(2) Method 2:
Consider air as control volume A and the computer components as control volume B.
Control Volume A:
Heat transfer occurs between the air and the electric components, Hence,
the heat transfer term is not zero. No work is done by the air or to
the air. The energy balance is:
From method 1, the enthalpy change is determined to be 120.6 W.
Hence, the heat transfer is given by
= 120.6 W
The positive sign means that the heat transfer is from the computer components to the air.
Control Volume B:
Heat transfer occurs between the air and the computer components. Hence,
the heat transfer term is not zero. Work is done to the components, and
given by the sum of the power of the fan, the CPU and other
components, and the the PCBs. According to the assumptions, no
enthalpy change occurs in control volume B. Thus, the energy balance is:
The amount of heat transfer from the PCBs to the air is the same as
the heat transfer from the air to the PCBs, hence,
The negative sign indicates that heat is transferred from the system.
The power supplied to the PCBs can be determined by
-120.6 - (-20 - 60) = -40.6 W
Note that fan
have negative sign since power is applied to them.
The result ob is the obtained same as in method 1, hence the number of PCBs is 4.
Based on the results from both methods, for the given fan, Alex can only install up to 4 PCBs.