I came across to an interesting problem when I was having difficulty implement a computing cluster in my small office. Although my attempt was unsuccessful, at least I had the chance to observe a situation described graphically below.

As seen in the figure, there is an insulated case. In the case there is a heat source. The objective is fixing the ambient temperature of the case by selecting a fan having an appropriate power at the outlet (blue rectangle at the upper right corner).

Assumptions are:

- case is fully insulated
- indoor temperature remains fixed
- case ambient temperature has no temperature gradient (it instantly adapts to average temperature)
- efficiency of the elements are assumed to be 100%

Question is what factors affect the fan selection process?

Factor suggestions:

- initial case ambient air temp
- target case ambient air temp
- indoor temp
- outdoor temp
- heat source power
- case interior volume

My answer is only “the difference between target case ambient temperature and indoor temperature” and “heat source power”. Interesting isn’t it?