Ducting Optimization A Case Study

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1 C. Hemalatha 1, T. Radhakrishnan 2 & K. Sankaranarayanasamy 3 1&2 Bharat Heavy Electricals Limited, Tiruchirappalli, India 3 National Institute of Technology, Tiruchirappalli, India hema@bheltry.co.in, ksnsamy@nitt.edu Abstract - Ducting system in a power plant is a conduit connecting one system to another through which either cold air or hot air or flue gas with ash is conveyed for continuous operation of the plant. To perform this function properly, careful design of ducting is required to take care of the stresses and forces exerted by various loadings and thermal expansion of the ducting and equipment it connects under given pressure and temperature. Ducting can be of any configuration viz. square, rectangular, circular, straight or bend, elbow or any other irregular shape. The design of large air and gas ducts is an extremely complex task which should meet both mechanical and structural design criteria. The purpose of ducts is to efficiently convey the air or flue gas from one location to another while maintaining the pressure drop and temperature drop as low as possible. This paper discusses about the optimization in the design engineering of the duct system. Keywords - CFBC, Primary Air, Secondary Air, Duct, Stiffeners, aspect ratio, optimum, weight, cost I. INTRODUCTION Engineering Design optimization both shortens design cycle time and finds new designs that are not only feasible, but also optimal based on the design criteria. Traditional engineering design processes involve strategies such as trial and error, use of previous experience etc., until the requirements are either met or changed to fit the performance. Often, the process is time consuming and does not produce the best design but just a feasible one. Since the process of design and engineering of ducting is time consuming, the process is automated by developing software. The software generates complete set of manufacturing and erection drawings for Circulating Fluidised Bed Combustion Boiler [1]. For the development of software the duct design is required to be parameterised and optimised. Engineering Design Optimization can both reduce the cycle time for the design iterations and find the best (optimal) design for the specifications. This process differs from the traditional process in that the iteration loop is computerized. An optimization problem is posed for which the design variable, the design objective and all constraints are specified. The analysis code performs the test phase of the iteration loop. The optimizer may function by perturbing each design variable to determine how each affect the performance and then seek a solution that optimizes the objective. While considering the design engineering of ducting system, the analysis can be done in various areas like fixing duct configuration or sizing, selection of material thickness, sizing, selection of expansion joint, maximum size for shipping etc., Among these, this paper deals with duct configuration, material thickness and and sizing. Since, the total weight of the ducting and hence the cost for the customer mainly depends on the above parameters, they are taken up for optimization. II. DUCT DETAILS Any duct assembly is a combination of straight duct, transition duct, bend duct, metallic expansion joints or non metallic expansion joints, supports, damper, gates and man hole doors [2]. Straight duct can be of square, rectangle or circular cross section with adequate s on outside. For air duct and gas duct, carbon steel is used as duct material. Normally, square corner expansion joints are provided in the ducting assembly and while carbon steel is used as expansion joint material for air ducts, corten steel (carbon steel with minimum percentage of copper) is used for gas ducts. Wherever removal of duct is required for maintenance purpose flanges are provided with packing rope and in all other places ducts are welded permanently with each other and the gas tight joint is ensured. Manhole doors are provided in the ducting assembly preferably in a straight duct portion with appropriate access from the nearby floors. Care must be 31

2 taken that these doors are provided considering the location of duct s. Ducts are supported either from bottom or from top or in combination. Fixed support and sliding supports are type of bottom supports. The basic difference is that in fixed supports, the support is directly welded with bottom supporting member and in sliding support the support member is placed over the slide bearing plates and allowed to slide over the slide bearing plate to accoodate the movements of the ducts due to thermal expansions. The slide bearing plates are welded with the bottom support plate and then welded with the supporting structural member. The top support can be of either with hanger rod assembly or with variable / constant load hangers. Restraints are provided in the supports to restrict the movement of the support and duct [3]. III. CFBC DUCTING Ducts connect pieces of major equipment, but are not necessarily considered part of the equipment. Ducts may be circular or rectangular in shape or sometimes they may be unusual in shape as they transition into a piece of major power plant equipment like fan, air heater or electrostatic precipitator. To accoodate temperature changes, ducts are usually independent structures which float on slide bearing plates or suspended by a hanger system. In a fluidized bed boiler system, (Fig.1) air ducting system consists of a combination of Primary Air (PA) fan and Forced Draft (FD) (also called Secondary Air (SA)) fan thatt supply the air needed for combustion to the boiler. This duct extends up to the air pre heater. In boiler, the outlet flue gas usually passes through an air pre heater where it is cooled; an electro static precipitator or bag filter where most of the fly ash particles are removed from the flue gas; the ID fans vent flue gas to atmosphere through chimney. Induced Draft (ID) fans are located within the duct system in the downstream of the boiler between the precipitator and chimney. They suck the flue gas out of the boiler and through the precipitator and out to the atmosphere through the chimney or stack. Fig.1: Schematic of Duct arrangement of CFBC In CFBC boiler different zones are at different temperature and pressure. The temperature profile of the CFBC ducting system is plotted as shown in Fig.2. Fig. 2: Temperature profile of ducting system IV. CONCEPT OF OPTIMIZATION Optimization is to make the system as perfect or effectivee as possible with available features to get the most or use the best of the system.[4]. In duct optimization, different duct types, expansion joint, toggle details, various support types and support components, stiffening elements and splitting requirements are compared with practical design knowledge and site working experience. Based on these knowledge and according to the manufacturing feedback, the use of various duct sheet thickness, the sizing and with duct sizing cases are analysed. As per the operating condition of the plant, the design parameters like temperaturee and pressure are considered for any specific system. Details regarding various duct configuration, and sizing, weight of the duct with s and duct design cost and manufacturing cost are studied. Graphically plotting the calculated values, the analysis is made to find the optimu value for duct sheet thickness, number of s, size, weight and cost. V. DUCT DESIGN OPTIMIZATION While doing duct engineering, there are various types of ducts used depending on flow area, velocity and location. Circular duct is considered to be the best while considering performance. But because of the difficulties encountered during the manufacturing of the circular duct and difficulty in providing supports, at most of the situations the circular duct is avoided, wherein the square duct is preferred. In places where there is limitation of space rectangular duct is used. Depending upon the size of the duct and thickness of the duct plate, the sizing and of the s vary. This paper discusses about parameterizing and optimizing the use 32

3 of and sizing with the use of duct size and configuration so as to reduce the total weight of ducting system. Objective of function: Duct size and aspect ratio are required to be analysed to have the optimum duct configuration. The effect of thickness of the duct plate on sizing and are analysed considering the manufacturing feasibility, because increase in duct weight almost due to duct sheet thickness and s. Duct sheet thickness lies within the limited range and number of s required depends on the thickness of the plate. Based on the maximum calculated, number of s is found and the size suitable to stiffen the duct size is selected. Number of s decides the number of welding which decides the maximum portion of the duct cost. So it is required to reduce the total cost by optimally selecting the duct plate thickness, size and. Case study: To explain the optimization procedure followed in the present analysis, a portion of the ducting in CFBC Boiler, Induced Draft fan outlet duct is taken as a case study. Parameters considered for optimization: Duct sheet thickness Duct configuration - aspect ratio Number of Stiffeners Stiffener size Duct material cost Duct fabrication cost While designing the duct panel the loads considered are, Positive and negative pressure load from mechanical design data. (PPL & NPL) Dead load according to the system (DL) Wind load (WL) Ash load(al) Live load (LL) The limiting conditions considered for the present case are Design temperature: 200 deg C Positive Pressure Load (PPL):+20 mbar Negative Pressure Load(NPL): -10 mbar Dead load (DL) : 100 kg per m 2. Wind load (WL) : 200 kg per m 2 Ash load (AL): 300 kg per m 2 Live load (LL): 100 kg per m 2 By calculating the occurrence of different possible combinations of the loads the numerical maximum is taken as the design load of the respective panel. A duct shell is considered to consist of at-least four duct panels (two sides, top and bottom). Each of the four panels may be designed independently to achieve the most economical design for a given application. If different is considered for all the panels then the minimum of the values is considered as the maximum. Stiffener sizing will vary depending upon the load acting over the top, side and bottom walls of the shell. Always the bottom wall is highly loaded because of the ash load and water wash load. For economic size, different sizing and shall be maintained for the different side of the duct shell walls. The span is calculated using Stress criterion (Eqn.1), Static Plate deflection (Eqn.2), Dynamic plate deflection (Eqn.3) and Vibration criterion (Eqn.4) methods [5]. Stress Criterion: S=2t Ys 2P Static Plate deflection: +2 (1) Et S = 4 +2 (2) P Dynamic plate deflection criterion: 3 Δ384( t 12) E S = (3) 2 ' (1 γ ) P Plate vibration criterion: S = 893t f E where S (4) 33

4 t - plate thickness Ys- yield strength P - design pressure E - Young s modulus γ = Poisson s ratio = plate deflection p =expected pressure pulsation f = natural frequency Among the values from these four criteria, the minimum value is identifiedd as the maximum. Then for each size, the span (depth or width) to be stiffened can be calculated using stress criterion (Eqn.5), Stiffener deflection (Eqn.6) and Stiffener vibration methods (Eqn.7). Stiffener stress criterion: L Stiffener deflection criterion: L MB S + PS EIK 5x464xPS Stiffener vibration criterion: L 1834 EI A f (5) (6) (7) Where M= combined section modulus of (plate + ) B = allowable bending stress L = Maximum allowable length I = combined moment of inertia of (plate +) K = length factor; k=1 for uniform length, k=2 for multiple lengths A = combined cross sectional area (plate +) A table has been prepared which gives size of the for the give span of the panel (both channel and I sections) for various duct plate thickness. Table 1: Thick ness in For each individual panel of the duct shell, for different channel / beam size, the size is selected from the table. Taking a duct made of 4 m plate for analysis, and sizes are calculated for a maximum transport length of duct as Applying Eqn.1,2,3 and 4 for Stress criterion, Static Plate deflection, Dynamic plate deflection and Vibration criterion for the specified conditions the values obtained are 1962, ,,1195 and. The minimum value is found to be the maximum which is for the case considered for analysis. a) If uniform is considered, Stiffeners = length/max_sp=>round(3.5)=4 Stiffener (S1) = 2500/4 = 625 Number of s (n) = 3 Fig.3: Selection of channel size for span of duct size C75 C100 C 125 Channel size C150 C 200 C250 C Duct panel with uniform b ) If non-uniform is considered, Max. Stiffener (N1) = 630 N2 = 620 Number of s (n) = 3 C

5 Fig.4: Duct panel with non-uniform Similar way the requirement of s for other sizes were computed for different cases of plates traditionally used for fabrication of ducts. Table 2 is prepared for number of s required to stiffen the duct span keeping the duct length as The dataa are found for uniform and non- uniform. Table 2: Stiffener requirement and for various duct plate thickness Sl. No 1 3 Thickness Max. Stiffener Spacing (calculated) 553 Uniform (S1), number of s (n) N1 553 Sample1 N Non Uniform number of s n N1 550 Sample2 N2 425 number of s Length n m & 273 From Table2, it is observed that for ducts of 4, 5 and 6, even the maximum is different, while considering uniform/ non-uniform, number of s are same. For 8 plate thickness, no of s is reduced but duct sheet weight is more compared with others. In the case of 3 plate thickness, number of s has increased and this increases number of welding also. Moreover welding in 3m plate is found to be very difficult. For all the above cases, under above specified conditions, by changing the aspect ratio of the duct, suitable s are selected and keeping the uniform, total duct weight is calculated and tabulated. Considering a sample case of 4 thickness and for aspect ratio of 1.15, the size is identified from table1. Total duct weight is calculated which includes duct panel weight and s weight. Total cost is calculated based on material cost and welding cost. Table 3 lists the sample calculated data of total weight and cost for the duct plate thicknesss of 4 for specified aspect ratio. Table Table showing weight and cost for aspect ratio aspec t ratio a b _ a Stiffener_ b Wt. Panel _a Wt. Panel_b Total panel_wt no of s Total _stiff_wt Total weight total cost C100 C

6 Similarly the total weight and cost are calculated for various duct plate thickness. Since the difference between 3 m and 3.5 plate are very minimum and minimum plate thickness specified in literature is 3.5, for further analysis 3.5 plate is considered. Various combinations of data from table are analysed and graphs are plotted. VI. RESULTS AND DISCUSSION Fig.5 relates the aspect ratio (depth/width) of duct configuration with corresponding weight of the duct in tons, for various plate thicknesses. The curve pattern is similar for the plate thicknesses of 4,5,6 and 8. An aspect ratio of 1.15 results in least weight for 5,6 and 8 thicknesses. For 3.5 and 4 plates, square duct (aspect ratio 1) is found to be optimum. While comparing 3.5 and 4 plates, weight of duct made of 3.5 plate is more when comparing with that of 4, because of increased number of s. When comparing the weight of the other plates with various aspect ratios, though the optimum aspect ratio for 4 duct is 1, the space availability at different location demands different aspect ratios. It is found from the graph that 4 duct gives the least overall weight for the duct for alll aspect ratios. Fig. 5 : Duct aspect ratio with weight The cost of the duct for various duct thickness are plotted for different aspect ratio.(fig.6) ). This graph explicitly shows that 4 thickness plate is optimum for all the aspect ratios of the duct. Fig.6 : Duct thickness with cost Fig.7 shows the relationship between the cost of the duct with same flow area but with various aspect ratios for various duct thickness. From the graph, it is found that 3.5 plate is the worst considering the manufacturing cost which includes both material cost and s welding cost of 4 and 5 plates. But for all the aspect ratios, the 4 plate is found to be optimu compared with other plate thickness. Fig.7: Duct aspect ratio with cost VII. CONCLUSIONN It is evident from the mechanical design optimization approach adopted on a Induced Draft (ID) fan outlet duct system of a CFBC boiler that, While it appears that increase in duct thicknesss may decrease the number and size of s,, it is found from the analysis presented that 4 thickness and uniform is the best solution for the given design condition. Though various are in use from the analysis, it is prudent to conclude that the uniform is the most preferred one considering the overall weight of the ducting. The analysis shows that the ducting with plate thickness of 4, under design condition reduces the cost of ducting which is the prime objective of any manufacturing industry. REFERENCES [1]. Hemalatha C, Sundararajan S, Sankaranarayanasamy K, Automation of Duct Engineering. [2]. Patrick J.Brooks, Duct design fundamentals, ASHRAE Journal, Jan 2010, Vol.152. [3]. Ronald L.Schneider, The Structural Design of Air Gas Ducts for Power Stations Industrial Boilers Applications, Air and Gas Structural Design Coittee of the energy Division of The American Society of Civil Engineers, (ASCE),1995 [4]. Kerry Pennington, Worley Parsons, Economical Ductwork Design and Engineering, Power engineering, Oct 2008, Vol.12 (10) [5]. Raymond.J.Roark, Warren.C.Young, Roark s Formulas for stress and strain, Current Law Publishers, 2002, American Engineers, (ASCE), Society of Civil 36