating a cooler is almost 75% cheaper than an air conditioner. Once installed, it only requires water and electricity to run. It is seen that, even if the cooler is kept running for eight hours, it costs less than one dollar to run.

If you are conscious about the environment around you, then a cooler is definitely meant for you. It keeps you cool without harming the environment. As opposed to that, an air conditioner uses refrigerants which are ozone-harming agents. The next benefit of a cooler is that it keeps the air fresh, because it requires keeping the windows open to allow proper circulation of air. But, when you talk of an air conditioner, an airtight space is required for it to be effective.

The last reason why a cooler again outshines an air conditioner, is that it humidifies the air. By humidifying the air, problems such as dry eyes, skin, hair, and throat decrease. An air conditioner does just the opposite, it makes the air dry.
Air conditioner
Air Conditioner
I'm not saying that you should never opt for an air conditioner, it has its own set of advantages and can be definitely used effectively. But if the climate of your area permits, I would rather say, that opting for an evaporative cooler would be lighter on your wallet as well as the environment.