I have a relatively simple simulation of a vapor compression cooling loop using only water at low pressure. In the evaporator heat exchanger, the liquid water is vaporized at low pressure to derive cooling. When looking at the discharge temperatures, the temperature of the fluid being cooled goes below the temperature of the water-side discharge, which is mostly vapor. That is not possible unless a colder temperature was achieved somewhere in the heat exchanger profile. However, when looking at the computed temperature profile, the water side temperatures never go below the temperature of the fluid being cooled. Essentially, the simulation is saying that a fluid is being cooled by a fluid that is vaporizing and thus consuming heat but is always at higher temperature in the HX. This seems completely non-physical. Is there a fundamental issue with DWSIM's heat exchanger model or what am I missing?
I downloaded yoursimulation and it seems to work perfectly. Only thing I did was add a small pressure drop to the first exchanger and specify a percentage efficiency similar to the other exchangers.
Hope this helps
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Frank - thanks but I know the simulation converges and your changes did not affect the result significantly. What I don't understand is how it is physically possible for the glycol-water fluid that is being cooled to be discharged from the HX at a lower temperature than the water vapor that is discharged (plus minor liquid) that is the heat sink via phase change in the HX. The temperature profile shows no temperatures below 14.5°C that would be required to cool the glycol solution to the discharge temperature. The simulator is cooling the fluid with an inverse temperature gradient. This is physically impossible. Something is wrong in DWSIM's HX solver with cooling via phase change. Or at least I can't understand how the results are possible.
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I have had a closer look at your problem. I found that there is a temperature cross that makes the heat transfer impossible. Have a look. Check the box for Calculate Heat Transfer Profile on the Condenser details panel. If necessary, rerun the simulation, then check.
It is necessary to change the Heat Transfer Efficiency to about 10% in order to make it work.
Here is what I found. It is labeled R-2.
Comment ; I do not know why the calculation does not stop when the temperature crosses and the transfer surface would become Infinite. But you can clearly see from the heat curve that the majority of the heat transferred is the condensing of the water phase.
Hope this helps.
The temperature cross generates a warning message in the log window, as the heat exchanger model in DWSIM doesn't support phase change in most calculation modes.
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Thanks Daniel. I was not aware of the issues with phase change in most of the HX calculation modes. First thing that was important is to uncheck was the "Ignore LMTD Error" box. Doing that immediately generates an error message so alerts the user that there is a problem. After that, I switched the Calculation Type to Calculate Outlet Temperatures and now the outlet temperatures are reasonable with computed heat transfer efficiency of 70%. Because phase change in heat exchangers is an extremely common process modeling need, I hope these posts/discussions will be helpful to others. Users need to be very careful and double check the discharge stream conditions if doing phase change in DWSIM's HX object.
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I have a relatively simple simulation of a vapor compression cooling loop using only water at low pressure. In the evaporator heat exchanger, the liquid water is vaporized at low pressure to derive cooling. When looking at the discharge temperatures, the temperature of the fluid being cooled goes below the temperature of the water-side discharge, which is mostly vapor. That is not possible unless a colder temperature was achieved somewhere in the heat exchanger profile. However, when looking at the computed temperature profile, the water side temperatures never go below the temperature of the fluid being cooled. Essentially, the simulation is saying that a fluid is being cooled by a fluid that is vaporizing and thus consuming heat but is always at higher temperature in the HX. This seems completely non-physical. Is there a fundamental issue with DWSIM's heat exchanger model or what am I missing?
Thanks,
Pete McGrail
I downloaded yoursimulation and it seems to work perfectly. Only thing I did was add a small pressure drop to the first exchanger and specify a percentage efficiency similar to the other exchangers.
Hope this helps
should have posted solution. Here it is.
Frank - thanks but I know the simulation converges and your changes did not affect the result significantly. What I don't understand is how it is physically possible for the glycol-water fluid that is being cooled to be discharged from the HX at a lower temperature than the water vapor that is discharged (plus minor liquid) that is the heat sink via phase change in the HX. The temperature profile shows no temperatures below 14.5°C that would be required to cool the glycol solution to the discharge temperature. The simulator is cooling the fluid with an inverse temperature gradient. This is physically impossible. Something is wrong in DWSIM's HX solver with cooling via phase change. Or at least I can't understand how the results are possible.
I have had a closer look at your problem. I found that there is a temperature cross that makes the heat transfer impossible. Have a look. Check the box for Calculate Heat Transfer Profile on the Condenser details panel. If necessary, rerun the simulation, then check.
It is necessary to change the Heat Transfer Efficiency to about 10% in order to make it work.
Here is what I found. It is labeled R-2.
Comment ; I do not know why the calculation does not stop when the temperature crosses and the transfer surface would become Infinite. But you can clearly see from the heat curve that the majority of the heat transferred is the condensing of the water phase.
Hope this helps.
Hi Frank, thank you very much for helping me with providing support here in the forums, I really appreciate it ❤️
The temperature cross generates a warning message in the log window, as the heat exchanger model in DWSIM doesn't support phase change in most calculation modes.
Thanks Daniel. I was not aware of the issues with phase change in most of the HX calculation modes. First thing that was important is to uncheck was the "Ignore LMTD Error" box. Doing that immediately generates an error message so alerts the user that there is a problem. After that, I switched the Calculation Type to Calculate Outlet Temperatures and now the outlet temperatures are reasonable with computed heat transfer efficiency of 70%. Because phase change in heat exchangers is an extremely common process modeling need, I hope these posts/discussions will be helpful to others. Users need to be very careful and double check the discharge stream conditions if doing phase change in DWSIM's HX object.