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Imagine you have a box, initially fully sealed with an electronic component generating heat. You create a small hole at the bottom and a large hole at the top.

What effect does this have on airflow through the box?

My question relates to adequate non-forced ventilation of electrical enclosures. Will having a vent near the top of an enclosure allow warm air to escape at the top and draw cooler air in from a vent near the bottom? Would vent size play a factor in improving or degrading this?

A lot is said about forced ventilation methods but non-forced is less widely covered.

Many thanks, Chris.

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You are describing something called natural convection, which works like this: air gets warmed by contact with hot object > air heats up and becomes less dense > hot air rises because of resulting buoyant force > cooler air moves in to take its place > process continues.

It is common to cool electronic gear by natural convection, when the heat load to be dissipated is modest. Just as you describe, holes in the top allow heated air to rise and escape and holes in the bottom admit cooler air to take its place.

The forces involved are small, so lots of holes are needed to encourage the cooling effect, and both the top and the bottom of the enclosure must have free access to lots of air.

Modeling this process and furnishing a set of convenient equations to predict for example how many holes to use and what diameter, etc. is hard because small things have big effects when the forces involved are weak. It is easier to perform an experiment with thermometers inside and outside the case to determine if a forced-air cooling fan is needed, or if natural convection will be adequate.

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