Cooling Computer by a Fan

The engineering computer lab is planning to discard some old computer parts this summer. Alex plans to build a computer by himself using those parts but is worried about the cooling capacity of the fan he obtained since the electronic components of a computer can only work under a specific temperature range. For the given fan, he needs to determine how many printed circuit boards (PCBs) he can use in the computer before it overheats.

What is known:

• Each PCB dissipates 10 W of power
• The air enters the box at 20 ^oC and 1 atm
• The temperature at the exit can not exceed 32 ^oC
• The power of the fan is 20 W. CPU and other components other than PCBs dissipates 60 W of power
• The flow rate of the fan is 0.01 kg/s

Method 1: Consider the whole enclosure as a control volume.

Method 2: Consider the air as control volume A and the fan, the CPU, and PCBs as control volume B.