Portable Air Conditioners, with a single hose, pull air from within the room only, and expel warmed air and moisture outside. This creates a negative air pressure since you are pushing air out of the room. The result is air has to be replaced in the room and is consequently sucked in from cracks beneath doors or around windows. This warm air, forces the unit to work harder to keep the room cool. Single hose units are simple machines designed for smaller rooms.
Dual Hose
Portable Air Conditioners, with dual hoses, feature one air intake hose and one air out-take hose. These models take all their make up air from the outside so there should not be any negative pressure generated. The air intake pulls air from the room, cools it, and then returns it back into the room. This process heats up the unit, so another intake hose takes air from outside to cool the unit down. The second hose expels all warm air back outside. Dual hose systems are more efficient in larger settings.