example 5-2 - Example 5-2 A steam pipe with an OD of 6 in...

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Example 5-2 A steam pipe with an OD of 6 in. is at a temperature of 700°F. Two layers of insulation cover the pipe, as shown. The layer closest to the pipe can withstand high temperatures and has a thermal conductivity of 0.122 Btu/h·ft·°F. A less expensive type of insulation with a thermal conductivity of 0.0215 Btu/h·ft·°F is used for the second layer. This second layer can only withstand temperatures of 550°F or less. Air cools the exterior of the insulation with a heat transfer coefficient of 4.2 Btu/h·ft 2 ·°F. Using geometry data on the figure, a) Find the surface temperature, T 3 b) Determine whether the low temperature insulation fails under these conditions
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Solution: The relevant radii are 1 3in. 0.25ft in. 12 ft r = = 2 1 2in 0.417ft in 12 ft r r = + = 3 2 3in 0.667ft in 12 ft r r = + = The thermal resistances are o 2 1 1 1 o 0.417ft h ft F ln ln 0.666 0.25ft Btu Btu 2 2 0.122 h ft F r r R k L L L π π = = = o 3 2 2 2 o 0.667ft h ft F ln ln 3.48 0.417ft Btu Btu 2 2 0.0215 h ft F r r R k L L L π π
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