subcooling region heat transfer coefficient in steam condensers

[pastyl]

Hi everybody

I try to find a suitable correlation for the heat transfer coefficient of the shell side in the subcooling region of a shell & tube steam condenser.
I have found a equation from free convection which is the following:

Nu=0.138Gr^0.36(Pr^0.175-0.55)

(Perry's paragraph 5.13 eq.5.33a)

But the above equation give shell side heat transfer coefficient as high as in the shell side of condensation region (were I make use of the thin film consensation approach). I think that it should be lower.

Does anybody knows a well-known & proven equation for the shell side heat transfer coefficient in subcooling region.

Thanks in advance.

 

[rmw]

Yes, the method is knowing the heat transfer mechanism and beign able to calculate Reynolds numbers, LMTD, etc for that zone. Assuming that your S&T subcooling region is designed to do that via water flow across tube surface, then you have to know the velocity of the condensate across the tubes and start there. The basic rules of heat transfer work nicely in this region. If it is subcooling but not over heat transfer surface (droplets falling through non heated regions) then another whole different heat transfer regime is at play.

rmw

 

[davefitz]

Try TEMA or HEI standards, or the text books by Kern. According to Kakac (sect 11.2.4 chapt 11 by Butterworth in "boilers , evaporators, and condensers"), it is very difficult to accurately predict the heat transfer for the subcooler section of a condenser, partly because the low velocity, the baffles being arranged primarily for the condensing zone, the difficulty in knowing the liquid level thruout the lower zone of the condenser. Butterworth indicates that if a defined amount of subcooling is required per spec, then a separate subcooler be provided.