September 14, 2011


In movies and video games, ductwork seems to exist for the sole purpose of moving people through a building. In reality, it is used to move air. When a system is running, air is moving through it. If you are going to actually travel through a duct, you want to take this into account.

Engineers think of air velocity in ductwork in terms of feet per minute (FPM). A normal speed is 1000 FPM, but high velocity systems can run at 5000 FPM or higher. Converting to miles per hour, that is 11.3 MPH in most ductwork and 56 MPH in high velocity systems. These speeds would make travel through the ductwork difficult, but probably are not high enough to actually uncontrollably pull you. The ninja must have stumbled into a veryhigh velocity system.

When fan blades are exposed, there is often a guard screen to protect against large objects from entering the fan. Unfortunately for the ninja, this fan does not have that safety feature.

For the engineers in the audience, this fan lacks some critical details, such as bearing supports, but we doubt the ninja was thinking about that as he went through it.

Mark Robison, PE


  1. Joel says:

    Looks like your Ninja (aka Tom Cruise) has encountered another impediment in the ductwork that has slowed his progress and put an end to his mission. He must have missed the mandatory Lock Out Tag Out Training Session. Your next warrior must be more aware of obstrutions in Exhaust Ductwork such as UV Lamps that could blind him, or Electronic Precipitator Cells that not only are able to zap grease from the exhaust stream but Ninjas as well.

  2. David Robison says:

    As you will see, our ninja slept through all his safety classes.

    Thanks for the suggestions of additional dangers in ductwork–there’s a chance our ninja might run into these in the future!

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