# Rotary vane compressor lost motion?

In a rotary vane compressor, the vanes constantly move in and out. Does the work required to accelerate the vanes radially outward and then inward cancel out or is the kinetic energy of the vanes lost and completely converted to heat?

Q_Goest
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Hey Sid. I've had to think about this one a bit, I don't claim to be a rotary vane compressor expert, but I do a lot of recip design. I'm sure if my conclusion is wrong someone will speak up, but this is what I've come up with so far.

Let's assume the rotary vane compressor has a cylindrical housing that it rotates in. There are various types so I'll start with this most simple configuration.

From the perspective of the vane, it is not going in and out. That is to say, it rides on the ID of the housing at all times, so it is actually rotating in a circle. It is going in and out with respect to the hub in the middle which is off center, but it is not going in and out from the perspective of the vane. Further, it is actually slowing down and speeding up slightly as it goes around. If it didn't speed up or slow down, it's kinetic energy would remain constant but since it slows and accelerates, it will impart a very slight amount of force on the hub. Obviously if it is slowing and accelerating, any kinetic energy lost as it slows it makes up as it accelerates. As it slows, that energy goes into the hub and as it accelerates it gains it back. So for a single rotation, the kinetic energy lost equals the kinetic energy gained.

What's much more significant is frictional losses. It rubs on the hub and it rubs on the ID of the housing. This energy loss is much more significant and for a design engineer that does these things, I have to believe that even though that frictional loss is very small compared to the energy used for compression, it is taken into account during design. I'd also guess the kinetic energy loss/gain during a single rotation is neglected. When designing recips, that's generally been my philosophy.

Thanks a lot Q_Goest. :-)

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