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Molecular Weight Effect On Centrifugal Compressor Power
#1
Posted 24 February 2017 - 08:17 AM
I hope everyone will be fine. I am a process engineer in a refinery in Pakistan. I am currently working on a project i.e. flare gas recovery system (FGRS) to recover flare gas from refinery flare. The recovered gas will have a molecular weight of 14 which will be routed to a wet gas compressor of gas saturation plant.
Wet gas compressor is a two stage centrifugal compressor that currently operates at a gas molecular weight of 31.3 but with the introduction of flare gas , molecular weight will be reduced to about 29.73.Following data is given for compressor;
1st stage:
P1=3.72 kg/cm2 g
P2=8.21 kg/cm2 g
T1=41 oC
T2=83 oC
Z=0.97
k=1.146
Design polytropoc effeciency =80%
Considering the above design data, I want to calculate the required power with a molecular weight of 29.73. Please guide me through the formulas used to determine the required polytropic head, polytropic efficiency and polytropic power. Please also elaborate the effect on operating curve with respect to surging.
Best regards,
Qasim
#2
Posted 24 February 2017 - 09:52 AM
Surely as an engineer you can locate formulas to answer your question. A simple search will show many sources and online calculators Do your work, then show it if you want help.
Bobby
#3
Posted 24 February 2017 - 12:09 PM
Qasim,
Refer the link below for equations for power calculations:
https://www.cheresou...mpressor-power/
Regards,
Ankur
#4
Posted 26 February 2017 - 10:23 PM
Dear Ankur,
With the use of relations in the topic you referred, following are the results at design conditions. Design power and efficiency provided by compressor vendor, is also mentioned below;
Calculated Results:
1) Polytropic Head=62051 Nm/kg
2) Polytropic Efficiency=61.33%
3) Polytropic Power=424 kW
Design Results by Vendor:
1) Polytropic Efficiency=80.9%
2) Polytropic Power=376 kW
Note that the calculated results are at a molecular weight of 31.3 and mass flow rate of 15903 kg/hr. All remaining values are already mentioned in my post.
I want to know that why is there a huge difference in my calculations? Is there any error in my calculations? Please do let me know and guide me.
Best Regards,
Qasim
#5
Posted 26 February 2017 - 11:36 PM
If you had spent minimal time to look you would have found this online calculator from a major manufacturer. Just like the fish story.
http://www.elliott-t...ssor-calculator
Bobby
#6
Posted 27 February 2017 - 12:25 AM
Qasim,
Use the polytropic efficiency given by vendor since it is a specific machine dependent parameter. Different vendors use different polytropic efficiencies based on their own machine configuration. The polytropic efficiency calculated based on equation given in my blog entry is a conservative one and should be used only for initial estimation of power requirement during the development of the Basic Engineering Package.
Regards,
Ankur.
#7
Posted 27 February 2017 - 04:24 AM
If you had spent minimal time to look you would have found this online calculator from a major manufacturer. Just like the fish story.
http://www.elliott-t...ssor-calculator
Bobby
Thanks Bobby.
#8
Posted 27 February 2017 - 04:29 AM
Qasim,
Use the polytropic efficiency given by vendor since it is a specific machine dependent parameter. Different vendors use different polytropic efficiencies based on their own machine configuration. The polytropic efficiency calculated based on equation given in my blog entry is a conservative one and should be used only for initial estimation of power requirement during the development of the Basic Engineering Package.
Regards,
Ankur.
Dear Ankur,
My main concern for starting this post was to analyze the performance of my compressor with a lighter gas at a higher flowrate, which I analyzed that power will be increased. Is it possible that polytropic eff. is increased?
Moreover I also want to determine the the compressor curve with modified parameters.
Best Regards,
Qasim
#9
Posted 27 February 2017 - 05:40 AM
Qasim,
As I have mentioned in my blog entry, polytropic efficiency as a function of inlet volume flow rate, the polytropic efficiency will increase with increase in inlet volume flow rate. So if you are going to compress a lighter gas (lower relative density) then since the inlet volume flow flow increases for a given gas mass flow rate, the polytropic efficiency will increase. Also for a lower molecular weight gas the polytropic head (Hp) will also increase considering suction pressure and temperature remain the same.
The gas power equation is:
P = M*Hpoly / 3.6*106*ηpoly
From above it is clear that gas power is directly proportional to polytropic head and inversely proportional to polytropic efficiency.
Regards,
Ankur.
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