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Rules of Thumb for Mechanical Engineers Episode 2

Tham khảo tài liệu 'rules of thumb for mechanical engineers episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The equation for cylindrical coordinates is slightly different because the area changes as you move radially outward. As Figure 3 shows the temperature profile will be a sttaight line for a flat wall. The profile for the pipe will flatten as it moves radially outward. Because area increases with radius conduction will increase which reduces the thermal gradient. If the thickness of the cylinder is small relative to the radius the cartesian coordinate equation will give an adequate answer. Thermal conductivity is a material property with units of Btu hr foot F Figure 3. Temperature profile for flat wall and cylinder. Tables 3 and 4 show conductivities for metals and common building materials. Note that the materials that are good electrical conductors silver copper and aluminum are also good conductors of heat. Increased conduction will tend to equalize temperatures within a component. Example. Consider a flat wall with Thickness 1 foot Table 3 Thermal Conductivity of Various Materials at o c Material Thermal conductivity k W m- C Btu h-ft- F Metals Silver pure 410 237 Copper pure 385 223 Aluminum pure 202 117 Nickel pure 93 54 Iron pure 73 42 Carbon steel 1 c 43 25 Lead pure 35 20.3 Chrome-nickel steel 16.3 9.4 18 Or 8 Ni NonmetaUic solids Quartz parallel to axis 41.6 24 Magnesite 4.15 2.4 Marble 2.08-2.94 1.2-1.7 Sandstone 1.83 1.06 Glass window 0.78 0.45 Maple or oak 0.17 0.096 Sawdust 0.059 0.034 Glass wool 0.038 0.022 Liquids Mercury 8.21 4.74 Water 0.556 0.327 Ammonia 0.540 0.312 Lubricating oil SAE 50 0.147 0.085 Freon 12 Cd2F2 0.073 0.042 Gases Hydrogen 0.175 0.101 Helium 0.141 0.081 Air 0.024 0.0139 Water vapor saturated 0.0206 0.0119 Carbon dioxide 0.0146 0.00844 Source Holman 1 Reprinted with permission of McGraw-Hill. Area 1 foot2 Q 1 000 Btu hour For aluminum k 132 AT 7.58 F For stainless steel k 9 AT 111.1 F Sources 1. Holman J. p. Heat Transfer. New York McGraw-Hill 1976. 2. Cheremisinoff N. p. Heat Transfer Pocket Handbook. Houston Gulf Publishing .