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typical compressor specifications 1
- Thread starter asianx
- Start date
asianx,
That is truly a wide open question. When I was researching the same thing for a class I'm teaching in Denver next week, it took a LOT of digging, a LOT of reconciling conflicting claims, and a LOT of nonsense to wade through before I had a table that I beleived.
People use the word "isothermal" when they mean "adiabatic". People use isentropic constants for polytropic machines. People use the word "efficiency" without defining it (and the definitions are all over the map--if you are comparing a recip with clearance to a recip without clearance the "efficiency could be over 95%, if you are comparing power input to work output the number is lower, if you are comparing energy content of input fuel to work out the number is much lower still). People lie.
Maintenance and operating costs are another whole bag of worms. I had a client ask me to look at his flooded screw compressors because they were costing him $20k/month for replacement oil. I operated a fleet of flooded screws 15 times the size of this operation and spent closer to $2k/year on replacement oil. He was operating them without understanding them. So is the number $20k or $2k?
Good luck with this.
David
That is truly a wide open question. When I was researching the same thing for a class I'm teaching in Denver next week, it took a LOT of digging, a LOT of reconciling conflicting claims, and a LOT of nonsense to wade through before I had a table that I beleived.
People use the word "isothermal" when they mean "adiabatic". People use isentropic constants for polytropic machines. People use the word "efficiency" without defining it (and the definitions are all over the map--if you are comparing a recip with clearance to a recip without clearance the "efficiency could be over 95%, if you are comparing power input to work output the number is lower, if you are comparing energy content of input fuel to work out the number is much lower still). People lie.
Maintenance and operating costs are another whole bag of worms. I had a client ask me to look at his flooded screw compressors because they were costing him $20k/month for replacement oil. I operated a fleet of flooded screws 15 times the size of this operation and spent closer to $2k/year on replacement oil. He was operating them without understanding them. So is the number $20k or $2k?
Good luck with this.
David
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1
- #3
I agree with zadas. I have some of my opions and personal rule of thumbs that I use to get started. If I was an anal guy, I'd make a wall chart of these. having started with compressors when most of us used the charts in the GPSA data book, you get a real good feeling about efficencies of compressors. But her is what I do.
Centrifugal compressors can range from 60% to 75+% polytropic efficency. The approximate valiue is a function of each manufacturers actual design and you won't the exact value without a RFQ. But generally, the smaller and m ore off theshelf (no custom impellars) run around 68%. Larger custom machines will be closer to 75%. This is the values I use in the simulators to get the rough idea. One of the larger yet noncustom impellar compressors use to let you have their DOS program that gave better eff values. But you are at the mercy of most of the vendors I know.
recips. Recips are a little different. I use polytropic eff in modeling because the thermo efficency is a function of the not only ther estimated approach to isentropis, but adjustments for valve losses and other mechanical losses. Here, the assumed polytropic eff runs between 75% to 85%. For higher speed compressors with high mole weights I assume 76% and for low compression ratio low gravity gases I use .81%. Then I use different vendors programs to get the exact value, well they don't calculate eff, but you get the required Horsepower and then i tweak the polytropic eff in the model to get a matching HP. The crosss check is the discharge temp, this should also match.
With screws, its similar to recips. the range is 40% to 70%. the wide spread is a function of compression ratios. Compression ratios of 8 will have a 40% and with a ration of 2, the eff is closer to 70%. Again, you run the manufactures program that give the HP required. You can adjust the models. The last screw run I saw had adiabatic eff listed.
Maintenance is a little more difficult. maintenance philosophies can creat wild swings. Size of units, number on location... A mid range might look like this
$/HP-yr
cent 100
screw 120
Recip 150
all with engine drivers
no driver
cent 15
screw 20
Recip 30
I've seen way more and way less.
Centrifugal compressors can range from 60% to 75+% polytropic efficency. The approximate valiue is a function of each manufacturers actual design and you won't the exact value without a RFQ. But generally, the smaller and m ore off theshelf (no custom impellars) run around 68%. Larger custom machines will be closer to 75%. This is the values I use in the simulators to get the rough idea. One of the larger yet noncustom impellar compressors use to let you have their DOS program that gave better eff values. But you are at the mercy of most of the vendors I know.
recips. Recips are a little different. I use polytropic eff in modeling because the thermo efficency is a function of the not only ther estimated approach to isentropis, but adjustments for valve losses and other mechanical losses. Here, the assumed polytropic eff runs between 75% to 85%. For higher speed compressors with high mole weights I assume 76% and for low compression ratio low gravity gases I use .81%. Then I use different vendors programs to get the exact value, well they don't calculate eff, but you get the required Horsepower and then i tweak the polytropic eff in the model to get a matching HP. The crosss check is the discharge temp, this should also match.
With screws, its similar to recips. the range is 40% to 70%. the wide spread is a function of compression ratios. Compression ratios of 8 will have a 40% and with a ration of 2, the eff is closer to 70%. Again, you run the manufactures program that give the HP required. You can adjust the models. The last screw run I saw had adiabatic eff listed.
Maintenance is a little more difficult. maintenance philosophies can creat wild swings. Size of units, number on location... A mid range might look like this
$/HP-yr
cent 100
screw 120
Recip 150
all with engine drivers
no driver
cent 15
screw 20
Recip 30
I've seen way more and way less.
- Thread starter
- #5
vultureman
Chemical
One other thing to remember is tha application and its characteristics. If the loads are relatively steady, i.e., mole weight, flow, suction and discharge pressures won't change much day-to-day or month-to-month, then centrifugal compressors often have an advantage in reliability and/or lower maintenance cost.
But if the loads/conditions varely, the reciprocating compressors may have an advantage as they can be adjusted with pockets and/or driver speeds to meet the changing conditions.
But if the loads/conditions varely, the reciprocating compressors may have an advantage as they can be adjusted with pockets and/or driver speeds to meet the changing conditions.
I believe there are some good compressor schools available out there.
or papers on the subject can be purchased here
or papers on the subject can be purchased here
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