Recuperative counterflow furnaces (RCF) are the principal thermotechnical units in which ceramic materials are fired. They include tunnel, shaft, rotary, drum, and other furnaces, which are widelyused in the production of refractories, cement, and building materials.Despite the wide variety of design solutions, in all these furnaces there is a single heat exchange scheme which is ideal from the heat engineering point of view --counterflow with inversion of heat exchange between the heat transfer agents in the region of maximal temperatures. As a result of the use of this scheme, which is insufficiently realized in practice [1], the heat expended on heating the material (in the parts of the preheating and firing zones preceding the inversion point) is recovered by the air in the cooling section which operates as a recuperator, and the material and gases emerge from the furnace at low temperatures. This has the result that a thermodynamically ideal RCF operating in steady conditions can heat products with mutually compensated exothermic and endothermic effects without the expenditure of fuel [1, 2] --something which cannot be achieved by any other scheme of heat exchange. Recuperative counterflow furnaces have taken a leading part in world practice of firing ceramic materials; other furnaces such as batch-type units are used only when RCF are less suitable owing to the low throughput or complex heat treatment technology.The efficiency of a furnace as a thermodynamic unit has been assessed in the literature in various ways; the indices employed for this purpose are not universal and remain controversial. Most frequently used is the heat utilization efficiency ~ hue' which is the ratio of the power absorbed by the technological material to the power input [3, 4]: qm-~-qpc, ~l hue
VhQ1(1)where qm" is the heat absorbed by the technological material, in kJ/kg; qpc, total thermal effect of physicochemical processes occurring in the material, in kJ/kg; Vf, specific fuel consumption in m3/kg; and Q~, lower heat of combustion of the fuel in kJ/m 3 (for solid or liquid fuel, cubic meters are replaced by kilograms). Equation (1) cannot be applied to RCF, because in these plants qm" determines not the useful energy but the wasted energy lost with the discharged product. In operating RCF this item in the heat balance is small [5,6], and this is their undoubted advantage. We cannot use Eq. (1) by eliminating the term qm", because qpc > 0 leads to ~hue > 0, and this has no physical meaning. There is also no support for an attempt to interpret qm" as the heat content of the material at maximum firing temperature, because part of the heat is utilized in the cooling zone, so that we can have a case in which qm" + qpc > VhQ/c, i.e., ~hue > 1.On the basis of an ideal furnace model, in [7] the author suggested a method of thermodynamic assessmerit of RCF on the basis of the exergetic characteristics, which are an alternative to the energy approach, based purely on the starting point of thermodynamics. Since this approach has both suppo...
