Simple air cooling of the transformers is adopted in dry type transformers. The limit for this is reached by the time the rating is a few kVA. Hence air cooling is used in low voltage machines. This method of cooling is termed as AN(Air Natural). Air Blast(AB) method improves on the above by directing the blast of air at the core and windings. This permits some improvement in the unit sizes.
Substantial improvement is obtained when the transformer is immersed in an oil tank. The oil reaches the conductor surface and extracts the heat and transports the same to the surface of the tank by convection. This is termed as ON (Oil Natural) type of cooling. This method permits the increase in the surface available for the cooling further by the use of ducts, radiators etc.
OB(Oil Blast) method is an improvement over the ON-type and it directs a blast of air on the cooling surface. In the above two cases, the flow of oil is by natural convective forces. The rate of circulation of oil can be increased with the help of a pump, with the cooling at the surface remaining natural cooling to air.
This is termed as OFN (Oil Forced Natural). If now a forced blast of air is also employed, the cooling method becomes OFB(Oil Forced Blast). A forced circulation of oil through a radiator is done with a blast of air over the radiator surface. A substantial amount of heat can be removed by employing a water cooling. Here the hot oil going into the radiator is cooled by a water circuit. Due to the high specific heat of water, heat can be evacuated effectively. Next in hierarchy comes OFW which is similar to OFB except that instead of a blast of air a forced circulation of cool water in the radiator is used in this.