7 replies to this topic

[whatzit]

Hello; I have what is probably for you a relatively basic question, and I think I am missing a couple (surely obvious to everyone else) steps towards getting an answer. Below I have outlined the problem, what I am solving for, what I am trying to do, and why I am stuck. I look forward to any suggestions that'll help me through this... Thanks.

Situation: I have a bath (of volume V) of liquid nitrogen to evaporate using hot water heat exchange in steel pipe (because it is available), which comes in at pressure P. There is a known T_in and target T_out for the water. There is a maximum time t to evaporate everything.

Solving for: L, length of pipe.

Getting stuck at:
1) overall heat transfer coefficient. I can assume a diameter and wall thickness, but from there; where? I'm not sure whether the surrounding fluids or just pipe material will matter, and internet search hasn't made things any clearer.
2) I can get AN answer by finding
- the heat necessary to evaporate the nitrogen, then
- the minimum necessary rate of heat exchange using mc_p*ΔT, the maximum time possible, and the above result
- h = ΔQ/(A * ΔT * Δt) where h is the heat transfer coefficient
but this a- doesn't account for the water pressure and b- still has the problem with the heat transfer coefficient, described above.

My background: I did this in school 10+ years ago and can do thermo in general but am getting turned around on this because of the pressure and what seem to be too many unknowns. It is at the fringes of what I normally do, so I don't have a lot of personal experience to say if a result is reasonable or not.

[wojtar]

Your description of problem is unclear. Let us know where is that liquid nitrogen to be evaporated and how you want to do this evaporation. What do you mean saying: "heat exchange in steel pipe because it is available?" I imagine that hot water is inside the pipe. Could you attach a simple sketch of your situation?

Alpha coefficients and resulting overall heat transfer coefficient will depend on the geometry of your system -> you must assume that -> is it coil or something else?

[whatzit]

You are right, the hot water is in the pipe, the nitrogen in a tank, so cooling by conduction. There wasn't a specific geometry given to me; a coil seems fine. At this point, I'm really trying to get a first approximation, or even be sure I understand how to solve this.

Does that clarify the question? Thank you for your input.

[Zauberberg]

Essentially what you have there is a pool boiling application, and for calculating heat transfer coefficient (and individual film coefficients) you can use the procedure for thermal design of kettle reboilers - as presented in the file attached below from Coulson & Richardson.

As for the evaporation rate it is calculated based on the volume of liquid Nitrogen (latent heat required to evaporate pool of Nitrogen, kJ) and the maximum time given for the evaporation process (hr). You simply divide the two.

Attached Files

•  Kettle Reboilers.pdf   123.36KB   39520 downloads

[wojtar]

You could also try this: http://www.lv-soft.com/

[Zauberberg]

It's always better to do the hand calculations first, especially if they are not too complex. If one doesn't grasp the concept of a process by going through step-by-step calculation, he'll never feel confident with the results obtained by software.

[wojtar]

It's always better to do the hand calculations first, especially if they are not too complex. If one doesn't grasp the concept of a process by going through step-by-step calculation, he'll never feel confident with the results obtained by software.

Yes ... that's right... but those simple-look programs may help to organize your own excel workbook if you don't know how to start...then search the literature for equations.

[whatzit]

Essentially what you have there is a pool boiling application, and for calculating heat transfer coefficient (and individual film coefficients) you can use the procedure for thermal design of kettle reboilers - as presented in the file attached below from Coulson & Richardson.

As for the evaporation rate it is calculated based on the volume of liquid Nitrogen (latent heat required to evaporate pool of Nitrogen, kJ) and the maximum time given for the evaporation process (hr). You simply divide the two.

Thank you. I had gotten through the first steps okay, and your outline of the procedure confirms that. As for the boiling, the attached PDF is a good outline of the kettle reboilers; I am pretty sure I have never worked on these, so no wonder I was lost at that point.

Thanks everyone for the replies.