Electrical heating Furnace

[sivamech]

Hi,

I am designing a electric heating furnace having different layers of insulation following by a outer metal casing. Its a 150 kW furnace. I have the thermal conductivity & thickness details for all insulation layers. I would like to calculate the outer skin temperature of the casing. What is the amount of heat transfer to be considered for this calculation?
Inner temperature-1200 deg. C

 

[saplanti]

I trust that your target is to make the transferred heat as minimum as posible. Before everything you need to target the acceptable outside surface temperature. In case you need to touch with hands it should be limited to 60 degree C. Sometimes you may have to target a lot less which depends on your application and location of application.

The rest is changing the thicknesses of insulation layers to achieve the outside surface temperature and lowest heat transfer. It is a trial and error calculation for insuiation materials, their heat transfer coefficients and thicknesses. You may assume it is a wall heat transfer calculation in case the furnace is in rectangular cross section.

 

[3DDave]

If you continue to put 150kW in then the entire furnace will eventually be incandescent.

You will have as much heat transfer as the surroundings of the oven will take away, modulated by the thermal resistance of the insulation.

Q = K * delta T. K depends on the configuration, the insulation, the radiant properties, et al.

See

http://www.thermopedia.com/navigation/

 

[IRstuff]

How can you have "thermal conductivity & thickness details for all insulation layers" and not know what the skin temperature is? Conservation of energy says that your skin temperature MUST dissipate your 150 kW.

How big is your furnace? A 1 m^3 object would definitely be glowing hot

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[sivamech]

@saplanti
The skin temperature to be maintained at 50 deg. cen.

@IRstuff
I am doing back calculation for an old design of my company product. The thickness and thermal conductivity data of the insulation materials are available in the old design. However, shell temperature assumed, amount of heat transfer details used for thickness calculation are not available. Its a 5 m^3 furnace

@3DDave
Do you mean that the amount of heat transfer/rate of heat transfer depends upon the outside air physical properties?
If so, please clarify the factor "heat transfer coefficient of air". How much it should be considered for this?

 

[saplanti]

You almost have everything you need for the calculation in case the environment is still air. Please check the following for the calculation;

https://www.powderprocess.net/Tools_html/Thermodynamics/Heat_Conduction_Wall.html

It even has a spreadsheet calculation however it does only consider single layer of insulation. But you can manage the same with several layers of insulation by building your spreadsheet. So you can check the suitability of insulation by finding the temperature on the hotter face of each layer of insulations if they are different. In case the insulation materials the same you can consider them as a single layer.

 

[saplanti]

Here is another;

https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

 

[saplanti]

This looks better;

https://www.pdhonline.com/courses/m237/Heat-Transfer-Excel-Calculations-1.xls

 

[MintJulep]

Conservation of energy says that your skin temperature MUST dissipate your 150 kW.

No, it doesn't. Because we have also been told that the internal temperature is 1,200 C. This implies a temperature controller. In turn that opens the possibility that the power input is not 150 kW continuously.

150 kW seems to be the "name plate" rating. We do not know the operating point or profile.

Further, we have not been told if there is any exhaust taking away heat. We have not been told if this is a batch oven, or a continuous process oven. We have not been told anything about the product in or passing through the oven.