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Rotary vane compressors

  1. Aug 15, 2005 #1
    Why can't rotary vane compressors deliver high pressure ratios efficiently considering that the bending stress decreases as the vanes in the compressor slide down.
     
  2. jcsd
  3. Aug 15, 2005 #2

    Q_Goest

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    Actually, rotary vane compressors are fairly good at doing high compression ratios. They're typically used for vacuum pumps but also used for low pressure compression. A single stage can easily give you an 8 to 1 compression ratio when used above ambient pressures, and much higher compression ratios below ambient when used as a vacuum pump. They are commonly found in vacuum pump applications where the compression ratio can be as high as 1000 to 1. (ie: they can go down to less than a torr).

    Perhaps you mean, "Why aren't they used for high pressure?" I'm not an expert in rotary vane type compressors, but I'd guess as you point out, that it has to do with the pressure difference across the vanes. An 8 to 1 compression ratio from ambient to 8 atmospheres doesn't give you nearly as much stress on a vane as a 2 to 1 compression ratio starting out at 8 atmospheres.
     
  4. Aug 15, 2005 #3
    Thank you for the reply.

    Can't the pressure ratio be boosted to say 40 to 1 starting from 1 atmosphere. Surely that would be possible considering that the bending stress will be reducing on the vanes as the vanes move down.

    Also doesn't moving the vanes up and down cost a lot of energy as the vanes have to move against the high pressure air in their slots?
     
  5. Aug 15, 2005 #4

    Q_Goest

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    Consider how much energy is needed to compress a gas, and how much of that energy goes into the gas as heat or thermal energy. Here's a few things I think you'll agree with:
    1) The higher the pressure ratio, the higher the final temperature.
    2) For any given pressure ratio, the higher the initial pressure, the more energy is put into the gas as heat energy.

    This all seems fairly obvious, but …

    For any given pressure ratio, the higher the initial pressure, the larger the energy that is needed to compress the gas which does nothing more than create thermal energy.

    In other words, consider a compressor that has a given volumetric displacement V that it displaces per unit time t. We'll consider a compressor with a constant V/t. For the sake of argument, let's say we can put into this compressor as much power (from the electric motor) as needed to compress the inlet gas stream at V/t (ie: it's RPM is constant). As we increase the inlet pressure, the amount of heat energy created inside the compressor increases with pressure. The final temperature doesn't increase, but the total heat energy does. So the amount of energy we put into the compressor is a function of the heat created, and the higher the initial pressure, the more energy we need to input, thus the more heat energy created inside the compression chamber. That says a lot. As the inlet pressure increases, we need to put more and more energy into that volume. This assumes we maintain a given pressure ratio.

    Now consider this. The heat energy goes into warming the gas, so if we assume the gas is being compressed isentropically, we find the final temperature is roughly constant. A 40 to 1 compression ratio with an initial pressure of 1 torr will result in the same final temperature as a 40 to 1 compression ratio with an initial pressure of 10 atmospheres (assuming ideal gas behavior). But the amount of energy that is needed to compress that gas is much MUCH larger (ie: the amount of energy is a function of the initial pressure at some given temperature for any given compression ratio).

    Some compressors are designed with a large surface area in contact with the gas being compressed. The best example of this is a diaphragm compressor. It has a large diaphragm and head, and small volume with respect to surface area. A reciprocating compressor is just the opposite. It has a large volume to surface area ratio. Similarly, a vane compressor has a large volume per unit area. It is closer in volume/area to a recip.

    So if the final temperature is very high, and you have very little surface area per unit volume, there is a lot of energy in the fluid that results in a high temperature (isentropic compression) that is trying to find a way to "get out" so to speak. If you have lots of surface area, the heat flux is small and the subsequent temperature of the parts is small. If the surface area is small, the heat flux is large, and the subsequent temperature of the parts is high.

    Conclusion: the reason a compressor with a large volume per unit surface area can't produce as high a compression ratio at any given initial pressure when compared to a compressor with a small volume per unit surface area has to do with the temperature of the parts in contact with the fluid stream.

    Yes, the work input is a function of the frictional forces the compressor must overcome due to pressure as the compressor is rotating. Piston rings for example on a recip create friction, just as vanes create friction. The higher the pressure, the larger the forces on the seals, the higher the friction and the more energy needed to overcome that frictional loss.
     
  6. Aug 15, 2005 #5

    Q_Goest

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    Side Note

    If you look at high pressure compressors (ie: 3000 psi or above), you'll typically notice mostly recips and diaphragm types. The diaphragm types have large areas per unit volume so they often have high compression ratios whereas the recips have relatively low compression ratios. Vanes (generally) can't compete because they simply can't stand the stresses, though they are more simple and might overcome some issues if made to work.
     
  7. Aug 16, 2005 #6

    FredGarvin

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    Well said. Take a look at the attached article. Pay particular attention to the section on isentropic efficiency and the rapid temperature rise and the lack of time for heat transfer.

    http://www.mech.uwa.edu.au/courses/TF209/tf_notespdf/AT08b_RotComp.pdf
     
  8. Aug 16, 2005 #7
    Thank you for the replies Q_Goest and for the link FredGarvin.

    Two more questions -
    Some gas turbines deliver very high pressure ratio 30:1 starting from ambient. Moreover aircraft gas turbines don't even have an intercooling stage.
    Are the compression losses acceptable due to the higher engine efficiency at high compression ratios and/or the medium volume to surface area in axial compressors (and as FredGarvin's link's graph suggested, ideal polytropic efficiency is better than isentropic efficiency) or a little bit of both?

    Why would the stress on vanes in a 2 to 1 compression ratio starting at 8 atm be higher because due to volume reduction of gases during compression to 8 atm, the area of the vanes in the second compressor required to be exposed the air will be consequently 8 times lesser.
     
  9. Aug 17, 2005 #8

    Q_Goest

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    Fred brings up a good point. Given that compression generates thermal energy in the fluid, the faster a compression cycle operates, the less time the gas has to reject the heat and the hotter the gas will get. Recips and diaphgram machines operate around 500 to 1000 RPM, but vane compressors operate at a few thousand RPM's, giving them less time to disipate heat. Centrifugal and axial compressors operate at much higher speeds, generally well above 10,000 RPM and they can go up to almost 100,000 RPM. The faster they operate, the less heat transfer per compression cycle, and the hotter the gas is. Further, there is more heat energy per unit time. So with higher driving temperatures inside the compression chamber and more thermal energy to reject, the machinery can get much hotter as well.
     
  10. Aug 17, 2005 #9

    Q_Goest

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    Turbine compressors used to compress the inlet stream of air often operate at high temperatures, but that's a different story. There, the gas is being mixed with fuel and burned. I personally can't get into all the details regarding them though, perhaps Fred can give more insight.

    Ok, I'm stretching a bit here. Not enough coffee this morning and my brain is overloaded already... anyway. At the beginning of the compression cycle, the vane has zero psid across it. At the end of the cycle it has some pressure across it which is a ratio of the initial and final pressures. That ratio is a function of the number of vanes. So at any position in this cycle, each vane has some dP across it which is dependant on, and is a ratio of, the initial and final pressure.

    Let's assume in position 1 where the vane is just starting to allow a fresh load of gas into the compression chamber, the dP across the vane is 0. The vane starts out with 0 psid across it and as it continues around in a circle, the fluid is discharged and the vane sees the highest dP across it. At any position in between there is a pressure across it which is dependant on the inlet and discharge pressure. The highest dP across a vane is going to be discharge pressure minus inlet pressure, but is more likely a function of that pressure which is largely dependant on the number of vanes. For the sake of discussion, let's assume that ratio is 1:1 but it could be any value and the arguement still stands.
    Compressor 1: For the 8:1 compressor with the outlet pressure at 8 atm and inlet at 1 atm, the difference is 7 atm.
    Compressor 2: For the 2:1 compressor with the outlet pressure at 16 atm and inlet at 8 atm, the difference is 8 atm.
    It's the difference across the vane that creates the stress in the vane which is primarily a bending stress and some shear stress which is highest where it meets the wheel, so compressor 2 has the higher dP across it and the higher stress in the vane.
     
  11. Aug 17, 2005 #10
    Compressor 2 does have a higher dP accross it but the area on which the dP is acting is lower since the volume of an 8 atm gas is lesser than the volume of a 1 atm gas provided that the number of moles in both the gases are the same as is the case with two compressors acting in dual configuration.

    If say the vane in both comp 1 and 2 is of thickness L, then the shear stress is roughly equal to dP*height*length/(length*thickness) = dP * height/thickness.

    Since the height is decreasing as the pressure is increasing, the shear stress shouldn't increase by a lot atleast not by a factor of greater than 3 if pressure ratios remain within say 50:1.
     
  12. Aug 17, 2005 #11

    Q_Goest

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    Yes, if your assumption is that these two compressors have an equal flow rate, your absolutely correct, and the higher pressure compressor has less stress. But that's because the compressor is so much smaller.

    If on the other hand, the assumption is that these two compressors are the same in size and dimension, then the higher pressure compressor has more stress. That was the assumption I was working off of.
     
  13. Aug 17, 2005 #12
    Thank you for the help
     
  14. Aug 17, 2005 #13

    FredGarvin

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    Those turbines have the benefit of many stages of compression coupled with diffusion to create that pressure ratio. The higher the pressure ratio, chances are the more stages of compression. A single stage can not have too much of a delta P across it or huge losses occur and you stand to threaten the stability of the flow through the compressor (i.e. surge). An intercooling stage really doesn't provide enough benefit for an aero application due to the added weight. It may help in the limited sense of the compressor's efficiency, but not better for the overall engine efficiency. The added temperature helps due to less energy required for the burner to provide for the initiation of the combustion process.
     
  15. Nov 20, 2007 #14
    hi
    i am using rotary vane compressor make is MATTEI ERC2055 .when i starts the compressor it delivers 7 bars but temperature shoots up within 5 minutes above 110 celcius and it trips.i had even bypassed the thermostat but no sucess.Can anyone tell me what is happening.
     
  16. Nov 21, 2007 #15

    Q_Goest

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    What are you compressing? (water, air, other)
    What is the inlet pressure to the compressor?
    Is there an aftercooler on it? (should be for air or other gasses, otherwise discharge temperature will quickly rise)
     
  17. Nov 21, 2007 #16

    FredGarvin

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    The only time I have ever had this happen to me was when we ran a rotary screw compressor with the doors that covered the inlet to the aftercooler closed. The doors have the option to do so in cold climates to help the compressor heat up quicker.
     
  18. Nov 22, 2007 #17
    i am compressing air.
    inlet pressure is normal atmospheric pressure.
    yeah oil is acting as cooling agent and radiator is acting as aftercooler.
    i think for oil circulation through radiator , rotor stator assembly is not generating sufficient pressure.
    After running two to three times rotor seizes.
    i have changed already two rotor stator assembly.
    what do you suggest????
     
  19. Nov 22, 2007 #18

    Q_Goest

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    Hi viren'
    Lots of things come to mind. Have you talked to your supplier or the compressor manufacturer?

    Is this a new or old machine? If old, I'd assume it was recently rebuilt so you may need to check that it was properly assembled and that there aren't additional things wrong that may have been overlooked. You should verify oil level, cleanliness of oil and heat exchanger, oil flow rate, oil temperature, aftercooler inlet and discharge temperature, worn bearings or leaky valves, etc... Try going through the list of potential issues on this web page then come back with any new info or findings:
    http://www.tpub.com/content/construction/14050/css/14050_207.htm

    You might also try posting the question here:
    http://www.eng-tips.com/threadminder.cfm?pid=1036
     
  20. Nov 23, 2007 #19
    hi Q Goest

    Compressor i am reparing is third one and in earlier two case i simply replaced the rotor stator with new one provided by ECL our supplier.There occured no problem with those compressor and they are still running good.

    In this third compressor i did not changed the rotor stator but i had opened the rotor stator to check the blade conditions ,changed the shims of endcover with new one ,checked axial alignment ,cleaned oil injection and every other ports in R-S assembly ,checked the bushes ,replaced the servovalve ,replaced the intake ,pressure exhaust ,off load valves with new one.

    Meaning i had replaced every thing with new one except R-S assembly.

    i want to know if this old rotor stator is creating the problem of overheating ((given condition that every assembly is done accurately)) and if yes how???
     
  21. Nov 23, 2007 #20

    Q_Goest

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    I can't think of any reason the rotor stator would cause overheating. Try the engineering tips forum I mentioned, you may find someone with a bit more experience there.
     
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