30079_63b - Fig 63.26 End correction for regenerator heat...

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Fig. 63.26 End correction for regenerator heat transfer calculation using symmetrical cycle theory 27 (courtesy Plenum Press): A = 4HS(T + r,) = reducedlength U C 1 C + ^W 1 W 12H 0 (7" C -f T w ) ^ ^ TT = 5 ^ — = reduced period Cp 5 C/ 1 [1 0.1dl U ° = 4U + J where T w , T 0 = switching times of warm and cold streams, respectively, hr S = regenerator surface area, m 2 U 0 = overall heat transfer coefficient uncorrected for hysteresis, kcal/m 2 • hr • 0 C U = overall heat transfer coefficient C w , C 0 = heat capacity of warm and cold stream, respectively, kcal/hr 0 C c = specific heat of packing, kcal/kg 0 C of = particle diameter, m p s = density of solid, kg/m 3 phases are well distributed in the flow stream approaching the distribution point. Streams that cool during passage through an exchanger are likely to be modestly self-compensating in that the viscosity of a cold gas is lower than that of a warmer gas. Thus a stream that is relatively high in temperature (as would be the case if that passage received more than its share of fluid) will have a greater flow resistance than a cooler system, so flow will be reduced. The opposite effect occurs for streams being warmed, so that these streams must be carefully balanced at the exchanger entrance. 63.4 INSULATIONSYSTEMS Successful cryogenic processing requires high-efficiency insulation. Sometimes this is a processing necessity, as in the Joule-Thomson liquefier, and sometimes it is primarily an economic requirement, as in the storage and transportation of cryogens. For large-scale cryogenic processes, especially those operating at liquid nitrogen temperatures and above, thick blankets of fiber or powder insulation, air
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Fig. 63.27 A T limitation for contaminant cleanup in a regenerator. or N 2 filled, have generally been used. For lower temperatures and for smaller units, vacuum insulation has been enhanced by adding one or many radiation shields, sometimes in the form of fibers or pellets, but often as reflective metal barriers. The use of many radiation barriers in the form of metal- coated plastic sheets wrapped around the processing vessel within the vacuum space has been used for most applications at temperatures approaching absolute zero. 63.4.1 Vacuum Insulation Heat transfer occurs by convection, conduction, and radiation mechanisms. A vacuum space ideally eliminates convective and conductive heat transfer but does not interrupt radiative transfer. Thus heat transfer through a vacuum space can be calculated from the classic equation: q = 0-AF 12 (T 4 , - T 4 ) (63.10) where q = rate of heat transfer, J/sec cr - Stefan-Boltzmann constant, 5.73 X 10~ 8 J/sec • m 2 • K F 12 = combined emissivity and geometry factor T 19 T 2 = temperature (K) of radiating and receiving body, respectively In this formulation of the Stefan-Boltzmann equation it is assumed that both radiator and receiver are gray bodies, that is, emissivity e and absorptivity are equal and independent of temperature. It is also assumed that the radiating body loses energy to a totally uniform surroundings and receives energy from this same environment.
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