Heat/Fluid flow in electic heated Boiler

Hello

I'm currently working on a project to design a new boiler. The boiler is heated by an electrical heating element which is situated at the bottom of the boiler. My question handles the water/heat flow in the boiler in to situations.

How does the water in the boiler heat up when no water is added or released from the boilee. Is the temperature change uniform (and how long does this take) or does the top the boiler become warmer the the rest.

The second situation is when cold water is added at the bottom and warm water is released at the top. the same questions do accure.

Does anyone know a book ? or some information on the internet?

 

[poetix99]

Sounds like you need to become familiar with the basics. I hope that you don't actually attempt to design a boiler without the assistance of someone who has some experience (and postings on this website are not sufficient for what you need).

I suggest that you get the book "Steam/ its generation and use" by Babcock & Wilcox. This is mostly concerned with fired boilers, not electrically heated ones, but will give you some of the fundamental principles of the water-side design.

There have also been many other threads on related subjects.
- Check "Boiler and Pressure Vessel engineering" Forum of this site. Some of the threads have cited text references.
- Do a keyword search. Try: boiler, HRSG, steam generator, etc.

 

[jcll]

Dear Colleague:

I agree with the basic comment made by our other fellow 22th of this month. However some tips will be mentioned due I have been working some years ago in one similar project:

1.-First it will be usefull the carefull reading of one good book. I can recomend to you the chapter nº 20 of Kern's "Heat transfer process"(MacGraw Hill).

2.-Electrical heating coils provide (simplification) one constant power = Q(kW/m2) as per different manufacturers information.

3.-Boiler shell has some water volume (Mw) when heating starts with some phisical conditions (P1, T1).So Enthaply state nº1 is fixed h1(kJul/kg).

4.-Rated boiling conditions you desire have some phisical conditions (P2, T2)as saturated steam. So enthaply state nº2 is already fixed.

5.-Boiler steam production must be one of your design basis (Ms) thus the required selected coil size will be :

Ms x(h2-h1)/Q

6.-Regarding your concerns about what will happen when no water is added the situation is so that the phenomena is Pressure and temperature will raise (the use of Mollier diagram for steam is needed, draw different states)in one polytropic transformation and boiler could be destroyed. For this reason the system must have one control which provide heating stope if no water supply is made (quite easy with small PLC and PT, TT and water supply valve). However the codes and regulations for the boiler installation imposes the use and recomended layouts for the safety devices. At the same time one(or more as per codes) safety valve is required rated to design pressure of boiler vessel.

One aproach to the phisical phenomena is possible with some iterative calculations with computer:

(a)Steam production Ms = QxA/(h2-h1), consider time lapse of t and m is obtained(steam /time).

Ms/density nº2=V(volume)nº2/time

(b)Steam mass&volume is encreasing inside vessel so that pressure state will be nº3. Check pressure state nº3 against mass production of steam P=densRT --->h3. Go to (a)

These are only basis that must be analized by you but could be usefull.



7.-When adding the cold water to the bottom some spreading distribution is recomended.

Hope can be useful for you. And good luck

Regards/JCLL