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Show h apacitance Heat exchangers are frequently evaluated by comparing their effectiveness to t e c, be ratio of the two streams passing through the heat exchanger, Theoretical relationshIPs can By calculated for the various types of heat exchangers I, The counter flow heat exchangers gener~h provide the greatest amount of heat transfer and the raining bed heat exchanger approaches, e perfonnance of a counter flow heat exchanger. A farru,l y of curves can be d e f'I ne d for vanfol US values of the Number of exchanger heat Transfer Units (Nl)' Figure 7 displays the counter ow heat exchanger effectiveness (E) as a function of the ratio of thennal capacities of the gas to the batchlcullet for several values of N where: lu N =A~ Iu C . nun and 1 - e -Nlu(1-C . IC ) uun max £ = ------..;;-----~:------::-- 1-( C min / C max)e -Nlu(I-CrrunICmax ) Data points from Raining Bed Heat Exchanger tests have been plotted on this chart to evaluate the perfonnance of the heat exchanger. This chart shows the perfonnance of the Tecogen cullet preheater which maintained an effectiveness consistent with a counter flow heat exchanger with an N,U of 0.7 for more than 1200 hours of testing on a furnace owned by FosterForbes in Milford, MA.2,3 The batchlcullet mixture, tested in the recent trials, displays effecti veness consistent with an N,U of 2. Variations in the N,U value of the heat exchanger result from differences in the particle size of the solids falling through the exchanger and in the active height of the exchangers. The Foster-Forbes heat exchanger had an active height about half as high as the heat exchanger used in the recent trials and was heating particles with an average diameter ten times that of the recent trial. ECONOMIC EVALUATION To detennine preheater design requirements and to evaluate furnace perfonnance with oxygen-fuel combustion and culletlbatch preheating, heat balance analyses have been perfonned for four different furnace operating baseline configurations. Table 1 summarizes the furnace operating conditions used in the evaluations. Evaluations on Tank 4 were made for an existing air-fuel fired system and for a planned oxygen-fuel fired system. Figure 8 is a sketch of the furnace/preheater configuration used in the heat balance analysis. This configuration utilizes exhaust gas recirculation from the preheater outlet to dilute and cool the hot (+25?0IlF) exhaust gases leavi~g the furnace and enteri~g the pre~eater. With oxygenfuel combustIon the exhaust gases leaVIng the furnace must be dIluted to elIminate the possibilit of overheating the batch material causing preferential volatilization and material agglomerafI onY. This arrangement is preferred over the alternate approach, which cools the hot furnace exhau t gases with air, since it minimizes the gas flow which must then be handled by downstream s handling and pollution control equipment. Additionally, on extremely efficient furnaces gas furnaces with high levels of electric boost where the exhaust gases do not contain SUff?r, on energy to preh eat t h e b atc h to I' ts maX'Imuhm pre e at temperature, t h e exhaust gas recirc II Ct'e nt con fI"g uratlon provI' de s a s I'I g h t 1y ebt ter overa1 1 e ff"I CIency. Ifa'u dI'l utI' on is used in tu adtl On exhaust gas recirculation, under these low exhaust gas conditions, the supplemental bu s ea of mers must 4 |