Steve,

There's a great book published by the American Association of
Industrial Hygenists called "Industrial Ventillation". It contains not
only enough tables, charts, formulae,etc. to satisfy even a world class
geek, but some very good "real world" examples of what type of
configurations are "preferred" "good" "no-way". I've learned a lot
about vents from this book, but in no way do I profess to be an expert.
*CAUTION* I have noticed some self proclaimed experts that can
calculate losses when "blowing" through a pipe haven't a clue what
happens when you "suck" through the same pipe. Things get a little
wierd there, and the "run of the mill" HVAC guys many times haven't a
clue. The voice of experience speaks here.

My suggestion (other than buying the book AND reading it) is to
consider splitting up the systems. So what if there is one or five
blowers on the roof... I'm sure the pidgeons won't complain a bit! What
you will gain is a much better margin of error when the HVAC guy's
theories work on paper, but not in your plant. Variables like building
barometric pressure (changes seasonally with some heating/cooling
systems) can be positive (the building blows) or negative (the building
sucks). You can tell this when you open a door such as in a lobby. Most
facilities types like the building to be positive so they can control
the ingression of fresh air (remember, trust is good, control is
better). In addition to this margin, you also have lower overall cost
when you start getting up there in blower size as you are finding out.
Another BIG plus is that you run very little risk of water/steam ladden
air getting back down into your reflow or soldering system. That can
get pretty ugly, especially if they don't forsee this and do not put a
drip leg into the configuration that can handle the weight. I've seen
the results of a poorly desighned vent of this size trap nearly 100
gallons of water in the vent, and suddenly the structure (originally
intended to hold some sheet metal vent duct) decided that the weight of
the water wins. Not pretty at all.

So... to make this short (yeah, right) if you would like to discuss
this r/t, please feel free to call me.

Ed Popielarski
QTA Machine
714-581-6601

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