OK...maybe my sarchasm fogged the point. Where I was headed (or
intended to be) was the losses when sucking through a vent are
different than when blowing. Major contributors are corners. For
instance, a bend in a duct (commercially available 90 deg elbow)
creates an eddy current that reduces the effective diameter to <25% of
the duct. That is, if you needed a 4" duct, and put a 90 in, then what
you end up with is an effective <1" duct. The solution is simple, such
as using 2 45 deg, which increases the bend radius to >2.5X the duct
dia, keeping the efficiency somewhere around 80%, yielding 3.2"
effective diameter. The "typical" HVAC guy doesn't know this. Things of
this nature can sabotage the installation and cause major headaches if
several machines are on the circuit. Isolating each machine will
simplify rectifying errors.

Ed

You wrote:
>
>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
>

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