# Fan Selection Guide

Objective :

• Optimize air flow efficiency
• Minimize form factor and fit
• Minimize acoustic disturbance
• Minimize power consumption
• Maximize reliability and service life
• Justify the “total” cost Step 1 : Total Cooling Requirement

• Determine the total cooling requirements which will be needed to operate the system efficiently and which will provide the desired operating conditions to maximize the performance and life from all components within the system.
• Three critical factors to obtain total cooling requirements:
• The heat (∆T) which must be transferred.
• The heat transfer {W} in watts to offset (∆T).
• The amount of air flow (CFM) needed to remove the heat.
• The formula for determining the necessary air flow is:

Q=  (1760 × KW) / (∆T)

Where :

Q = required air flow in CFM

KW = heat to be dissipated

∆T = Allowable temperature rise

Example : Step 2 : Total System Resistance

• Define the total system impedance or System Characteristic curve of the enclosure or of the system which needs to be cooled.
• Once the load parameters are defined in terms of the heat which needs to be removed or the number of air changes per hour that are required, and the required air flow (CFM) is determined, it is necessary to define the static pressure characteristics which the moving air will encounter as it passes through, over and around components located within the path of the air flow.
• All these elements which impede the flow of air create a pressure rise within the system which restricts free flow and passage of air.
• This change in pressure [∆P] is the static pressure measured in inches of water.

The System Characteristic Curve formula is:

∆P = K Qn

Where :

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

• Solve the air moving challenge is the process of curve fitting which overlays your system characteristic curve on the air performance curves of selected, alternative, air moving devices.
• Points of intersection are “possible fits” as exemplified in the operating point graph.  