Heat Transfer and Friction Characteristics of Heat Exchanger Under Lignite Fly-Ash

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1 The 20th Cnferene f Mehanial Engineering Netwrk f Thailand Otber 2006, Nakhn Rathasima, Thailand Heat Transfer and Fritin Charateristis f Heat Exhanger Under ignite Fly-Ash Pipat Juangjandee 1*, Ass. Prf. Thawan uharitakul 2 1*, 2 Department f Mehanial Engineering, Faulty f Engineering, Chiang Mai University, Chiang Mai 200, Thailand. 1* Tel.: , Fax.: , pipat.ju@egat..th 2 Tel.: , Fax: , , thawan@dme.eng.mu.a.th Abstrat This researh wrk was t investigate heat transfer and pressure drp harateristis in rssflw heat exhanger under lignite fly-ash nditin. The tube rws f a bank were aligned and the plain tubes were seleted fr investigatin. The result was divided in t 2 items. Firstly, fr lean air nditin, the lngitudinal tube pith effets n heat exhanger effetiveness, nvetin effiient, Nusselt number, and pressure drp. Therefre, at a larger lngitudinal tube pith, the effetiveness, nvetin effiient, Nusselt number, and pressure drp values were higher. Finally, fr air-fly ash nditin, at slightly dust flw rate, the effet f dust is nt muh and slightly different with lean air nditin, but at high dust flw, they were redued and lwer than the values in lean air nditin. investigate the perfrmane f rss-flw heat exhanger under lignite fly-ash nditin. 2. Perfrmane data In this wrk, the rss-flw heat exhanger, the tube rws f a bank were aligned, and plain tubes were seleted fr investigatin. The tested data in eah mdel were rerded every 10 minutes, 3 hurs after mpleted heat sak. The experiment setup, aligned tube arrangements, dimensins f rss-flw heat exhanger, and fly-ash hemial mpsitin are shwn in figure 1, figure 2, table 1, and table 2 respetively. Keywrds: heat exhanger, lignite fly-ash mixture nditin, perfrmane 1. Intrdutin Heat exhanger is a devie that is used t transfer thermal energy frm higher temperature heat sure t lwer temperature heat sink. There are many types f heat exhanger appliable t revery f the waste heat suh as shell-and-tube heat exhanger, plate-type heat exhanger, and rss-flw heat exhanger. The rss flw type is very ppular due t lw st and easy t lean and lear. In lignite-fired pwer plants, many f rssflw heat exhangers were used i.e. primary air heater, enmizer, and flue gas desulfurizatin plant gas-gas reheater et. Nrmally, existing heat exhangers are perated under the high partiulate nditin whih are fly-ash frm the mbustin press and tend t derease its perfrmane. Unfrtunately, there is lak f data abut the perfrmane dereasing due t this nditin. Therefre, the bjetive f this researh wrk is t Figure 1 Experiment setup Figure 2 In-line tube arrangements f rss-flw heat exhanger.

2 Table 1 Dimensins f tested rss-flw heat exhanger. Categries alues Tube utside diameter (m) Tube inside diameter (m) Tube length (m) 0.5 Tube material Cu Tube thermal ndutivity (W/m K) Transverse tube pith (m) 1.33 D ngitudinal tube pith (m) 1.33 D, 2.66 D, 3.99 D Number f tubes 64 Number f tube rws 4 The symbls were listed in nmenlature item. Table 2 Fly-ash hemial mpsitin. Categries alues (%) Na 2 O 1.46 MgO 3.41 Al 2 O io P 2 O O K 2 O 2.44 CaO TiO MnO Fe 2 O The airflw arss the heat exhanger was generated by an eletrial air blwer with the ntrllable range f kg/s by using a frequeny inverter. The mass flw rate f air stream was measured by a standard flw meter and an inlined manmeter with ±0.5 Pa auray. The inlet and the utlet temperatures f air stream were als measured by anther set f K-type thermuple mesh. Nte that all f thermuples have been alibrated t ±0.1 ºC auray. The pressure drp arss the heat exhanger was als measured by the inlined manmeter with ±0.5 Pa auray. The inline tube arrangements are tested in this study. The effets f air flw and air-fly ash flw n the air side perfrmane are examined ardingly. 3. Theretial analysis In this experiment, ld air flws arss the bank f tubes but the ht fluid flwing inside the tubes and transfers heat t the air whih plaed in the utside f the tubes, and the heat transfer rate (Q& ) an be alulated as equatin (1) and (2). & (1) Q = m& h ( T p, h hi T h ) Q & = m& ( T T ) (2) p, The heat transfer rate an be alulated in the frm f lg mean temperature differene methd as equatin (3). i Q & = UAΔ (3) T lm The verall heat transfer effiient area f the heat exhanger an be evaluated in the term f thermal resistane as equatin (4). 1 1 ln = + UA h A 2πk ( D / D ) t i + 1 hi A i Fr the aligned tube arrangement, the maximum velity urs at transverse plane A1 n figure 2, an be alulated as equatin (5). = T D (4) max (5) T ynlds number arss the bank f tubes are defined as equatin (6). ρ max D D, = (6) μ The tube side heat transfer effiient an be estimated by Dittus-Belter equatin, [2], in the term f Nusselt number and ynlds number as equatin (7) and (8) respetively. 0.8 n = 0.023, Pr (7) Nu D i where n =0.4 fr heating, n=0.3 fr ling. 4m& D, i = (8) π D μ Nte that Nusselt number and Prandtl number in this wrk are defined as equatin (9) and (10) respetively. i

3 h D Nu = (9) k p μ Pr = (10) k Pressure drp in rss-flw tube arrays, whih may be expressed as equatin (11), [3]. 2 p ρ N f max Δ = 2 f χ (11) where fritin fatr, and rretin fatr, χ may be reeived frm figure 7.13 in [3]. 4. sults and disussin The seleted rss-flw heat exhanger was tested under lean air and air-dust mixture nditins in the frm f inline tube arrangement. Fr lean air nditin, the test was divided int 3 ases suh as ase 1;, ase 2;, and 33 = D = 1. D = 2. ase 3; D. Therefre, the test result and disussin were shwn as fllwing. Cnvetin effiient Clean air nditin, Inline ase 1 ase 2 ase 3 Figure 3 latinship between nvetin effiient and frntal velity under lean air nditin. fer t figure 3, shws learly that nvetin effiient values were inreased while frntal velities were inreased. This test result has dne under lean air nditin. It was fund that the lngitudinal tube pith effets n nvetive heat transfer effiient, at a larger lngitudinal tube pith, the nvetive heat transfer effiients were higher. A lw flw resistane results in a dereased heat transfer effiient. Therefre, this figure shws that the heat transfer effiient dereases as lngitudinal tube pith dereases. Effetiveness Clean air nditin, inline 0.25 ase 1 ase 2 ase 3 Figure 4 latinship between effetiveness and frntal velity under lean air nditin. Figure 4, shws learly that the heat exhanger effetiveness values were redued while frntal velities were inreased. This test result has dne under lean air nditin. It was fund that the lngitudinal tube pith effets n effetiveness, at a larger lngitudinal tube pith, the effetiveness values were higher. Nusselt number Clean air nditin, inline ynlds number ase 1 ase 2 ase 3 Figure 5 latinship between Nusselt number and ynlds number under lean air nditin. fer t figure 5, shws learly that Nusselt number values inreases as ynlds inreases. It was fund that at a larger lngitudinal tube pith, the Nusselt number values were higher. The fatrs gverning resistane t flw als determine heat transfer. Therefre, this fatrs affeting the Nusselt number t. Pressure drp (Pa) Clean air nditin, inline 0 ase 1 ase 2 ase 3 Figure 6 latinship between pressure drp and frntal velity under lean air nditin.

4 Figure 6, shws learly that pressure drp values arss the bank f tubes inreases as frntal velity inreases. It was fund that at a larger lngitudinal tube pith, the pressure drp values were higher. The pressure drp dereases as the tubes are brught lser tgether. This result was urred beause the reduing the area f turbulene between the tubes has a remarkable effet n the fritin fatr and the pressure lss. Air-fly ash nditin, Inline Cnvetin effient Air-fly ash nditin, inline lean air dust flw 0.00 kg/s dust flw kg/s dust flw kg/s Heat transfer (J/s) lean air dust flw kg/s dust flw kg/s dust flw kg/s Figure 7 latinship between heat transfer and frntal velity at lngitudinal tube pith equal t 2.66 D under air fly-ash mixture nditin. Figure 7 and 8, In the lngitudinal tube pith equal t 2.66 D and under air fly-ash mixture nditin, the heat transfer values and nvetin effiient values were inreased while the frntal velities were inreased. At slightly dust flw rate, the effet f dust is nt muh and slightly different than the values in lean air nditin, but at high dust flw rate, the heat transfer values and nvetin effiient values were lwer than the values in lean air nditin respetively. fer t figure 9, shws relatinship between effetiveness and frntal velity under air-fly ash mixture nditin at lngitudinal tube pith equal t 2.66 D, the effetiveness values were redued while frntal velities were inreased. At slightly dust flw rate, the effet f dust is nt muh and slightly different with lean air nditin, but at high flw rate f dust, the effetiveness values were lwer than the values in lean air nditin. Figure 8 latinship between nvetin effiient and frntal velity under air-fly ash mixture nditin. Effetiveness Air-fly ash nditin, Aligned lean air dust flw kg/s dust flw kg/s dust flw kg/s Figure 9 latinship between heat exhanger effetiveness and frntal velity at lngitudinal tube pith equal t 2.66 D under air fly-ash mixture nditin. Nusselt number Air-fly ash nditin,inline 3,000 4,000 5,000 6,000 7,000 8,000 9,000 lean air dust flw kg/s ynlds number dust flw kg/s dust flw kg/s Figure 10 latinship between Nusselt number and ynlds number at lngitudinal tube pith equal t 2.66 D under air fly-ash mixture nditin.

5 Figure 10, In the lngitudinal tube pith equal t 2.66 D and under air fly-ash mixture nditin, the Nusselt numbers were inreased while the ynlds numbers were inreased. At slightly dust flw rate, the effet f dust is nt muh and slightly different with lean air nditin, but at high dust flw rate, the Nusselt numbers were lwer than the values in lean air nditin. 5. Cnlusin It an be nluded as fllwing. 5.1 Fr lean air nditin, the lngitudinal tube pith effets n heat exhanger effetiveness, nvetin effiient, Nusselt number, and pressure drp. Therefre, at a larger lngitudinal tube pith, the effetiveness, nvetin effiient, Nusselt number, and pressure drp values were higher. 5.2 Fr air-fly ash nditin, at slightly dust flw rate, the effet f dust is nt muh and slightly different with lean air nditin, but at high dust flw, they were redued and lwer than the values in lean air nditin. Aknwnledgments The authrs gratefully aknwledge the supprt prvided by the Eletriity Generating Authrity f Thailand and Chiang Mai university. ferenes [1] Drisll, J.M. et al, AME perfrmane test des test de fr air heaters, UA, [2] Hewitt, G.F., hires, G.. and Btt, T.R., Press Heat Transfer, Ba Ratn, CRC Press, UA, [3] Inrpera, F.P. and Dewitt, D.P., Intrdutin t heat transfer, 4 th editin, Jhn Wiley & ns In., UA, [4] hah, R.K., ekuli, D.P., Fundamentals f heat exhanger design, Jhn Wiley & ns In., Canada, [5] Juangjandee, P., uharitakul, T, and Nuntaphan, A., Perfrmane analysis f primary air heater f al-fired pwer plant under partiulate nditin, The 3 rd Cnferene n heat and mass transfer, Chiang Mai, Thailand, Nmenlature A area (m 2 ) p speifi heat at nstant pressure (J/kg K) p, speifi heat f ld medium at nstant pressure (J/kg K) p, h speifi heat f ht medium at nstant pressure (J/kg K) D diameter (m) f fritin fatr h heat transfer effiient k thermal ndutivity (W/m K) length (m) m& mass flw rate (kg/s) Nu Nusselt number p pressure (Pa) Pr Prandtl number heat transfer rate (W) Q & D T T U ynlds number transverse tube pith (m) lngitudinal tube pith (m) temperature ( C) verall heat transfer effiient velity (m/s) Greek symbl μ dynami vissity (Pa s) ρ density (kg/m 3 ) χ rretin fatr ubsript a air side ld h ht i inner r inlet max maximum uter t tube