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30523_Ward's World+MGH Heat Transfer.2

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emission, absorption, transmission, and reflection are a func- tion of the surface of the material and are described by the ma- terial emissivity, absorptivity, transmissivity, and reflectivity. The net energy transferred by radiation is equal to the difference between the radiation emitted and the radiation absorbed by the surface. Radiation heat transfer has spectral characteristics, meaning that radiation is a function of wavelength. Radiation from solids is typically a continuous spectrum that spans many wave- lengths (for instance, from the visible to the infrared regions). As the temperature of the object increases, the radiation emis- sion shifts to shorter wavelengths associated with higher ener- gies. For example, as the temperature of an iron bar is raised to about 800 K (approximately 525°C or 1000°F), the radiation from the iron shifts from long infrared regions, which are invis- ible to the human eye, to the visible wavelength region and the iron appears to glow dark red. As the temperature is increased further, the intensity of the radiation increases and the color appears more orange/yellow. This process is also apparent in the filament of an incandescent light bulb. When the bulb is operated at a lower voltage, the light emitted from the filament appears red. As the voltage is increased, the filament tempera- ture increases and the light appears progressively more yellow. Liquids and gases absorb and emit radiation with more distinct spectral characteristics. In other words, liquids and gases can be transparent to some wavelengths of radiation and highly absorptive at other wavelengths. A key example is molecular oxygen (O2) present in the Earth's atmosphere. The O2 in the Earth's atmosphere absorbs nearly 100% of the harmful radia- tion from the Sun in the vacuum ultraviolet (<200 nm). Simi- larly, greenhouse gases such as carbon dioxide and methane are effective infrared absorbers and responsible for global warming. Many liquids, especially organic liquids, have selec- tive absorption bands in the infrared and ultraviolet regions. Design considerations The principles governing the three modes of heat transfer can be used to design and analyze systems. Proper selection and fabrication of materials can allow heat transfer for specific purposes, including cooling or heating of specific regions for steady or transient conditions. All three modes of heat transfer can be applied in one process, and conduction, convection, and radiation are commonplace in many familiar systems. In summer, the roof on a house becomes hot because of radiation from the Sun, even though a cool breeze is transferring some of the heat away from the roof by convection. At the same time, conduction transports some of the heat through the roof where it is distributed to the attic by convection. The prudent householder attempts to reduce the heat transferred to the attic by reducing the heat that is absorbed by the roof by paint- ing the roof white. The householder also installs insulation to the underside of the roof to reduce the transfer of heat through the roof. Further, the heated air in the attic can be vented through louvers in the roof. While all three modes of heat transfer may be present, one mode may dominate the total heat transfer. Importantly, con- vection and conduction heat transfer rates are both propor- tional to the temperature difference between the objects of interest. Radiation heat transfer is proportional to temperatures to the fourth power. This non-linear dependence on tempera- ture often causes radiation to surpass conduction and convec- tion heat transfer rates in high temperature systems such as fire and combustion applications. Additionally, radiation is the only mode of heat transfer present in vacuum systems. Heat exchangers Heat transfer is a goal in many industries. Devices used for the purpose of transferring heat between two substances are called heat exchangers. Some of the most prevalent types of heat exchangers use two working fluids that are physically separated by a heat-exchange surface in the form of plates or tubes. A car radiator, hot-water heater, steam or hot-water radiator, steam boiler, condenser and evaporator on a refrigera- tor or air conditioner, and even the ordinary cooking utensils in everyday use are all heat exchangers. In power plants, oil refineries, and chemical plants, two com- monly used designs are tube-and-shell and double-pipe heat exchangers. The first consists of a bundle of tubes inside a cylindrical shell. One fluid flows inside the tubes and the other fluid flows between the tubes and the shell. The double-pipe heat exchanger consists of one tube inside another tube with one fluid flowing inside the inner tube and the other flowing in the annular space between tubes. In both cases, the tube walls serve as the heat-exchange surface. Heat exchangers consisting of spaced flat plates with the hot and cold fluids flowing be- tween alternate plates are also in use. Each of these exchangers essentially depends upon convection heat transfer through the fluid on each side of the heat-exchange surface and conduc- tion through the surface. Countless special modifications, often also utilizing radiation for heat transfer, are employed for a variety of purposes. In these exchangers, the fluid streams may flow parallel concur- rently or in mixed flow. In most cases, the temperatures of the various streams remain essentially constant at a given physical location, and the process is said to be a steady-state process. As Heat Transfer (continued) + ward ' s science

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