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Thread: Looking for air flow number

  1. #1
    Associate Engineer
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    Jan 2021
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    Looking for air flow number

    Hi - I am trying to determine what "Moderate Air Flow" means in regard to this table:

    https://www.engineersedge.com/heat_t...nts__13378.htm

    Does anyone know of another reference range for "moderate" in CFM?

    I am trying to specify the CFM for a heatsink designed with the coefficient labeled:

    "Forced convection, moderate speed flow of air over a surface"

    from the table above.

    Best
    Kevin.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Well, no easy answer as nobody knows what geometry, material, environment your system is operating in.

    First thing you need is the a rough estimate of the airflow needed to dissipate a given amount of heat at say sea level. It should be noted that the mass of air, not its volume, governs the amount of cooling.

    If using an off-the-shelf heat sink there is usually specifications on which fan or other you need. If not the following is a guide but not the definitive answers on what you might need.

    Establish Cooling Requirements

    Before a fan and the CFM specified, the airflow required to dissipate the heat generated has to be approximated. Both the amount of heat to be dissipated and the density of the air/fluid must be known.

    The basic heat transfer equation is:
    q= Cp x W x DT

    where:
    q = amount of heat transferred
    Cp = specific heat of air
    DT = temperature rise within the system
    W = mass flow

    Mass flow is defined as:

    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 = 3.16 x Watts / DT (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 expressed 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. For most forced air cooling application, the system curve is calculated by:

    P = KrQn

    where:

    P = static pressure
    K = load factor
    r = Fluid Density
    Q = Flow
    n = constant; Let n=2; approximating a turbulent system.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Thanks Kelly - you are spot on, this is in review of a heat sink that was designed/modeled for us using the coefficient from the linked table. What we are trying to figure out is the minimal air flow required to keep the board cool. The problem is that in a typical PC chassis, we are seeing the devices overheat.

    I will likely follow up with more questions as I piece together the info above.
    Kevin.

  4. #4
    Principle Engineer Cragyon's Avatar
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    There's a "Forced Air Flow Convection Cooling. Fan Air Flow Required for Heat Sinking Formulas and Calculator" on Engineers Edge.

    See: Fan Air Flow Required for Heat Sinking Formulas and Calculator

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