Underground Vault Ventilation

Why Use a Blower?

Forced ventilation is required for underground transformer vaults to remove the heat dissipated by the transformers. The heat loss on different transformers vary slightly. Most transformers lose approximately 1½% of their capacity to heat, when operated at full rated capacity. When transformers are overloaded, their heat loss is considerably more.
The heat generated from transformers can be removed by exhausting heated air or conducting the heat through the vault wall. On only the smaller installations can enough heat be dissipated through the walls to cool the transformers satisfactorily. On large transformer vaults, approximately 15% of the heat is dissipated through the vault walls. A powered air-moving system is required to remove the rest of the heat.

Why Blow Air into a Vault?

Forcing air into a vault benefits the transformer in two ways:

Why Vent Size is Important

The vent sizing should be large enough to allow air movement without an excessive pressure drop. If the inlet and outlet vent sizes are too small, the volume of air will decrease and cause the transformer to heat vault up. During periods when the transformers are operating at capacity, this excessive heat will cause the transformers to operate less efficiently and will shorten the life of the transformers.

Where to Locate the Inlet and Outlet

The inlet and outlet should be located on opposite side of a transformer vault. However, with a Swing out Blower, air is forced into the vault at high velocity and turbulent air is pushed all around the vault. With this type of blower, the inlet and outlet can be located on adjacent walls if necessary.
The inlet of a Swing out Blower is normally located 6' to 8' above the floor where the high velocity air will reach the floor easily. The exhaust vent should be located as near the roof as possible.

Typical Mounting Arrangements

The plenum arrangement has the advantage of making any vault easily converted to a blower system.
The vent system allows the underground transformer vault to be installed without concern for location of the inlet and outlet. The inlet and outlet locations can be changed by using different lengths of vent piping. This allows the inlet and outlet to be located outside heavy traffic areas. The inlet and outlet can also be located in dyer ares to keep the vault dry.