Exergy analysis of tunnel kilns

Self Cite

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...

Section: Qf =(Wa--wgi(ra--ro)mentioning

confidence: 99%

mentioning

confidence: 98%

Utilization of energy in recuperative counterflow furnaces

Abbakumov

1980

Self Cite

“…where q=q(tw)=(z(t w) (tw--ta), W/m z; (6) r 1 is the annual operating resource of the equipment in sec/yr, ~ is the consumption of introduced energy per unit of it lost through the lining, ~ > i, Cij is the j-th item of costs related to the i-th material of the lining in rubles/m a, m is the number of items of costs taken into consideration referred to the materials (cost of material, installation, foundation, etc. ), ~(f) is a function taking into consideration amortization deductions and costs for technical servicing in i/yr (here the dimension of r corresponds to a year), and t w is the temperature of the outer surface of the lining in ~ The introduction into the cost function of the value ~ reflects the nonequivalence of the energy introduced into the furnace and that lost through the lining, which results from the energy and balance analyses of the named values [6].…”

mentioning

confidence: 99%

“…), ~(f) is a function taking into consideration amortization deductions and costs for technical servicing in i/yr (here the dimension of r corresponds to a year), and t w is the temperature of the outer surface of the lining in ~ The introduction into the cost function of the value ~ reflects the nonequivalence of the energy introduced into the furnace and that lost through the lining, which results from the energy and balance analyses of the named values [6]. The value ~ depends upon the type and operating conditions of the equipment and for high temperature tunnel kilns it is within limits of 1.5-2 (conversion of the data of [2] gives ~ -1.54, which corresponds to the determination given).…”

mentioning

confidence: 99%

Construction of economical linings of production equipment

Abbakumov¹,

Tsibin²,

Новиков³

1987

Self Cite

Effective utilization of material and energy resources is an important problem of the national economy to which much attention is being devoted at present. The problem of optimization of production linings installed in furnaces and other equipment, which are important technical, thermal engineering, and constructional functions, is one of its aspects. The lining must eliminate uncontrolled gas exchange of the furnace with the atmosphere, possess the necessary strength and chemical resistance at the working temperatures and under constant and variable thermal and mechanical loads, and have a high thermal resistance. In the final analysis these factors are integrated into the index of economy of the lining, the total combination of capital and operating costs related to it.Problems of optimization of a lining may be solved by minimization of some target function reflecting the influence of the primary production factors on costs. Depending upon the specific approach and assumptions the structure of the target function is not the same for different authors, which naturally leads to a difference in the results obtained.In [i] the target cost function I I in rubles/m 2 is taken as the sum of the cost of the energy lost during the period of the interrepalr campaign and the construction costs for thermal insulation:where q is the heat flow in W/m z tg--t a q= ; (2)is the length of the interrepair campaign, sec; Cq, Cin , and Cin , costs of heat, rubles/J, of thermal insulation, rubles/m s, and of installation of it, rubles/mZ; 61n, thickness of the thermal insulation layer, m; t s and ta, temperatures of the furnace gases and the surrounding air, ~ as, coefficient of convective heat exchange from the furnace gases to the wall, W/(m2"K), and a s -f(Cs, As, Ws, deq); ~w, coefficient of heat exchange from the wall to the surrounding medium, W/(m2"K); Cs, As, and Ws, specific heat, J/(mS'K); the thermal conductivity, W/(m'K); and the velocity, m/sec, of the furnace gases; d.q, equivalent diameter for the channel of gases, m; As thermal conductivity of the i-th layer of the lining, W/(m'K); and n, number of layers in the wall.From the condition dL =0 with constant thermophysical characteristics was determined u6i n the optimum thickness of thermal insulation ~opt in m: In the method of optimization considered the cost of the lining is taken into consideration insufficiently fully and the temperature factor, the minimized function, is decreased as the result of both positive and negative factors (for example, a decrease in q and r).

“…The absence In the book of a reference in the text to original source material makes it difficult to achieve the required accuracy; the list of references is incomplete (e. g., Table 4 was taken from [10] but this is not acknowledged). A tunnel kiln with a pulsing movement of the contents of the carriages is called an intermittent kiln (p. 237) although it is actually a continuous process and only the products emerge in batches.…”

mentioning

confidence: 99%

Book reviews

Abbakumov¹

1979

Self Cite

The book under review deals with the bringing about and the heat-engineering tests of drie:~ ~z~d kiln equipment; the planning and mastery of heat equipment; and the drawing up of plans to improve the equipment. The contents of the book have d~termined its predominantly practical nature; the authors have striven to generalize their accumulated industrial experience and to produce a tool which will be of use to specialists in the refractory industry.