All of designers in electronic fields which need fan to dispel the heat must determine the air flow needed to dissipate heat from a given system. A required air flow is determined by knowing the power consumption in the system and the amount of air needed to remove sufficient heat from the system to limit its rise in temperature. In fact, the service life of a system will be typically decreased by insufficient cooling system. The designer should also know that the price or the sales will be reduced if the service life of the system does not meet the user’s expect.
 
To choose the correct ventilation fan, the following items should be considered:
The high efficiency of air flow
The most suitable size
The minimum noise
The lowest power consumption
The Maximum reliability and The longest service life
The most reasonable price

The following steps can help you to reach above-mentioned goals :

Step one: To know the total cooling demand, three key factors which can influence the total cooling demand are as follows:
1. The heat need to be transferred  ( DT  difference temperature )
2. the watt of converted heat must be Offset  (W)
3. the needed amount of air flow in order to move the heat (CFM)
The total cooling demand is very important to operating the system effectively. Efficient systematic operation must offer the desired operation condition , enable the component in all systems to give full play to function and the longest service life .

Step two: After getting the total cooling demand , the air flow can be worked out by the following methods :
1. Calculate the heat produced inside of the equipment.
2. Determine the temperature rise range which can be  allowed inside of the equipment
3. Calculate the necessary wind amount from the equation.
If distribute the heat and rising amount of total temperature of allowing within the known system equipment, the cool wind amount equipment can be got. 

The following is a basic hot conversion equation: H = Cp * W * △T
 H=The amount of heat transferred   Cp = specific heat of air   △T = Temperature rise  inside the equipment   W = Air weight   Obviously  W  = CFM * D    D = The air density   , we can receive the following heat dissipation equations by substitution after knowing the specific heat and the density of the sea level air : CFM =3160 * Kilowatt /／△℉
Example one: the power consumption inside of the equipment is 500w, The DT is 20 degrees Fahrenheit, the following is its result of calculation:

or
 
Example two: the power consumption inside of the equipment is 500w, the DT is 10 degrees Centigrade: 
or
     
Conversion table of the wind amount 
 
conversion table of Static pressure