IJRME - International Journal of Research in Mechanical Engineering ISSN: 2349-3860

DESIGN AND ANALYSIS OF HIGH PRESSURE FEEDWATER HEATER Suhas D. Ambulgekar1 | Prof. Dr. B. S. Gawali2 | Prof. Dr. N. K. Sane3 | Vinaykumar Kabra4 | Ajit Gavali5 1(Dept. of Mechanical Engineering, Walchand College of Engineering, Sangli, India, [email protected]) 2(Dept. of Mechanical Engineering, Walchand College of Engineering, Sangli, India, [email protected]) 3(Dept. of Mechanical Engineering, Walchand College of Engineering, Sangli, India, [email protected]) 4(Power plant and , Walchandnagar Industries Limited, Pune, India, [email protected]) 5(Power plant and Boiler, Walchandnagar Industries Limited, Pune, India, [email protected]) ______

Abstract— Most of the electricity generates throughout the world is from power plants. The plant efficiency can be improved by regeneration which is done by using feedwater heater. It is a shell and tube used for waste heat recovery from steam power plant. This paper represents mathematical model of high pressure feedwater heater. The heater is divided into three zones: desuperheating, condensing and, subcooling for the modeling purpose. The same input parameters are given to the Aspen FRAN software and results are obtained. It is found that the analytical results are close compared to results given by software.

Keywords— Feedwater heaters, Aspen FRAN, heat transfer zones. ______

1. INTRODUCTION being regenerative. The feedwater heaters are integral portions of the powerplant thermodynamic cycle. Today most of the electricity generates throught Normally, there are multiple stages of feedwater heating. the world is from steam power plants and the ideal cycle Feedwater heaters allow the to be brought up for vapor power plants is the . In simple to the required temperature gradually. This minimizes the Rankine cycle, steam is used as working fluid generated in inevitable irreversibility associated with heat transfer from saturated liquid water. This saturated steam flows and hence improves the thermodynamic efficiency of a through the turbine, where its internal energy is converted turbine cycle. This decreases operating costs and also helps into mechanical work to run an electricity generating to avoid thermal shock to the material of the . The system. The Rankine cycle efficiency can be improved by greater the number of extraction stages, the lower the regeneration, which uses sensible heat of small amount of amount of thermal energy required to heat up the steam taken from high pressure turbine. Basically this feedwater. A beneficial by-product of the energy extracted enables internal transfer of high temperature steam to the by the feedwater heaters is the reduced rate of rejection of low temperature condensate (feed water). Regeneration is energy to the environment. accomplished in all large scale modern power plants through the use of feedwater heaters. Figure (1) shows the The feedwater heaters mainly reduce the required Schematic of feedwater heater heat supplied to the plant by utilizing the waste heat leaving the plant. It also increases the overall plant cycle efficiency and lowers the size of boilers. Most of heaters are of a standard shell and tube configuration, it consists of a bundle of parallel tubes inside a shell. One fluid flows through a number of tubes, while the other fluid with different temperature flows over the shell but outside the tubes to transfer heat through the tube walls. The two fluids are separated by the walls of the tube so that they never mix. Due to this design, a large number of tubes are used to Fig.1.Schematic of feedwater heater enlarge the heat transfer area and transfer heat efficiently. A feedwater heater is a heat exchanger designed In a word, it is an efficient way to use waste heat in order to preheat boiler feedwater by means of condensing steam to conserve energy. bled from a . An open feedwater heater is So the main objective of this work is to set a basically a mixing chamber, where the steam extracted or proven design methodology to design high pressure bled from the turbine mixes with the feedwater exiting the feedwater heater. The design of shell and tube configured pump. Another type is closed feedwater heater, in which feedwater heater requires the knowledge of a number of heat is transferred from the bled steam to the feedwater factors on both shell side and tube side. An exhaustive list without any mixing taking place. The heating process by of these items and existing standards to select these are means of extraction steam from turbine is referred to as well documented.

IJRME - International Journal of Research in Mechanical Engineering Volume: 03 Issue: 03 2016 www.researchscript.com 1

IJRME - International Journal of Research in Mechanical Engineering ISSN: 2349-3860

NOMENCLATURE: i inlet A area, m o outlet 2 C specific heat, kJ/kg.K s shell side d p tube diameter, m t tube side

D shell inner diameter, m 2. PROBLEM DEFINITION g s gravitational force, m/s Table I gives process design data and Table II 2 gives geometric design data required of feedwater heater. h heat transfer coefficient, W/m K With consideration of these process and geometric 2 h latent heat of vaporization, kJ/kg parameters, the feedwater heater is designed. k fg thermal conductivity, W/m.K TABLE I. PROCESS DESIGN DATA L length of the tube, m Sr. No. Parameters Unit Value 1 Tube side fluid flow rate kg/sec 26.66 m mass flow rate, kg/sec 4 Shell side fluid flow rate kg/sec 3.12 3 Tube side fluid inlet temperature °C 115 Q heat transfer rate, W 4 Tube side fluid outlet temperature °C 187 R resistance, m . K/W 5 Shell side fluid inlet temperature °C 343 6 Shell side fluid outlet temperature °C 125 2 t f temperature of hot fluid, °C 7 Tube side fluid inlet pressure bar 16.7 8 Shell side fluid inlet pressure bar 117.6 th temperature of cold fluid, °C 9 Tube side fluid inlet velocity m/sec 1.6 tc saturation temperature, °C TABLE II. GEOMETRIC DESIGN DATA tsat wall temperature, °C Sr. No. Parameters Unit Value 1 Shell material SA 516 GR 70 twall film temperature, °C 2 Tube material SA 556 GR A2 u�ilm velocity, m/sec 3 Tube outside diameter m 0.0158 4 Tube wall thickness m 0.0016 dynamic viscosity, kg/m.sec 5 Tube inside diameter m 0.0126 6 Tube pitch m 0.021 µ density, kg/m 7 Tube layout Triangular (60°) 3 ρ Dimensionless: 3. DESIGN PROCEDURE Design is an activity aimed at providing complete Hg Hagen number description of the feedwater heater as a component. This Lq Leveque number description can be accomplished using a well-defined design methodology. The thermal design is directed to N Nusselt number calculate an adequate surface area to handle the thermal Nu number of tubes duty for the given specification while the hydraulic analysis determines the pressure drop of the fluids flowing Prt Prandtl number in the system. Re Reynolds number A. Mathematical model X ratio of the diagonal pitch to the tube outside Mathematical model helps to determine ∗ diameter unmeasured parameters of the heater. The first attempts to d provide methods for calculating pressure drop and heat X ratio of the longitudinal pitch to the tube transfer coefficient is the well known Kern method [5]. ∗ outside diameter l Now considering the desuperheating zone, X ratio of the transverse pitch to the tube outside condensing zone, and the subcooling zone of feedwater ∗ diameter t heater separately,

1) Desuperheating zone (DS): In the desuperheating Subscripts: section, the superheated steam is cooled in a contact with dry tubes of the U-tube bundle to the saturation c cold fluid (water) temperature. The heat transfer is written as, h hot fluid (steam)

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IJRME - International Journal of Research in Mechanical Engineering ISSN: 2349-3860

Q = m C (t - t ) = m C (t - t ) (1) . Hg = 1.25 + + 0.2 1 ( . ) . ∗ 3 I s ps hi ho t p ci co 0 6 Xl t ∗ 1 08 ∗ The properties of tube side and shell side fluids turb 0.005�� X1t −0 85Re .� �X t − � − (10) ∗ 3 are calculated at the arithmetic mean of fluid temperatures Xt 1 75 ∗ at the inlet and outlet of zone. �Xl − � � s Leveque Number is expressed as, Logarithmic mean temperature difference, ( ) L = 0.92 Hg. Pr ( ) ( ) ∗ ∗ (11) LMTD = ( ) (2) 4Xl Xt ⁄π −1 ∗ ∗ thi−tco( − t ho−) tci q � Xl Xd � thi−tco 푙푛 Nusselt Number can be calculated with help of Leveque tho− tci a) Thermal analysis for tube-side: number as follows,

Number of tubes: Nu = 0.404 L (12) 1 �3 The flow rate inside the tube (m ) is a function of the q The heat transfer coefficient for∗ the shell-side is expressed density of the fluid ( ), the velocity of the fluid (u), cross- t as follows: sectional flow area of the tube (A ), and the number of t tubes (N ). ρ c h = Nu . (13) t s m = ρ . u . A . N (3) k s d0 Overall heat transfer coefficient: t t t c t Tube side Reynolds number: The overall heat transfer coefficient is given by

Re = (4) U = (14) ρtut di . . . 1 µt o 1 ro ro ro ro 1 +Rfo+ ln + Rfi+ Tube side Nusselt number: hs kt ri ri ri ht Now total heating surface area can be find out from, The Nusselt number is a function of Reynolds number (Re) and Prandtl number (Pr). Q = U . A . T (15)

I I I ∆ mI ( ) Length of tube in this zone can be calculated using, Nu = (5) . . ( ) ( ) f⁄2 RetPrt t 1⁄2 2⁄3 L = (16) 1 07+12 7 f⁄2 Prt −1 Where f is the friction factor which can be calculated from AI I π doNt Shell Diameter can be expressed as, [5] f = (1.58 ln Re - 3.28) (6) −2 D = 0.637 ( ) (17) Tube side heat transfer coefficient is given as, 2 CL A0PR d0 2 s �CTP LI h = Nu (7) Where, kt t di CTP is tube count calculation constant, PR is tube pitch b) Thermal Analysis for Shell-Side: ratio given as , CL is tube layout constant, and A is Shell side heat transfer coefficient: outside heat transferPt area and it can be calculate from do o Martin correlation of Hagen number for flow normal to πd N L. staggered plain tube bundles is given as [6], 2) oCondensingt zone (CD): In the condensing zone, the steam condenses in a contact with tubes where it changes Hg = Hg + Hg [ 1 exp (1 )] (8) its phase from saturated vapor to saturated liquid and Res+200 releases the latent heat of vaporization. lam turb − − 1000 Where, Heat transfer in condensing zone is latent heat exchange,

. ( . ) . Q m . h m C t t ) Hg = 140Re (9) = = ( - (18) ∗0.5 2 Xl (−0 6 +0 75) ∗ ∗ II s fg t pt ci co lam s ∗1 6 4Xt Xl Xt π −1

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IJRME - International Journal of Research in Mechanical Engineering ISSN: 2349-3860

The driving force for condensation is the . .( ) . h = 0.725 3( ) (22) difference between the temperature of cold wall surface kf ρf ρf−ρv g hfg and the bulk temperature of the saturated vapor. cond � µf Tsat−Twall d0 � The tube side heat transfer coefficient h and overall heat T = T T (19) transfer coefficient U can be calculate similar to the t desuperheating zone. ∆ Driving sat − wall II The viscosity and other properties used in the condensing The heat transfer is written as, correlations are evaluated at the film temperature, a weighted mean of the cold surface (wall) temperature and Q = U . A . T (23) the vapor saturation temperature. II II II mII Length of tube in condensing∆ zone can be calculated using, T = T (T T ) (20) [5] 3 �ilm sat − 4 sat − wall L = (24) Where, T = t +t (21) co ci AII Tsat+ 2 II π doNt wall 2 Now the heat transfer coefficient for condensation is given as, [7]

Fig.2. Results obtained by Aspen FRAN software 3) Subcooling zone (SC): In the subcooling zone, the heat transfer coefficient, area of zone and length of tube tube side and shell side fluids are liquids i.e. single phase can be calculate using same equations as that of the and the heat exchange occurs in form of sensible heat desuperheating zone. transfer. The values of heat transferred, LMTD, overall

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IJRME - International Journal of Research in Mechanical Engineering ISSN: 2349-3860

The same input conditions shown in TABLE I changes along the heater because of phase change. The and TABLE II are given to the Aspen FRAN software variation is also seen due to change in properties of fluid and the high pressure feed water heater is designed. The with change in properties of fluid with change in results obtained by analytical calculation and software temperature cannot be considered in analytical method. are compared. The figure (2) shows results obtained by 5. CONCLUSIONS Aspen FRAN software. Sizing of high pressure feedwater heater is done 4. RESULTS AND DISCUSSION analytically and by using Aspen FRAN software to A) Temperature profile of fluid along the length of perform required heat duty. feedwater heater The following conclusions are drawn, Figure (3) shows the temperature variation for both shell and tube side along the length of heater. The • The maximum amount of heat duty is handled superheated steam shows a large drop in temperature in in the condensing zone of feedwater heater. the desuperheating zone. The horizontal line in the • Shell side heat transfer coefficient has a large condensing zone indicates constant temperature and influence on overall heat transfer coefficient finally a gradual decrease in temperature in the and hence area of feedwater heater. subcooling zone. While the temperature of tube side fluid rises gradually along the length of the tube. • Fouling factor affects in calculation of surface area to a large extent. tube side 350 shell side • Condensing zone accounts for maximum amount of area of feedwater heater than other 300 two zones. 250

C) • Pressure drop on shell side is maximum in ⁰ 200 ( subcooling zone as compared to other two zones. While pressure drop on tube side of 150 feedwater heater is negligible. 100Fluidtemperature 0 2000 4000 6000 8000 10000 12000 ACKNOWLEDGMENT Length of tube (mm) The authors acknowledge the support facility Fig.3. Fluid temperature variations along the length of provided by “Walchandnagar Industries Limited” and heater also thankful for providing the valuable guidance and encouragement to carry out the work.

REFERENCES B) Three zones of feedwater heater:

In feedwater heater, the heat is transferred from [1] M. Alvarez Fernandez, Luis del Portillo Valdes and C. A. hot fluid on shell side to cold fluid on tube side. The Tristan, “Thermal analysis of closed feedwater heaters in major heat is transferred in the condensing zone due to nuclear powerplant”, Applied thermal engineering, Vol. 68, latent heat transfer while sensible heat transfers in pp. 45-58 (2014). desuperheating and subcooling zone. [2] B. R. Becker and R. E. Pearce, “Life management of feedwater heaters at KCPL”, American society of Figure (4) gives areas of three zones of heater. mechanical engineers, October 1996. It shows that condensing zone accounts for maximum [3] R. Mukherjee, “Effective design of shell and tube heat area than other two zones. exchangers”, Chemical engineering progress, pp.1-8, (1998). Analytical Software 80 [4] Heat exchange institute, Inc., “Standards for closed feed water heaters”, eighth edition, (2009).

60 [5] D. Q. Kern, “Process Heat Transfer” Tata McGraw-Hill, ) 2 (2004). 40 [6] R. K. Shah, “Fundamentals of Heat Exchanger Design” 20 Edition, John Wiley & Sons, Inc. (2003). Area(m [7] Y. A. Cengel, “Heat and Mass Transfer”, Third edition, 0 Tata McGraw-Hill, (2010). SC CD DS Zones of heater [8] Sadik Kakaq and Hongtan Liu, “Heat exchangers: selection, rating and thermal design”, second edition, (2004). Fig.4. Area of three zones of feedwater heater [9] Standards of the tubular exchanger manufacturers Variation in analytical and software results is association, ninth edition, (2007). seen due to change in velocity of fluid on shell side IJRME - International Journal of Research in Mechanical Engineering Volume: 03 Issue: 03 2016 © Researchscript.com 5