Heat transfer from inside to outside

In summary, at 1000 degrees Celsius, a layer of mineral wool blocks (k=.091 W/m-c) followed by fiberglass boards (k=.0425 W/m-c) will provide the best insulation. The interface between the materials should not be greater than 400 degrees Celsius, and the outside surface should not be greater than 55 degrees Celsius.
  • #1
JSBeckton
228
0
An insulation system is to be selected for a furnace wall at 1000 deg C using first a layer of mineral wool blocks (k=.091 W/m-c) followed by fiberglass boards (k=.0425 W/m-c). The outside of the insulation is exposed to an environment with h= 15 W/m^2-c and temp of 40 deg C. Claculate the thickness of each material such that the interface between to insulators is not greater than 400 deg C and the outside surface is not greater than 55 deg C. What is the heat loss per unit square meter?

I have been going over this question that sounded pretty simple at first, but I soon realized that I have no idea where to start. I cannot calc the overall heat loss because I don't know the thicknesses, and I can't calc the thicknesses because I don't know the overall heat loss. Catch 22? I'm sure that it has something to do with solving equations simo but cannot figure out where to start w/o any additional info, can anyone help me out?

Thnaks
Josh
 
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  • #2
Did you make a thermal resistance network?
 
  • #3
Not sure what you mean by that but if you are referring to the resistances between the inside and outside:

Temp inside->
R conduction from mineral blocks ->
Temp interface ->
R conduction from fiberglass ->
R convection from fiberglass ->
Temp outside

Where do I begin?
 
  • #4
(1) Draw a picture and upload it to www.imageshack.cc[/URL]
 
Last edited by a moderator:
  • #5
You have 2 equations.

1. The temp difference between the interior and the interface, the interior material thickness and conductivity.

2. The Temp difference between the interface and the exterior, the exterior material thickness and conductivity.

Solve.
 
  • #6
I tried that but I have 3 unknowns:

1) Thickness of mineral blocks
2) Thickness of fiberglass
3) Heat flow per unit area.

I uploaded my work here

[URL=http://imageshack.us][PLAIN]http://img126.imageshack.us/img126/1482/img020yu7.jpg[/URL][/PLAIN]
 
  • #7
Let me know if you can't read it and I will try again.
 
  • #8
A 3rd equation would be the one relating the total thickness, the total resistance and the internal and external temperatures.

You are also given the temperature of the environment, this could give you a net heat loss with an assumption on the mechanism, ie pure radiative loss or perhaps a convective loss. With either you would need to know something about the emissivity of your final surface.
 
  • #9
Thanks, I think that I figured it out.
 

Related to Heat transfer from inside to outside

1. How does heat transfer from inside to outside occur?

Heat transfer from inside to outside occurs through three main mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact between two objects, while convection is the transfer of heat through the movement of fluids. Radiation is the transfer of heat through electromagnetic waves.

2. What factors affect the rate of heat transfer from inside to outside?

The rate of heat transfer from inside to outside is affected by several factors, including the temperature difference between the inside and outside, the type and thickness of the materials involved, and the presence of insulation or barriers that can slow down heat transfer.

3. How can I reduce heat transfer from inside to outside in my home?

One way to reduce heat transfer from inside to outside is by using insulation materials such as fiberglass, foam, or cellulose to slow down the transfer of heat through conduction and convection. Sealing any cracks or gaps in doors, windows, and walls can also help prevent heat from escaping.

4. Why is heat transfer from inside to outside important to consider?

Heat transfer from inside to outside is important to consider because it can affect the temperature and energy efficiency of a building. In colder climates, heat transfer can result in higher heating costs, while in hotter climates, it can increase the need for air conditioning. Reducing heat transfer can help save energy and lower utility bills.

5. How does the type of material affect heat transfer from inside to outside?

The type of material and its properties can greatly affect heat transfer from inside to outside. For example, materials such as metals are good conductors of heat, while materials like wood and plastic are insulators. Materials with a higher thermal resistance can slow down the rate of heat transfer, while materials with a lower thermal resistance will allow heat to pass through more easily.

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