I've built a lot of large electrical equipment enclosures for the chemical processing industry...
From a common sense perspective I always thought the fan & filter should draw air into the cabinet for two reasons:
1. This maintains a positive pressure which keeps dirt etc from entering in through gaps or cable entry holes - this also means air has to come through the filter.
2. The positive pressure means there's more air particles to cool down the equipment inside. This is also done in (for example) tube transmitters where the anode is cooled with a blower to remove more heat than ambient pressure could remove.
Cheers,
Matthew
