The preceding steps indicated the necessary procedure to estimate the required airflow in order to obtain the desired overall air temperature rise T. However, it was also indicated that the actual operating airflow is determined by the intersection of the fan curve and the system resistance curve. There are three options available for estimating this operating point: (1) experimental measurement using a thermal/mechanical mockup of the system, (2) calculation of the operating point using airflow network methods [3], or (3) calculation of the system airflow using computational fluid dynamics software (available from commercial software companies).

The experimental procedure can be used to measure the total airflow for specific fans or several pressure-airflow data pairs can be measured to develop a complete system resistance curve. The latter experimental method will then require the Engineer to superimpose the selected fan pressure vs. airflow curve and system resistance curve to obtain the operating airflow.

The airflow network procedure provides adequate results when the geometry is simple and the flow path within the cabinet is known or a rough estimate can be made. In many practical applications, however, the Designer deals with complex three dimensional flow paths that are not known from the very first instance. In these situations, CFD software can be used. The fan performance curve can be supplied as an input to the CFD software and the software system allowed to determine the operating point and system resistance. CFD works by numerically solving the governing equations of flow and heat transfer in three dimensions and takes into account the effects of turbulence and gravity. CFD can be used to study the performance of fans in series and parallel arrangements as well as optimize the location with respect to other objects inside the cabinet. Both of the computational procedures require a static pressure vs. airflow curve for the fan in question.

Irrespective of which method is chosen to estimate the system airflow, all packaging systems are characterized by a system resistance curve of the type shown in Figure 3. System resistance curves may usually be expressed as a non-linear expression of pressure vs. airflow:

where
	P = system pressure loss
	K = a load factor specific to the system
	þ = density of air
	G = airflow rate
	N = a constant which varies between 1 and 2 depending on whether the flow is completely laminar (N=1) or completely turbulent (N=2)

If the estimated value of the actual airflow is significantly less than the required value, the packaging system should be examined for regions where the airflow resistance could be reduced. Should it fail to provide an answer a different fan or perhaps even multiple fans should be considered. The search for a different fan is a simple matter of reviewing the catalogs of the various fan vendors. The consideration of multiple fans is a little more complex.

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