Boiling impact on level guage

[jamesbanda]

Dear sirs,

We have a kettle steam generator. We have increased rates by 35 %. I think I need to adjust down my calibration density on the device becuase the overall level will increase. Can anyone advise of a link to calculte this?

I've tried

Density cal * g height
=
g height(water only) * density water
+ g height(mixed phase) * density water * Fract(Wat)
+ g height(mixed phase) * density steam * Fract(steam)

I may be missing something obvious but i can cannot see a way to remove time from my equations.

if have mass in / mass out but how to covert to residence time volumes...

Any advice really apprecaited.

 

[djack77494]

james,
You seem to be treating the shell side of the kettle as having a region of 100% liquid and a second region of mixed phase water/steam. As you increase rates, do you expect those regions to change geometry? I would not. I would expect the same liquid level regardless of my steam generation rate. Ideally, the steam would separate from the tubes and quickly rise to the vapor space as soon as it was generated; practically the two phase density may change some as the generation rate changes. Maybe my approach is too simplistic, but I've always treated these type of calculations by assuming 100% liquid to my overflow level (weir) and 100% vapor above that. I truly think this is an adequate approach.
Doug

 

[25362]

Entrainment would tend to increase with rate for a given geometry.
Sometimes the boiling water surface is kept lower than the upper row of tubes to try to eliminate entrainment by vaporizing liquid mist/droplets.

Often the tube bundle is not circular, by leaving a flat tube row at the top of the bundle. Following experts' advice, liquid level should not be greater than 2" above the top horizontal tube and never less than 12" between the liquid level and the top centerline of the shell.

I think the actual inventories cannot be accurately assessed. There are special instruments for that purpose.
Anyway, to estimate the froth density, if there is a froth, the Hepp correlation:

[φ] = 1 - 0.62 v ([ρ]_v)^0.5​

where v = vapor velocity, ft/sec
[ρ]_v = vapor density, lbm/ft³

may be of use.

[φ] = froth density, lbm/ft³/clear liquid density, lbm/ft³​

 

[jamesbanda]

Thankyou for your post.

Would you be able to advise a reference for this correlation ie text book...i've tried a few searches but not found this correlation..

 

[25362]

I found the correlation in old notes made once on kettle-type reboilers. Sorry, I cannot refer you to any particular source.

 

[insult2injury]

did a google search....its on page 12 of this reference which to a paper by Hepp from 1963

http://students.washington.edu/~mpb...4.pdf#search="Hepp correlation froth density"

I2I

 

[25362]

I found a similar site:

http://students.washington.edu/~mpb...iam L. Luyban, Joseph P. Shunta 1985/DCC4.pdf

 

[jamesbanda]

Thankyou again for your posts.

To calify when you state lbm/ft3

Is this pounds mass (lbm) or
do you mean lb / ft3 *10^-3.
Ie in those units

density of water is approx 62 lbm /ft3 or 62000lb/ft3

 

[25362]

m = mass, better written as subindex.

For example, at saturation:

[ρ]_water @ 100^oF = 62 lb_m/(ft)³
[ρ]_water @ 210^oF = 59.88 lb_m/(ft)³
[ρ]_water @ 400^oF = 53.65 lb_m/(ft)³
[ρ]_water @ 500^oF = 49.02 lb_m/(ft)³