2 replies to this topic

[A mukherjee]

Hi everyone,

I am in doubts regarding the values of the heat transfer coefficient which I am getting during design of a vaporizer. In the vaporizer, liquid nitrogen is flowing through the tubes. The tubes are submerged in a water bath. The water bath is heated by steam distributed through spargers. The liquid nitrogen enters in the saturated state & leaves as superheated gas. But for all the zones (saturated liquid to vapor & saturated vapor to superheated vapor), I am getting forced convective heat transfer coefficient (around 250-300 W/m²-K) of nitrogen much lesser than natural convective heat transfer coefficient (around 700 W/m²-K) of water.

Is this result erroneous?

[Art Montemayor]

A mukherjee:

First of all, I have to presume that you have done the calculations without employing a simulator (since you have posted in this Forum and not in the Simulation Forum).

I interpret your statement as saying that after calculating both the forced convective heat transfer coefficient and the natural convective heat transfer coefficient, your results show that forced convection yields a LOWER coefficient that the natural convection. The results do not make sense. The values should normally be reversed. There is something affecting your calculations and since you have not submitted your detailed calculations (in an Excel Spreadsheet), we can’t decipher where the error might be.

If you upload your detailed, Excel spreadsheet calculations our members would probably succeed in detecting what is wrong with the calculations. [Be sure to differentiate between the film coefficients (h) and the Overall Heat Transfer Coefficient (U)]

[A mukherjee]

I am actually facing some problem in uploading the file.Also many of the properties have been calculated using add ins.But surely I'll try my best to upload it.

I would like to clarify that,I am getting Forced convective coefficient higher than natural,for the zones where Sat.liquid is converted to sat. vapor & then to superheated vapour.
For the zone where sub cooled liquid is converted to saturated liquid,Natural convective coefficient is higher.