Establishing Cooling Requirements

Before a fan can be specified, the airflow required to dissipate the heat generated had to be approximated. Both the amount of heat to be dissipated and the density of the air must be know.

The basic heat transfer equation is:

q= Cp x W x D T

where Q = amount of heat transferred
Cp = specific heat of air
D T = temperature rise within the cabinet
W = mass flow

W = CFM x Density

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

CFM = 3160 x Kilowatts / D T x ° F

This yields a rough estimate of the airflow needed to dissipate a given amount of heat at sea level. It should be noted that the mass of air, not its volume, governs the amount of cooling.

Determining System Impedance

After the airflow has been determined, the amount of resistance to it must be found. This resistance to flow is referred to as system impedance and is expresses in static pressure as a function of flow in CFM. A typical system impedance curve, in most electronic equipment, follows what is called the "square law", which means that static pressure changes as a square function of changes in the CFM.

Static pressure through complex systems cannot be easily arrived at by calculation. In any system, measurement of the static pressure will provide the most accurate result.

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