2 replies to this topic

[rmarzo]

Dear all,

 

I am following to study the behaviour of centrifugal compressor further than the design initial conditions.

 

Trying to understand what happens when sution pressure varies, I take the following assumptions:

 

- The centrifugal compressor curve is the same (Really is not true, because the initial curve available is given at specific suction conditions)

- Mass flow change

- Discharge pressure is impossed by the system (since a big volume absorber downstream)

- Suction temperature change

- Gas composition is fixed

- Compressible factor is considered unaltered.

 

Therefore, with the previous curve and the new Pressure suction, I iterated to get the same value for Hp with the ecuation of politropic transformation and for Hp with the ecuation curve.

 

I obtained the attached table.

 

 

 

Thus, higher suction pressure, higher suction temperature (obviously)

Higher suction pressure, lower pressure ration and greater volume reducction. Therefore lower politropic head and higher volume flow.

 

Everything is comprehensible till here. But there are something I don't get... If the mass flow increases, the friction looses shall be higher on the system curve, and the system curve should be steeper... Is it?

 

Feel free to comment whatever assumption or calculation, It would be very helpfull in my analysis.

Attached Files

•  Sin título.png   23.38KB   21 downloads

Edited by rmarzo, 24 August 2014 - 03:53 AM.

[PingPong]

Normally a compressor has a process control system that makes sure the compressor is doing what the process wants it to do. Without at least a P&ID, compressor datasheet(s) and driver type and size, the rest of the world has no idea how that works with your compressor, so nobody can tell what happens with the mass flowrate if the suction pressure changes.

 

Without the actual Hp-curve of the compressor it is completely unclear what you are doing in that table.

Also unclear is why T1 should change when P1 changes.

 

Note also that the compressor will hit the "stonewall" if the Q increases too much.

Moreover the driver may trip on too high power (electric motor drive), or speed will decrease (steam turbine drive).

Edited by PingPong, 24 August 2014 - 07:26 AM.

[trinhduchanh]

Hi rmarzo,

Regarding your question above then i have some advice to you.In centrifugal compressors, suction pressure will proportional with driver power requirement, an increased suction pressure will raise the discharge pressure and increase the horsepower.If the suction pressure is lowered, the centrifugal machine will not compress to the desired discharge pressure as well as mass flow rate shall proportional with driver power. Mass low rate only vary if you change gas density by variable suction pressure and/or variable volume flow rate.  For  a  fixed mass flow  rate  and  gas composition,  temperature  has  a  small  effect  on gas density because the temperature term is very large. Conversely, increasing compressor suction pressure  will  significantly  increase  gas  density and reduce the gas volume. 

 

Please take a look on page no 3 in my document attachment. It will help you easy understand effect of suction pressure with compressor performance. 

 

Attached Files

•  How to Use the Performance Curves to Evaluate Behavior of Centrifugal Comp.pdf   538.83KB   422 downloads