Danfoss
When changing a fluid’s thermal state, there has to be a temperature difference between the two mediums for heat transfer to take place, and the heat lost by the hot medium is equal to the amount of heat gained by the cold medium, except for losses to the surroundings.
To continually transfer heat between two mediums, heat exchangers are used.
In a direct heat exchanger, both mediums are in direct contact with each other and the indirect heat exchanger, both mediums are separated by a wall through which heat is then transferred.
Outside of a heat exchanger, the predominant heat transfer is air, whilst internally ranges from water, with or without additives, to natural and synthetic refrigerants, steam and even oils, to name the most common.