Hi there Stve I am an HVAC/Mechanical Engineer I will appreciate to help
you if you will.
Ruben E. Irizarry
Mechanical Engineer
[log in to unmask] on 05/12/98 11:37:54 PM
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cc: (bcc: Ruben Irizarry)
Subject: [TN] ASSY: Reflow, Wave, and Cleaner Exhaust Issues...(again!)
Hi Folks!
I hate to keep beating a dead horse so to speak, but I've still got a
few issues with the exhaust system I'm trying to set-up in our start-up
facility. I should say it's not me that has the issue, but others that seem
a little hesitant with what I want to do.
I did get some more quotes, and was able to cut that $11,000 one
almost in half...the latest one was for $5,960, and the gentleman that came
out and did the quote said that there shouldn't be any problems with the
exhaust fan I have...
BUT, I guess since this is one of the first times I've actually dealt
with the "nuts and bolts" of this stuff, a few other people that don't know
anymore than I do about these things, have gotten involved with this
project...talk about too many cooks in the kitchen! SHEESH!
If you remember, I have a Dayton 5,700 CFM 2-hp, belt-driven,
centrifugal ventilator fan up on the roof (that's less than 2-years old)
that was left by the previous tenants. They also left a large portion of
the ducting inside the building as well. We're going to have 4 pieces of
equipment to exhaust; a Heller reflow oven (500 cfm req.), a Soltec wave
solder (700 cfm req.), a Trek Triton IV cleaner (1800 cfm req.), and a
Roilguard water evaporator (has it's own blower @ 1600 cfm) coming off a
ultrasonic stencil cleaner.
The evaporator is going to have it's own duct to the roof because it
has it's own blower. One of the "Guru's" here on the TechNet pointed out
to me to exhaust the evaporator by itself as it would lower my main fan
efficiency if I tried to plumb it in with the main duct, and that makes
sense, so that's what I'm a doin'...
But what seems to be not setting well with others, is that I want to
use one fan to exhaust the other three machines. They seem to feel that
because I'm going to plumb the cleaner into the same duct system that will
be exhausting the reflow oven and wave, the moisture in the air that will
be coming from the cleaner is going to somehow work it's way against the
flow of air, backwards, and fill the reflow and wave ducts full of water.
I know about the condensation that could occur, and how you need to
orient and position the duct run to prevent a problem like that from
occurring. That's exactly why I'm placing the cleaner right beneath the
exhaust fan, so that the exhaust duct will be making a shot straight-up
thru the roof (which is only about 12-feet).
The exhaust fan has a rectangular plenum beneath it which is around
2-ft X 2-ft X 4-ft. The cleaners exhaust duct will tap into the bottom of
the plenum, and there will be one duct that will tap into the side of the
plenum and run horizontally with a very slight downward angle towards the
plenum to keep any water (if it should happen to condense in the duct) away
from the ovens...I really don't think that'll happen though.
This horizontal duct will run above the wave and reflow, and each
machine will have one duct with a volume damper that will tap into that.
Right now, I'm just trying to use my common sense with this project,
and I've worked at companies before where we've had multiple machines
(reflow and cleaners) sharing an exhaust system without any sort of
problems. I will readily admit that I'm not a HVAC engineer, but I'm trying
to learn fast though, I went and bought a book called; "Elements of
Thermal/Fluid System Design" and been studying that for the past few days
and have learned more about different fan types and figuring loss
coefficients for duct elbows and such, than I ever thought I would need to
know. But I guess I haven't come to the chapter yet that talks specifically
about setting-up an exhaust system for a printed circuit manufacturing
floor.
Every book I've looked at talks mostly about air conditioning. I
haven't been able to find a book that was recommended to me by a Technetter
called; "Industrial Ventilation; A Manual of Recommended Practices" from
the American Association of Industrial Hygenists...
Oh, another curve was tossed at me today too. Someone had said that
they had heard that is was very detrimental to mix the exhaust from a
cleaner with exhaust from reflow or wave because the moisture in the
cleaner exhaust would react with the vapors coming from reflow and wave and
then make some sort of toxic substance...I know better than that, but
trying to find something published that says "It's okay to exhaust a
aqueous cleaner, reflow oven, and wave solder together in the same duct
system." Is pretty tough if ya' know what I mean...
I'm sorry I'm going on and on about this, but does it sound like I'm
doing things logically? Or am I way off base? I've called Trek by the way,
and sorry to say they've not been much help at all...
I know that if it were a perfect world and we had a lot of money I
would've said to pay the $11,000 and be done with it, but we're just
starting out and you all know how that is.
I would appreciate any feedback at all...and don't be shy, if I'm
screwing up let me know please!
Thanks in advance!!
-Steve Gregory-
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