Standard Vented Portable Air Conditioning Units
These are the more commonly accepted type and they work by passing warm air extracted from the room through a refrigeration process similar to your domestic fridge and expel chilled air that cools the room. There is a by-product of this chilling process which is the hot exhaust air produced by the heat exchange system.
That hot exhaust must be vented to the outside through a suitable opening in an external wall. It can be a dedicated extraction port, or an open window with the gap blocked (to keep the hot outside air out). So even though these units are labelled as "portable", they still need to be sited close to an outside wall and their vent properly set up so that the hot exhaust does not come back into the room (or you'd end up with a net increase in ambient temperature).
Vent Free Coolers
The "other" type of portable aircon unit is actually not an air conditioner at all (in the same sanes as the type covered above). It is actually an evaporative air cooler (often referred to as a "swamp cooler") and works on a completely different principle.
There is no refrigeration process, no heat exchange and no refrigerant gasses inside these units. There is, instead, a water reservior, a porous membrane and a fan. This is a simplistic explanation, but accurate in any case.
These exhaust free air conditioners work in a similar way that your skin does to keep you cool in hot weather by sweating. When you sweat, moisture covers your skin, which is evaporated by a breeze and feels cold. the evaporation reduces the temperature of the skin and the same principle applies to an evaporative cooler.
Water is soaked into the membrane, the fan blasts air through it which is chilled as it evaporates, resulting in a chilled breeze that cools the room. Because of the device's simplicity, it uses very little power to run the fan and many of these small, portable units will run on about the same amount of power as a 60w light bulb.
Disadvantages of a Swamp Cooler
The only real downside is these devices don't work very well in climates that have high humidity. That's because the more saturated with water vapor the atmosphere in a roombecomes, the less effective the cooling evaporation process will be.
So it depends on where you live whether or not you can take advantage of the low running cost of a swamp cooler at all. If you have dry heat with low average humidity during the hot summer months, then a swamp cooler will be perfect for keeping you cool indoors if you're on a budget and want a cheap and environmentally friendly way to beat the heat.
And that's pretty much why one of these coolers is so cheap to run when compared with a similar sized aircon unit.
