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        All of designers in electric and electronic fields determine the air flow needed to dissipate heat from a given system. A required air flow is determined by knowing the power consumed in the system and the amount of air needed to remove sufficient heat from the system to limit its rise in temperature. In fact, years of experience has shown that the service life of a system is typically decreased by insufficient cooling system. The designer should also know that the price or the sales may be reduced if the service life of the system is not user expected.

        To select a right air moving device, one has to consider the objectives in the following list .

l        Optimize air flow efficiency

l        Minimize size and fit

l        Minimize acoustic disturbance

l        Minimize power consumption

l        Maximize reliability and service life

l        Justify the total cost

Step 1: The Total Cooling Requirements

        The first step is to recognize three critical factors to obtain total cooling requirements. There are:

l        The heat (DT ) which must be transferred

l        The heat transfer (W) in watts to offset DT.

l        The amount of air flow (CFM )needed to remove the heat

       The total cooling requirements are critical to operate the system efficiently. An efficient operating system is to provide the desired operating conditions that maximum the performance and life from all components in the system.When making the selection of the fan motor for ordinary use, the following methods are used .

l        Determine the amount of heat generated inside the equipment.

l        Decide the permissible temperature rise inside the equipment.

l        Calculate the air volume necessary from equation.

l        Estimate the system impedance in the unit.

l        Select the fan by performance curve shown in the catalogue or data sheet.

        The volume of air flow required to cool an equipment can be determined, if the internal heat dissipation and the total rise in temperature allowable are known .

                          H=Cp ×W ×△T



Obviously we have

W= CFM × D

Where, D= Density

By substitution, We obtain :

Q(CFM)= Q / (Cp ×D×T)

By incorporating conversion factors and specific heat and density for sea level air, the heat dissipation equation is arrived at :

CFM =3160×千瓦/ °F

Then, we obtain the following equations :

Q(CFM)=3.16×P/ Tf = 1.76×P/ Tc

Q(M3/Min) = 0.09×P/Tf = 0.05 ×P/ Tc

Q: Required air flow

P: Internal heat dissipation

Tf: Allowable temperature rise in °F

Tc: Allowable temperature rise in °C




Step 2 : Total System Resistance / System Characteristics Curve

        In order to specify the cooling per slot in watts , the system designer / manufacturer must not only have a valid air flow curve to determine the maximum air flow , but must also know the system air resistance curve . There is a loss of air pressure due to resistance of components inside the enclosure. This loss varies with air flow and is known as system resistance .

The System Characteristic Curve formula is :


K= system characteristic constant

Q= air flow , CFM

n = turbulence factor , 1< n < 2

Laminar Flow , n= 1

Turbulent Flow , n=2  

Step 3 : System Operating Point

The intersection point of system characteristics curve and air performance curve of selected air moving device is named System Operating Point that is the best air moving device for your application .



At the point , the change in slop of the air performance curve is minimized while the change in slop of the system characteristics curve is at its lowest . Note that the static efficiency (air flow times static pressure divided by power) is also

Designing Considerations :

1.      Keep the air flow path as unobstructed as possible . This air intake and outlet should be kept free for air flow .

2.      Guide vertical air flow through your system , it will assure the flow moves more smoothly and increase cooling efficiency .

3.      If a filter is required , you should consider the additional resistance to air flow .


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