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Vac Pump Traps


G’day; I have noticed a number of postings referring to vacuum
casting, which contain comments to the effect that water is
literally boiled out of the casting mould material. (Water boils
at room temperature if the external pressure is low enough, like
it’s not easy to make tea on Mount Everest; the water never gets
hot enough!) One thing I haven’t heard mentioned is what happens
to the water that boils off and then condenses back in the pump,
where it hangs around busily corroding the works until the pump
gets used again. At one time in the (distant) past I had a great
deal to do with high vacuum (very low pressure) systems, and we
invariably had a trap inserted at the input side of the pump.
This was a simple and often very cheap lab-made device. In my
case (but only for what we called a low vaccuum - only down to
5mm mercury pressure. it consisted of a heavy-walled pyrex flask
with a couple of glass tubes and a rubber bung bored to take the
tubes, one short at the output and one long at the input side.
Another way (when I couldn’t get such a flask) consisted of a
half pint milk bottle - which in Britain and NZ are of very thick
glass with a 1" dia mouth, holding same sort of fitting. These
vessels were immersed in an ice and salt mix, which condenses out
all the water before it reaches the pump. For REAL vacuum
systems - 10 to the minus 5 of a centimetre of mercury
(0.00001mm) and better we used a special bit of sealed glassware
especially designed for the job (I used to have to do the
glassblowing) and immersed in liquid nitrogen at a temperature of
minus 140C. However, if you don’t want to mess with the ice and
salt mix, then fill the jar/bottle with LUMPS of calcium chloride
or even quicklime (calcium oxide, CaO) or properly dried silica
gel to absorb the water. And get the silica gel with the blue
indicator - you can easily regenerate it in the microwave oven
when it goes pink. A lot of messing around, you reckon? Well,
your expensive pump will last longer. Especially if you empty and
dry the trap & contents before re-use. Cheers, –

   / /    John Burgess, 
  / /
 / //\    @John_Burgess2
/ / \ \

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