A Study of Natural Circulation in the Evaporator of a Horizontal-Tube Heat-Recovery Boiler

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1 ISSN , Thermal Engineering, 0, Vol. 6, No. 7, pp Pleiades Publishing, Inc., 0. Original Russian Text P.V. Roslyakov, K.A. Pleshanov, K.V. Sterkhov, 0, published in Teploenergetika. STEAM BOILERS, POWER-GENERATING FUEL, BURNERS AND BOILER AUXILIARY EQUIPMENT A Study of Natural Circulation in the Evaporator of a Horizontal-Tube Heat-Recovery Boiler P. V. Roslyakov, K. A. Pleshanov, and K. V. Sterkhov Moscow Power Engineering Institute National Research University, Krasnokazarmennaya ul., Moscow, 50 Russia Abstract Results obtained from investigations of stable natural circulation in an intricate circulation loop with a horizontal layout of the tubes of evaporating surface having a negative useful head are presented. The possibility of making a shift from using multiple forced circulation organized by means of a circulation pump to natural circulation in vertical heat-recovery boilers is estimated. Criteria for characterizing the performance reliability and efficiency of a horizontal evaporator with negative useful head are proposed. The influence of various design solutions on circulation robustness is considered. With due regard of the optimal parameters, the most efficient and least costly methods are proposed for achieving more stable circulation in a vertical heat-recovery boiler when a shift is made from multiple forced to natural circulation. A procedure for calculating the circulation parameters and an algorithm for checking evaporator performance reliability are developed, and recommendations for the design of heat-recovery boiler, nonheated parts of natural circulation loop, and evaporating surface are suggested. Keywords: natural circulation, vertical heat-recovery boiler, evaporator, burnout, circulation ratio, circulation rate, steam quality. DOI: 0.3/S X The plan for development of Russia s power industry up to 030 foresees construction of new power units based on the technology of combined-cycle plants (CCPs) firing natural gas as the main fuel. Such a decision has been adopted in view of the following circumstances: the time taken to construct a CCP with heat-recovery boilers (HRBs) is much shorter than that taken to construct large-capacity thermal power plants of other types, and the efficiency of electricity production by a CCP (55 60%) is significantly higher than that of steam power units (35 %). An HRB, which serves for recovering the heat of flue gases exhausted from power-generating gas turbine units (GTUs), is an integral part of a CCP. According to their layouts, HRBs are subdivided into apparatuses of vertical and horizontal types. One essential advantage of horizontal HRBs is the possibility to organize natural circulation of medium in the evaporators. One of the key advantages of vertical HRBs is that they occupy an essentially smaller inplan area. For setting up circulation of steam water mixture in vertical HRBs, their evaporation tubes are connected to circulation pumps (CPs) (Fig. ). The need to use CPs is one of the main disadvantages of such boilers because it entails complication of the design and higher capital as well as operational costs due to increased consumption of electric energy and payments for maintenance and repair of CPs. The aim of this work is to develop a vertical HRB made with using a horizontal layout of the heat-transfer surface tubes and with organizing natural circulation in the evaporator without using CPs. Stable circulation is an essential condition for reliably cooling the evaporator tubes provided that there is no stagnation/reversal of flow, free level (which may occur in a circulation loop under certain conditions []) and unstable flow modes as well as for organizing efficient heat transfer without burnout for a two-phase medium. In fossil-fueled power-generating boilers and in horizontal HRBs it is possible to set up reliable natural circulation by organizing a significant difference between the height elevations in the evaporator (a useful head). If the available height difference is insufficient for setting up reliable natural circulation in the evaporator, multiple forced circulation by means of CPs is used. In what follows, we consider and propose criteria for characterizing performance reliability and efficiency of a horizontal evaporator formulated taking into account the theoretical principles of hydrodynamics and field experience [], because the recommendations on securing stable circulation set out in [] (the stagnation and flow reversal safety margins) 65

2 66 ROSLYAKOV et al b g a g = Fig.. Sketch of the circulation loop in the Pr heat-recovery boiler. () Drum, () downtake tubes, (3) circulation pump, () evaporator, and (5) removal pipes. b g a g are the gas conduit sizes. can be used only if there is a positive useful head in the evaporator.. Reaching circulation rates preventing stratification of steam water mixture in a horizontal tube. In this way, overheating of metal is prevented in the furnaces of conventional boilers. In HRBs, which are characterized by moderate heat absorption densities, it is necessary to prevent not only scale formation, but also possible occurrence of temperature pulsations in the tube wall metal, which have an adverse effect on its longterm strength.. Securing the necessary circulation ratio (K c ) preventing the possibility of degraded heat transfer due to high values of steam quality, which may give rise to temperature pulsations and inefficient use of heattransfer surface (this measure is taken only if criterion is not fulfilled). 3. Maintaining the circulation rate at a level of higher than 0. m/s at the nominal load to prevent sludge deposition in horizontal and slightly inclined tubes [].. Keeping the velocity of steam water mixture lower than 0 m/s to limit flow-accelerated corrosion of horizontal tubes in bend locations [3]. 5. The higher the circulation ratio, the more reliable the circulation is. The main disadvantage of the natural circulation loop comprising a horizontal evaporator is that stable motion of medium may become less reliable. In turn, loss of circulation in a horizontal evaporator may lead to the following negative phenomena (the occurrence of some of these phenomena is difficult to predict): Burnout may occur, entailing less efficient operation of the convective evaporator due to a drastically dropped coefficient of heat transfer from medium to tube walls inside the tubes. The temperature of evaporator tube walls may rise due to stratification, stagnation, or reversal of medium flow. Vibration may occur during unstable motion of medium. Pulsations and temperature stresses may occur. In carrying out investigations in the course of this work, the design of the Pr (PK-79) heatrecovery boiler (operating on the exhaust gases from a General Electric PG658B gas turbine unit) produced by ZiO for the Chelyabinsk TETs- cogeneration station (Fig. ) was considered as the initial one. In the THERMAL ENGINEERING Vol. 6 No. 7 0

3 A STUDY OF NATURAL CIRCULATION IN THE EVAPORATOR b g a g = Fig.. Sketch of the circulation loop with evaporator tubes inclined at 7. () Drum, () downtake tubes, (3) evaporator, and () removal tubes. case of using a CP, the calculated circulation ratio in the boiler was determined to be K c = 3.0 at the nominal load N GTU = 00% and outdoor air temperature t o.a = C. Increasing the difference between the height elevations of the evaporator s distribution and collection headers is one of possible ways for setting up stable circulation in a vertical HRB. In this work, we investigated an evaporator with the tube slope angle from 0 to 7 C (Fig. ); in doing so, we increased the drum placement height by 7.3 m. The numerical experiment was carried out at the GTU load N GTU = 00% and outdoor air temperature t o.a = C. The calculations were carried out for the case of using downtake and removal tubes with the standard size d δ = 73 0 mm. If was found from the calculation results that exclusion of CPs from the prototype HRB causes the circulation ratio K c to drop from 3.0 to. (Fig. 3). With such low circulation ratio, normal operation of the evaporator will be impossible. Therefore, it is not possible to do away with CPs unless the loop configuration is suitably altered. With the slope angle increased to 7 C, K c increases by approximately a factor of.5 (see Fig. 3); in this case, the drum placement height has to be increased from 6.5 to 3.8 m, and the useful head in the evaporator increases from 6 to 3 kpa, which is still insufficient for securing reliable operation of the evaporator. In addition, the use of such design solution entails a growth of metal intensity and complication of the design of not only the evaporator, but of the entire boiler. An analysis of the obtained results showed that the medium in the evaporator can move only under the effect of useful head developed in the removal system. This circumstance generated the need to estimate the effect the removal system height has on the circulation parameters with a horizontal layout of evaporator tubes. It can be seen from the calculation results shown in Fig. that the effect exerted by the removal system height on the circulation ratio is similar to the effect produced by changing the evaporator tube slope angle. A significant growth of useful head from to 89 kpa occurs in the removal system and not in the evaporator itself. THERMAL ENGINEERING Vol. 6 No. 7 0

4 68 ROSLYAKOV et al Evaporator tube slope angle, deg (b) Removal system height, m (c) Number of tubes in the coil Fig. 3. Dependences of K c on the evaporator tube slope angle (a), on the removal system height (b), and on the number of tubes in the evaporator coil (c). () Calculated values and () approximating curve. One of the ways in which a higher flow rate of medium in the loop can be obtained is to decrease flow friction. Smaller flow friction in the evaporator can be obtained by increasing the number of tubes both in the coil (the number of passes) and over the gas conduit width, and by arranging the evaporator along the gas conduit s larger side. (а) The latter option entails significant complication of the HRB design and can be resorted to if the use of other methods does not allow the required circulation parameters to be obtained. Attempts to increase the number of passes in a coil encounter limitations connected with the technology of its manufacture. Modern technologies (specifically, the technological possibilities presently available at the ZiO Works) allow heating surfaces to be made with the number of tubes in a coil equal to 7 8 without using intermediate headers. Figure 5 shows the results from calculations of natural circulation with the number of passes in coils varied from to 7. It can be seen that the effect from increasing the number of tubes in the coil in terms of increasing the circulation ratio is essentially more pronounced than that from increasing the evaporator tube slope angle or from increasing the removal system height (see Figs. 3 and ). Thus, making an evaporator with U-shaped tubes (Fig. 6) having a larger flow section area is more rational than making it with the use of multiloop coils, because with such solution it is possible to decrease the length of tubes and the number of their bends. Increasing the gas conduit width also results in a larger flow section area in the evaporator; however, certain limitation is imposed on the gas conduit depth-to-width ratio (around.5) from the conditions of ensuring aerodynamically stable motion of combustion products. Figure 7 shows the results from calculation of K c with the gas conduit width varied from 3. to.6 m and with the number of tubes over the gas conduit width varied from 33 to 50. An analysis of the presented results shows that the effect from increasing the gas conduit width in terms of increasing the circulation ratio is approximately a factor of two more pronounced than that from increasing the removal system height or from increasing the evaporator tube slope angle. Increasing the number of passes in the evaporator coil and increasing the gas conduit width are the boiler design modification methods that do not lead to a growth of excessive boiler metal intensity. Therefore, the subsequent work was focused on studying the combined effect of these methods on the circulation ratio (Fig. 8). With simultaneously increasing the number of evaporator coil passes from to 7 and the gas conduit width from 3.00 to.55 m, the evaporator flow pass section in the studied HRB increased from 0.5 m in the initial version to 0.99 m. This made it possible to obtain the circulation ratio equal to.5, which is almost equivalent to the sum of circulation ratios in the case of separated use of the methods (see Figs. 5, 7, and 8). This means that the combined use of the THERMAL ENGINEERING Vol. 6 No. 7 0

5 A STUDY OF NATURAL CIRCULATION IN THE EVAPORATOR b g a g = Fig.. Sketch of the circulation loop. The notation is the same as in Fig.. above-mentioned two methods does not lead to decrease of their efficiency. It can be noticed from a comparison of the efficiency of different methods for achieving more reliable operation of the circulation loop (Fig. 9) that the effect produced by increasing the flow pass section on the flow rate in the downtake link D dt is much more pronounced than that from increasing the removal system height or from increasing the evaporator slope angle. In addition, the use of the two last methods invariably entails an increase of boiler height and, consequently, a growth of boiler metal intensity. Increasing the evaporator flow pass section must be regarded as the only sound method of achieving the required circulation parameters and securing reliable operation of the evaporation system. The following changes were made in the circulation loop design of the c PK-79 heat-recovery boiler for the Chelyabinsk TETs- cogeneration station, which was taken as the prototype: (i) The configuration of downtake tubes was modified; the number of downtake tubes was increased from to, and the number of uptake tubes was increased from to 5. Concurrently with this, the circulation pump was excluded from the loop, and the recommendations suggested in [] were implemented (Fig. 6). (ii) The number of tubes in the coil was increased from to 7, and the number of tubes over the gas conduit width was increased from 3 to (а) Gas conduit width, m (b) Evaporator flow section area, m Fig. 5. Graphs of K c as a function of HRB gas conduit width (a) and of evaporator flow section area (b). The notation is the same as in Fig. 3. THERMAL ENGINEERING Vol. 6 No. 7 0

6 70 ROSLYAKOV et al. Δp dt, kpa G dt, kg/s in each subsequent row of convective evaporator tubes, due to which the steam quality in them has different values. The absence of burnout in tube rows was checked after subdividing the evaporator into rows and determining the steam quality x i at the heating surface outlet (Fig. 0). The calculation results are summarized in Table. The heat absorption values for all tubes in a row were taken equal to each other. In accordance with [], the check for burnout was carried out for the tube coil with the maximal heat absorption. The steam quality at the outlet from the first coil of tubes is equal to 39% and exceeds the value equal to 30% recommended in []. The need to check this tube row for stratification of the flow arises. Fig. 6. Pressure difference in the downtake tubes vs. coolant flow rate with varying the circulation loop parameters. (iii) The gas conduit width was increased from 3.00 to.55 m. The resulting characteristics of the evaporator made of finned tubes and of the entire HRB (Table ) were determined from a thermal hydraulic calculation carried out using the Boiler Designer software system []. It is obvious that the temperature of gases in the evaporator decreases as they pass each subsequent tube row; therefore, the temperature difference and coefficient of heat transfer from the side of gases are decreasing. This leads to a decrease of heat absorption Calculations showed that the mass velocity in the first tube row along the flow of gases ρw calc = 3. kg/(m s) is lower than the boundary mass velocity ρw bnd = 350 kg/(m s) above which stratification of medium will not be observed. For solving this problem, the finning pitch in the first row of tubes along the flow of gases was increased from 3.6 to.70 mm (Fig. ). We see from the calculation results that increasing the finning pitch in the first row of evaporator tubes caused the steam quality in it to drop from 39.0 to 30.%, and the following conditions took place: The mass velocity in the first row of evaporator tubes along the flow of gases [ρw calc = 35.6 kg/(m s)] exceeds the minimal mass velocity at which stratifica- Table. Design features of the heat-recovery boiler final makeup version Indicator Steam superheater Evaporator Economizer Gas-water heater Outer diameter d out, mm 38 Tube wall thickness S t, mm.5 Tube transverse pitch S, mm 90 Tube longitudinal pitch S, mm Convective gas conduit width b g, m.55 Convective gas conduit depth along the tube a g, m Number of tube rows along the gas flow n g 5 0 Number of tube passes n pass 7 Number of tube rows over the gas conduit width n g.w Thermal efficiency ϕ Tube length in the heated zone L t, m 65 0 Type of fins Spiral Segments Segments Segments Fin height h fin, mm Fin thickness d fin, mm Fin pitch S fin, mm Fin metal.5 EN kp-OM-T-Sh--A as per GOST THERMAL ENGINEERING Vol. 6 No. 7 0

7 A STUDY OF NATURAL CIRCULATION IN THE EVAPORATOR ρ_w = kg/(m s) Нgeod = m x = 39.0% Evaporator _ Rem_tub L x =.7% Collecting header x =.% Evaporator _ Нgeod = m dpho = MPa dplev = 0.0 MPa CircRat = 7.75 Evaporator 3_ Mixing s/w_37 x = 5.8% x = 0.8% Evaporator _ MPa t/h J/kg С x = 8.0% x = 6.3% Evaporator 5_ Evaporator 6_ Нgeod =.6000 m dpho = MPa dplev = 0.06 MPa ρ_w = kg/(m s) Mixing s/w_35 Нgeod = m x = 5.5% Evaporator 7_ Downtake tub x = 0.3% Evaporator _ Evaporator _ Evaporator 3_ Evaporator _ Evaporator 5_ Evaporator 6_ Evaporator 7_ w_ho = 0.39 m/s w_ho = 0.8 m/s w_ho = 0.68 m/s w_ho = 0.58 m/s w_ho = 0.77 m/s w_ho = 0.86 m/s Separ. s/w_33 Separ. s/w_3 w_ho = 0.93 m/s Distributing head (a) Rem_tub ρ_w = kg/(m s) L x =.8% Нgeod = m Collecting header x = 30.% x = 6.0% Evaporator _ Evaporator _ Нgeod = m dpho = MPa dplev = 0.0 MPa Evaporator 3_ Mixing s/w_37 x = 6.8% x =.5% Evaporator _ CircRat = 7.73 MPa t/h J/kg С x = 8.% x = 6.6% Evaporator 5_ Evaporator 6_ Нgeod =.6000 m dpho = MPa dplev = 0.06 MPa ρ_w = kg/(m s) Mixing s/w35 Нgeod = m x = 5.8% Evaporator 7_ Downtake tub x = 0.3% Evaporator _ Evaporator _ Evaporator 3_ Evaporator _ Evaporator 5_ Evaporator 6_ Evaporator 7_ w_ho = 0.3 m/s w_ho = 0.6 m/s w_ho = 0.67 m/s w_ho = 0.56 m/s Separ. s/w_33 w_ho = 0.76 m/s w_ho = 0.85 m/s Separ. s/w_3 w_ho = 0.9 m/s Distributing header (b) Fig. 7. Analysis diagrams of the circulation loop in the Boiler Designer computer program before (a) and after (b) increasing the finning pitch. THERMAL ENGINEERING Vol. 6 No. 7 0

8 7 ROSLYAKOV et al. Table. Circulation parameters calculation results Indicator Number of U-shaped evaporator coil Evaporator surface area, m 35 Medium flow rate, kg/s Evaporator heat absorption, kw Mass velocity in the evaporator, kg/(m s) Medium velocity at the tube row inlet, m/s Steam quality at the evaporator outlet, % tion of medium will not be observed in horizontal tubes ρw bnd = 350 kg/(m s). The velocity of medium at the inlet to the first row of evaporator tubes along the flow of gases (w d = 0.3 m/s) is higher than the minimal velocity at which stratification of medium will not be observed in horizontal tubes (w bnd = 0. m/s). The boundary steam quality at which dryout occurs is x bnd = 65%, which is higher than the maximal steam quality in the first row of tubes along the flow of gases (x bnd = 30.%). Thus, calculations have shown that the evaporator operating mode is reliable and efficient. CONCLUSIONS () Natural circulation in horizontal evaporators with negative useful head can be set up after optimizing the circulation loop. () More stable natural circulation in vertical HRBs is achieved in the most efficient manner by installing a U-shaped evaporator with the maximal possible number of tubes in the coil. (3) Reliable and efficient operation of the evaporator is secured in all tube rows. In making a shift from multiple forced to natural circulation we have succeeded in increasing K c from 3.0 to 7.7. () The commonly adopted criteria for stability of natural circulation are not suitable for determining reliable operation of a horizontal heater []; therefore, indirect parameters have to be used. The velocity of medium at the inlet to horizontal evaporator tubes must be higher than the minimal value from the conditions of avoiding sludge deposition and stratification of two-phase medium. The circulation ratio must be higher than the minimal value at which burnout does not occur for a two-phase flow. REFERENCES. O. M. Baldina, V. A. Lokshin, D. F. Peterson, I. E. Semenkovker, and A. L. Shvarts, Hydraulic Design of Boiler Units (a Standard Method), Ed. by V. A. Lokshin, D. F. Peterson, and A. L. Shvarts (Energiya, Moscow, 978) [in Russian].. Yu. P. Kurochkin, N. S. Galetskii, P. A. Berezinets, and A. L. Shvarts, About the possibility of shifting chimney-type heat-recovery boilers of combined-cycle plants to operate with natural circulation, Energetik, No. 6, 9 (006). 3. R. B. Dooley and R. Tilley, Guidelines for Controlling Flow-Accelerated Corrosion in Fossil and Combined Cycle Plants, EPRI Document (EPRI, Palo Alto, USA, 005).. G. I. Doverman, B. L. Shelygin, A. V. Moshkarin, and Yu. V. Mel nikov, Calculation of Boiler Units Using Modern Software Products: A Handbook (IGEU, Ivanovo, 007) [in Russian]. Translated by V. Filatov SPELL:. Baldina,. Shvarts THERMAL ENGINEERING Vol. 6 No. 7 0

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