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Technical Note

Introduction

    Selecting the right fan for your application is a challenging task. The objective function,
    for members of a group with the right mind, is to maximize benefit to the society, as is the case with any design activity. For this objective, immediate explicit factors that we should be aware of include:
  1. Enough air flow
  2. Size that fits
  3. Longer life
  4. Lower Cost
  5. Less power consumption
  6. Lower electro-magnetic interference
  7. Lower sound noise

Air Flow through System

    For determining necessary airflow to accomplish enough cooling for our system, we first model the macroscopic temperature map.

    We have a rough equations for the above four quantities as follows:
    Q (CFM)=1.76 W (watts)
    Ti (°C) - To (°C) (eq 1)

    The equation reveals with more heat dissipation, we need higher airflow to keep the same inbox temperature. In most cases, we know the heat dissipation and target inbox air temperature and the equation gives the necessary air flow.

Fan Selection

    Once we know the necessary air flow from the previous step, we move on to find the right fan. Looking up the free airflow column can be misleading. Free airflow, or maximum airflow is what the fan accomplishes when there is no pressure difference across the fan. In the extreme case of no air holes other than the fan outlet, there is no airflow and the static differential pressure maximizes.

    The Static Pressure column in the fan table lists this value for each fan. The fan performance curve shows how much air flow each fan produces as static pressure varies. The unit, on the other hand, has a characteristic that static pressure increases with the airflow through it. When plotted together, the two curves meet at the operation point when the fan is installed in the unit. The diagram below shows the case where a 115CFM fan operates at 75CFM on a system with the plotteed characteristics. The static pressure at operation is 0.14inchesAq.

    The fan performance curve is available from the fan manufacturer. The unit characteristic curve is not, and it is not easy to find unless you have a wind tunnel with an orifice and pressure gauges, and you will have to struggle designing a fixture to mount the unit on the wind tunnel.

    An alternative to directly measuring the unit characteristic curve is tu use (eq 1). Thermocouples are inexpensive and in most cases, readily available in any thermal labs. Knowing (or having a good estimate of) total heat dissipation W and measuring temperature values with different fans installed allows you to estimate the unit characteristic curve in the neighborhood of operation. Here is an example:

    We have a design condition of 100W total heat dissipation, 50°C maximum inbox at 25° ambient temperature. From (ew 1), we find required airflow of:
    1.76 *100/(50-25) = 7.0 (CFM).

    With free airflow of 8.0 and 10.0CFM, we sampled HDF4012L-12HB and HHB. Installing these fans on the box and measuring average inbox temperature resulted in 66°C and 52°C respectively with the HB and HHB fans. Close, but not quite at target. Plugging these values back in (eq 1), we find the airflow at 1.76* 100/(66-25)=4.3(CFM) and 1.76* 100/ (52-35) =6.5(CFM). Plotting these points on the fan curves, we can estimate the unit characteristic curve with the red dotted line. In order to reach 7.0CFM with the system, we need, HDF4020L-12HHB or HDF6015L-12HB. Note that HDF5010L-12HB and HDF4020L-12HB even lowers the air flow through this particular unit.

    If you need a quick starting point before having to go through any testig or calculation, you may want to use the attached equation as a conservative guideline.
    Q (CFM)=0.23 W (watts) (eq 2)

W (watts)102050100200500
Q(CFM)2.34.611.502346115