We built a new air handler for a large office building.  The air handler
had a 200 HP fan motor, which delivered 80,000 CFM at high pressure (5"
static).  I got involved when we had to install a variable frequency drive
to run the 200 HP motor, and interlock it with the HVAC controls.

Now this was one of the bigger air handlers the contractor had built.
Picture this, an airtight box, maybe 50 feet long, with a fan at one end
sucking air out of the box, various filters and cooling coils inside, and
a big wall of dampers at the other end.  The dampers selected between
outside air and return air, and the controls picked whichever one used the
least energy to cool.

Now one or the other damper *must* be open at all times, and one of my
jobs was to build the interlock that made this happen.  My stuff worked
fine, but the contractor put tiny little motors on the dampers.  I was
*inside* the air handler when the inevitable occurred.  The 200 HP fan
kicked on, and both dampers remained closed.  The walls began buckling,
and the whole wall of dampers began to move toward me with a noise like a
bulldozer was coming through them. .  I had to hit the door (which opened
OUT) with my shoulder a few times to get out of there alive.  I ran is
slow motion like one of those dream sequences in the movies to slam the
panic switch on the motor controls and shut the whole system down.


-- Lawrence Lile

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