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Venturi Vacuum thoughts


#1

Be advised, this is a first-time post from a new lurker who is not a
jeweler - but some of my work crosses-over where techniques and
equipment apply.

  1. Without knowing the size of your castings or the volume involved
    in each cast/ time & strength of vacuum application needed, it seems
    Mr. Grandi’s advice is on-target. The salient point is: there is no
    vacuum “pulled” unless there is air movement through the venturi.
    (Example: perfume atomizer - when you finish squeezing the bulb it
    stops drawing perfume up the tube and into the airflow.)

  2. However, IF you work in or have access to a facility providing
    "shop air" in piping with a massive compressor at a distant location,
    this might be feasible. Your capacity would be limited by the
    strength of vacuum produced at the venturi, and you would still have
    the problem of constantly-exhausting shop air through the venturi at
    your workstation, to keep the vacuum active. See 3

  3. Perhaps if you attached a hose to the output of the venturi (IF it
    has a provision for this) and left its open end (if a "compressor"
    hose with fitting you would have to defeat the fitting or attach a
    blowgun or similar pass-through tool) a sufficient distance away from
    your workstation, the exhaust air and its noise/ moisture might be
    tolerable…by the way, there may be a lot of noise at the venturi
    body itself! If it is a closed system you might be able to muffle it
    under a pile of towels or household insulation. If it’s open with the
    exhaust-air right there you are stuck with the exhaust. If you can
    attach a hose, the hose and any attached tools would of course have
    to have sufficient inside diameter to allow for the passage of the
    air volume with little frictional loss or the venturi would no longer
    function fully; its output would have a resistance load. I’m not an
    engineer; I just have some practical experience using air tools.

  4. Are you still reading? Then here’s an idea that might make it all
    work anyway: maybe the venturi and its exhaust don’t have to operate
    continuously. If it produces a high enough vacuum that vacuum might
    be maintained by adding a solenoid-operated air valve between the
    casting machine and the venturi. There are many such valves available
    through Grainger, McMaster-Carr, etc. and some would operate on 12
    volts or even 6 volts that you could supply with batteries. Once the
    vacuum is pulled the valve could be closed (OK even a manual valve
    would do this) and then the maintenance of sufficient vacuum would
    depend upon how leak-free your vacuum plumbing is and the volume of
    air you are trying to remove from the casting machine. An
    intermediate vacuum tank, using the common “freon-tank conversion
    kit” (intended for compressed air) might provide a large enough
    vacuum volume to drop the casting machine pressure enough, and for
    sufficient time, for the casting process to complete. If not, when
    the vacuum decreases you can restart the compressed-air venturi…

  5. Note: this is wild speculation and might never work well enough
    for your application! I don’t really recommend pursuing this route
    unless you love to tinker, consider it a challenge, and could throw $
    150 at it without being upset at failure…

  6. FYI - other industries use vacuum pumps of high capacity; maybe
    you can find one used but still very serviceable. Specifically,
    automobile air-conditioner servicing requires such a pump as does
    neon-sign tube manufacturing.

Good luck! If you pioneer a way to produce such vacuum less
expensively than the $500 pump, do tell. I’ve wanted to make some
art-neon for years but the equipment expense stopped me.

Michael Miron


#2

Hi all from sunny South Africa, Some more comments on the venturi
vacuum. I have a self built vacuum caster which uses a PIAB mini
vacuum pump to create the vacuum. Although it is called a pump it is
actually just a multi channel venturi. It operates with a very low
feed pressure of 0.4 Mpa which I get from a small home use
compresser. To ensure a decent vacuum at the point of casting I have
a small reservoir (about half a cubic foot) which stores a vacuum.
The pumps are inexpensive and if you have a source of compressed air
then there is no need to buy a compressor. The total cost of building
my vacuum caster was less than $ 250.00 which included the
compressor.The pump itself was about $ 80.00 and has a five year
guarantee.

The pump produces a vacuum of - 760mm mercury which is the same as a
shop bought vacuum caster costing $ 500.00.

PIAB have a website at www.piab.com.

Shaun Pearton


#3

Shaun Can you let us know the Model number of the pump that you are
using and also the reservoir and if you have a switch for the
vacuum. This will be pretty interesting for readers who have
compressors. I checked the site and they make so many pumps it is
confusing. Looking forward to your reply. Kenneth Singh.


#4

Hi All, I thought I might chip in with the fact that running water
will pull a vacuum using the Venturi prin. I first saw this working
in a industrial chem. lab, we had a simple metal tube that screwed on
to a standard spigot. It had a tube coming out at right angles which
you could hook up to anything needing a vacuum, turn the water on
full blast, and presto! 28in vac. (a bit wasteful of water though).
Some years later I bought a used , factory made vacuum casting table
that worked on this principal. It had a hose that you screwed on to
any spigot. I tried it, and it will pull a good vacuum. It has a
pretty small bell jar though, one flask at a time.

I’m sure that lab supply houses still probably have the spigot
attachment I mentioned. Shouldn’t cost much either. Bob
Robert Wise Studio


#5

Hi All, Although Vacuum by venturi does work,I must mention that you
can only cast very small flasks with something like this and if you
were planning on investing (plaster)then, again, you can only do one
small flask at a time. If you were to use a larger venturi system,
it is possible to do slightly larger flasks one at a time, however,
the larger venturi’s will use a lot of air. Lets assume you were
thinking about doing this… now total up your costs involved for
being able to invest and cast a small flask… Let’s take a wild
guess at about $400 for your minibell jar, piping, valves( not
cheap), filter, trap, casting platform or chamber… assorted other
small items… If your pretty handy and understand all the
mechanics involved , you may get it done for around $400.

The systems that are available from gesswein, contenti and a few
others that use mechanical vacuum pumps and will do investing of
more than one large flask and be able to cast any size flask costs
New about $750 + and can be had used from a number of different used
machinery dealers ( even the jewelry supply companies have used
ones) for anywhere from $250-- To about $400… definitely a better
deal than building a system that may not do all that you want to do.

In any case, Have fun , whichever way you go…

Daniel Grandi we do casting ,finishing, model work, molds, cnc
soldering,handpolishing and a lot of other things for people in the
trade. Please contact us off list at sales@racecarjewelry.com


#6
    Hi All, I thought I might chip in with the fact that running
water will pull a vacuum using the Venturi prin.  I first saw this
working in a industrial chem. lab, we had a simple metal tube that
screwed on to a standard spigot..... 

G’day; I have never seen a chemistry lab without several! Nearly 60
years ago when I took a lab technician’s exam we had to do scientific
apparatus glassblowing and a “Buchner Pump” was one of the items we
had to make - in glass. A good glassblower could turn out 6 in an
hour and guarantee they all worked. The lab pumps were commonly used
to increase the rate of filtration; hence they were also known as
filter pumps.

We measured vacuum differently though. What we measured was the
actual pressure of gas left in a system at maximum vacuum. Our
little glass water pumps if properly made, would leave a pressure of
only three millimetres of mercury. What that means in practical
terms is if one took a long glass tube closed at one end and filled
it with mercury, inverted it and attached it to a vessel being pumped
down, the mercury level would fall to a height of 3 mm instead of the
atmospheric pressure of 760mm; a much more accurate way to measure
vacuum. Putting it another way, normally the atmosphere will exert
enough pressure to hold up a column of mercury about 760 mm high (it
varies with the weather). The pumps exhausted the air from a closed
vessel, to the extent that was was left would only hold up a column
3mm long. – Cheers for now,

John Burgess; @John_Burgess2 of Mapua Nelson NZ