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Referring to the pasted in ductwork diagram, if an inline ventilation fan with a high CFM airflow was used to circulate air within a sealed ductwork system, would the fast moving airflow result in a significant pressure drop within the ductwork and cause a section of it to implode?

Or since it is a sealed system and air can neither leave nor enter it, would the result be that the fan would essentially just be circulating pressurized air around the ductwork and there would thus be no pressure drop within the ductwork? Referring to the ductwork diagram again, would static and dynamic pressure (in PSI) at points A, B, C, and D be approximately the same if measured first with the fan turned off and then measured again with the fan turned on?

Although this appears to be a homework question, it is not. I am contemplating running ductwork from my house to my garage and I want to make sure that what I construct will not implode.

Ductwork diagram

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    $\begingroup$ It's probably worth noting that neither the heating coil nor the air conditioner condenser were sized for what contemplating. Those units have ratings and limits on what they can do to condition the air. You're likely going to negatively affect the rest of the building by adding a substantial amount of ductwork to the system. $\endgroup$
    – user16
    Dec 13 '18 at 2:24
  • $\begingroup$ Check the fan rating: many fans are designed to work with essentially zero back-pressure (think of a fan pulling air out of a duct into the outdoor atmosphere) and will be much less efficient in a closed system like this. $\endgroup$
    – Carl Witthoft
    Dec 13 '18 at 19:05
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Closed or not, a fan will always raise the pressure in front of it, and lower the pressure behind it. In order for the ducting to implode you would need a relatively strong vacuum. What is needed would be based on the rigidity of the ducting. However, a couple points that are important to remember:

  • Its a closed system, so mass is conserved. Regardless of where the air density is at any given point, the air is available.

  • While a fan will slightly raise the pressure on the outlet side, even a powerful fan, is not a compressor. It may move the air but it will not compress the air significantly, so the "make up air" will be readily available on the intake side.

  • Natural convection dictates that an increase pressure gradient will increase air flow. Even if there was compression, in a closed system, the rate of compression would be compensated by the rate of airflow.

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Would the pressures be the same with the fan On/Off? No.

The pressure in front of the fan would be higher than that behind it - that is to say, the pressure would be highest at A, and then drop gradually around the loop B, D, C.

Would it implode? No. - The pressure at C would be lower with the fan on than off, but in order to implode, the pressure would have to drop to below atmospheric pressure, so unless the system is already at a very low pressure, and on the point of failure, this would not push it over the edge. This is not a concern for ductwork between house/garage!

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