I have an Evaporator, which goes evaporates R290 into an superheated vapor. I have available from Experimental Results the inlet and outlet temperatures and the surface area. I'd like to estimate the overall heat transfer coefficient through some analytic method, so to compare it with our simulations.
I thought about using the LMTD method, but maybe it doesn't suit me. Is there any other way?
When there is only phase change (evaporation or condensation) on one (or more) sides of a heat exchanger, the NTU-effectiveness method reduces to the LMTD method; so the calculation is the same. Q=UA*LMTD will be adequate for this. There are several empirical correlations, which will work for this arrangement, connecting thermo-physical properties to the fluid contributions to U. The others (1/U= 1/hinside +1/hwall +1/houtside…) can also be estimated. You can consider fouling in this way. If the exit is superheated vapor, then you must subdivide the heat exchanger into the saturated and superheated sections, as these will be different. While the properties are different, the process isn't different from a once-through steam generator in a combine cycle power plant heat recovery steam generator. You can easily handle this with an Excel spreadsheet. I attached one that calculates condensation and boiling heat transfer coefficients for a variety of refrigerants plus two that solve a feed water heater with sub-cooled, saturated, and superheated sections. Here's a link to a free refrigerant property Excel AddIn that has R290 and dozens more. http://dudleybenton.altervista.org/software/Refrig100.zip I have a lot more free stuff if you're interested.
Thank you so much Dudley J Benton! I'll definitely have a look in your material.
If the LMTD method is used in performance calculations for which
both outlet temperatures must be determined from knowledge of the
inlet temperatures, the solution procedure is iterative.
For both design and performance calculations, the effectiveness-NTU
method may be used without iteration. NTU stands for Number of Transfer Units.
Depending on what type of heat exchanger you have, you may use Table 11.3 (Heat Exchanger Effectiveness Relations) in the Incropera textbook to solve this problem.
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