Kettle Boiler: shellside velocity criterion 1

[Leclerc]

Dear All,
There is a process condenser which generates steam in a kettle-type boiler.

What should the (average)vertical velocity of generated steam be so that a normally accepted level of disentrainment is achieved? What should the velocity be for flowsheet, and what should be the maximum allowable velocity? I would assume that the criterion would be a value for the term rho.v2.

The condenser/ boiler is fitted with a demister, but I feel that the demister should not have to carry out disentrainment below a certain small droplet size.

 

[tgschaef]

Can't say I did any calculations, but we have a very similar operation, except we're just cooling, not condensing tube side. We keep the level enough to just keep the tubes submerged. So the disengagement area is just 30inx19ft at the water surface with ~7-8" from the surface to the nozzles. The take off is 3 3" nozzles, which expand to a 6" common header. We have produced upwards of 8,000 lbs/hr of 100psig steam and haven't seen excessive trapping out in the steam header (to me indicating no water carry over). I'll leave the calculations to you, how this compares with your application.

I hope this helps.

 

[srfish]

Would you clarify why you mention vertical vapor velocity instead of horizontal velocity ?

In my experience, the vertical vapor velocity is used for an evaluation of the situation near CHF. The horizontal velocity is used in a seperator type equation. A good discussion of seperators can be found in the Engineering Data Book of GPSA, Vol 1, section 7.

 

[Leclerc]

I mention vertical velocity because the reboiler has a short, fat tube bundle, with dia about 3m and tube length 7m. The reboiler shell is about 5 m dia, thus liquid surface area is approx 5m x less than 7m: nearly square.

 

[srfish]

OK, I see your problem.

One suggestion is to refer to a Art Montemayor post in the Heat Transfer & Thermodynamics engineering forum of several years ago. In his post he discusses both The Souders-Brown and the gravity seperation method.

Another suggestion is to log on to:

http://www.amisto.com/products/eliminators.

They have guidance on what vapor load factor in using the Souders-Brown method.

 

[srfish]

Correction to my last post.
The URL should contain amistco, not amisto

 

[whylie]

HTRI program runs on allowable entrainment of < 2% wt Entrainment has 2 major is the adverse effects:

1.Excessive pressure drop
2.Effect on bottom tray efficiency,of the vapor is part of a reboiler circuit in distiilation..etc

Up to 5% wt vapor is allowed as long as your DelP is <0.2 psi/100 ft

When you calculate DelP ,use 10% wt liquid as a safety margin

Regards

 

[25362]

My 2 [&cent;] pledge. One typical problem with this type of boilers resides in the measurement of level. The s.g. of the water in the external indicator is ~1.0, while that inside the boiler may be perhaps ~0.5 due to the steam bubbles.

That is one reason for keeping the top of the kettle well above the top tube of the bundle (say, doubling the height of the top tube) which is itself covered with boiling water.

Sometimes domes with demisters are placed on top of the kettle to improve steam quality and to reduce entrainment of boiler feed water (BFW) in the steam.

If the steam -not generated from clean condensate and containing BFW- is to be further superheated, solid deposits due to entrainment may cause the tubes to fail. However, if the steam is used to reboil a nearby vessel, entrainment is, in general, not considered a major problem.