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  • fan in intake or exhaust?

    For my solar air heater box, should I install the fan in the intake from inside the house, or in the hot air exhaust blowing into the house??? The box is 4'x 6' and the computer fan is 100 CFM.Thanks!!!!!!

  • #2
    There are some differences - what is important to you?
    a) Bearings / Motor will run hotter in the exhaust, cooler at the intake
    b) Slightly higher CFM at the exhaust (pull), slightly lower CFM at the intake (push)
    c) Outside air is sucked in through leaks with fan in exhaust, inside air is pushed outside through leaks with fan in intake.

    I think ...
    a) Put the fan at the intake = runs cooler
    b) Tolerate slightly lower CFM
    c) Seal the leaks

    Is the CFM of your fan affected by static pressure?
    Last edited by NEOH; 09-20-2017, 09:02 PM.

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    • #3
      Push the air though box. You'll move more air mass. The blower will run cooler and last longer. That's probably how it was designed to work anyway.

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      • #4
        Originally posted by J.P.M. View Post
        Push the air though box. You'll move more air mass. The blower will run cooler and last longer. That's probably how it was designed to work anyway.
        You want turbulent air flow in the box to maximize heat transfer for a given bulk air flow rate.
        Putting the fan at the intake is more likely to boost turbulent flow.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Great info!!!!! Sure glad you guys replied. Fan in intake is the plan. Thank you!!!!

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          • #6
            Originally posted by inetdog View Post

            You want turbulent air flow in the box to maximize heat transfer for a given bulk air flow rate.
            Putting the fan at the intake is more likely to boost turbulent flow.
            The blower ought to be on the inlet, but either on the inlet or outlet, the blower's location may, or may not, increase the film coefficients and flow distribution of the air.

            The collector's efficiency will depend on those parameters which are not necessarily f(blower location). If you are referring to entrance effects and the perhaps increased film cofficients that may result, they are more f(inlet shape, location, and any diffusion schemes). For the same inlet shape or form. the flow characteristic will be about the same whether the air is being pushed, or pulled through the inlet.

            FWIW, and climbing into the wayback machine, seems like what goes around, comes around: Looking for information about the very flow distribution we're discussing here was the precise matter of a question I asked of an M.E. grad student in spring of 1976 as he sat monitoring his solar air heater gathering data for his thesis. I was building my first air cooled solar collector and was wondering about such things. The (non) answer I got amounted to a 10 minute dressing down for not understanding what a Reynolds Number was, or its significance and power, and until I did, I was wasting my time.

            Appreciating that he was correct, but not appreciating the non answer, I returned to school for the 3d time the following fall. I now understand what a Reynolds number is.

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