There are two main types of portable air conditioners—those that use two ducts and those that only use one. The two-duct models operate based on the same principle as a window-mounted air conditioner but tend to be less efficient. As for air conditioners with a single duct, the hot air expelled by the device is replaced by outside air that leaks in through the openings in the building, for instance under doors or through cracks. Using a second duct to refresh the air prevents this inconvenience.
Even though different types of air conditioners (window-mounted or one- or two-duct portable models) may have the same rated capacity (e.g., 10,000 BTU/hr), their actual capacity is not the same. We recommend that you choose a window-mounted air conditioner but if you need a portable model, opt for one with two ducts.