 To effectively dissipate the heat from we system, we must find the suitable fan with correct CFM value. This guide will explain how to calculate the needed CFM for your system.

The basic idea behind the CFM calculation is the total heat expelled by the fan will equal to the sum of the power of your system. Therefore, we can write the equation as below:

Then we need to link the total heat with the CFM. Here’re the few things we need to know before the calculation:

1. CFM means the cubic feet per minute blew by the fan. It can represent as Q/min.
2. CMM is cubic meter per minute, also the unit of the air flow. 1 CMM=35.3CFM
3. The specific heat capacity of the air at constant pressure is 0.24 kCal/Kg°C
4. The relative density of the air is 1200g/m3
5. 1 calorie equals to 4.2 joule

Now we can link all the things together:

Now we organize the equation to leave only CMM on one side:

To represent in CFM, the equation can be rewritten as:

After deriving the equation, we can try few examples now:

Ex1. Say a computer system consumes 200W power, the technician plans to add 2additional 1W fans to help cool down the system. The room temperature is 25°C and the recommended working temperature of the system is 65°C.

What CFM spec should he choose?

Solution:

Ex2. Johnathan bought a new CPU to upgrade his computer. The CPU is 65W TDP and Johnathan wants to find a new cooler for it. The room temperature is 30°C. Best working temperature of the CPU is 40°C.

What CFM spec should he choose?

Solution: