THEORETICAL ANALYSIS OF THE PERFORMANCE OF DUAL PRESSURE CONDENSER IN A THERMAL POWER PLANT

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 6, Issue 2, February (2015), pp IAEME: Journal Impact Factor (2015): (Calculated by GISI) IJMET I A E M E THEORETICAL ANALYSIS OF THE PERFORMANCE OF DUAL PRESSURE CONDENSER IN A THERMAL POWER PLANT K.K.Anantha Kirthan* S. Sathurtha Mourian* P. Raj Clinton* *Department of Mechanical Engineering (SW) PSG College of Technology (Autonomous Institution) Coimbatore ABSTRACT The condenser is a large shell-and tube type heat exchanger. This is positioned next to the turbine in order to receive a large flow rate of low pressure steam. This steam in condenser goes under a phase change from vapour to liquid water. In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency, and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water. A Dual -pressure condenser results in efficiency improvement because the average turbine back pressure is less compared with that of a single-pressure condenser. The average exhaust pressure of the LP turbines becomes lower than the single pressure condenser. This paper presents the theoretical analysis that shows the significant of dual pressure condenser application in thermal power plants for improved efficiency over the conventional single pressure condensers. Condenser performance is discussed in detail and the analytical results are given with respect to dual pressure operation of condensers. In India, the subcritical and supercritical units of thermal power plants which were supplied by Chinese have adopted with the dual pressure condenser technology. This report introduces the theoretical approach to the above technique with a brief description. Keywords: Thermal Power Plant, Condenser, Single Pressure and Dual Pressure Condenser, LMTD, Steam Cycle LP Turbine Efficiency, Condenser Effectiveness 37

2 NOTATIONS C Mass flow rate of cooling water Condensing or saturation temperature specific heat Q Heat transfer U Overall heat transfer coefficient 1 Cooling water inlet temperature at Low pressure condenser 2 Cooling water outlet temperature at Low pressure condenser and inlet to the high pressure condenser 3 Cooling water outlet temperature at high pressure condenser TTD Terminal temperature difference t Diff between cooling water inlet & outlet temperature δt Difference between the steam saturation temp & cooling water outlet temp ln Natural logarithm Log mean temperature difference (LMTD) h fg T m Specific enthalpy of steam 1. INTRODUCTION Condenser is a heat exchanger apparatus and is vital for a thermal power plant because the efficiency of the turbine is fully dependant on it. The thermal efficiency of a power plant can be increased by enhancing the heat transfer rate of a condenser; this can be achieved by reducing the turbine exhaust pressure. The condenser is positioned next to the turbine in order to receive a large flow rate of low pressure steam. This steam in the condenser goes under a phase change from vapour to liquid water. External cooling water is pumped through thousands of tubes in the condenser to transport the heat of condensation of the steam away from the plant. Upon leaving the condenser, the condensate is at a low temperature and pressure. Removal of this condensate may be considered as maintaining the low pressure in the condenser continuously. The phase change in turn depends on the transfer of heat to the external cooling water. The rejection of heat to the surroundings by the cooling water is essential to maintain the low pressure in the condenser. In condenser, by lowering the back pressure a little bit increases the work of the turbine, increases the plant efficiency, and reduces steam flow for a given output. 2. DUAL PRESSURE CONDENSER Large power plant condensers are usually 'shell and tube' heat exchangers where the two fluids do not come in direct contact and the heat released by the condensation of steam is transferred through the walls of the tubes into the cooling water continuously circulating inside them. In a Dual pressure condenser the two shells operate at different pressures. The bottom sides (hot well) of both the condensers are connected. 38

3 Figure-1 Dual pressure condenser arrangement For the convenience of cleaning and maintenance, cooling water flows through the tubes and steam condenses outside the tubes. These condensers can be classified based on the cooling water flow as single or multi pressure, depending on whether the cooling water flow path creates one or more turbine back pressures and thereby increasing the turbine efficiency. Dual pressure condensing system is incorporated with two condensers. These condensers have different internal pressure from each other and are installed below the each low pressure turbines LPT-A & LPT-B as shown in the figure-1. SERIES TYPE COOLING WATER FLOW ARRANGEMENT In case of a serial configuration, the total cooling water mass flow rate enters the low pressure condenser and upon exiting enters the high pressure condenser as in figure-2. Structurally, the unit can be thought of as two separate condenser shells, although the actual design is often a single shell with a pressure tight partition separating the high pressure condensing region (A) from the lower pressure region (B). Figure-2 Cooling water flow arrangement series type While part of the steam is expanded to a lower pressure and temperature (T CB < T C ), the remainder is discharged at a somewhat higher temperature (T CA > T C ). Different condenser pressures are achieved (Figure-3). 39

4 Figure-3 Temperature profile of dual pressure condenser Assuming equal mass flow rates for the cooling water in both configurations, the average condenser pressure in the serial arrangement is lower. In general, the lower the condenser pressure the higher the efficiency of the overall plant. CHARACTERISTICS OF CONDENSER Double-shell: Steam side divided by a sealed partition plate into two parts, shell A and shell B, respectively Cooling water in B is colder than A Pressure in B lower than A as well Average pressure is lower at same heat transfer area and cooling water flow, higher vacuum and thermal economy Single-pass: once-through ADVANTAGES OF DUAL PRESSURE CONDENSER The Dual pressure condenser is configured such a way that to obtain the Temperature of the condensate can be increased considerably. Both the hot wells are interconnected at bottom which is having different temperature due to difference in saturation pressure. A Dual -pressure condenser results in efficiency improvement because the average turbine back pressure is less compared with that of a single-pressure condenser (Which is determined by the highest circulating water temperature). The average exhaust pressure of the LP turbines becomes lower than the single pressure condenser. (P A + P B )/2 < P S The TTD can be made larger in dual pressure condenser thereby the cooling area can be reduced. Cooling water flow is reduced because it flows once through both condensers consecutively in series. As a result, cooling tower size can be reduced and also, the pumping cost can be reduced. CONDENSER OPERATING CHARACTERISTICS Relationship of condenser vacuum, steam load, cooling water inlet temperature, circulating water flow rate and the TTD is called operating characteristics, or thermal characteristics of condenser The corresponding variation curve is called curve of condenser operating characteristics Vacuum of condenser corresponds to condensing water temperature Tc. So the factor which affects the condenser vacuum also affects Tc 40

5 FACTORS AFFECTING THE OPERATING CHARACTERISTICS These following 3 factors affect the vacuum in condenser Tw1 - cooling water inlet temperature t- (Tw 2 - Tw 1 ) diff between cooling water inlet & outlet temperature δt temp diff between steam saturation temp & cooling water outlet temp Cooling water inlet temp (Tw1) If cooling water temperature (Tw1) decreases, saturation temperature of condensing steam (Tc) also decreases, then vacuum will increase If cooling water temperature (Tw1) increases, saturation temperature of condensing steam (Tc) also increases, then vacuum will decrease Cooling water Temp rise ( t ) If t 1, δt and exhaust steam constant, t inversely proportional to cooling water flow. So, water flow, t, vacuum ; water flow, t, vacuum Steam load & δt t proportional to exhaust steam, So, exhaust, t, vacuum ; vice versa In case of t 1 and t are constant, δt, vacuum ; δt, vacuum 3. ANALYSIS OF DESIGN & TEST PARAMETERS OF CONDENSER The Surface condenser - double back pressure, divided water box - double shell & single pass condenser is considered for this analysis. Table-1 Design & test data s Description Unit Design Data Test Data Unit load MW Total active area of the condenser m Numbers of pass and shell 1/2 1/2 Net heat carried away under the condition of circulating water kj/s * Circulating water flow kg/hr Cooling water inlet temp (Tw1) Cooling water outlet temp (Tw3) CW inlet / outlet pressure in the water chamber MPa / 0.22 Cleanness factor Condenser inlet steam temp 48 Steam flow at condenser inlet kg/hr (LP turbine outlet) Design TTD of the condenser 6.69/ /3.39* Condenser pressure KPa Condensate (Hot well) temp * calculated values 41

6 DATA ANALYSIS Heat flow is one of the most critical parameters required for condenser performance monitoring. Two methods are available to calculate this value: the water side Q and steam side Q. In theory both value should be identical if it is assumed that no heat energy is lost from the steam side through radiation from the condenser shell. Parameters furnished in the TEST data column in the above table are considered for this analysis of condenser performance. 4. PERFORMANCE ANALYSIS OF DUAL PRESSURE CONDENSER If cooling water is supplied to condenser in series flow arrangement, the performance of condenser can be: Let as assume that basis construction of both the condensers are same. Performance of low pressure condenser Figure - 4 cooling water flow in dual pressure condenser Water side parameters of Low Pressure Turbine (B) at rated load of 600 MW: Tw1 =34.4 T w2 =? m C = 63,390 m 3 /hr = 17,608 kg/sec Heat gained by cooling Water in Condenser-B Cooling waterside Q is calculated from the relationship: Q Gain = m C X C p (Tw2 - Tw1) (A) Where Tw1 is the inlet cooling water temperature ( ) Tw2 is the outlet cooling water temperature ( ) m C is the cooling water flow rate (kg/sec) C p is the specific heat of water, which may be assumed to be 4.19 kj/kg = 17,608 X 4.19 X (Tw2-34.4) = X (Tw2-34.4) kj/sec (1) Steam side Parameters of Low Pressure Turbine (B) at rated load of 600 MW Total steam flow (at 2 LP Turbines) = 1146 T/hr (The steam flow has been taken as half of the total flow but in actual condition it may vary due to dual pressure at condensers) 42

7 Steam inlet at LP turbine (B) = = = Therefore Heat value of steam at condenser inlet From steam table, h fg = 2387 kj/kg Q lost by steam = 159 X 2387 = kj/sec (2) Heat gained by cooling water = Heat lost by the steam From Equation (1) and (2) X (Tw2-34.4) = , Tw2 = = 39.5 Cooling water temperature at outlet of low pressure condenser-b (Tw2) = 39.5 Performance of high pressure condenser Cooling water inlet temperature at high pressure condenser is 40 Hence, heat gained by the cooling water at high pressure condenser Q Gain = C P (Tw3 - Tw2) = 17,608 x 4.19 x ( ) = x ( ) = kj/sec Details of enthalpy gained by cooling water in series flow configuration Hence, heat lost by steam at condenser-a = kj/sec = 159 kg/sec 159 X h fg = Q lost, h fg = / 159 = 2134 kj/kg The saturation temperature can be fixed for specific enthalpy of evaporation kj/kg & 90% dryness fraction, the saturated steam temperature would be 54.5 Calculation of LMTD for series flow configuration LMTD of LOW pressure condenser ( T m ) - B: Tw1 is the inlet cooling water temperature ( ) = 32 Tw2 is the outlet cooling water temperature ( ) = 39.5 Let us assume that the inlet temperature of steam & the out let temperature of condensate below the condenser are same (since the latent heat alone is removed in condenser). At 12.9 kpa pressure Tm (LP condenser-b) = =.... = LMTD of HIGH pressure condenser ( T m )-A Tw2 is the inlet cooling water temperature ( ) = 39.5 Tw3 is the outlet cooling water temperature ( ) = 44.1 Let us assume that the inlet temperature of steam & the out let temperature of condensate below the condenser are same (since the latent heat alone is removed at condenser). T m (HP condenser-a) = ln = 5.97 Average LMTD of series flow cooling water & 5.97 = 8.41 Effectiveness of condenser in series flow Tw2 Tw (For design value) = = = Ts Tw Effectiveness of condenser in series flow (For Test data) = =

8 5. RESULTS AND DISCUSSIONS All the calculations during this analysis have been performed on the basis of available plant data and first law of thermodynamics. The intention of this analysis is to observe the causes which affect the performance of the condenser. Based on the following assumption, the above calculations were carried out and the results are tabulated as below: The quantity of steam inlet at condenser is same as the design value The dryness fraction of the steam at condenser inlet is 90% Table-2 Calculated data of dual pressure condenser Condenser B Condenser A Heat lost by steam kj/sec Saturation temperature of steam entering the turbines Cooling water inlet temperature 34.4 (Tw1) 40 (Tw2) Cooling water out let temperature (Tw3) Cooling water temperature difference TTD Saturated steam temperature Corresponding saturation pressure at kpa LMTD Average LMTD 8.41 Effectiveness in dual flow From the table, the followings are inferred: It could be seen that the heat gained by the cooling water at condenser A is less than condenser-b by 11%. This is due to higher CW inlet temperature at condenser-a. Different saturation temperature of the steam at condenser inlet confirms the dual pressure operation of the condenser. Cooling Water inlet temperature at condenser inlet is higher than the design value. Cooling water temperature is changing with weather conditions in particular region, and cannot be changed in order to achieve better condenser performance. However, the test (9.7ºC) is almost equal to the design value (10ºC). This can be increased by means of supplying more quantities of cooling water. The cooling water supplied to the condenser for rated load is less than the design value. Due to higher cooling water inlet temperature, the condenser pressure is high. Due to large LMTD, rate of heat transfer is more in dual pressure condenser which will increase the work output of turbine. The analysis is show that TTD is higher than the design value. This rise in TTD is due to increase in cooling water inlet temperature. The TTD can be made larger in dual pressure condenser thereby the cooling area can be reduced. 44

9 Performance test confirmed that the effectiveness of the condenser equaled or surpassed the design value. 6. CONCLUSION The technological investigations and analysis of various important characters affecting the dual pressure condenser performance and other related factors responsible for deviation from ideal working of condensers have been discussed. Also, the analytical results of dual pressure condenser are presented in detail. From the analysis, it can be concluded that the causes which effecting the performance of condenser are: Deviation due to cooling water inlet temperature Deviation due to water flow rate and Deviation due to condenser pressure In general, the lower the condenser pressure the higher the turbine efficiency and hence, the overall plant cycle efficiency is improved. This performance test confirmed that the effectiveness of the condenser is equaled or surpassed the design value. However, the following measures are suggested to achieve the design performance and to ensure the sustained performance. Cooling water flow can be increased to increase the condenser heat transfer rate for the given cooling water temperature. The total efficiency of the power plant can be increased considerably by overcome above deviations in the condenser. ACKNOWLEDGMENT The authors would like to express their profound gratitude and deep regards to Mr.R. VENKATESH for his exemplary guidance, monitoring and constant encouragement throughout the course of this thesis. The blessing, help and guidance given by beloved principal shall carry them a long way in the journey of life on which they are about to embark. REFERENCES 1. R.K. Kapooria, K.S. Kasana and S.Kumar, Technological investigations and efficiency analysis of a steam heat exchange condenser: conceptual design of a hybrid steam condenser, Journal of Energy in Southern Africa, Vol 19 No 3, 35-45, Aug Zhang C., Souse A.C.M., Venart J., The Numerical and experimental study of a power plant condenser, J.Heat Tran 115 (1993) Heat Exchange Institute, standards for steam surface condensers, 9 th edition, Heat Exchange Institute, Inc., Ohio, USA, R.P.Roy, M.Ratisher, V.K.Gokhale, A computational model of a power plant steam condenser, J. Energy Resour. Technol.123 (2001) 81 c M.M. El-Wakil, Power plant technology, McGraw-Hill Book Company, New York 6. Obert, E.F: Concepts of Thermodynamics. McGraw-Hill Book Company, New York, Chakrabarti S., 2005, A case study on availability losses in a condenser for a 210 MW thermal power unit in India. Journal of Energy & Environment, 5: Arman J. and Ghosal A.K., 2007, Performance analysis of finned tube and unbaffle shell and tube heat exchangers, International Journal of Thermal Sciences, 46: Process heat Transfer by D.Q.Kern, Edn Modern Power Station Practice British Electricity International Volume G: 45

10 11. Manjinder Bajwa and Piyush Gulati, Comparing The Thermal Power Plant Performance At Various Output Loads by Energy Auditing (A Statistical Analyzing Tool) International Journal of Mechanical Engineering & Technology (IJMET), Volume 2, Issue 2, 2011, pp , ISSN Print: , ISSN Online: S. Paliwala, H.Chandra and A. Tripathi, Investigation and Analysis of Air Pollution Emitted From Thermal Power Plants: A Critical Review International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 4, 2013, pp , ISSN Print: , ISSN Online: Vivek singh, Dr. A.C. Tiwari, Rajiv Gandhi Proudyogiki Vishwavidyalya, Performance Analysis of Electrostatic Precipitator In Thermal Power Plant International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp , ISSN Print: , ISSN Online: AUTHORS DETAILS Name Degree Field Institution City State Country : ANANTHA KIRTHAN K K : Bachelor o f Engineering : Mechanical Engineering (SW) : PSG College of Technology : Coimbatore : Tamilnadu : India Name Degree Field Institution City State Country : SATHURTHA MOURIAN S : Bachelor o f Engineering : Mechanical Engineering (SW) : PSG College of Technology : Coimbatore : Tamilnadu : India Name Degree Field Institution City State Country : RAJ CLINTON P : Bachelor o f Engineering : Mechanical Engineering (SW) : PSG College of Technology : Coimbatore : Tamilnadu : India 46