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Show Accepting this validation as a working conclusion at this time, the equations can now be used to indicate some of the expected changes in engineering behavior, and notably the influence of stoichiometry. The governing equation of relevance here is Eq. 15. Since the multiplier on the Excess Air factor in Eq. 15 is (1 - uo,sO), there is an important change in sensitivity in the calculations from this equation to change in excess air depending on the initial (stoichiometric) value of the Intrinsic Efficiency, Uo"o. If the value of Uo .. o is near unity (as in a boiler), the multiplier is small; but it increases rapidly with decreasing value of Uo .. o. The engineering significance of this is in the impact on increased fuel rate if excess air is increased, where the impact varies widely depending on the type of thermal operation involved. For boilers, where Uo,s° is approaching unity, the increased fuel rate is quite minor. For a processing furnace, however, the impact can be substantial. For such furnaces, the value of Uo .. o may be about 0.5. For this special case, the correcting ratio: f1.o .. o/fJ.o~o = 1/(1 - E%/I00). This can be approximated by expansion of the reciprocal to two terms by: (1 + E%/I00); and this last is the correction factor used in Ref 3 to correct Lobo and Evans data for the radiant section of a petroleum heater. Consequently, the percentage increase in firing rate from stoichiometric is roughly the same as the excess air percent. Even an increase from 25% excess air to 50% excess air would require about a 20% increase in firing rate. This simple expression, or its nore accurate reciprocal form, is thus considered to be a reasonable first-approximation corrector for process furnaces when estimates are needed of the change in fuel rate with excess air, and it again emphasises the substantial penalty on fuel demand if excess air is not controlled. References 1. Hudson, lC.: The Engineer, 70,449,483,523 (1890) 2. Orrock, G.A.: Trans. A.S.M.E., 48, 218 (1926) 13 |