Heat Transfer and Thermal Radiation

## Question:

I need some help on three problems. I would appreciate any help and explanation you can give.

1) The space between an inner wall and outer wall that measures 10 ft x 8 ft is insulated with foam having an R-value of R-13.

a) What is the rate of heat transfer through the insulated space when the temperature difference is 20 degree Fahrenheit?

b) How thick is the insulation?
2) What is the intensity of the radiation being emitted by an object with an emissivity of .40 at a temperature of 100 degree Celsius?

3) Compare the R-value of a thermopane window with 1/8 inch glass panes and a 1/4 inch air space with that of a single pane window with at thickness of 3/16 inch.

## Answer:

Hi Gloria,
1a) H=A*(T2-T1)/R = 80*20/13= 123 BTU/hr

1b) R=L/k. If you know the k for the foam you can calculate L. Otherwise I do not know.

2) Power radiated is P=5.67e-8*Area*0.4*100^4. P/Area=5.67e-8*0.4*100^4=2.27 watts/m^2. The 5.67e-8 is just one of those constants that is invoked to make the units all come out right. The equation is Stefan's law.

3) R=L/k for each layer, where L is the thickness of the layer and k is the thermal conductivity. Then you have to convert all the units from metric to English because R is expressed in English units.

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