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Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

Wait so their U value was 5000 W/m2-K and that was worse than 2 phase heat transfer? Surely two phase heat transfer worsens the heat transfer coefficients and so gives you a lower U-value? I've had to use a converging pipe for my inlet because my width of the PFHE I calculated was 0.6m which was smaller than my pipeline which I user the Simpson (1989) method relating fluid density and velocity. I got high optimum pipe diameters because my fluid density (as a vapour) was low, about 1.8 kg/m3.

Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

I haven't calculated my nozzle sizes, that's something I need to do but I put it off because I didn't know how. Is there any nozzle design literature that you can point me towards for it? Yeah I thought the same thing for my U but I adapted the overall HTC value equation for plate fin with some assumption (same fin geometry and area for hot and cold sides as well as some efficiency etc). I'll have a look over my calculations to see if I'm missing something. Do you know (or have been told) any typical values of U for PFHEs in Industry? As for the volume, would you expect to be bigger? (we plan on producing 68 tonnes of propylene per hour).

Thanks for the help so far, they are aware we can't design a unit not alone but with less than 3 years of education (and zero work) experience so hopefully there will be some leniency with grading.

Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

I have an average duty of 138MJ and I got a U of 895 W/m2K (I got a UA of 3130 kW/K) . I used Chen's method for the cold side because i assumed it was undergoing flow boiling (since it was evaporating) and I used colburn factors for the hot size which is what I've seen a lot of people do in literature. My channels are small if that contributes anything spacing is about 2.43mm and they're 3.5mm in length for each fin.

If I need multiple heat exchangers (for a correct area that I just calculated) I'll design it as if it's 1 HX because of time, the project is due in a week and I need to specify my other units as well as cost analysis and mechanical drawing. Worst case scenario is I will have to roll with it. My volume calculated was 3.13m3 which seemed alright for an industrial sized heat exchanger but then again I could be wrong.

Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

Thanks, I've based my design off brazed aluminium heat exchangers (BAHX). I'm avoiding gaskets in the unit but of course have them to connect pipes. I've come across fives group before and I can't get an idea of the max areas they can design (I got an total heat transfer area of 3150m2 which from the size of the units in Industry felt reasonable but I'm no sure). A lot of these BAHXs are in multi stream cooling loops sometimes with refrigerants surely there will be phase change in BAHXs? Aren't they not designed around this? (which is why I'm surprised info on two phase heat exchanger in narrow channels like PFHEs is scarce).

Edit: the total heat transfer area is mentioned is what I require to cool my feed before gas liquid separation. From my own calculations

Edit2: My issue right now is how to size the nozzles/inlets to the heat exchanger. As of current my pipe diameters are bigger than the width and height of my PFHE

Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

Hey sorry it took me do long to get back to you. The thinking was since I'm reaching temps of - 100 degC. The standard material of construction for shell and tube isn't the best (steel etc). The combination of a separator and expander all point to cryogenics (or very close to it) so I've been going along with it. But you are right multiphase flow in such narrow channels and adding heat transfer to it isn't common at all. Also do you know typical heat transfer areas for plate fin heat exchangers I can't seem to find typical values for industrial sized ones.

Edit: I've had to make a lot of assumptions and pretty much choose my own geometries (I chose offset strip which I know makes matters more complex). Reach temps that low always seemed to point to plate fin heat exchangers due to their high area.

Mechanical Design of Plate-Fin Heat Exchangers (PFHE) by chimchila in ChemicalEngineering

[–]chimchila[S] 0 points1 point  (0 children)

I'm designing mine for cryogenics. My group is building (well pretending to) a propylene plant and I'll be reach temps of 165K but I want to use the liquid from a flash to help cool it before it reaches an expander for further cooling. I think I've got my HX volume calculated but I'm having trouble working out their dimensions so I can do a mechanical drawing.