Besides the common and traditional central evaporative coolers, designed to provide fresh air to entire homes, there are now window units, and the so called mobile coolers, for outdoor uses; and also new two-stage evaporative coolers, and a new line of low-capacity units (portables) designed to cool just a room...
When buying, pay attention to the type of cooler, and its goals. All types of evaporative coolers are designed for dry climates. They aren't like AC. While air conditioners remove humidity, evaporative coolers add humidity. They basically cool by adding water to the air… That’s why they perform so badly in humid climates.
Small portable evaporative coolers
Most small portable evaporative coolers have a capacity of 400-800 CFM (Cubic Feet of air per Minute), which is a small fraction of the capacity of traditional models (at least 3.000-6000 CFM).
They are cheap and can perform reasonably in moderate climates (with low levels of humidity) but they are not efficient enough in warmer weather conditions, even in dry climates. They can reduce the room’s temperature up to 10-15°F or so, but do not expect much more.
They haven't to be connected to the plumbing system or installed in the roof, or in a window or wall, which is a huge advantage. But you will have to fill the unit with water manually...
Two-stage coolers are a new technology trend, at least in theory (common coolers are of one-stage type). They include a pre-cooler, high-effective pads and better motors. They are often called energy-efficient evaporative coolers. Unfortunately, in practice, this technology is not meeting the expectations. They do not seem a credible alternative, at least for the time being. Their high prices do not also help them either: they amount to the cost of central air conditioners…
High-Capacity mobile evaporative coolers
Some manufacturers are now offering a special type of portable evaporative coolers, that they call mobile coolers. Contrary to common portable coolers, they are high-capacity units, designed to provide fresh air in situations where air conditioners can’t be cost-effective: out-of-door spaces (patios, job sites) and large open spaces like garages and warehouses.
They are typically more expensive than residential coolers for the same capacity – that’s, namely the case of Port-a-Cool evaporative coolers (Port-a-cool is the main manufacturer of this type of units). These unit make little sense from a performance standpoint.
Window evaporative coolers
Window and wall evaporative coolers come in different capacities and involve different goals. They are more powerful than portable coolers, and most are designed to heat just one room. But there are also units designed to heat small homes and garages.
They are easier to install than ducted coolers, and their capacity (often in the range of 3.000-4.000 CFM) provides a much better performance than portable units.
Central evaporative coolers
As already mentioned, central evaporative coolers (ducted or un-ducted) are the most traditional type of evaporative coolers.
Their design varies a lot, but they are typically powerful units, able to provide 20, 30 or 40 changes of the home’s air per hour. Capacities between 3.000 and 25.000 CFM are common.
They require the windows to be opened (or specialized ventilation openings), and also routine maintenance (several times during the cooling season, and a major annual cleaning).
Many central systems are ducted systems, which is important in homes with closed rooms or with walls and hallways that block air circulation. These units are installed in the roof, or outside, in a wall on on the ground (image, from PGE).
But there are also many non-ducted systems, with the evaporative cooler installed indoors, in a central location, usually in the roof or on a wall. The cooled air is, in this case, blown directly into the room where the cooler is installed.
Evaporative coolers, whatever their type, do not make sense in low-energy homes in cold and moderate climates, or in hot-wet climates. Low-energy homes rely on very high levels of insulation and sealing, and in small heating and cooling HAVC systems.
Central home evaporative coolers are the best choice for those who want a large multi-room cooling system. Their installed prices are 1/2-1/3 of that of air conditioners, and they consume 1/3-1/4 of their electricity. That can make them an advantageous choice, in dry-hot climates.
But you may prefer a window evaporative cooler. They are a cheaper option for small houses and small cooling needs… They cost less and consume less electricity (and water) and are easier to install.
As to portable units they are designed for mild climates, or mild-hot weather conditions; they do not perform well in hot-arid dry climates where evaporative coolers are traditonally used… And they are intended to cool one or two rooms, never more than that. They only make sense in very specific situations.