Main Question or Discussion Point
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.
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: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.
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.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?
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.Q_Goest said: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.
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.Q_Goest said: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.
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.Some gas turbines deliver very high pressure ratio 30:1 starting from ambient. Moreover aircraft gas turbines don't even have an intercooling stage.
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.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.
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.Q_Goest said: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.
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.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.
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.sid_galt said:Some gas turbines deliver very high pressure ratio 30:1 starting from ambient. Moreover aircraft gas turbines don't even have an intercooling stage.
What are you compressing? (water, air, other)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 am compressing air.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)
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.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???