Film boiling in heat exchangers 1

[Buchi]

Hi gangs,

can some one explain to me the concept of film boiling in heat exchangers?
cheers guys,

Buchi

Buchi

 

[PlantEng]

When vaporizing fluid in a heat exchanger around tubes, there are three types of boiling. With Tube temp @ saturation, you get pool boiling or without bubbles. This in mainly convective. (The film coefficient would be the samed as convection)

At temperatures greater than saturation temperature, you get nucleate boiling or what you see when boiling water on the stove (bubbles forming).

At 200F or higher than the saturation temp, the whole surface of the tube is covered by a film of vapor. Convective film coefficients do not work as well for this mode.

The main point is that each type of boiling has a different characteristic of heat transfer. The more film there is, the less easily heat is transferred by convection.

 

[rmw]

Good job, Planteng. Can I chime in with the same thought in some different words? In nucleate boiling, with the formation and rising of the bubbles, (back to the pan on the stove,) as the bubbles rise, it causes agitation, and constantly replaces the liquid that is changing to vapor with new liquid to be boiled. Good turbulence, good convection.

The film, however, notwithstanding that it is hot as the dickens, (200 degrees over saturation) becomes an insulator or you might say isolator, but whatever you call it, it is a resistance to heat transfer, retarding overall heat transfer, because of having a detrimental effect on the convection part of heat transfer.

I don't think I said anything different than you did, but just in a different way.

rmw

 

[MJCronin]

buchi,

There is an excellent discussion of the types of boiling in the book: "Steam" by Babcock & Wilcox....

Find an older mechanical/power engineer and look on his bookshelf or contact a local university library

My thoughts only....


MJC

 

[25362]

Bubble formation is influenced by the wetting properties of the liquid and the finish of the surface. To avoid the insulating effect that vapour bubbles have, a wetting agent is added to certain liquids, such as magnesium in mercury boilers.

Bubbles start at specific sites. A rough surface will have more sites where bubbles can start than a polished surface, and therefore has a higher coefficient of heat transfer.

This is one reason for using evaporators with finned surfaces for simple clean liquids that do not scale up, as the refrigerants R11 and R115.

In most cases however, smooth surfaces are preferred, to reduce the risk of scale formation and to make cleaning easier. Evaporators for spent sulphite liquor have polished surfaces.

The HTC rises with increasing temperature differences up to a maximum value, h_max, at a "critical" temperature difference, and decreases at higher temperature differences. The increase is due to the greater number of bubbles giving more interface between bubbles and liquid, and the increased agitation from the rising bubbles. This is called 'pool' boiling.

The insulating or blanketing effects of the bubbles at the HT surface at higher temperature differences has already been explained by PlantEng and rmw.

The critical temperature difference for film boiling is a function of the "reduced" pressure p_r of the boiling fluid, namely the ratio of the prevailing pressure to the critical pressure, p_c of the boiling compound.

Designs of industrial evaporators aim at tep. differences well below the critical temperature difference to avoid film boiling.

Graphs show that the critical temperature difference for film boiling increases with lower reduced pressures.

Some examples. For acetone boiling at atmospheric pressure (p_r=0.021) the value for the critical temp. difference would be around 69^oF. For pentane (p_r=0.03): 63^oF. For ethanol boiling at atmospheric pressure (p_r=0.0157), the difference would be 76^oF. For water (p_r=0.0045) it would be about 115^oF. Water boiling at 22.1 atmospheres (p_r=0.1) the crit. temp. difference would drop to 47^oF. These data were worked out from Chilli and Bonilla: "Heat transfer to liquids boiling under pressure" Trans.A.I.Ch.E., 41, No.6, 755 (1945).

Although to my knowledge there is still no general equation for estimating the HTC for all boiling liquids there are some for pure compounds boiling over horizontal tubes. Even these give only ballpark estimates, or orders of magnitude.