Design of Air Pre-Heater for Blast Furnace Gas Fired Boiler R.Venkateshkumar, Kishor Kumar, B.Prakash, R.Rahul Department of Mechanical Engineering, M.Kumarasamy College of Engineering, Thalavapalayam, Karur 639 113. Tamilnadu. India Abstract In most of the boiler, the input energy is not fully converted into useful output energy. This paper is about to decrease the fuel consumption of the boiler thereby increasing the combustion air temperature. By proper designing of waste heat recovery system we can increase the overall efficiency of the boiler and also improve the controllability of the system. As per BEE evaluation, an i ncrease of 20 o C in the air temperature increases the boiler efficiency by 1 %. This paper is about to increase the efficiency of the boiler, thereby reducing the fuel cost to lower the amount that is spent for the purchase of fuel. This project is focused on study of performance of the boiler, possibilities to improve efficiency and study of air pre-heater.. An air preheater is incorporated in the Blast furnace gas fired boiler that is made use of in power plant. The heat required for the functi oning of air pre heater is obtained from the heat content of the Blast furnace flue gases. Keywords Air pre-heater; Blast Furnace Gas fired boiler I. Introduction Heating combustion air can raise boiler efficiency about 1% for every 22 C in temperature increase. The most common way to preheat the air is with a heat exchanger on the flue exhaust. There are two types of air pre-heaters for use in steam generators in thermal power stations One is a tubular type built into the boiler flue gas ducting and the other is a regenerative air pre-heater. These may be arranged so the gas flows horizontally or vertically across the axis of rotation. Another type of air pre-heater is the regenerator used in iron or glass manufacture. Many new circulating fluidized bed (CFB) and bubbling fluidized bed (BFB) steam generators are currently incorporating tubular air-heaters offering an advantage with regards to the moving parts of a rotary type. With the increasing price of fuel and technology improvements, the size of a boiler that can be economically equipped with a pre-heater should become smaller. Although still a technology most applicable to large boilers, high energy prices will certainly motivate innovative new applications for economical combustion air pre-heaters on ever smaller boilers. Air heaters can also use extraction steam or other sources of energy depending upon the particular application. The hot air produced by air heaters enhances combustion of all fuels and is needed for drying and transporting the fuel in pulverized coal-fired units. Industrial units fire variety of fuels such as wood, sewage sludge, industrial waste gases as well as coal, oil and natural gas. In the small units tubular, plate and cast iron heaters are widely used. Fuels fired on stoker grates, such as bituminous coal, wood and refuse, don t require high air temperatures, therefore water or steam coil air heaters can be used. Tubular pre-heaters consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. Inside the ducting, the hot furnace gases pass around the pre-heater tubes, transferring heat from the exhaust gas to the air inside the pre-heater. Regenerative air heaters are relatively compact. Air to gas leakage can be controlled by cold-pre-setting axial & radial seal plates to minimize gaps at the hot operating conditions, or using sacrificial material. Computational studies are performed to investigate the effect of various operating parameters on entrained flow through air pre-heater tubes.cfd is a versatile tool. A large variety of problems with different levels of complexity can be solved. In a typical tubular air heater, energy is transferred from the hot flue gas flowing inside many thin walled tubes to the cold combustion air flowing outside the tubes. The unit consists of a nest of straight tubes that are roll expanded or welded into tube sheets and enclosed in a steel casing. In the vertical type tubes are supported from either the upper or lower tube sheet while the other (floating) tube sheet is free to move as tubes expand within the casing. An expansion joint between the floating tube sheet and casing provides an air/gas seal. Intermediate baffle plates parallel to the tube sheets are frequently used to separate the flow paths and eliminate tube damaging flow induced vibration. Carbon steel or low alloy corrosion resistant tube materials are used. 53
The tubes which range from 38 to 102 mm in diameter and have wall thicknesses of 1.24 - to 3.05 mm.larger diameter, heavier gauge tubes are used when the potential for tube plugging and corrosion exists. The most common flow arrangement is counterflow with gas passing vertically through the tubes and air passing horizontally in one or more passes outside the tubes. A variety of single and multiple gas and air path arrangements are used to accommodate plant layouts. 1.1 Air Pre-Heater Introduction Air Pre-heater has an important role to improve the efficiency of the high thermal of the steam boiler by utilizing the thermal energy with low temperature flue gas from the steam boiler before being discharged into atmosphere. Air Pre-heater doing heat transfer between flue gas with combustion air. Air Pre-heater cooling the flue gas air for every 40o F (22o C) will increase the overall steam boiler efficiency is about 1%. With the utilization of heat from flue gas combustion air will be hotter so it can save fuel requirement needed. Air Pre-heater requires a large amount of heat transfer surface per unit of heat recovered because of the relatively small difference between the flue gas temperature and temperature of combustion air. Air Pre-heater is typically located behind steam boiler, where it receives hot flue gas form the economizer and cold combustion air from the force draft fan. The hot air produced by Air Pre-heater to enhance combustion of all fuel and is needed for drying fuel like coal in stoker steam boiler. The Air Pre-heater is usually the last heat trap in the steam boiler. Air Pre-heater exit gas temperature should be higher than the corrosion limit of the AH internals and the downstream equipment. Air heaters generally operate in a temperature range of 450 120 C on the gas side and 150 450 C on air side Air Pre-heater Types On the basis of construction features, AHs can be classified as shown in Table 1. 1. The heat transfer in tubular AH (TAH) is direct as in any HX. 2. In rotary AHs (RAHs), it is indirect. The hot gases heat a rotor filled with baskets of corrugated sheet steel, which then moves into the cold-air stream to give up heat in a moving rotor design. The heat exchange is, thus, through the medium of steel baskets subjected to hot and cold air streams alternately on a continuous basis. In the rotating hood design, the rotor remains stationary and the hoods for gas and air rotate to achieve the same results. Table 1. Air Pre-heater Classification Action Recuperative Regenerative Type Tubular air Pre-heater 1. Vertical tubes 2. Horizontal tubes Rotary air Pre-heater 1.. Rotating baskets (Ljungstrom) 2. Rotating hood (Rothemuhle) 1.2 Performance Analysis Of Air Pre-Heater Temperature of flue gas inlet, thi = 180 C Temperature of flue gas outlet, tho = 140 C Temperature of air inlet, tci = 40 C Temperature of air outlet, tco = 150 C LMTD=([thi-tco]-[tho-tci])/(ln[(thi-tco)/(tho-tci)]) =([180-150]-[140-40])/(ln[(180-50)/(140-40)]) LMTD=58.14 K 54
Overall heat transfer co-efficient =29W/m 2 K (Assume) Outer diameter of tube, do =0.065 m Inner diameter of tube, di =0.060 m Thickness of tube, t =0.0025 m Air mass velocity =8 kg/sm 2 Density of air, ρ =0.815 kg/m 3 Provisional area(a): mcp T =UA (LMTD) 8.76 x 1.004 x (150-40 ) =0.029 x A x 58.14 A =573.79 m 2 To find Number of tubes (n): vg 1 =RT 1 /p =(0.287 x 453)/101.325 vg 1 =1.283 m 3 /kg n=(mg x 4 x νg1)/(π x di^2 x vg 1 ) =(20.218 x 4 x 1.283)/(π x 0.060^2x 14) n=655 tubes To find Length of tubes (L): A=nπdoL L=A/nπdo L=573.79/(655 x π x 0.065) L=4.3 m Single tube cross-sectional Area=π/4do 2 =π/4 x 0.0652 =0.003318 m 2 All tube cross sectional area =0.003318 x 655 =2.173 m 2 2. Properties Of Air: Viscosity of air,µ =24.5 x 10-6 Ns/m 2 Thermal conductivity of air, Kf=0.0364 w/m K Specific heat of air, Cp=1017 J/kg K Heat transfer factor(jk)=0.0028 Reynolds number, Re=ρVdo/µ Re=(0.815 x9.815 x 0.065)/(24.5 x 10-6) Re= 21222.4 Prandtl number,pr=(µcp)/k =(24.5 x 10-6 x 1017)/0.0364 Pr =0.6845 Tube side heat transfer co-efficient(hi): Nusselt number,nu= (hi x di)/kf Nusseltnumber,Nu=jnRePro.33(µ/(µw))0.14 (µ/(µw)) =1 hi=(0.0028x21222.4x(0.6845)0.33x1x0.0364)/0.060 55
hi=31.8 w/m 2 K PRESSURE DROP IN TUBE SIDE( Pt): Pt =Np[8jf(L/di) (µ/ (µw))^(-m) +2.5] (ρu^2)/2 Where Pt is tube side pressure drop in N/m2 L is length of tube = 4.3 m Np is No. of tube side passes = 1 u is tube side velocity= 9.815 m/s di is inner diameter= 0.060 m jf = 0.0028 -m = 0.14 Pt =161.15 N/m 2 Pt =15.95 mmwc 3. Shell Side Co-Efficient Properties Of Flue Gas: Density of flue gas, ρ=0.2776 kg/m3 Specific heat of flue gas, Cp=1307.9 J/kg K Viscosity of flue gas,µ=0.000047 Ns/m2 Thermal conductivity of flue gas, Kf=0.0787 w/m K Prandtl number,pr =(µcp)/k =(0.000047 x 1307.9)/0.0787 Pr =0.78108 Cross sectional area =length of pipe x thickness of pipe =4.3 x 0.0025 =0.01075 m 2 Velocity of flue gas=(mass flow rate of flue gas)/(density of flue gas x area of cross section) =11.458/(0.2776 x 0.01075) =3839.55 m/s Reynolds number, Re=ρVdo/µ =(0.2776x3839.55x0.065)/(0.000047 ) Re = 1474060.43 SHELL SIDE HEAT TRANSFER CO-EFFICIENT (ho): Nusselt number,nu=(ho x di)/kf Nusseltnumber,Nu=(c(a/b)^p Re^mPr^n(Pr/Prd)^0.25 x Kf)/di c(a/b)^p=0.27 Pr = 0.729164 Prd = 0.8792 m = 0.63 n = 0.36 ho=(0.27x(1474060.43)^0.63x(0.7810839)^0.36x (0.739164/0.8792)^0.25 x 0.0787)/0.060 ho = 2379.07 w/m 2 K 4. Overall Heat Transfer Co-Efficient: 1/Uo=1/ho+1/hod+(doln(do/di))/(2kw)+(do/di)x1/hid+(do/di)x1/hi Where 56
COLD AIR IN HOT AIR OUT South Asian Journal of Engineering and Technology Vol.2, No.23 (2016) 53 59 Uo is the overall heat transfer co-efficient on the outside area of tube ho is the outside film co-efficient = 2379.07 w/m 2 K hi is the inside film co-efficient = 31.8 w/m 2 K hod is the outside dirtco-efficient = 10000 w/m 2 K kw is the thermal conductivity of tube wall material = 40 w/m 2 K di is the inside diameter of tube = 0.060 m do is the outside diameter of tube = 0.065 m 1/Uo= 1/2379.09+ 1/10000+ (0.065ln (0.065/0.060))/ (2 x 40)+ (0.065)/0.060 x 1/10000+ (0.065)/0.060 x1/31.8 =4.203 x 10-4 + 1 x 10-4 + 6.503 x 10-5 + 1.0833 x 10-4 + 0.03406 1/Uo = 0.03482 Uo = 28.71 ~ 29 w/m 2 K It is concluded that both the values of calculated overall heat transfer value is equal to assumed overall heat transfer value. So, the pipe selection is correct. 5. Air Pre-Heater Design Parameters: Outer diameter of tube, do 0.065 m Inner diameter of tube, di 0.060 m Thickness of tube, t 0.0025 m Provisional Area, A 573.795 m 2 LMTD 58.14 K Number of tubes, n 655 tubes Length of tube, L 4.3 m Single tube cross-sectional Area 0.003318 m 2 All tube cross sectional area 2.173 m 2 Pressure drop in tube side( pt) Tube side heat transfer co-efficient(hi) Shell side heat transfer co-efficient(ho) Overall heat transfer co-efficient(uo)f 15.95 mmwc 31.8 w/m 2 K 2379.07 w/m 2 K 29 w/m 2 K 6. Different Views Of Aph: a)top VIEW FLUE GAS IN 57
b) side view of aph 7. Conclusion In order to increase the efficiency, a brief study is done to recover the heat by utilization of flue gases. An air pre-heater is designed so that the inlet air is pre-heated by using exhaust gas heat which directly increases the efficiency of the boiler. The pre-heater is located in the exhaust flue gas duct connected by flange. The primary air is diverted to one end as input, where the air is pre-heated and passes to the combustion chamber. By implementing air pre-heater, the Blast furnace gas cost is saved up to Rs 8359562.88 /- for one year and the burning rate of fuel is increased and increase in boiler efficiency is achieved. 8. References: 58
[1] Staseik J.A., Experimental studies of heat transfer and fluid flow across undulated heat exchanger surfaces, Int. J. Heat Transfer. Vol.41 Nos. 6-7, 1998, pp. 899-914. [2] T. Skiepko, Effect of reduction in seal clearances on leakages in a rotary heat exchanger, Heat recovery system CHP 9 (6), 1989, pp. 553-559. [3] Rakesh Kumar & Sanjeev Jain, Performance Evaluation of air pre heater at off design condition, Dept of Mech. Engg., IIT, New Delhi, pp.1-4. [4] Steam Book, The Babcock & Wilcox Company, 2006, pp.20-7. [5] Donald Kern, Process Heat Transfer, 2004 Tata McGraw-Hill Publication, pp. 701. [6] Rodney R. Gay, Power Plant Performance monitoring, 2004, pp.433 59