Calculation of heat conduction

This post was last edited by wiseboy on 2011-6-26 13:14. Simply, no one wants to answer. Asbestos board is originally a thermal insulation material with a very low thermal conductivity.

Reply to 2# wiseboy, but you didn’t answer the question I asked

This post was last edited by arpcd on 2011-7-21 at 13:39. How did you arrive at the figure of 10,000 times? In terms of surface heat dissipation alone, the heat transfer between the surface and the surrounding air depends on both radiation and convection. Convection is related to wind speed, the heat transfer area, and the surface temperature; it has nothing to do with the thermal conductivity of the surface material. As for radiative heat transfer, it is related to the emissivity of the surface material. For example, a surface coated with black paint has the highest emissivity, around 5.5, while polished aluminum has an emissivity of only about 0.3. The emissivity of asbestos board lies somewhere between these two values, with a maximum difference of only 20 times – it certainly isn’t 10,000 times different. So to answer your question, you first need to provide the algorithm and formulas; otherwise, it’s impossible to know how you did the calculations, let alone determine whether they are correct or not. Even in cases of convective heat transfer with large differences, such as insulating materials applied to carbon steel steam pipes with a thickness of 100 mm, replacing that 100 mm of asbestos with steel plate won’t result in a difference of 10,000 times; the thermal conductivity of steel is around 50 W/m·K, while that of asbestos is approximately 0.05 W/m·K. The heat transfer coefficient is equal to the thermal conductivity divided by the thickness, and the ratio of their thermal conductivities is only 50/0.05 = 1,000. 1,000 times is already a significant figure – I really don’t understand where the figure of 10,000 comes from. Please check the data carefully to see if you might have missed a zero

The question is as follows: The inner diameter of a regular steel pipe is 150 mm, the wall thickness is 5 mm. The temperature of the fluid inside the pipe is 100°C, while the temperature of the outer wall of the pipe is 95°C. What is the heat loss per meter of pipe length? An 40 mm thick asbestos insulation layer is applied around the steel pipe; the temperature of the outer surface of the asbestos is 40°C. What is the heat loss per meter of pipe length? The thermal conductivity of steel pipes is 45 W/(m·K), while that of asbestos is 0.1 W/(m·K). My calculation results are 21,901.5w and 86.32w respectively. But what I want to ask is, assuming the temperature of the outer surface of the steel pipe is 40°C, does that mean the wall thickness of the pipe has to be 1.8 meters?