No replies to this topic

[chacha]

Hi,

Short:
In my Aspen Simulation, everything including mass, molar and enthalpy balances seem ok. Except for 2 compressors, they have ~12% relative error. Why is this? What can I do? (everything converges, tolerances are minimized...)

Long:
I have been working on a simulation for a refrigeration cycle. I have used data from a patent, and managed to simulate the process. I minimized tolerances, checked all mass and mole balances and everything seems fine until here.

My problem is with enthalpy balances, especially for one compressor and two coolers.

I have about 35 components, and when I check the relative error for enthalpy balance (Hin-Hout):
~ 20 have 0% relative error (valves, mixers, some heat exchangers)
~ 5 have %1-5E-14 relative error (mixers and some heat exchangers)
~ 7 have %1-3 relative error (some compressors and phase separators)
~ 3 have %9-13 relative error (propane compressor and coolers) (Hin=800 kW, Hout=900kW)

When I compared the one compressor to the other ones with small errors, I saw the problematic one had a much larger pressure ratio. So I tried using 7 compressors in a row (with intermediate pressures, without interstage coolers) instead of just 1, now the error is only 1.8% and outlet temperature and required work is a bit higher..

And I couldnt do anything about the coolers..

I tried different property methods, playing with tolerances and convergence options, but couldnt manage to eliminate or minimize errors..

Any suggestions?
Thanks
Cha

Edited by chacha, 11 June 2012 - 05:03 PM.