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# Centrifugal Compressors in Series

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## Centrifugal Compressors in Series

(OP)
Hi All,

In my plant we have two centrifugal compressors in series. The first one is called CP1 and the second one is called CP2. Both the compressors are of the same size except that they are operated at different RPMs. CP1 operates at 6343 RM while CP 2 operates at 3583 RPM. I have the performance curves for both the compressors in terms of Discharge pressure ( Y axis) and Flow in CFM ( X-axis.)

We have a hyrdaulic software called AFT Arrow that needs the curve to be entered as Differential pressure( Y axis) vs Flow (X-axis). For CP1 it is easy to enter this data as I know the inlet pressure, take difference between the discharge pressure and the inlet pressure for each flow.

Now for CP2 I need the discharge pressure of CP1 which acts as the inlet pressure for the flow that CP2 will operate at. Now since the RPM of CP2 is less than CP1, I am finding it difficult to get the discharge pressure of CP2 and use it to get the differential pressure of CP2 to enter into the software.

Can anyone suggest a solution to this problem.

Thanks and Regards,
Pavan Kumar

### RE: Centrifugal Compressors in Series

The performance curves you have are valid for fixed inlet conditions at CP1 and CP2 respectively.

If the conditions at outlet of CP1 are going to change, for example because you vary the speed, the speed of CP2 will change to the same ratio (assuming CP1 and CP2 are mechanically coupled via a step down gear unit) HOWEVER the PROBLEM you have is that because CP1 outlet conditions have changed, they ipso facto INVALIDATES the curves you have for CP2 at least when expressed in terms of <pressure vs. flow>.

To understand the variation of pressure at outlet of CP2 versus any speed variation and/or change of interstage conditions, you need a combined map that includes CP1 and CP2 in a single performance envelope (stacked). In this case you could do a substraction: At any point of operation, you would have the outlet pressure for CP1 (that map is still valid) and you would have the outlet pressure for CP2, being the shaft line spinning at same speed ratio.

SIDE NOTE: if you have process stuff / equipment in interstage, a combined map is not very meaningfull END OF SIDE NOTE

To obtain an overall map, if you do not it have already, you would need a special software (I could advise one) but my recommendation is to contact the OEM to obtain it.

### RE: Centrifugal Compressors in Series

Addendum to previous post:
In the event the shafts CP1 and CP2 are not mechanically connected, the reasonning is the same in the sense that changes at outlet of CP1 will invalidate the maps available for CP2. Except that in this case there no such a thing as "combined" map because machines are not mechanically coupled. In this case the correct approach is either to involve OEM or use special software I mentionned which would simulate behavior CP2 on change of CP1 and also define the speed ratios.

In case you have fixed speed drivers, you are controlling process by means of throttling valve and/or spill back or blow off valve (I skip variable IGV"s). Here again, in order to understand operating point changes following action on control element you need a tool to make some sort of simulation.

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