Heat transfer radiation exchange

There are three heat-transfer modes, ie, conduction, convection, and radiation, each of which may play a role in the selection of a heat exchanger for a particular appHcation. The basic design principles of heat exchangers are also important, as are the analysis methods employed to determine the right size heat exchanger. 

The simplest configuration for a recuperative heat exchanger is the metallic radiation recuperator (Fig. 27-57). The inner tube carries the hot exhaust gases and the outer tube carries the combustion air. The bulk of the heat transfer from the hot gases to the surface of the inner tube is by radiation, whereas that from the inner tube to the cold combustion air is predominantly by convection. 

The subject of heat transfer refers to the process by which energy in the form of heat is exchanged between objects, or parts of the same object, at different temperatures. Heat is generally transferred by radiation, convection, or conduction, processes that may occur simultaneously. 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

Heat transfer in the furnace is mainly by radiation, from the incandescent particles in the flame and from hot radiating gases such as carbon dioxide and water vapor. The detailed theoretical prediction of overall radiation exchange is complicated by a number of factors such as carbon particle and dust distributions, and temperature variations in three-dimensional mixing. This is overcome by the use of simplified mathematical models or empirical relationships in various fields of application. 

In many of the applications of heat transfer in process plants, one or more of the mechanisms of heat transfer may be involved. In the majority of heat exchangers heat passes through a series of different intervening layers before reaching the second fluid (Figure 9.1). These layers may be of different thicknesses and of different thermal conductivities. The problem of transferring heat to crude oil in the primary furnace before it enters the first distillation column may be considered as an example. The heat from the flames passes by radiation and convection to the pipes in the furnace, by conduction through the... 

Siegel, R. and Howell, J. R. Thermal Radiation Heat Transfer, 2nd edn (McGraw-Hill, New York, 1981) Sparrow, E. M. and Cess, R. D. Radiation Heat Transfer (Hemisphere Publishing, New York, 1978) Taylor, M. (ed.). Plate-fin Heat Exchangers Guide to their Specification and Use (HTFS, Harwell, 1987). Tohloukian, Y. S. Thermophvsical Properties of High Temperature Solid Materials (Macmillan, New York. 1967)... 

Dunkle, R.V. Radiation Exchange in an Enclosure with a Participating Gas in Rosenhow. W M. and Hartnett, J.P.. eds. Handbook of Heat Transfer (McGraw-Hill, New York, 1973)... 

Convective heat exchange, natural or forced Radiation heat transfer, e.g. in furnaces Evaporation Condensation... 

The vial heat transfer coefficient is the sum of heat transfer coefficients for three parallel heat transfer mechanisms (1) direct conduction between glass and shelf surface at the few points of actual physical contact, Kc (2) radiation heat exchange, Kr, which has contributions from the shelf above the vial array to the top of the vials, Krt, and from the shelf upon which the vial is resting, Krb and (3) conduction via gas-surface collisions between the gas and the two surfaces, shelf and vial bottom, Kg ... 

At present, waste heat exhausted from the ICE is removed with any efficient radiator system through direct apparent heat exchanging. On the contrary, organic chemical hydrides can recuperate the chemical energy of endothermic reaction heat during exhausted heat removal. Heat transfers accompanying the phase change of evaporation and condensation of aromatic products and unconverted reactants will certainly facilitate the removal of heat from the ICE parts, with adoption of any new radiator system compelled. 

Boundary layer models take a similar approach but attempt to extend the parameterization of gas exchange to individual micrometeorological processes including transfer of heat (solar radiation effects including the cool skin), momentum (friction, waves, bubble injection, current shear), and other effects such as rainfall and chemical enhancements arising from reaction with water. 

Radiation heat transfer is usually not important in ordinary heat exchanger design and analysis, unless significant temperature differences are present. 

In process operations, simultaneous transfer of momentum, heat, and mass occur within the walls of the equipment vessels and exchangers. Transfer processes usually take place with turbulent flow, under forced convection, with or without radiation heat transfer. One of the purposes of engineering science is to provide measurements, interpretations and theories which are useful in the design of equipment and processes, in terms of the residence time required in a given process apparatus. This is why we are concerned here with the coefficients of the governing rate laws that permit such design calculations. 

Heat can be defined as a portion of the total energy flow across a system boundary and is caused by a temperature difference between the system and the surroundings. Heat can be exchanged by conduction, convection and/or radiation. We can evaluate heat transfer by use of the energy balance, which will be discussed later. 

In a series of papers, Derby and Brown (144, 149-152) developed a detailed TCM that included the calculation of the temperature field in the melt, crystal, and crucible the location of the melt-crystal and melt-ambient surfaces and the crystal shape. The analysis is based on a finite-ele-ment-Newton method, which has been described in detail (152). The heat-transfer model included conduction in each of the phases and an idealized model for radiation from the crystal, melt, and crucible surfaces without a systematic calculation of view factors and difiuse-gray radiative exchange (153). 

Heat transfer from a process requiring controlled cooling takes place by conduction, convection, and radiation, although radiation plays a minor role with most heat exchangers. Conduction of the heat away from the process can often be achieved most practically by the use of a heat exchanger, where a heat-conducting metal wall separates the cooling water from the hot process liquid, gas, or vapor. 

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