7 replies to this topic

[oomme]

Is there a maximum compression ratio for compressors (centrifugal, reciprocating etc)?

In our design, we have boosted gas pressure from 6 bara to 30 bara in a single stage centrifugal compressor to meet the requirement for the inlet pressure at the main compressor header. The exit temperature is below 200 deg.C as specified in DEP but a compression ratio of 5 seems high to me.

Please, I need your professional contributions as soon as possible.

Thank you

Obioma

[Art Montemayor]

Obioma:

Yes, there are limits placed on the maximum compression ratio available from a given compressor - be it a reciprocating one or a centrifugal type. The limits are placed on the machine primarily by the manufacturer (and sometimes the user) based on the maximum discharge temperature that is tolerable for the process and the mechanical equipment. Additionally, the manufacturer might have mechanical restrictions imposed on the machine related to the compression ratio - like for example, the MAWP (Maximum Allowable Working Pressure) for the casing or cylinder - whichever is the case.

200 ^oC seems hot to me; however you don't give specific process data or information, so we can't speculate. What do you call a DEP? Do you mean Design Engineering Procedures? We all don't work for Shell Oil, so we can't guess. The best way to know what are your limits is to communicate with the manufacturer of your compressor and work directly with him/her.

[oomme]

Dear Art,

Thank you for your enlightenment.

DEP is Design and Engineering Practice for Shell.

Here are some process data from our simulation:

Mass Flow = 28389 kg/hr
Molecular weight = 35.1kg/kmol
Molar volume = 22.41 Nm3/kmol
Ratio of specific heat in = 1.184
Ratio of specific heat out = 1.207
Compressibility in = 0.9476
Compressibility out = 0.8831
Gas Constant = 8.314 kj/kmol.K
Isentropic efficiency = 0.757
Mechanical efficiency = 0.98
Pressure in = 6 bara
Pressure out = 30 bara
Temperature in = 36.21 deg. C

Also, I got from DEP that compressor casings may be horizontally split design up to a maximum allowable working pressure MAWP of 50 barg (725 psig).

If there are anyother information you need for your speculations, please let me know.

Thank you

Obioma

[engware]

Hi there:

Here are a few free online calculators that you might find useful when dealing with compression (pressure and volumetric compression ratios need to be specified).

http://members.aol.c...gware/calc3.htm
http://members.aol.c...gware/calc4.htm

Thanks,

Gordan

[Emetovweke Ochuko]

Hello Obioma,

To me the compression ratio you are using is too high.
I have read in a literature (though I can't recall the name of the literature now) that stated that the maximun compression ratio should not exceed 4 to 4.5. and even DEP of shell have its own maximun compression ratio.

I would suggest that you use two stage compression with cooling inbetween the stages. This will reduce the load that would have been placed on one compressor.

[engware]

Hi there:

Yes, I would agree that the suggested compresion ratio is somehow too high ...

Thanks,

Gordan

[JoeWong]

oomme,

Few observations...

i) Discharge temperature & Polytropic efficiency
Based on information you have provided and polytropic efficiency (Pe) of 0.7 (typical and for high MW), discharge temperature is around 170 degC which is far below 200 degC as you claimed. In order to obtain discharge temperature of 200 degC, your Polytropic efficiency would be around 0.6. What polytropic efficiency you have used ? Are you really using such low efficiency machine ?

ii) High Pe compressor
If my estimation is correct (Pe you have used is 0.6) and you are sourcing new compressor, i would suggest you to source for high efficiency compressor (e.g Dresser-Rand - just an example that i can t have used and recalled now...it does not implies other machine is not good). If you can get high efficiency machine, you may bring down discharge temperature and boost up your compression ratio.

iii) Compressor limitation
Are you serving OWNER, DESIGN CONTRACTOR, TRADER or COMPRESSOR SUPPLIER ? I guess you are serving DESIGN CONTRACTOR. aren't you ? Personally i wouldn't pay too much attention on the compression ratio but the discharge temperature (although they are inter-related).

Mr. Montemayor clearly indicated that the temperature limit is on the compressor itself. It could be limited by the MAWP (Maximum Allowable Working Pressure) for the casing or cylinder, it could be the maximum allowable working temperature of seal, etc...Ultimately it set by the compressor itself.

Now if you specified some operating condition which will give you discharge temperature of 200 degC @ Pe=0.6 (or 170 degC @ Pe=0.7), you may encounter problem in sourcing a compressor which can serve your demand. General guideline for discharge temperature would be around 130-150 degC in order for you to get a reliable compressor with reasonable cost. On the other, this does not stop you to expedite compressor operate at high discharge temperature.

I re-iterate Mr. Montemayor statement here "The best way to know what are your limits is to communicate with the manufacturer of your compressor and work directly with him/her."

iv) DEP is just a guideline
You mentioned that "Also, I got from DEP that compressor casings may be horizontally split design up to a maximum allowable working pressure MAWP of 50 barg (725 psig).". DEP is just provides some guidelines. Ultimately, MAWP is set by the compressor casing design. This mean it set by the compressor manufacturer. Again, please discuss with compressor manufacturer.

Hope this help.
JoeWong

[oomme]

Dear All,

Thank you for your contributions, i have learnt so much.

Our mechanical team is discussing with manufacturer on that. I thought there could be a guideline/standard or recomended practice somewhere that specifies such as we have for maximum discharge temperature.

My concern is that "does it mean compression ratio could be as high as possible so long as discherge temperature remains below the recommended value?".

Your advice is the best - discuss with manufacturer.

Thank you

Obioma