Friday, May 05, 2006

Crawling Thru Ductwork: A Primer

From Mark Steyns review of Mission Impossible I,

Incidentally, it's lucky for Tom that, even for its state-of-the-art secure vault, the CIA continues to install air ducts the entire Radio City Rockettes could comfortably tap-dance down. A couple of years ago, at a loose end in a skyscraper, I tried crawling down an air duct and got as far as my shoulders. Possibly you could have got Kate Moss down there if you'd greased her up with Vaseline first. But, in the CIA duct, there's room for Tom and his accomplice, a camera team and probably a couple of other film crews: "Arnie! Quelle surprise! What are you doing here?" "'I'm breaking into the CIA for my new movie The Eraser, Tom. Vot brinks you?"

Bad guys never crawl thru ducts, it's always the good guys being clever who do it. Let's say you're a typical good guy agent trying to outwit your advisary and escape thru the air conditioning ducts.... The bad guys left you locked in a room, I'll help you escape.

In most office buildings the supply air is delivered by ceiling mounted diffusers, the amount of air varies depending on the cooling load in the room. It is extremely rare to find a diffuser with a throat size larger than 10" in diameter, 8" would be the norm. Stick you hand in one and start crawling.


The journey begins here. Posted by Picasa

These diffusers are fed by flexible aluminum duct. They in turn are connected to a VAV box; short for variable air volume. It's a butterfly valve located in the air stream that constantly adjusts the quantity of air. You would have to crawl through it to get to the bigger ducts and these things are generally 8" or 10" in diameter. The air is moving at about 1200 feet per minute upstream of the box.


Watch out for those flow sensors. Posted by Picasa

Lets say you got through diffuser, through ten feet of 8" flex then through the VAV box, thru the 10" supply duct and then into the truck duct, all the time bucking a head wind. Your good; (nobody does it better).

Depending where you are in the distribution system, the trunk duct may only be marginally larger than the 10" duct upstream of the VAV box. Ceiling space is not rentable so the depth of such space is always a minimum; generally 20". The ducts can never be deeper then 16" or they won't pass over lighting fixtures, sprinkler heads, speakers, smoke, flash and fire detectors, cabling, conduits or under column caps, beams or water and drainage lines serving the floor above.

Wow, you made it into the 16” high trunk duct (makes me feel sad for the rest). You can either play Alice and go back down to another VAV box where everything gets smaller again or head upwind to the mechanical room. What ever your choice, there’s going to be a lot of popping and banging noises coming from the ceiling space. If there’s a verticle shaft you could be screwed. A drop or rise is really tough to negotiate (and really spooky to look down, they’re not lit). If there is a shaft there will also be a fire damper and a balancing damper; opposed blade type, four to six inches on center.


Typical damper. Posted by Picasa

You were lucky, there weren’t any reheat coils or humidification wands soaking you and amazingly you made it thru the dampers and made the climb inside the most friction free environment in the building and you’re actually in the mechanical room. However you’re still inside the duct, which is now maybe 48" wide and 24” high so you can’t quite crawl and it’s really windy; your hair is a mess and you can't hear a thing. Up ahead is the reason for your bad hair; the air handling unit. They usually contain one or two four foot diameter ‘squirrel cage’ fans. The blades are four inches apart and spinning at you between 200 and 800 RPM. Your timing will be critical.


The end of your journey. Posted by Picasa

O.K. you got thru the fan; I’m impressed (Nobody does it quite as good as you). Now just a matter of opening the air handling unit fan compartment access door which is double latched on the outside; so you can’t. That means you have to crawl under, around or thru the cooling coil and you can't do that either so you’re screwed. The walls are 2” thick insulated sandwich panels; metal on both sides. It’s windy, dark, cold, damp and there’s a 60 horse power fan screaming beside you.

I hate to say it, but after getting this far you’ve failed the mission. If you’re dumb enough to even think about escaping through ductwork you’re too stupid to be in any big organization with the possible exceptions of Paramount or Al Queda. Just go back to where you started and take what’s coming to you 'cause baby, you ain't the best.

Sheesh.

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