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Compressor Suction Throttling

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technologist:
We have a centrifugal compressor in MEG plant which is recycling gas flow thru the system. Makeup is added at suction & product is removed from the loop.

Compressor is a constant speed machine. Which is giving higher pressure at discharge say 19 Kg abs. We dont need 19 Kg in the system so there is a pressure killing device which is reducing it to 17 Kg abs. Further system drop is 3 Kg. So at compressor suction it is 14 Kg.

Now somebody proposed to install IGV i.e. instead of dischrge throttle it should be suction throttle.

I am not able to understand how this is going to reduce the power consumtion in the compressor.

Because,
1. Will the pressure ratio change in this case, I suppose so bcoz suction pressure is different now.

2. If it changes, it will increase or decrease. My opinion - it will increase mathematically.

3. IGV control will reduce flow or not, bcoz we dont want to reduce flow. (My plant capacity will go down.)

4. How to calculate suction pressure and power consumption in this case.

Thanx

eugenedakin:
Hello technologist,

You have a very good question.  In my opinion, there will be a decrease in power consumption, BUT, it is going to be small.  I'll answer your questions first, and then provide an explanation:

1.  the pressure ratio will change.  Instead of the pressure going from 14(intake)-19(post compressor)-17(post throttle)-14(back to intake), it will go 14 (intake)-12(post throttle-*guess*)-17(post compressor)-14 (back to intake).
2. when the throttle is on the intake side, the pressure will decrease post-throttle.
3. I am not sure... it would depend where the gas is removed from the system.  Could you draw a very basic flow diagram and post it?  It really depends on a few critical factors ...
4. Most programs are able to determine the suction pressure (given a pipe size, restrictor (throttle valve) opening, etc.  The power consumption will decrease.... albeit -very very slightly .. my guess, close to the 2nd decimal point.  The reason is that less pressure (less atoms being moved and allowed to have friction), whereas more atoms (higher pressure, more atoms to create friction) has more friction.  If the system has an overall pressure differential which is lower after the modifications, then the pump efficiency should increase.

I have about 10 years experience on refrig plants.  They rank as one of my many favorite plant types.

I hope that I explained this well.  Feel free to comment on the parts which do not make sense ...

Sincerely,

Eugene

mbeychok:
technologist:

I am a bit confused by your pressure units of "Kg".  Did you mean kilograms per square centimeter (i.e., kg/cm2) ? 

Usually, the clearer you can be when asking a question, the better will be the answers you receive.

Regards,

Milt Beychok
(Visit me at www.air-dispersion.com)

eugenedakin:
Hi technologist,

Ah yes, you noticed me using oilfield slang terms.  I was using the terms in which you originally asked your question, to minimize the confusion.  Slang terms for pressure in psi is commonly referred to as pounds or lbs, whereas Kg/cm2 should be referred to as Kgf/cm2 (Kilograms force per square centimeter), and not the slang term, Kg.

I'll try to not use 'technology slang' in the future.

Thanks for pointing that out to me Milt.

Sincerely,

Eugene

technologist:
Thanks Eugene & Milt.

Kg was not used as technical slang. It was just to make life simpler & faster. I know what "Pressure" units are & how they are used.

Anyway Would like to thank you Eugene for very reasonable points.

1. Yes, U r right in your guess also. ( I hope now u wont mind SM slangs). Suction will reduce somehwhere close to 12 Atm. (Ya U noticed it again).

2. Well Now would like to ask U if u have already predicted that change will be at second decimal point, what do you expect if I Use IGV in place of suction throttle.

Thanks Again, I was looking for this kind of expertise level.

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