500mm浮頭式換熱器浮動(dòng)管板工藝夾具設(shè)計(jì)【鉆φ25孔】【說(shuō)明書+CAD】,鉆φ25孔,說(shuō)明書+CAD,500mm浮頭式換熱器浮動(dòng)管板工藝夾具設(shè)計(jì)【鉆φ25孔】【說(shuō)明書+CAD】,500,mm,頭式,換熱器,浮動(dòng),工藝,夾具,設(shè)計(jì),25,說(shuō)明書,CAD ORIGINALWorachest Pirompugd Somchai WongwisesChi-Chuan WangA tube-by-tube reduction method for simultaneous heat and masstransfer characteristics for plain fin-and-tube heat exchangersin dehumidifying conditionsReceived: 19 August 2004/ Accepted: 24 November 2004/Published online: 4 March 2005? Springer-Verlag 2005Abstract This study proposed a new method, namely atube-by-tube reduction method to analyze the perfor-mance of fin-and-tube heat exchangers having plain finconfigurationunderdehumidifyingconditions.Themass transfer coefficients which seldom reported in theopen literature, are also presented. For fully wet con-ditions, it is found that the reduced results for bothsensible heat transfer performance and the mass transferperformance by the present method are insensitive tochange of inlet humidity. Unlike those tested in fully drycondition, the sensible heat transfer performance underdehumidification is comparatively independent of finpitch. The ratio of the heat transfer characteristic tomass transfer characteristic (hc,o/hd,oCp,a) is in the rangeof 0.6?1.0, and the ratio is insensitive to change of finspacing at low Reynolds number. However, a slight dropof the ratio of (hc,o/hd,oCp,a) is seen with the decrease offin spacing when the Reynolds number is sufficient high.This is associated with the more pronounced influencedue to condensate removal by the vapor shear. Corre-lations are proposed to describe the heat and massperformance for the present plate fin configurations.These correlations can describe 89% of the ChiltonColburn j-factor of the heat transfer (jh) within 15% andcan correlate 81% of the Chilton Colburn j-factor of themass transfer (jm) within 20%.Keywords Fin-and-tube heat exchanger Dehumidifying Sensible heat transfer performance Mass transfer performanceNomenclatureAfSurface area of finAoTotal surface areaAp,iInside surface area of tubesAp,oOutside surface area of tubesbpSlope of the air saturation curved between theoutside and inside tube wall temperaturebrSlope of the air saturation curved between themean water temperature and the inside walltemperaturebw,mSlope of the air saturation curved at the meanwater film temperature of the fin surfacebw,pSlope of the air saturation curved at the meanwater film temperature of the tube surfaceCp,aMoist air specific heat at constant pressureCp,wWater specific heat at constant pressureDcTube outside diameter (include collar)DiTube inside diameterfiIn-tube friction factors of waterFCorrection factorGmaxMaximum mass velocity based on minimumflow areahc,oSensible heat transfer coefficienthd,oMass transfer coefficienthiInside heat transfer coefficientho,wTotal heat transfer coefficient for wet externalfinIoModified Bessel function solution of the firstkind, order 0I1Modified Bessel function solution of the firstkind, order 1iaAir enthalpyia,inInlet air enthalpyia,mMean air enthalpyia,outOutlet air enthalpyigSaturated water vapor enthalpyW. Pirompugd S. Wongwises (&)Fluid Mechanics, Thermal Engineering andMultiphase Flow Research Lab. (FUTURE),Department of Mechanical Engineering,King Mongkuts University of Technology,Thonburi, Bangmod, Bangkok, 10140, ThailandE-mail: somchai.wonkmutt.ac.thTel.: +66-2-4709115Fax: +66-2-4709111C.-C. WangEnergy and Resources Lab.,Industrial Technology Research Institute,Hsinchu, Taiwan, ROCHeat Mass Transfer (2005) 41: 756765DOI 10.1007/s00231-004-0581-ximMean enthalpyir,inSaturated air enthalpy at the inlet water tem-peratureir,mMean saturated air enthalpy at the meanwater temperatureir,outSaturated air enthalpy at the outlet watertemperatureis,fmSaturated air enthalpy at the fin mean tem-peratureis,fbSaturated air enthalpy at the fin base tem-peratureis,p,i,mMean saturated air enthalpy at the mean in-side tube wall temperatureis,p,o,mMean saturated air enthalpy at the meanoutside tube wall temperatureis,wSaturated air enthalpy at the water film tem-peratureis,w,mMean saturated air enthalpy at the meanwater film temperature of the fin surfacejhChilton-Colburn j-factor of the heat transferjmChilton-Colburn j-factor of the mass transferK0Modified Bessel function solution of the sec-ond kind, order 0K1Modified Bessel function solution of the sec-ond kind, order 1kfThermal conductivity of finkiThermal conductivity of waterkpThermal conductivity of tubekwThermal conductivity of water filmLpTube length_ maAir mass flow rate_ mwWater mass flow rateNNumber of tube rowPPressurePlLongitudinal tube pitchPrPrandtl numberPtTransverse tube pitch_QHeat transfer rate_QaAir side heat transfer rate_QavgAverage heat transfer rate_QtotalTotal heat transfer rate_QwWater side heat transfer rateRRatio of heat transfer characteristic to masstransfer characteristicRHRelative humidityriDistance from the center of the tube to the finbaseroDistance from the center of the tube to the fintipReDiReynolds number based on inside diameterReDcReynolds number based on outside diameter(include collar)ScSchmidt numberSpFin spacingTaAir temperatureTwWater temperatureTw,mMean temperature of the water filmTp,i,mMean temperature of the inner tube wallTp,o,mMean temperature of the outer tube wallTr,mMean temperature of watertFin thicknessUo,wOverall heat transfer coefficientVAverage velocityWaHumidity ratio of moist airWa,mMean air humidity ratioWs,p,o,mMean saturated air humidity ratio at the meanoutside tube wall temperatureWs,wSaturated air humidity ratio at the water filmtemperatureWs,w,mMean saturated air humidity ratio at the meanwater film temperature of the fin surfaceywThickness of condensate water filmeFin factorgf,wetWet fin efficiencylDynamic viscosityqMass density1 IntroductionThe most widely used heat exchangers take the form offin-and-tubeconfigurationinassociationwiththeapplication of air-conditioning and refrigeration sys-tems. The heat exchangers can be applicable to con-denser and evaporators. In the evaporators whichtypically use aluminum fins and the surface temperatureof the fins is generally below the dew point temperature.As a result, simultaneous heat and mass transfer occursalong the fin surfaces. In general, the complexity of themoist air flow pattern across the fin-and-tube heatexchangers under dehumidifying conditions makes thetheoretical simulations very difficult. Accordingly, it isnecessary to resort to experimentation.Many experimental studies have been carried out tostudy the heat and mass transfer characteristics of thefin-and-tube heat exchangers under dehumidifying con-ditions. For instance, McQuiston 11, 12 presentedexperimental data for five plate fin-and-tube heatexchangers, and developed a well-known heat transferand friction correlation for both dry and wet surfaces.Mirth and Ramadhyani 13, 14 investigated the heatand mass characteristics of wavy fin heat exchangers.Their results showed that the Nusselt numbers were verysensitive to change of inlet dew point temperatures, andthe Nusselt number decreases with an increase of dewpoint temperatures. Similar results were reported by Fuet al. 7 in dehumidifying heat exchangers having alouver fin configuration. They reported a pronounceddecrease of the wet sensible heat transfer coefficientswith increases of inlet relative humidity. On the con-trary, the experimental data of Seshimo et al. 19 indi-catedthattheNusseltnumberwasrelativelyindependent of inlet conditions. Wang et al. 23 study757the effect of the fin pitch, the number of tube row, andinlet relative humidity on the heat transfer performanceunder dehumidification, and concluded that the sensibleheat transfer performance is relatively independent ofinlet humidity. The difference in the existing literatures isattributed to the different reduction methodology.Even though many efforts have been devoted to thestudy of the wet-coils, the available literature on thedehumidifying heat exchangers still offers limited infor-mation to assist the designer in sizing and rating a fin-and-tube heat exchanger. This can be made clear fromthe reported data were mainly focused on the study ofthe sensible heat transfer characteristics, little attentionwas paid to the mass transfer characteristics. Therefore,the objective of the present study is to provide furthersystematic experimental information relevant to themass transfer performance and propose a new reductionmethod to determine the air-side performance of fin-and-tube heat exchangers under dehumidifying condi-tions. The effects of fin spacing and the inlet relativehumidity on the mass transfer characteristics are exam-ined in this study.2 Experimental apparatusThe schematic diagram of the experimental air circuitassembly is shown in Fig. 1. It consists of a closed-loopwind tunnel in which air is circulated by a variable speedcentrifugal fan (7.46 kW, 10 HP). The air duct is madeof galvanized sheet steel and has an 850550 mm cross-section. The dry-bulb and wet-bulb temperatures of theinlet air are controlled by an air-ventilator that canprovide a cooling capacity up to 21.12 kW (6RT). Theair flow-rate measurement station is an outlet chamberset up with multiple nozzles. This setup is based on theASHRAE 41.2 standard 3. A differential pressuretransducer is used to measure the pressure differenceacross the nozzles. The air temperatures at the inletand exit zones across the sample heat exchangers aremeasured by two psychrometric boxes based on theASHRAE 41.1 standard 2.The working medium or the tube side is cold water. Athermostatically controlled reservoir provides the coldwater at selected temperatures. The temperature differ-ences on the water side are measured by two precali-bratedRTDs.Thewatervolumetricflowrateismeasured by a magnetic flow meter with a 0.001 L/sprecision. All the temperature measuring probes areresistance temperature devices (Pt100), with a calibratedaccuracy of 0.05?C. In the experiments, only the datathat satisfy the ASHRAE 3378 1 requirements,(namely, the energy balance condition,_Qw?_Qavg?=_Qavg; is less than 0.05, where_Qwis the water-side heattransfer rate for_Qwand air-side heat transfer rate_Qa),are considered in the final analysis. Detailed geometryused for the present plain fin-and-tube heat exchangersis tabulated in Table 1. The test fin-and-tube heatexchangers are tension wrapped having a L type fincollar. The test conditions of the inlet air are as follows:The test conditions approximate those encounteredwith typical fan-coils and evaporators of air-condition-ing applications. Uncertainties reported in the presentinvestigation, following the single-sample analysis pro-posed by Moffat 15, are tabulated in Table 2.3 Data reduction3.1 Heat transfer coefficient (hc,o)Basically, the present reduction method is based on theThrelkeld 20 method. Some important reduction pro-Fig. 1 Schematic ofexperimental setupDry-bulb temperatures of the air:270.5?CInlet relative humidity for theincoming air:50% and 90%Inlet air velocity:From 0.3 m/s to 4.5 m/sInlet water temperature:70.5?CWater velocity inside the tube:1.51.7 m/s758cedures for the original Threlkeld method is described asfollows.The total heat transfer rate used in the calculation isthe mathematical average of_Qaand_Qw; namely,_Qa _ ma(ia;in? ia;out),1_Qw _ mwCp;wTw;out? Tw;in;2_Qavg_Qa_Qw2:3The overall heat transfer coefficient, Uo,w, is based onthe enthalpy potential and is given as follows:_Qavg Uo;wAoDimF;4where Dimis the mean enthalpy difference for counterflow coil,Dim ia;m? ir;m:5According to Bump 4 and Myers 16, for thecounter flow configuration, the mean enthalpy isia;m ia;inia;in? ia;outlnia;in? ir;out?ia;out? ir;in?ia;in? ia;outia;in? ir;outia;in? ir;out ? (ia;out? ir;in;6ir;m ir;outir;out? ir;inlnia;in? ir;out?ia;out? ir;in?ir;out? ir;in)(ia;in? ir;out)ia;in? ir;out) ? ia;out? ir;in;7where F in Eq. 4 is the correction factor accounting forthe present cross-flow unmixed/unmixed configuration.The overall heat transfer coefficient is related to theindividual heat transfer resistance 16 as follows:1Uo;wb0rAohiAp;ib0pAoln Dc=Di2pkpLp1ho;wAp;o.b0w;pAo?Afgf;wet.b0w;mAo?;8whereho,w1Cp;a.b0w;mhc;o? yw=kw;9ywin Eq. 9 is the thickness of the water film. Aconstant of 0.005 in. was proposed by Myers 16. Inpractice, (yw/kw) accounts for only 0.55% compared to(Cp,a/bw,mhc,o), and has often been neglected by previ-ous investigators. As a result, this term is not included inthe final analysis.In this study, we had proposed a row-by-row andtube-by-tube reduction method for detailed evaluationof the performance of fin-and-tube heat exchanger in-stead of conventional lump approach. Hence analysis ofthe fin-and-tube heat exchanger is done by dividing itinto many tiny segments (number of tube row numberof tube per row number of fin) as shown in Fig. 2. Inthe analysis, F is the correction factor accounting for asingle-pass, cross-flow heat exchanger for one fluidmixed, other fluid unmixed that was shown by Threlkeld20.The tube-side heat transfer coefficient, hievaluatedwith the Gnielinski correlation 8,Fig. 2 Dividing of the fin-and-tube heat exchanger into the smallpiecesTable 2 Summary of estimated uncertaintiesPrimary measurementsDerived quantitiesParameterUncertaintyParameterUncertaintyReDc=400UncertaintyReDc=5,000_ ma0.31%ReDc1.0%0.57%_ mw0.5%ReDi0.73%0.73%DP0.5%_Qw3.95%1.22%Tw0.05?C_Qa5.5%2.4%Ta0.1?Cj11.4%5.9%Table 1 Geometric dimensions of the sample plain fin-and-tubeheat exchangersNo.Fin thickness(mm)Sp(mm)Dc(mm)Pt(mm)Pl(mm)Rowno.10.1151.088.5125.419.05120.1201.6310.3425.422.00130.1151.938.5125.419.05140.1152.1210.2325.419.05150.1202.3810.3425.422.00160.1151.128.5125.419.05270.1201.588.6225.419.05280.1151.958.5125.419.05290.1203.018.6225.419.052100.1302.1110.2325.422.002110.1151.1210.2325.419.054120.1151.4410.2325.419.054130.1152.2010.2325.419.054140.1302.1010.2325.422.004150.1301.7210.2325.422.006160.1302.0810.2325.422.006170.1303.0310.2325.422.006759hifi=2ReDi? 1000Pr1:07 12:7ffiffiffiffiffiffiffiffifi=2pPr2=3? 1?kiDi;10and the friction factor, fiisfi11:58ln ReDi? 3:282:11The Reynolds number used in Eqs. 10 and 11 is basedon the inside diameter of the tube and ReDi qVDi=l:In all case, the water side resistance is less than 10% ofthe overall resistance.In Eq. 8 there are four quantities (bw,m, bw,p, bpandbr) involving enthalpy-temperature ratios that must beevaluated. The quantities of bpand brcan be calculatedasb0ris;p;i;m? ir;mTp;i;m? Tr;m;12b0pis;p;o;m? is;p;i;mTp;o;m? Tp;i;m:13The values of bw,pand bw,mare the slopes of satu-rated enthalpy curve evaluated at the outer mean waterfilm temperature at the base surface and at the fin sur-face. Without loss of generality, bw,pcan be approxi-matedbytheslopeofsaturatedenthalpycurveevaluated at the base surface temperature 23. The wetfin efficiency (gf,wet) is based on the enthalpy differenceproposed by Threlkeld 20. i.e.,gf,weti ? is,fmi ? is,fb;14where is,fmis the saturated air enthalpy at the meantemperature of fin and is,fbis the saturated air enthalpyat the fin base temperature. The use of the enthalpypotential equation, greatly simplifies the fin efficiencycalculation as illustrated by Kandlikar 10. However,the original formulation of the wet fin efficiency byThrelkeld20wasforstraightfinconfiguration(Fig. 2a). For a circular fin (Fig. 2b), the wet finefficiency is 23,gf;wet2riMT(r2o? r2i)?K1(MTri)I1(MTro) ? K1(MTro)I1(MTri)K1(MTro)I0(MTri) K0(MTri)I1(MTro)?;15whereMTffiffiffiffiffiffiffiffiffiffiffi2ho;wkftr;16The test heat exchangers are of Fig. 3c configura-tion. Hence, the corresponding fin efficiency is calcu-latedbytheequivalentcircularareamethodasdepicted in Fig. 4.Evaluation of bw,mrequires a trial and error proce-dure. For the trial and error procedure, is,w,mmust becalculated using the following equation:is;w;m ia;m?Cp;aho;wgf;wetb0w;mhc;o?1 ? Uo;wAob0rhiAp;ib0pln Dc=Di2pkpLp# !? ia;m? ir;m:17An algorithm for solving the sensible heat transfercoefficient hc,ofor the present row-by-row and tube-by-tube approach is given as follows:1. Based on the measurement information, calculate thetotal heat transfer rate_Qtotalusing Eq. (3).2. Assume a hc,ofor all elements.3. Calculate the heat transfer performance for eachsegment with the following procedures.3.1. Calculate the tube side heat transfer coefficient ofhiusing Eq. 10.3.2. Assume an outlet air enthalpy of the calculatedsegment.3.3. Calculate ia,mby Eq. 6 and ir,mby Eq. 7.3.4. Assume Tp,i,mand Tp,o,m.3.5. Calculate b0rAo?= hiAp;i?andb0pAoln Dc=Dihi=h2pkpLp?.3.6. Assume a Tw,m.3.7. Calculate the gf,wetusing Eq. 15.3.8. Calculate Uo,wfrom Eq. 8.3.9. Calculate is,w,mby Eq. 17.3.10. Calculate Tw,mfrom is,w,m.Fig. 3 Type of fin configurationFig. 4 Approximation method for treating a plate fin of uniformthickness7603.11. If Tw,mderived in step 3.10 is not equal that isassumed in step 3.6, the calculation step 3.73.10 will be repeated with Tw,mderived in step3.10 until Tw,mis constant.3.12. Calculate_Q of this segment.3.13. Calculate Tp,i,mand Tp,o,mfrom the insideconvection heat transfer and the conductionheat transfer of tube and collar.3.14. If Tp,i,mand Tp,o,mderived in step 3.13 are notequal that is assumed in step 3.4, the calculationstep 3.53.13 will be repeated with Tp,i,mandTp,o,mderived in step 3.13 until Tp,i,mand Tp,o,mare constant.3.15. Calculate the outlet air enthalpy by Eq. 1 andthe outlet water temperature by Eq. 2.3.16. If the outlet air enthalpy derived in step 3.15 isnot equal that is assumed in step 3.2, the cal-culation step 3.33.15 will be repeated with theoutlet air enthalpy derived in step 3.15 until theoutlet air enthalpy is constant.4. If the summation of_Q for all elements is not equal_Qtotal, hc,owill be assumed a new value and the cal-culation step 3 will be repeated until the summationof_Q for all elements is equal_Qtotal.3.2 Mass transfer coefficient (hd,o)For the cooling and dehumidifying of moist air by a coldsurface involves simultaneously heat and mass transfer,and can be described by the process line equation fromThrelkeld 20:diadWa Ria? is;wWa? Ws;w ig? 2;501R;18Where R represent the ratio of sensible heat transfercharacteristics to the mass transfer performance.R hc;ohd;oCp;a:19However, for the present fin-and-tube heat ex-changer, Eq. 18 did not correctly describe the dehu-midification process on the psychrometric chart. This isbecause the saturated air enthalpy (is,w) at the meantemperature at the fin surface is different from that at thefin base. In this regard, a modification of the process lineon the psychrometric chart corresponding to the fin-and-tube heat exchanger is made. The derivation is as fol-lows.From the energy balance of the dehumidification onecan arrive at the following expression:_ madiahc;oCp;adAp;oia;m? is;p;o;m hc;oCp;adAfia;m? is;w;m:20Note that the first term on the right-hand side de-notes the sensible heat transfer whereas the second termis the latent heat transfer. Conservation of the watercondensate gives:_ madWa hd;odAp;oWa;m? Ws;p;o;m hd;odAfWa;m? Ws;w;m:21Dividing Eq. 20 by Eq. 21 yieldsdiadWaR ? ia;m? is;p;o;m R ? e ? 1 ? ia;m? is;w;mWa;m? Ws;p;o;m e ? 1 ? Wa;m? Ws;w;m;22wheree AoAp;o:23By assuming a value of the ratio of heat transfer tomass transfer, R and by integrating Eq. 22 with aniterative algorithm, the mass transfer coefficient can beobtained. Analogous procedures for obtaining the masstransfer coefficients are given as:1. Obtain Ws,p,o,mand Ws,w,mfrom is,p,o,mand is,w,mfrom those calculation of heat transfer.2. Assume a value of R.3. Calculations is performed from the first element tothe last element, employing the following procedures:3.1. Assume an outlet air humidity ratio.3.2. Calculate the outlet air humidity ratio of eachelement by Eq. 22.3.3. If the outlet air humidity ratio obtained fromstep 3.2 is not equal to the assumed value of step3.1, the calculation steps 3.1 and 3.2 will be re-peated.4. If the summation of the outlet air humidity ratio foreach element of the last row is not equal to themeasured outlet air humidity ratio, assuming a new Rvalue and the calculation step 3 will be repeated untilthe summation of the ou