AN: Natural cooling by atmospheric circulation, without any special devices. The
transformer core and coils are open all round to the air. This method is confined to very small units at a few kV at low voltages.
AB: The cooling is improved by an air blast, directed by trunking and produced by a fan.
OB: The cooling of an ON-type is improved by air blast over the outside of the tank.
ON: The great majority of transformers are oil-immersed with natural cooling, the heat developed in the cores and coils is passed to the oil and hence to the tank walls.
OFB: For large transformers artificial cooling may be used. The OFB method comprises a forced circulation of the oil to a refrigerator, where it is cooled by air-blast.
OFN: The oil is circulated by pump to natural air coolers .
OW: An oil-immersed transformer of this type is cooled by the circulation of water in cooling-tubes situated at the top of the tank but below oil-level.
OFW: Similar to OFB, except that the refrigerator employs water instead of air blast for cooling the oil, which is circulated by pump from the transformer to the cooler