The statement
The equipment shall be cooled by a fan.
has a passive verb form – "shall be cooled".
We can understand the "by" as a passive subject, or as "by means of"
or "using".
We can add to it
The equipment shall be air cooled by a fan.
It is still the same verb, just clarified with the type of cooling. There is no confusion, as the form
The equipment shall be air, cooled by a fan.
is clearly wrong. We can add some more
The equipment shall be forced air cooled by a fan.
The alternative constructions
The equipment shall be forced, air cooled by a fan.
The equipment shall be forced air, cooled by a fan.
must be wrong, so we extract the "forced air" as a subject, and are left with
Forced air shall cool the equipment by means of a fan.
"Forced air" is treated as an embedded subject – caught between the verb auxiliary and the verb.