Centrifugal compressor recycle cooler sizing
Centrifugal compressor recycle cooler sizing
(OP)
Hello all,
I have a 3 stage variable speed centrifugal compressor, compressing natural gas. The first stage has a recycle cooler and discharge cooler and the 2nd stage has a discharge cooler. The compressor discharges into a manifold fed from another set of compression trains.
I am trying to come up with a basis for sizing the recycle cooler. My thoughts were to simulate the compressor (with compressor curves included) at full recycle with the discharge pressure at the operating manifold pressure to replicate a start-up, where the speed is varied to achieve the discharge pressure. The remaining variable is then the recycle flowrate that I am now unsure how to choose. A recycle flow rate for the 1st stage, using the design flowrate would appear conservative as it should not be operated at design flow in recycle. Alternatively, a lower flowrate could be used with a good margin above the surge point.
Does anyone have an opinion on this or point me to a good reference?
I have a 3 stage variable speed centrifugal compressor, compressing natural gas. The first stage has a recycle cooler and discharge cooler and the 2nd stage has a discharge cooler. The compressor discharges into a manifold fed from another set of compression trains.
I am trying to come up with a basis for sizing the recycle cooler. My thoughts were to simulate the compressor (with compressor curves included) at full recycle with the discharge pressure at the operating manifold pressure to replicate a start-up, where the speed is varied to achieve the discharge pressure. The remaining variable is then the recycle flowrate that I am now unsure how to choose. A recycle flow rate for the 1st stage, using the design flowrate would appear conservative as it should not be operated at design flow in recycle. Alternatively, a lower flowrate could be used with a good margin above the surge point.
Does anyone have an opinion on this or point me to a good reference?





RE: Centrifugal compressor recycle cooler sizing
So in short, control line as baseline plus beware the fact that all the stages are mechanically connected which may thermodynamically constraint the 1st stage (ending up with extra flow recycle).
RE: Centrifugal compressor recycle cooler sizing
RE: Centrifugal compressor recycle cooler sizing
Is the subject valve specified for anti-surge protection only or is it foreseen for "dual" purpose, means anti-surge protection AND continuous recycle ?
I am raising this point to trigger some thinking / discussion... if the valve is for anti-surge protection only, I would suspect cooler must be sized correspondingly so that the max calculated Cv among all operating cases ((surge line intersecting MCS & mos speed curves) is covered - as minimum. But I can't state this for sure. Hope someone can help and provide further direction here.
Also when looking at the valve selected Cv, this one is normally selected between 1.8 to 2.2 times the maximum calculated Cv. The multiplier in question (1.8 to 2.2) does not mean that the flow through the valve is expected to run higher than the compressor surge flow. It is more of a measure to have the valve dynamic response cope with any surge event thereby ensuring machine is continuously protected.
I am tented to say use max calculated Cv case as sizing case for the recycle cooler when anti-surge protection applies only, but frankly it can be misleading. Like I said at this point, do not take this as granted but instead as a way to trigger discussion.
RE: Centrifugal compressor recycle cooler sizing
If it is meant for antisurge, then size for surge flow x 1.1 at max operating speed. If it is meant for capacity recycle, then size for the flow seen at min operating speed x 1.1.
Obviously you would take credit for the J-T effect across the recycle valve to get to the required stage 1S feed gas temp. So another set of parameters that would be applicable is to use the lowest gas mol wt and lowest stage 1D pressure. The gas with lowest mol wt may in some cases be that which is produced when the compressor is in full recycle with no fresh feed gas ( gas lean out effect when compressor is in full recycle).
Am assuming this stage 1R cooler gets its feed from upstream of the stage 1D cooler ( ie directly from compressor discharge). So another parameter to include would be some deterioration in polytropic eff in the stage 1 compressor that would appear after several years of operation or if the gas fouls the compressor impellers / diffusers.
It will be a lot less painful to get these cases studied if you have a process simulation program where you could load up the speed - polytropic head - flow curves.
RE: Centrifugal compressor recycle cooler sizing
Mass flow through the machine ( and hence the recycle gas flow ) at stage 1 will be highest at max stage 1S pressure - this is another parameter to take into consideration also.
RE: Centrifugal compressor recycle cooler sizing
No specs/DS for AS valve at the moment, that will probably come after I am off the job although could certainly be considered at the next phase. The recycle will be only used for AS in normal operation although it is not inconceivable they will operate at reduced flow rates far in the future where it is used as a recycle but currently that is not the design intent.
I have the curves in a simulator so I can simulate it in full recycle with the 3rd stage at discharge pressure (minimum and maximum), 10% above surge flow at each stage and the compressor at maximum speed, I think that is a reasonable design basis.
RE: Centrifugal compressor recycle cooler sizing