What is the difference between a single hose and double hose?

Portable air conditioners can either have a single hose or double hose (1 for intake of air and 1 for exhaust).

A single hose is going take the air from a room where it is located and return almost all of that air r back into the room as cold conditioned air.

On the other hand, a dual hose is going to take the air from an external location via one of the hoses and would use that air to cool the model, and so, that warm air is released via the vent hose. Dual hose employs the use of warmer unconditioned air In order to cool down the compressors, which achieves a marginally less efficient air and the use of two internal fans causing higher energy consumption.

There are no studies as to which is better, a single or dual hose, but several factors can determine if it is better for an individual user or not.

Is it fine to lengthen the exhaust hose?

As a precaution, one should not lengthen the vent or exhaust hose of a portable AC, since it could lead to a violation of warranty and overheating of the portable air conditioner.