Radiation & Convection Heat losses

[AndreChE]

Hi all,

We have 8 furnaces, 2 of them are going to be revamped because coils and casing are old and in bad shape.

I am considering 2% heat losses for the newest furnaces and 2,5% for these 2. (Of total heat input).

I have measured the casing temperatures and I am calculating heat losses with Langmuir and Stefan-Boltzman equations, respectively by Radiation and Convection heat loss with average temperatures, real wind speed and ambient temperature.

In these furnaces I get 8,5-9,5 W/m2 of heat loss and for the others 5,0-6,0 W/m2. Areas are similar.

Are these 2 and 2,5% a good aproximation?

Thank you all.
AndreChE

 

[25362]

1. Heat loss percentages

2% (or less) as assumed by you is considered OK for fired heaters with heat releases beyond 10 million kcal/h. Smaller heaters may reach a level of up to 5% !

2. Heat fluxes

A difference of 40-50^oC between wall temperatures and ambient air was until recent times considered state of the art. Assuming the ambient temperature, Ta=70 deg F, and the vertical wall at 150 deg F, with a 10 mph wind, Robert D. Reed (Furnace Operations-Gulf) estimated a loss of 445 Btu/(ft².h) which is about 1400 W/m².

Floors and roofs might not suffer from same wind velocity effects as vertical surfaces do, depending on their geometry and elevation, however convection HT coefficients for surfaces facing up and down are about 1.28 and 0.64 times those for vertical surfaces, respectively.

In view of the above, kindly comment.

 

[jalvarez]

AndreChE:
2% for a new furnace is OK.
Maybe 2.5% for a very old one is optimistic. I evaluated in the past 4% for a Linde ethane furnace. I can imagine the possibility of doing a survey, measuring wall temperatures over a grid and applying more consistently your calculations, but I can't see how to evaluate hidden issues like air intakes, which will be measured only with the flue gas composition. But its influence will depend in wich point you have the leaks.
The assumptions and approach of 25362 (by the way, 25362 has a very good bibliography!!) and the order of magnitude of the result -1400 W/m²- seems OK for me.
AndreChE, you may consider to do a termography on the old furnace, you will detect not only the areas with refractory problems, but also points of possible air intake. This is worthy only if you are in the process of deciding the refurbishment, or if this thermo is part of a wider campaign in your plant and this is only an addition.
You have a Linde plant, so you may also try to contact Selas to have some support for all these evaluations. They are in general very supportive. I have a very good experience with them.
Good luck
J. Alvarez

 

[steveen]

Consider coating walls and tubes with radiant efficient surfaces. There is a firm in Louisiana that offers this service. The benefit is cooler tubewall temps and more efficient heat transfer. Big Savings at today and tomorrow's fuel costs.

 

[jalvarez]

Steveen
I'm intereseted in this coating.
May you give more details?
Have a safe day
J.Alvarez

 

[steveen]

CALL ME Tuesday, 713-374-7900. ask for Steve

 

[silverlilly1]

It's textbook material that black bodies radiate the most heat. Painting a unit with silver paint (not any special coating) is enough to significantly reduce heat loss.

 

[steveen]

J alvarez,

The coating applied is a ceramic based type. There are two important benefits.
1. Heat flux increases and tube wall temperatures are cooler. Important on certain units.
2. The ceramic coating keeps the surfaces pristine. There is very little loss of heat flux due to fouling of the firebox side. This typically is not an issue with gas fired heaters, but oil fired heaters -- external tube fouling is a fact of life.

This web site does not permit commerical postings. Please call for vendors particulars...

 

[jalvarez]

OK, steveen, thanks.
I don't see a commercial issue or problem but I recognize that your precaution is correct. So, I'll call.
Have a safe day
J.Alvarez