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 Click here to return to the Engineering Archives

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