Hello Technetters!

     I've got a problem that hopefully somebody can teach me a little
something about. It has to do with exhaust requirements and our Trek Triton IV
that we'll be getting in a week or so…(at least I got that make-up DI water
issue taken care of...thanks to all of you!

     The next problem is; I just got the quote for the exhaust duct-work that
I need to get done, I gave it to my boss, who then promptly had a cow.

     What the moo'ing was all about is that the quote is around 11K. Both he
and I are having a hard time understanding why the company is quoting us a new
exhaust fan to replace the one we already have up on the roof...and yes, it's
still there after the day before yesterday (we're in Sunnyvale, California
about a mile or two from where the tornado was).

     I should probably describe what I currently have, what equipment I'm
going to be exhausting, and what the quote is actually for.

     I wanted to exhaust 4 machines from one exhaust fan up on the roof. There
will be a Heller 1700 convection oven which the specs say I need 500 cfm, a
Soltec Prisma wave solder (I can't remember what kinda' suck it needs, the
spec is on my desk at work), a waste water evaporator coming off a ultrasonic
stencil cleaner (which has it's own exhaust blower rated at 1600 cfm so it
really won't add a whole lotta load), and last but not least our Trek
Triton...and that seems to be the culprit behind the big buck expenses.

     Trek spec's say that it needs a -1.5 in. hg. negative pressure in the
duct, the HVAC company that quoted us says the fan we currently have doesn't
suck enough, but I think it sucks pretty good. They want to completely replace
the fan we have which means a crane rental, and cutting a different hole in
the roof with all the associated work that goes along with that.

     Before we had the company quote our duct work, me and our facilities guy
checked to see how much the fan we have sucks. We went up on the roof and
checked to see what kind of unit we had. It's a Dayton unit, so we checked
with Grangers (they sell Dayton), and based on a 2-hp, 1725-rpm motor with a
3.5" dia. pulley, and a 24.5" centrifugal ventilator type fan with a 9"
pulley, they said we would get around 5700 cfm which we thought was plenty,
but they never mentioned anything about negative pressure...we didn't ask
though', I was mainly concerned about cfm until I was told that the fan needed
to be replaced.

     This is the first time I've been aware of a negative pressure requirement
inside the exhaust duct of any equipment that I've worked with...but I only
know enough about ducts and exhaust fans to be dangerous. I know that if the
exhaust system isn't set up correctly, you can have all kinds of process
problems, that's about it.

     The company that was in this building before us, left the exhaust fan and
a good portion of the duct work up, and they were exhausting 4 reflow ovens, 2
wave solders, and a ultrasonic stencil cleaner evaporator. I know that because
we found a layout drawing of the production floor they had, plus I also knew
some of the people who worked for the company that was here. So that's adding
to my confusion of why we need a different type of exhaust fan.

     The only horizontal duct runs from the main manifold will be to the wave
and the reflow, the moisture laden exhaust from the cleaner and evaporator
will be making more or less a straight shot up through the roof as they will
be located beneath the exhaust fan...that much I know about, you don't want
any chance for the moisture to re-condense inside the duct and start
collecting there, or to start to run back inside the machines.

     So what am I missing? I've kinda' heard that this could be the case of a
union shop just charging us way too much, but like I said, I'm not a guru on
this stuff. Can anybody enlighten me?

                               Thanks again!!!

                              -Steve Gregory-

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