Sucking rather than blowing has more to do with leaks into the room rather
than leaks on the exhausting system and loss of CFM. If you blow exhaust out
and your ducts leak the flux ladden gas blows into the room whereas if you
suck the leaks pull air from the room. You would much rather have air sucked
from the room than to breath the exhausted gases.

Here at ETS we use only one exhaust motor and we manufacturer reflow ovens.
At any given time we are running 12 reflow ovens each exhausting 300 CFM. It
is possible to build an effective exhaust system using one motor for
multiple systems. Most important is to integrate an exhaust controller
(damper) which connects at the reflow oven exhaust outlet and to provide
twice the suck at the exhaust controller as is needed by the oven. This
allows the exhaust controller to regulate at 50% or the damper half open
which leaves a lot of room for dips and spikes on the suck.

Another important factor we consider at ETS is the load your exhaust system
puts on the airconditioner/environment control. The air you exhaust from an
oven originally came from the room. This air must be replaced somehow.

Just my 2 cents.

Brian


>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
>
>################################################################
>TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
>################################################################
>To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
>To subscribe:   SUBSCRIBE TechNet <your full name>
>To unsubscribe:   SIGNOFF TechNet
>################################################################
>Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional
information.
>For the technical support contact Dmitriy Sklyar at [log in to unmask] or
847-509-9700 ext.311
>################################################################
>
>
>

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
################################################################