A free cooler is basically a type of heat exchanger, specifically a finned pack exchanger, commonly employed for the cooling of a fluid flowing through the tubes using a gas (air) that touches the outside of the tubes and the thermal exchange fins.
It is classified as a thermal machine due to the presence of electrical fans, already installed when supplied, that provide the amount of air flow necessary to achieve the thermal duty (the thermal work to achieve): they have to move and gather the air required to dissipate the heat load.
These machine are named after the application they are employed in. They are usually called free cooler when employed for cooling leveraging the low temperatures of the external air in substitution of a chiller, achieving high energy savings.
They are called dry cooler, instead, when employed in substitution of evaporative towers (when allowed by climatic conditions), eliminating the thermal exchange achieved by evaporation that characterizes the cooling towers, thus cooling in a ‘dry way’.