This calculator determines the airflow and fan requirements for a forced-convection (fan-driven) system,
based on information about
the target air temperature rise through the system and system impedance. When the calculator is invoked,
the user is prompted for the following information:
- Ambient air temperature (deg. C): The (worst-case) air temperature of the room in which the
system will be operating.
- Altitude (m): The (worst-case) altitude above sea level at which the system may operate.
- System power (W): The total power dissipated as heat inside of the system.
- Air temperature rise (deg. C): The worst case allowable difference between the average system exhaust
air temperature and the ambient air temperature.
When the "Compute" button is pressed, the calculator determines air density (in kg/m^3) and the required
airflow (in CFM) to achieve the target air temperature rise. The equation used to perform this calculation is
[airflow] = [power] / ( [density] * [temperature rise] * [Cp] )
where Cp is the heat capacity of air. The user is now prompted for some more information about the
system to determine fan requirements:
- System impedance (in. water, CFM): A quadratic relationship is assumed between pressure drop across
the system components and airflow generated by the fans. By knowing the pressure drop at one airflow rate, the
entire impedance curve can
be extrapolated. For best results, the system pressure drop should be input at an airflow close to the
required system airflow.
- Fan type: A selection of typical fans used in electronics systems is presented here. The fan specifications
are supplied based on an average of at least 5 different manufacturers' specifications. Further revisions of
this calculator may allow for user specified fans.
- Maximum # of fans in parallel: In this calculator, it is assumed that the fans are arranged in arrays of fans
(trays) that are placed one after another in the air stream. This number is the maximum number of fans that are allowed
in an array.
- Use max # of fans only: By default, the calculator will determine the minimum number of fans (per array)
required to cool the system. If this box is checked, the calculator will report the airflow only for the number of fans
per array selected above.
- Maxium # of fans in series: This is maximum number of arrays of fans. It is assumed that all of the airflow passes
through each array. It is also assumed that airflow from an upstream array does not affect the performance of a
downstream array.
When the "Compute" button is pressed for a second time, the minimum number of fans required to cool the system
is determined (unless the "Use max" box is checked). Estimates of airflow, fan noise, and electrical power are
calculated for the given configuration. If it is not possible to cool the system within the constraints, the
calculator will report "Not Feasible".
- RPM: The first calculation is performed using a nominal fan speed (RPM). After the first calculation
is run, an additional input box is supplied if a different speed fan is desired.
This calculator allows the user to get an approximate understanding of airflow and fan requirements. Due to the
complexity of electronics systems, it may not supply accurate results for all types of systems. For more accurate or
more detailed results, please contact the engineers at Applied Thermal Technologies directly.
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