Condensating heat transfer coefficient

[Guido]

In literature, one can find heat transfer coefficients for different heater sections such as the desuperheating or the condensing section.

In the case of feedwater heaters; what causes the condensing heat transfer coefficient to have such a big value when compared to the convective heat transfer coefficient of the water that is heated?

Many Thanks!

 

[Gianfranco]

Ciao Guido,

There's a change of phase Gas -> Liquid
The heat transfer coefficient greatly increases because it involves the Latent Heat of Condensation (or change phase heat).

The condensation heat transfer coefficients depend on the arrangement of the flow and orientation (horizontal, vertical), shape of surfaces (round, plain, tilt, cavities or/and protrusions), properties of the surfaces (finishing, liquid-surface interaction), fluid flow (laminar or turbolent) and its working conditions (pressure, temperature, cleanliness of fluid).

For instance the typical values for steam at 1 bar, are:
Film condensation on horizontal tubes
h = 9 000 - 25 000 W/m2K

Film condensation on vertical surfaces h = 4 000 - 11 000 W/m2K

Dropwise condensation h = 60 000 - 120 000 W/m2K

For details see one of heat transfers reference books, cited frequently the forum, chapter on Boiling-Condensation.

If you have any ref. books on heat transfer available, see the very helpful Lienhard's e-book
at

http://web.mit.edu/lienhard/

http://www/ahtt.html

Regards.

Gianfranco