3 replies to this topic

[kybele39]

Dear All,

In API 560, burner clearance given for Natural draft for various type of fired heaters and from that point you can calculate the necessary radiant section width for the heater out to out of the shell checking across burner flame diameter. See attached document.

I am wondering:

• What base do you use determining of a cabin heater radiant section width for the forced draft burner application?
• Where can I find required or acceptable burner clearance for the forced draft burner applications?
• What is the rule of thumb for the furnace width for direct fired cabin heaters as configured horizontally with wall burners or floor burners?

Most burner companies does not know about the clearance. They can only give flame diameters. After all rest is depends your calculations. I am trying to be safe in my designs and not to cause to flame impingement plus I would like to stay under 12 ft. radiant width for the shipping reasons.

heat duty is 68 MMbtu/hr.
efficiency is 81%,
flux rate is 12500btu/ft2-hr.
Radiant duty split is 63%
Pipe is 6NPS and 4 passes
Convection section is 4 tubes per row and 6NPS pipe size.
I am using one burner for 95MMbtu/hr.

What would be the radiant section width of the horizontal cabin heater? Please see attached sketch.

PS: cannot use API560 Natural Draft applications chart to start calculating the width of the heater. My firing rate is so high and I am using only one wall burner.

Thank you to all

Kybele

Attached Files

•  Doc1.pdf   61.8KB   252 downloads
•  Doc2.pdf   100.85KB   183 downloads

Edited by kybele39, 30 July 2010 - 01:27 PM.

[ankur2061]

Kybele,

I am not a fired heater specialist but check out these links. These might be of help:

http://www.heaterdes...com/page4_1.htm

http://www.heaterdes...com/page4_2.htm

http://www.heaterdes...com/page4_3.htm

http://www.heaterdes...com/page4_4.htm

Regards,
ankur.

[kybele39]

Dear Ankur,

Thank you very much for the links. I base my calculations almost the same literature that was given on the links. However, I guess I missed the Ar refractory area calculation. Tomorrow I will check my calcs against the links that you copied above. Hopefully, I will have results. Seems to me on the board noone is expert on this topic. I will concentrate on this topic more and enrich the site more.

Thanks again.
Have good week at work.

Kybele.

[kkala]

Interesting to deal with burner clearances, instead of leaving it to furnace supplier through a more general ‘Process Duty Spec’. Involved only in steam boiler ‘Process Duty Specs’ (not for furnaces), opinions below (not based on experience) are just indicative.
Available data treated can be found in the attached ‘furnace.xls’, as well as some details.

1. Burner clearances have not been found for forced draft furnaces. References (1) and (2), reported in ‘furnace.xls’, apparently mean natural draft type whenever they mention furnaces.
For want of more precise info (that would be welcomed from others!), it is assumed that burner clearances required for natural draft are also applicable for forced draft. This might not be far from reality concerning distances B or C of API 560, Table 12.
Clearances A=burner (tip) distance from roof tubes and B=burner (axis) from wall tubes are presented in the diagram of ‘furnace.xls’, in function of burner max heat release (see sketch in furnace.xls). Available data concern burner heat releases in the range of 2-18 MM Btu/h, extrapolation is applied to approach the burner of 95 MM Btu/h = 23.9 MM kcal/h.

2. Radiant section width for one 95 MM Btu/h burner is estimated more or less at 2x8=16 m per API, or 2x5.99=12 m per Reference (1), according to the above. Length of the chamber would be about same and height much more. Even though above extrapolation is nothing more than a wild guess, it indicates a too big combustion chamber (i.e. radiant section). If one burner of 95 MM Btu/h is not a mandatory requirement (why be?), a series of smaller burners would result in a chamber of lower volume.

3. For instance, heat supply could be satisfied by 10 vertical burners of 10 MM Btu/h each, placed in a radiant chamber of approx 3.7 m x 14.5 m x 9.5 m, as estimated in ‘furnace.xls’.

4. Other points.
α) Distance B is actually flame radius and A flame length, eventually with some margins. Consultation from burner manufacturers on flame size could supplement available information (e.g. covering forced draft or bigger burners). Manufacturers’ data for a specific burner may be more precise, yet API is generally more conservative than them (experienced for flare flames, etc); so some additional margin may be considered if approach to ‘API safe level’ is desirable.
β) Typical width for cabin style furnace is 4-6 m per Reference (1) (see ‘furnace.xls’). Required w = 12 ft = 3.7 m is close to 4 m; nevertheless a chamber of 3.7 m x 14.5 m x 9.5 m (mentioned above) seems not possible to transfer by truck as a whole, due to long rest dimensions.

Attached Files

•  Furnace.xls   37KB   260 downloads
•  Furnace.doc   25KB   160 downloads