TABLE OF CONTENTS Oil filled transformers can generally take care of themselves, since they are seldom built into other enclosures. Dry types, however, are often purchased as core and coils with no enclosure and built into equipment. The burden then falls on the equipment designer to provide adequate cooling for the transformer. When the transformer is purchased with integral fans, these are usually located at the bottom of the assembly, and they direct cooling air up through the coils. The only further requirement is to provide sufficient makeup air for cooling.
Transformers that rely on forced air from an external source or are convection cooled may require some attention to airflow. If no air is admitted to the enclosure, the internal air will become heated as it rises through the coils and will tend to stratify with the hot air accumulating at the top of the compartment. The only heat exchange is then by radiation and convection from the enclosure, so the obvious thing to do is to admit air at the bottom of the enclosure and exhaust it at the top. But this may not end the matter. The coils represent a high head loss to airflow, whereas the surrounding space offers little. Cooling air that enters the bottom vent may simply flow directly up to the top vent, bypassing the coils in the process. Figure 4.25 illustrates the solution of adding a horizontal skirt to force the cooling air up through the coils. The skirt can be made of...
