Fan Engineering Blastphemy

I had my Fan Engineering book on my desk today and a friend who has never seen one before commented that with its black bound cover and gold lettering it looks like a bible.

“Uh, I think that was the intent.”, I said. “In fact, it is called the Fan Engineering Bible” and a good companion for any person working with fans. (Robert Jorgensen did a nice job on that book).

Anyways my friend said, “Okay if there is bible then what would be fan engineering blasphemy be? Or, should I say blastphemy” in a mildly mocking tone.

Hmmm. Good question I thought from someone who is an English major.
Got me thinking.

So I want to list some of the mortal sins that a person may do that is so wrong that excommunication by AMCA is a real possibility — or at the very least — being publicly asked to step out of the buffet table line at a PowerGen hospitality suite.

Faux Pas No. 1

Thinking That Fans Decide What’s Going on

Fans are not that smart. They might be cute, but they’re not that smart.

When air isn’t moving as per the required amount all eyes generally go to the fan and the comment is said, “There’s something wrong with the fan.”

More often then not it is not the case. The system and its pressure drop “decides” what the flow of air will be. Poor duct design is often a major culprit to an underperforming fan/system.

When trying to figure out why a fan is underperforming look at the fan and its system. For instance look for sharp bends that create eddies and turbulent flow. Also look for abrupt increases and decreases in diameter. Look for high fan outlet velocities – typically over 4000 fpm — that lacks an evase that would help to “regain” some of the static pressure.

Or just ask your friendly neighborhood fan guy.

Faux Pas 2.

Thou Shall Not Exceed Tip Speed Limitations

Fans can be connected to a 30 hp or 3000 hp motor. Either way that is a lot of energy in a relatively small container. It is all safely contained as rotational energy. But here’s the thing … that kinetic energy can become a little bomb if the fan wheel stresses exceed its designed yield strength.

Speeding up a fan without checking what the maximum tip speed limitations is no joke.

Also if you are replacing a fan wheel please don’t go a ordinary fabrication shop.

Use an approved fan manufacturer who understands stresses inherent in fan wheel design. The static components are different then a wheel as they don’t see the same stresses.

Don’t take chances on a fan wheel.

Faux Pas No. 3

Thou Shall Not Run the Fan Backwards.

A centrifugal fan running backwards can still push air — except of course a whole lot less. If you have a problem and you are troubleshooting the first thing you check is rotation. I personally came across two main exhaust fans running backwards. Everyone naturally assumed the two fans couldn’t be running backwards. One perhaps, but two, no way. Except of course it was.

Reminds me of a scene in the movie Slingblade where they are trying to figure out why a gas mower won’t start. One of the characters (Scooter) said he can’t figure out why it won’t run. He changed the points and the sparkplug.

The Billy Bob Thornton character (Karl) squats down, unscrews the gas cap and says, “The gas tanks empty”.

Moral, check the simple things first and go from there.

Well I can write a whole lot more but these if three are abided by it will mean you will have a good chance of staying in the buffet line right up to the roast beef.

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