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Valves
Name: Mike Z.
Status: Student
Age: 12
Location: N/A
Country: N/A
Date: N/A
Question:
Can a valve be made that only lets air through and not water?
Replies:
An interesting challenge! There are probably a lot of ways to accomplish
this, but the one I'll describe below is simple and has few if any moving
parts. There are some unknowns here, specifically, what is the flow rate and
what is the volume of material being handled (capacity). I will assume that
the flow rate and is "small (whatever that means)" and the capacity is
small.
General principle of operation: Make use of some property that is very
different for air and water. There are many, but the property I'll use is
density, whose difference is large for air and water.
With drawings this simple device is easily perceived, but it is a bit more
cumbersome to do strictly verbally. Here goes:
1. The separator is a reservoir that serves as a holding tank. It could be
any shape, but let's assume it is a vertical cylinder, whose volume is large
compared to the pipes carrying the air/water mixture. Three pipes are
attached to the cylinder: A pipe in the top face of the cylinder, call it
"Ae" (for Air Exit); a pipe in the bottom face of the cylinder, call it "We"
(for Water Exit); a pipe about 1/4 the distance from the bottom of the
cylinder, call it "AWi" (for Air/Water input).
2. The air/water two-phase mixture enters at AWi. Because of the difference
in density, the air will accumulate in the top of the cylinder, and the
water will accumulate at the bottom of the cylinder.
3. The flow rate of the incoming air/water mixture will determine the height
in the cylinder where the air water interface reaches a steady state. This
could be controlled with a valve on the input pipe. For the most versatile
control there should be a valve on all three pipes attached to the cylinder.
4. The pressure generated by the incoming air/water mix will continuously
push the air out of the top of the cylinder, and the water out of the bottom
of the cylindrical reservoir.
5. In the event the cylinder is metal rather than glass, there are several
devices to monitor where the level of the interface is in the cylinder
without actually seeing it. One idea is the type of "stud finder" you can
get at a hardware store. It may work, I'm not sure.
Vince Calder
There are materials that have microscopic holes, that
would allow only gases to pass, but not water. The
outside layer of diapers is made of such a material.
There are valves that contain filters made of this
material.
Snorkels contain a tube and a buoyant ball, which when
a diver goes under water, the ball would block water
from going down into the tube. When the diver
surfaces, the ball drops, and air can flow easily.
However, some water does manage to pass by the ball.
Hope this helps
-Wil Lam
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Update: June 2012
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