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The first and least costly option in portable AC units is the evaporating cooler. This unit works by pushing hot air through a medium that is water-logged so that evaporation reduces indoor temperatures. It is not necessary to ventilate these units, but they do have a water basin that you have to regularly refill.
It is also important to clean this type of air conditioning unit or it will start to smell musty over time. These machines tend to work best in arid regions, where evaporation is the most efficient manner of operation and in areas where increased indoor humidity is not going to be problematic.
The next option is a unit that relies on refrigerant as opposed to evaporation. You do have to ventilate these units as they are designed to remove hot air from the room through an exhaust tube. Units with dual hoses have both an intake tube and an exhaust tube, whereas single-hose designs just have a tube for exhaust.
The intake tube on dual-hose models serve the function of drawing in air from outside or from another part of the building so that air pressure is evenly distributed. Single hose units can create a difference in the air pressure, which causes warm air from high pressure spaces to get pulled down into the area that has be cooled.
Single hose designs work well in small-sized rooms, but if your room is over 500 square feet, you should look for a dual-hose option instead.