[Jewelry making - Article 00063] - [Orchid] Do it Yourself Ventilation

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    Dear all, I am once again trying to stay out of the fray as much as
    possible. Like I have said before, this website can be addictive.  I
    am responding because I was asked specifically so don't complain
    because I talk too much. There are some particular things that you
    need to know about hoods for the investment casting and jewelry
    industry that I have not seen in this thread. They may have been
    there but I don't read this every day.  Please forgive me. 

    The hoods for particulate in the investing areas are of a slightly
    different design than hoods for fumes.  The use of hoods that are
    really designed to remove heat (kitchen ventilation hoods) are not
    really suitable for our industry.  There are certainly ways to
    increase the effectivness of these types of hood designs but in
    general, any hood that has an up-draw system is not for us.  These
    types of up-draw hoods pull fumes and or particulates, up and through
    our breathing zones.  Generally we are standing in front of these
    things as we work with some fan or air conditioning vent blowing
    around the air in the room.  Most of the hoods that I have seen used
    like this are simply hung above a work station, most times attached
    to a wall. Two sides and the front of the area under the hood are
    open to the room with no limitations.  This means that as the hood
    draws, it is drawing  from all sides but the wall that it is attached
    to.  If you calculate the area that is available to draw from and the
    cfm (cubic feet per minute) of air that your hood can draw, you will
    see that the velocity of the air that you can move is dang near
    nothing.  What I am saying is that the force of someone simply
    walking by the "vented" area has more force than the hood above your
    area.  Air velocity is a much more important thing to consider than
    just cfm alone.  Now if you restrict the area from which the air can
    be drawn, then you can increase the velocity of the draw and increase
    the effectivness of the hood. 

    In order to modify these types of hoods so that you can use them in
    the safest manner, you would need to make sure that the system is
    closed on 3 sides of the hood all the way down to your table.  Then
    you would need to limit the size of the opening in the front from the
    top down using some clear plexiglass or lexan.  Clear so you could
    still see what your doing. Basically this would start to change your
    hood from an up-draw to a back draw system.  You could make one more
    modification by putting a full width baffle from the inside front of
    your kitchen hood that extends back and down about 3/4 of the
    distance between your hood and the table top.  You would leave
    adequate space in the back of this arrangment to create a back draw
    hood.  This baffle would have a few 1/2 inch holes drilled at the
    top, near the origin at the hood, so that any fumes that did rise due
    to heat that were missed by the back draw velocity could also be
    captured there.  I hope your following this descriptioin.  I just
    know someone is going to ask for drawings but please remember that
    this is what I do for a living.  If I keep giving everything away, I
    will continue to get poorer and poorer.  I just don't want any of you
    to get hurt. 

    Most of the kitchen hood designs depend on heat rising and you being
    a reasonably good cook.  I would dare say that even with the best
    kitchen hood, you can smell what your cooking all over your house.  I
    just hope that your cooking smells better than mine.  If you can
    smell food cooking all over the house, imagine having various
    chemicals wafting about your shop or factory.  Even worse is the
    silica in the investment powder.  It doesn't smell but it sure can
    cause some damage to your lungs.  Here I go again with the lung
    thing.  What I have just described is okay for plating purposes and
    some light chemical work but is still no good for investing or
    devesting. 

    Particulate hoods for investing and devesting need to be very strong
    and very targeted because particulates are solids and can collect
    while fumes tend to dissapate.  This means that when you are pouring
    your investment powder into your mixing bowl or machine, you need a
    back draw vent practically sitting on the lip of the bowl so that any
    powder (silica), cloud is immediately drawn back into the vent and
    away from your breathing zone.  This couldn't be any more important
    when you are "quenching".  The steam from quenching carries a much
    smaller silica particulate into the air that can be breathed deeper
    into the lungs.  Again, the use of a hood system that is closed on
    all sides except for a limited area in the front and draws to the
    back over your quench tank is imperative.  Some of you will say that
    you wear a mask but I can assure you that unless you are mask savey,
    clean shaven, and vigilantly clean they don't help that much.  Beside
    that, like I said, the particulates collect and can float around
    through the general ventilation system to affect everybody.  Better
    to get it out of the building. 

    Now I am not exactly handing you fish, but I am at least telling you
    things to consider as you go out fishing. 

    One book that is about $69.00 is called, Laboratory Fume Hoods: A
    User's Manual by G. Thomas Saunders 

	http://www.amazon.com/exec/obidos/ASIN/0471569356/theganoksinpr-20

    Also check out this website.  It's free
    http://www.pp.okstate.edu/ehs/LINKS/Fumehood.htm 

Now let me go back to my cave....
Best Regards,
J. Tyler Teague
JETT Research

[Edited Ton: Add Amazon link]


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