Rotary vane compressor lost motion?

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Discussion Overview

The discussion revolves around the mechanics of rotary vane compressors, specifically addressing the work and energy dynamics of the vanes as they move in and out during operation. Participants explore the implications of kinetic energy changes and frictional losses in the context of compressor design.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the work required to accelerate the vanes radially outward and inward cancels out or if the kinetic energy is lost as heat.
  • Another participant describes the motion of the vanes as not being in and out from their perspective, but rather rotating in a circle, which involves slight acceleration and deceleration that affects the hub.
  • This participant suggests that the kinetic energy lost during deceleration is regained during acceleration, leading to a balance over a single rotation.
  • Frictional losses are highlighted as a significant factor, with the participant noting that these losses occur at the hub and the inner diameter of the housing, and they are likely accounted for in design despite being small compared to the energy used for compression.
  • A separate participant expresses interest in understanding the defects of vane compressors, indicating a broader inquiry into their performance and limitations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the energy dynamics of the vanes, with differing views on the significance of kinetic energy changes versus frictional losses. The inquiry into defects of vane compressors remains open and unresolved.

Contextual Notes

The discussion does not clarify specific assumptions about the operating conditions of the rotary vane compressor or the definitions of energy loss and gain. The complexity of the energy interactions and the impact of design choices are acknowledged but not fully resolved.

sid_galt
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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?
 
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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. :-)
 
Hello. I want to know the defect of vane compressor . Do you know? Thanks.
 

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