Overall heat transfer coefficient with phase change
Simulate chemical processes using advanced thermodynamic models
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The problem I is the way it calculates the overall heat transfer coefficient in a heat exchanger, in this case an evaporator. The inlet is sub-cooled liquid and the outlet is two-phase. Therefore, the LMTD needs to have a correction. From a manufacturer of heat exchangers, we have got a value for the overall heat transfer coefficient. But this value is not met due to a different value for the LMTD. Due to the fact that the LTMD is calculated over the whole heat exchanger, and not separated for the sub-cooled part and the two-phase part, the LMTD is calculated almost two times too high. To illustrate this, I have created a simple model with the same in- and outlets of the heat exchanger, but on one side the single phase and two-phase parts are combined, and on the other side they are separated. The results show a LMTD on the left side of 6.06 and on the right side of 6.85 for the small single-phase part and 2.06 on the two-phase part, which is more in line with the data received from the manufacturer.
This so-called pre-heater solves the problem for this moment, but when running other scenarios, it is time consuming to keep finding the saturation point of the working fluid, and it is also difficult to test the simulation to reality due to the different LTMD and overall heat transfer coefficient. Is there a way to correct this error? Or is this just the way DWSim performs a heat exchanger calculation?
I have also tried to correct this with other software to model heat exchangers, called HTRI. HTRI should also be a CAPE-OPEN compliant software, just as DWSim. It is installed on the same computer as DWSim, yet it can not be found as a CAPE-OPEN Unit Operation from within DWSim. Is there any experience with people who have coupled the software of HTRI with DWSim and how this problem could be fixed?
Thank you in advance!
This could only be solved if I was add a user-defined parameter which multiplies the LMTD. You'll have to wait for the next update if possible.
As for HTRI, I've never worked with its cape-open unit operation so I can't help you with that.
Thanks for the help Daniel, I will wait for the next update!
Jaap, did you try to couple your preheater HEX model with a DWSim heater block (where you can better specify the outlet stream thermodynamic condtions ie at boiling point) and then add a spec (or an adjust) block linking the heater outlet temp to the HEX outlet temperature? This would avoid you the trial&error procedure of finding the boiling point ...
Hope this helps.
Luca