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:
1. The vault is pressurized with a positive pressure that keeps
sewer gas and other gasses from collecting in the vault
2. The blower discharges high velocity turbulent air around the
transformers accelerating the rate of heat transfer, this turbulent
air gives considerable added cooling to the transformers. The
effect is similar to installing fans on pad-mounted transformers
for additional cooling.
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.
