We have a reciprocating compressor which compresses inert gas (mainly N2) from 0.5 bar to 19.5 bar over 3 stages (4 cylinders). Can we use the same compressor to compress CO2 to the same pressures? This subject is quite new to me and I was wondering if the larger atomic weight of the CO2 would affect compressibility. Thanks in advance for any help.
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Recip Compressors
- Thread starter MadDog88
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I haven't done these calculations for a while but they are outlined in the GSPA Engineering data book (one source, any compressor book will have them). I would talk to your compressor vendor, they'll be able to run these numbers for you really quickly, I haven't worked with C02 but I know it has some funny characterics though 19.5 bar isn't that high.
Recip compressors are constant volume machines so from that point of view, the higher MW of C02 isn't a factor, the first stage cylinder will 'sweep' a constant volume of gas and a constant MMscfd.
The final discharge pressure will be set by the process the compressor is installed, it will (subject to mechanical limits and/or temperature, PSV settings, etc) put out whatever pressure is needed.
Essentially, the suction pressure to the first stage cylinder(s) will rise or fall until the incoming suction flow from the process = the capacity of the machine to move this amount of gas through all the cylinders (same principle applies to the interstage pressures, they are 'set' by the compressor's capacity: eg. the capacity of the next higher pressure cylinder = the capacity of the previous lower pressure cylinder). It's somewhat like a bucket of water being passed along in a bucket brigade.
The Cp/Cv ratio of CO2 is slightly less than that of N2 which will mean a lower discharge temperature compared to compressing nitrogen. That's good as that avoids a potential issue with high discharge temperatures. At your higher pressures, I don't know if the Cp/Cv ratio for C02 is still less than that of N2, compressibility changes between the two gases could also be a factor so hence my suggestion to talk to your vendor.
I 'think' the Hp also won't change. I had a spreadsheet that modeled a recip compressor that I did work on several years ago but unfortunately, I don't have it at work. This didn't seem obvious to me that a change in MW would not affect Hp (it's not true for a centrifugal) but I believe when I worked through the numbers, the reason is that because a recip is really a constant dP machine rather than a constant head machine as for a centrifugal, the change in MW, while increasing the mass flow through the compressor also decreases the head produced by the compressor and the two cancel each other other as they are both dependent on the MW.
Recip compressors are constant volume machines so from that point of view, the higher MW of C02 isn't a factor, the first stage cylinder will 'sweep' a constant volume of gas and a constant MMscfd.
The final discharge pressure will be set by the process the compressor is installed, it will (subject to mechanical limits and/or temperature, PSV settings, etc) put out whatever pressure is needed.
Essentially, the suction pressure to the first stage cylinder(s) will rise or fall until the incoming suction flow from the process = the capacity of the machine to move this amount of gas through all the cylinders (same principle applies to the interstage pressures, they are 'set' by the compressor's capacity: eg. the capacity of the next higher pressure cylinder = the capacity of the previous lower pressure cylinder). It's somewhat like a bucket of water being passed along in a bucket brigade.
The Cp/Cv ratio of CO2 is slightly less than that of N2 which will mean a lower discharge temperature compared to compressing nitrogen. That's good as that avoids a potential issue with high discharge temperatures. At your higher pressures, I don't know if the Cp/Cv ratio for C02 is still less than that of N2, compressibility changes between the two gases could also be a factor so hence my suggestion to talk to your vendor.
I 'think' the Hp also won't change. I had a spreadsheet that modeled a recip compressor that I did work on several years ago but unfortunately, I don't have it at work. This didn't seem obvious to me that a change in MW would not affect Hp (it's not true for a centrifugal) but I believe when I worked through the numbers, the reason is that because a recip is really a constant dP machine rather than a constant head machine as for a centrifugal, the change in MW, while increasing the mass flow through the compressor also decreases the head produced by the compressor and the two cancel each other other as they are both dependent on the MW.
Hi mrichards (Mechanical)
Is your compressor an API 618?
Please go th the Paluin Research Group, the Home on FE-PIPE AND BOS FLUIDS
Leonard@thill.boz
Is your compressor an API 618?
Please go th the Paluin Research Group, the Home on FE-PIPE AND BOS FLUIDS
Leonard@thill.boz
Capacity and Power will not be problems with this conversion. I am not sure of the compressor configuration you have. That should be reviewed to assure it is fitted with trim suitable for CO2 compression. Also, the cylinder valve lift may need adjustment. Feel free to forward questions with details, I am well versed in reciprocating compressors in process service. robinchandy@msn.com
Mark,
Yes, positive displacement compressor will give you the same volume (displacment), and I think that you can apply the N2 machine to CO2 service for the compression application keeping in mind the following:
1) Temperature:
N2 has a k-value (Cp/Cv) of 1.4, while CO2 is closer to 1.3. Each stage discharge temp will be (Temp. in Ranking and Pressure in Absolute):
Tout = Tin * ((Pout/Pin)^((k-1)/k)) / efficiency
You can see with this, if your k-value has increased, you'd probably need MORE stages of compression for the same differential pressure duty (but you're going from N2 to CO2, so it's gone down). Also note the general rule that you want to keep each stage discharge temp below 300F if it's an air-cooled compressor and 350F if it's a water-cooled (jacketed) compressor. You'll likely have lesser inter-stage cooler duty (but that may not be the case if you have increased your water content).
2) Power: For positive displacement machines, power will be about the same with the same ACFM and delta-P & any changing gas. So you're probably safe with your Motor HP.
3) Accoustics: You need to do a study, this is the only question. If you have a small HP unit, then you can probably crank it on and if you have problem, change orifices (get a local compressor expert to help you).
My big concern is CO2 corrosion. If 100% dry, then you should be okay. If you have water in your gas stream, then you may have other issues.... You probably need to go to SS valves (and if you're over-hauling the unit, go to SS rods).... Your bottles & other points where water will condense will be another issue... Find someone who has applied units in wet-CO2 service.... Start-up and shutdown proceedures should be reviewed.....
Also, keep in mind CO2 holds more water than what the partial pressures typically advise... So you may want to model this with Hysim or another process prediction program...
Yes, positive displacement compressor will give you the same volume (displacment), and I think that you can apply the N2 machine to CO2 service for the compression application keeping in mind the following:
1) Temperature:
N2 has a k-value (Cp/Cv) of 1.4, while CO2 is closer to 1.3. Each stage discharge temp will be (Temp. in Ranking and Pressure in Absolute):
Tout = Tin * ((Pout/Pin)^((k-1)/k)) / efficiency
You can see with this, if your k-value has increased, you'd probably need MORE stages of compression for the same differential pressure duty (but you're going from N2 to CO2, so it's gone down). Also note the general rule that you want to keep each stage discharge temp below 300F if it's an air-cooled compressor and 350F if it's a water-cooled (jacketed) compressor. You'll likely have lesser inter-stage cooler duty (but that may not be the case if you have increased your water content).
2) Power: For positive displacement machines, power will be about the same with the same ACFM and delta-P & any changing gas. So you're probably safe with your Motor HP.
3) Accoustics: You need to do a study, this is the only question. If you have a small HP unit, then you can probably crank it on and if you have problem, change orifices (get a local compressor expert to help you).
My big concern is CO2 corrosion. If 100% dry, then you should be okay. If you have water in your gas stream, then you may have other issues.... You probably need to go to SS valves (and if you're over-hauling the unit, go to SS rods).... Your bottles & other points where water will condense will be another issue... Find someone who has applied units in wet-CO2 service.... Start-up and shutdown proceedures should be reviewed.....
Also, keep in mind CO2 holds more water than what the partial pressures typically advise... So you may want to model this with Hysim or another process prediction program...
There is a lot of very cogent information above, but one thing that hasn't been mentioned is compatibality of any elastomers with CO2. I've seen cases where adding as little as 8% CO2 to a methane stream caused o-rings to fail. You need to make sure that any seals (on pre-filter cans for example) are compatible with CO2 before you make the switch. Other than that, it is a pretty easy switch.
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