mounting orientation for Meanwell enclosed power supply
If your enclosure has side and bottom vents then the advice you were given makes sense to me. Inverted with bottom and side vents.

i.e. Convection air intake rising and exiting up past hotspots to vent outside with thermal conduction to top cover from baseplate..

In a design I did with 180W open frame PSU in 1U high enclosure, I used side vents with thermal controlled fans using a thermistor epoxied to the hotspot, ferrite transformer.  It ran silently to 45'C up to 100W output to ext. load then fans started slowly to limit sense temp to 55'C but never ran full RPM at room temp.

If you mount it vertically then your convection air flow must not be allowed to rise over the hot spot to exit. Otherwise, a 40mm plenum fan may be needed.

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