Secondary Systems: Condensate/Feedwater Cycle

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1 Secondary Systems: Condensate/Feedwater Cycle K.S. Rajan Professor, School of Chemical & Biotechnology SASTRA University Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 8

2 Table of Contents 1 CONDENSATE/FEED WATER SYSTEM CONDENSER COOLING TOWER OTHER COMPONENTS COMPARISON OF OPERATING CONDITIONS OF THERMAL AND NUCLEAR POWER PLANTS REFERENCES/ADDITIONAL READING... 9 Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 8

3 In this lecture we shall discuss components of condensate/feedwater cycle of secondary system in a power plant utilizing steam from a typical water-cooled nuclear reactor. At the end of this session, learners will be able to (i) understand the role of condenser in secondary system (ii) appreciate the necessity of clean up system (iii) understand the role of low-pressure and high-pressure heaters (iv) differentiate the operating conditions of steam cycle of nuclear power plant from that of a thermal power plant In the previous lecture, the layout of secondary system of a typical nuclear power plant was presented. During the course of previous lecture, two sub-systems viz. (i) steam system and (ii) condensate/feedwater system in the secondary system were identified. The discussion on steam system was completed in the previous lecture. This lecture shall focus on the condensate/feed water system. 1 Condensate/feed water system Let us recall the components of condensate/feedwater system from the layout of secondary system discussed in the previous lecture. The important components are (i) condenser; (ii) cooling tower; (iii) condensate pump; (iv) cleanup system; (v) low pressure heaters; (vi) main feedwater pump and (vii) high pressure heater. 2 Condenser The condensate/feedwater system begins with the condensation of exhaust steam from low-pressure turbines in a condenser operating under vacuum. Condenser is a shell and tube heat exchanger with steam condensing on the outer surface of tubes, with coolant supplied through the tubes. The schematic diagram of a typical condenser is shown in Fig. 1. The important components of the condenser are (i) (ii) (iii) (iv) (v) (vi) (vii) tube bundle tube sheet inlet nozzle for steam hot well nozzle for condensate outlet nozzle for coolant inlet nozzle for coolant outlet Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 8

4 (viii) pass partition (ix) nozzle for ejector Fig 1. Schematic diagram of shell & tube condenser The condenser shown in Fig. 1 is a shell and tube heat exchanger, with two passes on the tube side and one pass on the shell side. Cooling water enters the condenser through inlet nozzle located near the bottom. The pass partition confines the cross sectional area available for coolant flow to 50 % of total cross sectional area of all the tubes. This arrangement ensures that the velocity of coolant flow inside the tubes is high enough to promote turbulence and enhance the rate of heat transfer. The tubes are arranged in a triangular pitch as shown in Fig. 2 and are held together at both ends using tube sheets as shown in Fig.3. The centre-to-centre distance between two tubes is called pitch, which is maintained between times the outer diameter of the tube. With the triangular pitch, more tubes can be accommodated per unit heat exchanger volume. In other words, higher heat transfer area per unit heat exchanger volume can be obtained with triangular pitch than that obtainable from a square pitch. However, the accessibility of outer surface of the tubes for cleaning becomes difficult. Tube sheets also provide physical barrier to prevent mixing of shell-side and tube-side fluids. Steam enters the condenser through a nozzle located at the top. The nozzle for steam inlet is larger in diameter compared to the nozzles for coolant inlet and outlet. The density of steam is lower than that of water. Hence larger diameter nozzle is required for steam inlet. Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 8

5 Fig 2. Arrangement of tubes in triangular pitch Fig 3. Tube sheets holding the tubes together at both ends Steam is condensed on outer side of the tubes by thermal contact with cooling water flowing inside the tubes. The tubes are generally made of stainless steel to overcome corrosion. The condensate water is collected in the hot well from which the same is pumped using a condensate pump for further chemical treatment and heating. The pressure inside the condenser is maintained below atmospheric by steam ejectors that remove non-condensibles. When steam condenses, its specific volume is reduced. This creates vacuum that draws the exhaust steam from the low-pressure turbines. Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 8

6 Lower temperature of cooling water is advantageous to achieve higher efficiency of power plant. The temperature of cooling water after passing through the condenser is increased by around C. When the cooling water for condenser is taken from a perennial water source like sea or river, the same can be dumped in to the source. The large volume of water in the source ensures rapid equilibration of temperature to ambient level. In case of power plants that are located away from such perennial water sources, cooling towers are employed to bring down the coolant temperature by evaporative cooling. 3 Cooling tower Cooling towers are available in several designs, configurations and sizes depending on the cooling requirements. Among different types of cooling towers, natural draft and induced draft cooling towers are widely used in industries. Natural draft cooling towers rely on the difference in densities of hot and cold air for flow of air through the tower. Cold air enters the tower at the bottom and is heated due to direct contact with water to be cooled, which is supplied in the form of fine sprays. The hot air, being denser rises upwards. This creates vacuum inside the tower. The relatively cool, ambient air at atmospheric pressure enters the tower due to this vaccum; gets heated upon direct contact with sprayed water and leaves at the top. The major advantage of natural draft cooling towers are the reduction in operating cost due to absence of mechanical equipment to induce the movement of air. Hyperbolic natural draft towers shown in Fig. 4 are commonly used in power plants. The shell of these cooling towers is hyperbolic in shape and hence called hyperbolic natural draft towers. This geometry facilitates the rapid movement of hot air towards the upper portions of the tower. These towers are fairly large in diameter as well as in height. The cooling water from the condenser is sprayed across the cross section of the tower at the bottom. The air entering the tower is unsaturated. In simple language, the water vapor content of unsaturated air is lower than the maximum water vapor carrying capacity of air at a given temperature. For instance, we often come across weather reports where temperature and relative humidity are mentioned. Air with relative humidity less than 100 % is unsaturated. When such unsaturated air comes into contact with water, the difference between partial pressure of water vapor in air and the vapor pressure of water at its temperature acts as the driving force for evaporation of water. Since the temperature of air is lower than that of water, the heat required for evaporation of water must be utilized from water itself. This causes cooling of water and hence the term evaporative cooling is used. The cooled water is Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 8

7 collected at the bottom of tower in a tank and is pumped back to the condenser as cooling water. Fig 4. Hyperbolic natural draft towers used in power plants Induced draft cooling towers use fan located at top of the cooling tower, to force the flow of air across the cooling tower coming into direct contact with water sprayed as droplets. The velocity of air at discharge is 3-4 times greater than air velocity at the tower inlet. This ensures that recirculation of exhaust air towards the inlet is not seriously high. These towers can be built with a wide range of sizes and are not as tall as the hyperbolic natural draft towers. The major disadvantage of induced draft cooling towers over the hyperbolic natural draft towers is the energy required for operating the fans. 4 Other components Having seen the major components of condensate/feedwater system, let us discuss the role of other components. Joint Initiative of IITs and IISc Funded by MHRD Page 7 of 8

8 The condensate pump takes inlet from the hot well of condenser and increases its pressure and pumps the condensate through clean up system and low-pressure heaters. Clean up system essentially removes the impurities that can form hard scales inside the tubes of steam generators. Such scales provide additional resistance to heat transfer, causing reduction in rate of heat transfer between primary and secondary coolants leading to reduction in steam generation. The condensate then passes through low-pressure heaters where the condensate is heated using the extraction steam obtained from low-pressure turbines. The condensate is then supplied to the suction side of main feedwater pump, which increases the pressure high enough to permit entry into the steam generator. Before water enters the steam generators, it is heated in high-pressure heater using the extraction steam obtained from high-pressure turbines. The heating of feedwater supplied to steam generator using the extraction steam increases the overall plant efficiency. 5 Comparison of operating conditions of thermal and nuclear power plants We shall compare some of the operation conditions of thermal and nuclear power plants. The focus here is on the generation of electricity from steam or the thermodynamic cycles, rather than the comparison of modes of steam generation and the thermal hydraulic characteristics. Table 1 is a compilation of characteristics of steam cycles of some water/heavy water colled nuclear power plants and thermal power plants. Table 1. Characteristics of steam cycles of some water/heavy water colled nuclear power plants and thermal power plants Characteristics Pressurized Boiling water Pressurized water reactor reactor heavy water (Westinghouse Co. ) (General Electric Co.) reactor (Atomic Energy of Canada Ltd.) 5.7 MPa 7 MPa 4.7 MPa Thermal power plant Steam pressure (MPa) Steam 273 C 288 C 260 C 525 C temperature ( C) Steam quality Wet steam Wet steam Wet steam Superheated steam Efficiency (%) 33.5 % 32.9 % 29.3 % Power (MWe) Joint Initiative of IITs and IISc Funded by MHRD Page 8 of 8

9 The following observations can be made from the above table: Ø Steam temperature: Higher for thermal power plant compared to nuclear power plant Ø Steam quality: Higher steam quality (superheated steam) in thermal power plant compared to nuclear power plant Ø Efficiency: Owing to higher steam temperature and better steam quality, thermal power plants are more efficient compared to nuclear power plant Ø Steam requirement: As a result of lower steam quality and steam temperature in nuclear power plant, higher mass flow rate of steam required per MW of electricity generated 6 References/Additional Reading 1. Nuclear Systems I: Thermal Hydraulic Fundamentals, N.E. Todreas, M.S. Kazimi, Taylor & Francis, Pressurized Water Reactor (PWR) Systems, Reactor Concepts Manual, USNRC Technical Training Centre Publications (Available at Joint Initiative of IITs and IISc Funded by MHRD Page 9 of 8

MODULE 6 HUMIDIFICATION AND AIR CONDITIONING

MODULE 6 HUMIDIFICATION AND AIR CONDITIONING LECTURE NO. 3 6.3 Humidification and dehumidification operations and design calculations Humidification operations: In this operation, water transfers from