# Rotary Vane Compressor Question

## Main Question or Discussion Point

Im quite new to this and having trouble in understanding the topic. - Rotary Vane Compressor.

Can someone please explain why the air mass flow rate reduces when the delivery pressure increases?

Thanks

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LURCH
Well, in a site full of physics-minded people such as this, you're going to get a lot of responses that say something along the lines of, "so that total energy is conserved." And this is a perfectly valid statement. If the total energy you're putting into the system (through the wall plug) remains the same, and potential energy (pressure) increases, then kinetic energy (flow) must decrease.

However, this answer is often somewhat unsatisfying. Another answer, won the deals more specifically with the mechanism by which this conservation is accomplished, would be to remind yourself that pressure is the result of resistance to flow. So, if pressure is going up, there must be some increased resistance somewhere down the line. This resistance, which is what is causing the pressure to increase, also causes the flow to slowdown.

Make sense?

Q_Goest
Homework Helper
Gold Member
Hi alexdg,
Note that the term "compressor" generally indicates the machine is increasing the pressure of a gas. The term "pump" is generally used to indicate the machine is increasing the pressure of a liquid. So by saying "compressor" you're indicating the use of a rotary vane machine to compress gas.

For a positive displacement (PD) compressor such as a rotory vane or reciprocating piston, the reasons for a reduction in flow with an increase in dP across the machine are different than the reasons for reduction in flow of a dynamic machine such as a centrifugal.

For a PD machine, there are 2 primary reasons for reduction in flow.
1. Void volume - There is always some small volume which can't be completely displaced inside the compression chamber. This volume has gas in it at the same state as discharge, or even higher in pressure due to dynamic pressure drop across the discharge valve or outlet. This volume re-expands during the 'suction' stroke, reducing the amount of fresh gas that can be inducted.
2. Seal leakage - A rotary vane machine doesn't have particularly great sealing between the walls and paddle. Leakage here increases as dP across the seal increases, reducing total throughput.

There's a third, somewhat less important reason.
3. On reciprocating compressors that have suction and discharge valves, these valves will leak, especially over time as wear deteriorates the sealing efficiency. This leakage is also proportional to dP, so flow drops as pressure increases. This isn't generally applicable to to rotary vane machines as they don't usually have any kind of valves.

Are you talking about a jet engine rotary compressor or the vane type rotary pneumatic pump?

If you are talking about a vane type rotary pneumatic pump, you might understand the situation a little better if you reverse the wording of the question but maintain the essence of the question to: Why does the pressure inside the pump increase when the exit air flow rate is reduced?

Let's say you have an air pump generating, say, 1000 psi and 1 cubic foot per minute that is allowed to exit through a tube. What happens when you pinch that tube? Suddenly the air flow is restricted. So what happens to the air being compressed inside the pump?

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