During normal operation, the active equipment dissipates heat to the air inside the enclosure which in turn
increases the air temperature inside the enclosure. The air then transfers this heat to the exterior of the enclosure
through convection, conduction and radiation (figure 2). A minor amount of the heat is transferred directly to the
enclosure from the active device through conduction since the equipment is in contact with the mounting (DIN) rail.
Heat flows from high temperature to low temperatures. So, the air temperature inside the enclosure will increase
until it exceeds the temperature outside of the enclosure. After a period (typical < 48 hours under continuous
operation) a steady state temperature is reached inside the enclosure. This internal air temperature must be at or
below the operating temperature specified for the components installed inside the enclosure. Since hot air is more
buoyant it will rise towards the top of the enclosure this creates a temperature gradient inside the enclosure. The
bottom area of the enclosure will be near the outside ambient temperature and the top area can be 10°C to 20°C
higher than the bottom of the enclosure.
Passive Natural Convection Cooling (Sealed Enclosure)
Figure 2: Idealized thermal (aka natural) convection cooling. Enclosure is 12" deep.
The door has been removed to see inside the enclosure.
Heat is lost to the
ambient air in
all directions
(outside of the
enclosure).
Air heated by the
Active Equipment flows
upward toward the top of
the enclosure.
Cooled air (heat lost to
the ambient) flows back
towards the bottom of
the enclosure.
Active
Equipment
Enclosure
Cool air flowing up
towards the active
equipment helps
cool it.
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