HIGH-EFFICIENCY CERAMIC RECUPERATORS TO GLASS FURNACES

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1 METALLURGY AND FOUNDRY ENGINEERING Vol. 3, 006, No. 1 Krzysztof Karczewski * HIGH-EFFICIENCY CERAMIC RECUPERATORS TO GLASS FURNACES Notation: α convection heat-transfer coefficient to air, W/(m K) α g heat-transfer coefficient of furnace gas, W/(m K) c a heat capacity of air, kj/(m 3 K) c g heat capacity of furnace gas, kj/(m 3 K) C o constant of radiation, W/(m K 4 ) D, d diameter, m ε emissivity F heating surface area, m ϕ surface area factor k over-all heat-transfer coefficient, W/(m K) λ thermal conductivity, W/(m K) s thickness of wall, m Δp pressure drop, Pa T, t temperature, K, C Q heat transfer rate, kw V volumetric rate of fluid flow, m 3 /h, m 3 /s ξ number of heat losses * Ph.D.; Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Cracow, Poland; dabrowsk@metal.agh.edu.pl 49

2 1. INTRODUCTION Furnace gases coming from glass melting furnaces contain a high amount of thermal energy. In high-efficiency glass melting furnaces there are 50% of total fuel consumption. The furnace gases are polluted by melted glass components. Temperatures of furnaces gas come up to 100 o C 1500 o C. For heat recovery of waste gas flow from glass melting furnaces metal recuperators, ceramic recuperators and regenerators [, 3] are used. Preheating in exchangers air is used directly in combustion of fuel. Thermal efficiency and service life of heat exchangers are major importance of optimization for this furnaces. Achieving the energy saving is determined by the heat recovery system. New heat exchangers to glass melting furnaces are from 15% to 5% more efficient than of the conventional heat exchangers used in furnaces some years ago. Hot air temperatures can be achieved: in metal recuperator up to o C [3, 4], in conventional ceramic up to 900 o C [6], in high efficiency ceramic up to 150 o C [7]. The conventional ceramic recuperators comprise the following designs: Lambot [6] Hermansen [6], Chapman Stein [5] made of chamotte, sillimanite or carborundum shapes. Improvement the recuperator s designs with higher air temperature, higher thermal efficiency, smaller weight and smaller over-all dimensions makes possible manufacture of new generation of fusion cast alumina-mullite-zirconia materials [7] and izostaticly pressed magnesia materials [7].. NEW GENERATION OF MATERIALS TO CERAMIC RECUPERATORS ZM Ropczyce for many years has specialized in the manufacture of basic refractory materials, including the ones for the glass-making industry. In 00 izostaticly pressed magnesia shapes Glasmag M98G was developed and implemented. This materials to ceramic recuperators for glass melting furnaces are used. Magnesia shapes have the following major parameters: chemical analysis: MgO 98%, Al O 3 0.1%, Fe O 3 0.3%, CaO 0.8%, SiO 0.%; open porosity: 15.4%; bulk density:.97 g/cm 3 ; cold crushing strength: 58 MPa; refractoriness under load: T 0.6 > 1700 o C. Similar class of materials used to ceramic recuperators are fusion cast alumina-mullitezirconia shapes. The cast shapes have the following major parameters: chemical analysis: Al O 3 50%, ZrO 33%, SiO 14%...; open porosity: %; bulk density: 3.3 g/cm 3 ; cold crushing strength: 350 MPa; refractoriness under load: T 0.6 > 1700 o C. 50

3 New generation of shapes has the following quality: high density, dimensional accuracy, thermal conductivity are multiples of those of conventional refractories, no decrease on account of corrosion, no shinhage or expansion, no phase changes, high mechanical strength, less sensitive to thermal stresses. The function thermal conductivity temperature [4, 7] for shapes are given in Table 1. Table 1. The function thermal conductivity temperature for shapes Temperature C Alumina-mullitezircona shapes W/(m K) Magnesia shapes W/(m K) Chamotte shapes W/(m K) The advantage of fusion cast heat exchangers shapes is that they may be combined with conventional burning refractories. It makes possible to install the fusion cast elements only in the most severally stressed areas. For example: in top layers of conventional ceramic recuperator checkers or before the metal recuperator to avoid burning through. These areas are exposed to flue gases and dust. For temperature ranges in glass melting furnaces do not evaporated melted glass components: silica, compound of aluminum, calcium and magnesium. The alcalies have the high volatilaty. Fusion cast shapes will practically do not decrease on account corrosion. Magnesia shapes will not react with alcalies. On account of material costs the ceramic recuperator should be calculated by mathematical model to achieve required construction and service characteristic for minimal number of shapes. 3. MATHEMATICAL MODEL OF HIGH-EFFICIENCY CERAMIC RECUPERATOR To calculate the heat of ceramic recuperators universal balance method is used. Additionally, the mathematical model has to be taken into account: untight furnace gas side, untight air side, heating surface area ratio and thermal resistance of the shape wall. Ceramic recuperators work in different arrangement of cross-flow air and furnace gas. Typical arrangements of fluid flow in ceramic recuperators illustrates Figure 1. 51

4 The heat calculates are separate for upper and lower row of recuperator checkers. The volumetric rate of air inlet to recuperator is given by equations: a Va = V, Δ Va = Va Va 1 n (1) n number of air untight, ΔV a absolute air losses. a) b) air furnace gas Fig. 1. Typical arrangement of fluid flow in ceramic recuperators: a) air cross-flow; b) furnace gas cross-flow The volumetric rate of furnace gas outlet recuperator is Vg = Vg +Δ Va () The heat transfer rate of recuperator equals Va + Va Qa = ca( ta ta) (3) V a volumetric rate of air with untight taken into account, V a volumetric rate of air, c a heat capacity of air, t a outlet air temperature, t a inlet air temperature. 5

5 The heat transfer rate of furnace gas inlet to recuperator is Qg =ξvg c g tg (4) ξ number of heat losses, V g volumetric rate of furnace gas inlet to recuperator, c g heat capacity of furnace gas, t g inlet furnace gas temperature. The outlet furnace gas temperature is given by formula ξqa Qa tg = Vg c g (5) where c g inlet heat capacity of furnace gas. The logarithmic mean temperature equals Δt Δt Δ tm = Δt ln Δt (6) The calculations of over-all heat coefficient are performed to upper row and lower row of recuperator checkers. The convection heat transfer coefficient to air is given by equation to counter-flow across inline banks of tubes [4] 0.65 wa α= ( ta ) 0.35 (7) D t a inlet or outlet air temperature, w a velocity of air, D outer diameter of tube. The heat-transfer coefficient of furnace gas is given by equation to flow inside tube [4] 0.8 g 0. w α g = ( tg) (8) d t g inlet or outlet furnace gas temperature, w g velocity of furnace gas, d diameter of tube. 53

6 The emissivity of furnace gas [4, 6] is described by equation ε g =βco ε CO +βh O εh O Δε g (9) ε CO emissivity of carbon dioxide, ε HO emissivity of water wapour, βco, βh O, Δε g correction factors. The effective emissivity of wall is [4] ε w + 1 ε w = (10) The radiation heat-transfer coefficient of furnace gas is given by system of equations T g εg ε wc o α r = Tg Tw k Tw = Ta + ( Tg Ta) α 4 4 Tw (11) C o T g T w T a constant of radiation, furnace gas temperature, wall temperature, air temperature. The heat-transfer coefficient of furnace gas is α g = α r + α c (1) The ratio s/λ is the thermal resistance of the tube wall. The surface area factor is [1] F ϕ= F a1 g1 (13) F a 1 heat surface area of shape to air side, F g 1 heat surface area of shape to furnace gas side. 54

7 The over-all heat-transfer coefficient to inlet or outlet of recuperator equals 1 k = ϕ ϕs α λ α g (14) The mean heat-transfer coefficient is k k + k = (15) The heat surface area of recuperator is given by equation Qa F = k Δ t (16) m The number of shapes in recuperator is F N = F a 1 (17) The number of rows in height of recuperator is given by equation Z = N { x y} (18) where { x y} arrangement of tubes in recuperator layer. Mathematical model is applied to calculations of the ceramic recuperator made of magnesia shapes. 4. THE CALCULATION OF CERAMIC RECUPERATOR TO GLASS TANK The ceramic recuperator uses a heat of waste gas from glass tank. Preheating in recuperator air is used directly in combustion of fuel. The recuperator works in cross-flow (Fig. 1a) and has the following parameters: volumetric rate of air V a = 1000 m 3 /h (0.78 m 3 /s), inlet air temperature t a = 0 o C, outlet air temperature t a C, volumetric rate of furnace gas inlet to recuperator V g = 100 m 3 /h (0.33 m 3 /s), inlet furnace gas temperature to recuperator t g = 1300 o C, chemical composition of furnace gas CO 10%, H O 18%, number of heat losses ξ =

8 The recuperator made of magnesia shapes by Figure, with the following dimensions: outer diameter of tube D = m, inner diameter of tube d = m, thickness of tube wall s = 0.01 m, total height h = 0.31 m, height of heat element h 1 = 0.3 m, pitch of shapes p = 0.11 m. Fig.. Magnesia M98G recuperator shape The volumetric rate of air inlet to recuperator is V 1000 V a a = = = n m 3 /h = 0.37 m 3 /s, untight of air channel n = 15%. The absolute untight of air equals Δ Va = Va Va = = 176 m 3 /h. 56

9 The volumetric rate of furnace gas outlet from recuperator is V g = = 1376 m 3 /h = 0.38 m 3 /s. Substituting the values into equation (3) we obtain heat transfer rate of recuperator Va + Va Qa = ca( ta ta) = 1.34 ( ) = 35.1 kw. The heat transfer rate of furnace gas inlet to recuperator is Qg = Vg cg tg = = kw. Substituting the values into equation (5) we obtain outlet furnace gas temperature ξqg Qa o tg = = = 761 C. Vg cg The logarithmic mean temperature equals Δt Δt Δ tm = = = 71 K, Δt 741 ln ln Δt 700 inlet Δ t = = 700 o C, outlet Δ t = = 741 o C. The recuperator row of shapes in layer is { x y} = { } 87. The calculations of over-all heat-transfer coefficient upper row The convection heat-transfer coefficient to air is 0.65 w 0.78 ( a α= + ta ) = ( ) = 11.1 W/(m K), D t a air temperature, t a = 600 o C, w a velocity of air

10 w a = Va Va f = 7 ph = = m/s, a D outer diameter of tube, D = m. 1 The convection heat-transfer coefficient of furnace gas equals wg 0.36 α c = ( t g) = ( ) = W/(m K), d t g temperature of furnace gas t g = 1300 o C, w g velocity of furnace gas w g Vg Vg 0.33 = = = = 0.36 f g Πd i Π m/s, d i diameter of tube d i = m. The emissivity of furnace gas by formula (9) is: CO per cent in furnace gas 10%; H O per cent in furnace gas 18%; layer thickness s = 0.9 d i = = 0.13; emissivity of carbon dioxide: ( p s) = = 1.3 Pa m CO t g = 1300 o C, ε CO = 0.04, p s emissivity of water vapour: ( ) = =.3 Pa m HO t g = 1300 o C, ε HO= 0.05, correction factors: β CO = 1.0, β H O = 1.05, Δε g = 0, ε g =βco ε CO +βh O εh O Δε g = = 0.066; 58

11 The effective emissivity of wall is ε w = = 0.9. The radiation factor is εg ε w Co = = 0.34 W/(m K 4 ). The radiation heat-transfer coefficient of furnace gas is given by system of equations: Tg εg ε wc o α r = Tg Tw k Tw = Ta + ( Tg Ta ) α 4 4 Tw, 8.0 Tw = 873+ ( ) = 1378K, 11.1 T g furnace gas temperature, T g = 1573 K (1300 o C), T a air temperature, T a = 873 K (600 o C), T w wall temperature, T w = 1378 K (1105 o C), ( 4 4 ) α r = = 43.9 W/(m K) The heat-transfer coefficient of furnace gas is α g = α c + α r = = 48.8 W/(m K). The thermal resistance of the tube wall equals s 0.01 = = (m K)/W, λ

12 thickness of the wall s = 0.01 m, thermal conductivity of magnesia shapes λ 1088 = 3.1 W/(m K). The heat surface area of shape is: air side Fa =π d 1 n h1 =π = m ; furnace gas side Fg =π d 1 i h1 =π = m ; surface area factor F ϕ= = = 1.8. F a1 g1 The over-all heat-transfer coefficient is 1 1 k = 8.0 ϕ ϕ s 1 = = αg λ α W/(m K). The calculations of over-all heat-transfer coefficient lower row The convection heat-transfer coefficient to air is 0.65 w 1.8 ( a α= + ta ) = ( ) = 13.4 W/(m K), D t a air temperature, t a = 0 o C, w a velocity of air w a Va Va 0.37 = = = = 1.8 m/s, f 7 ph a D hydraulic diameter D = m. 1 The convection heat-transfer coefficient of furnace gas is wg 0.4 α c = ( tg ) = ( ) = 4.3 W/(m K), d

13 t g furnace gas temperature, t g = 761 o C, w g velocity of furnace gas w g Vg Vg 0.38 = = = = 0.4 f g 56πdi 56 π d i diameter of tube d i = m. m/s, The emissivity of furnace gas to formula (9) equals: emissivity of carbon dioxide: ( ps) = 1.3 Pa m, t CO g = 761 o C, ε CO = 0.05; emissivity of water vapour: ( p s) =.3 Pa m, t g = 761 o C, ε HO= 0.04; HO correction factors: β CO = 1.0, β H O = 1.05, Δε g = 0; ε g =βco ε CO +βh O εh O Δε g = = The effective emissivity of wall is ε w = = 0.9. The radiation factor is εg ε w Co = = 0.47 W/(m K 4 ). The radiation heat-transfer coefficient of furnace gas is: Tg εg ε wc o α r = Tg Tw k Tw = Ta + ( Tg T a) α 4 4 Tw, 6.3 Tw = 93+ ( ) = 641 K,

14 T g furnace gas temperature, T g = 1034 K (761 o C), T a air temperature, T a = 93 K (0 o C), T w wall temperature, T w = 641 K (369o C), ( 4 4 ) α r = = W/(m K). The heat-transfer coefficient of furnace gas is α g = α c + α r = = 16 W/(m K). The thermal resistance of the tube wall equals s 0.01 = = λ 4.6 (m K)/W, where thermal conductivity of magnesia shape λ 343 = 4.6 W/(m K). The over-all heat-transfer coefficient is 1 1 k = 6.3 ϕ ϕs 1 = = α λ α g W/(m K). The mean heat-transfer coefficient is k k + k = = = 7. W/(m K). The heat surface area of recuperator is given by equation (16) F = Qa k Δ = t = m. m 6

15 The number of shapes in recuperator equals F 43.5 N = = = 333, admited 336. F a The number of rows on height of recuperator is given by equation (18) 336 Z = = The heat exanger chamber over-all dimensions are the following: length =.4 m, width =.1 m, height =.4 m. The heat and geometric parameters of recuperator are given in Table. The ceramic recuperator to glass tank using Glasmag M98G materials illustrates Figure 3. Table. Parameters of ceramic recuperator Parameter Upper row Lower row Furnace gas temperature, C Air temperature, C Volumetric rate of furnace gas, m 3 /h Volumetric rate of air, m 3 /h Velocity of furnace gas, m/s Velocity of air, m/s Convection heat-transfer coefficient to air, W/(m K) Heat-transfer coefficient of furnace gas, W/(m K) Thermal resistance, m K/W Over-all heat-transfer coefficient, W/(m K) Wall temperature, C Heat transfer rate, kw 35.1 Logarithmic mean temperature, K 71 Mean heat-transfer coefficient, W/(m K) 7. Surface heat area, m 45.3 Recuperator checkers {8 7 6} Number shapes

16 Fig. 3. Ceramic recuperator to tank furnace 5. INFERENCES Ceramic recuperators using energy of flue gases from glass melting furnaces on account of high-temperature, dust, and thermal shocks work in very difficult conditions. The service life of glass melting furnaces is determined by failure-free operation and the effectiveness of recuperators. Mathematical model of high-efficiency ceramic recuperator using new generation of shapes makes possible to define recuperator s design and service characteristics: overall dimensions, heat surface area, recuperator checkers, heat transfer ratio, temperatures of fluids and walls. The heat exchangers using Glassmag M98G shapes need to be made entirely of magnesia materials. The advantage of cast shapes heat exchangers is that they may be combined with chamotte or sillimanite materials. It makes possible to install cast materials only in the most severally stressed areas. High efficiency ceramic recuperators using magnesia materials or casted alumina-mullite-zirconia materials are from 15 to 5% more thermal efficient than of conventional ceramic heat exchangers. In glass melting furnaces the heat exchangers are capable to provide to the user a reduction of total fuel consumption about 15 5%. 64

17 REFERENCES [1] Hobler T.: Ruch ciep³a i wymienniki. WNT, Warszawa, 1979 [] Karczewski K.: Metallic Recuperators to Glass Tanks. Kosice, Technical University of Kosice, 1998, 83 87, ISBN [3] Karczewski K.: Foundry and Metallurgy Engineering, 9 (003) [4] Karczewski K.: Obliczenia cieplne rekuperatorów metalowych dla pieców przemys³owych. UWND AGH, Kraków, 004, SU 1667 [5] Redr M.: Vyu iti odpadniho tepla v hutnictvi eleza. Praha, SNTL 1971 [6] Senkara T.: Obliczenia pieców grzewczych w hutnictwie elaza. Œl¹sk, Katowice, 1968 [7] Catalog: Glass-Making Industry. ZM Ropczyce, 004 [8] Catalog: Heat Exchangers. Motim Hungary, 199 Received December

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