What is the difference between usual valves and reed valves
In: Engineering
Reed valves are a particular kind of one-way check valve, which is itself a particular kind of valve. In general, valves control the amount of flow of a fluid. One-way check valves preferentially allow fluid to flow in one direction more than the other. Reed valves do this with (generally tiny) flaps of something flexible over holes. When fluid is trying to exit the hole, the flap (reed) is lifted and the fluid flows mostly freely. When the fluid is trying to enter the hole, it presses the reed down and closes the valve, preventing more flow.
Reed valves are check valves. They only allow the fluid to flow one way. Most valves are two-way, forming a constriction but not impacting flow direction.
“Reed” Valves refer to the original choice of material for such valves. They are a kind of [Check Valve](https://beta.spiraxsarco.com/learn-about-steam/pipeline-ancillaries/check-valves) or one-way valve that is intended to be able to open and close rapidly. They were also originally developed from reed instruments which have existed for thousands of years.
The valve consists simply of a block of material with a hole or set of holes drilled in it, and a thin flexible sheet of material covering the holes, which is held in place with screws or rivets on one side. The sheet of material is called a “petal” or petals. (Often the fastenings are made such that, in the absence of any outside air flow the petals sit slightly ajar from the block holes.)
Air pressure in one direction can blow the petal or petals and make them flex away from the holes allowing air or liquid to flow. However if the flow tries to reverse direction, the petals flex shut and are held tightly against the valve block by the back pressure, preventing flow in the opposite direction.
Surprisingly, wood or reed canes can often outperform metals in this application. Wood is immune to the problem of [fatigue](http://news.mit.edu/2017/metal-fatigue-laminated-nanostructure-resistance-fracturing-0309) and is highly vibration resistant. It has a high strength to weight ratio. Modern reed valves often use a composite material made of fiberglass and resin. It’s an advantage with the petals to have a light material because bending open and slamming shut millions of times can produce tremendous forces on the material from it’s own mass.
One of the most common applications for reed valves is for two stroke gasoline engines. This allows air to enter the cylinder via the carburator, but prevents hot exhaust from flowing back through the carburator, only to be ingested again the next cycle instead if fresh air. This could cause misfiring and stalling.
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