Power transformers use several standard cooling methods defined by codes such as ONAN, ONAF, OFAF and OFWF. Oil natural air natural relies on natural convection of oil and ambient air. Oil natural air forced adds fans to blow air over radiators. Oil forced air forced uses pumps for oil plus fans for air. Oil forced water forced circulates oil through water cooled heat exchangers. Dry type transformers use air or air forced cooling. The chosen system depends on rating, ambient conditions, noise limits and available space at the installation site.