Dell OptiPlex 330 Guide De Référence Rapide page 14

• If your computer is installed in a corner on a desk or under a desk, leave at
least 5.1 cm (2 in) clearance from the back of the computer to the wall to
permit the airflow required for proper ventilation.
NOTICE:
Do not install your computer in an enclosure that does not allow airflow.
Restricting the airflow impacts your computer's performance, possibly causing it to
overheat.
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Quick Reference Guide