Performance comparison of heat exchangers using sextant/trisection helical baffl

时间：2024-05-22

Yaping Chen *,Hongling Tang2,Jiafeng WuHuaduo GuShifan Yang

1 Engineering Research Center of BEEE,Ministry of Education,School of Energy and Environment,Southeast University,Nanjing 210096,China

2 Hefei Yidu Education Consulting Co.Ltd,TAL Education Group,Hefei 230031,China

Keywords:Heat transfer Computational fluid dynamics Convection Helical baffle heat exchangers Sextant helical baffles

ABSTRACT The shell side flow fields of both sextant and trisection helical baffle heat exchangers are presented on meridian and multilayer hexagon slices.It verifies that the performance of sextant schemes is better than those of the other kinds of helical baffle heat exchangers.The main mechanisms are due to the restricted leakage flow in the minimized gaps with increased baffle number and by one row of tubes dampen the leakage flow in the circumferential overlapped area of the adjacent helical baffles.The performance features were simulated on two different angled sextant helical heat exchangers and each compared with two trisection ones of either identical helical pitch or identical incline angle.The results verified that the performances of helical heat exchangers are mainly determined by the helical pitch rather than the baffle incline angle.The average values of comprehensive indexof the trisection helical schemes T-24.1° and T-29.7° are correspondingly 3.47% and 3.34%lower than those of the sextant ones X-20°and X-25°with identical helical pitches.The comparison results show that the average values of shell side h.t.c.hoand comprehensive indexof the optimal dual helix sextant scheme DX30°are respectively 7.22%and 23.56%higher than those of the segment scheme S100.

1.Introduction

The tube-and-shell heat exchangers are the most widely used heat exchange equipment used in petroleum,chemical industry,medicine,metallurgy,power generation and some other industries,because of their simple and robust structure,mature design method,easy manufacture and installation,wide range of material selection,sustainable to high temperature and high pressure,easy to clean in tube-side,strong flexibility suitable for almost all heat and mass transfer processes[1-6].The helical baffle heat exchanger is a hotly studied and applied type in replacement of the traditional segmental baffle one[7-11],aiming at overcoming the drawbacks of the segmental baffle schemes,such as,lower heat transfer coefficient with flow stagnant zones,greater pressure drop,vibration and fouling.

Since Lutcha and Nemcansky[12]put forward the quadrant helical baffle heat exchangers,the helical baffle structures have caused wide public interest by domestic and foreign scholars[13,14].The conventional arrangement of the baffles is either end-to-end or axial overlap that the adjacent baffles touch at peripheral or at middle of the straight edges[15,16],as shown in Fig.1(a)and(b).However,the shortcut leakage problem of such arrangements of the quadrant helical baffles is serious in either the V-notches of an end-to-end scheme or the Xnotches of an axial overlap scheme.Wang et al.[17]studied continuous helical baffle heat exchanger,as shown in Fig.1(c),to eliminate the shortcut leakage problem.Nevertheless,the obvious drawback of the continuous baffle scheme is its difficult manufacture.Wen et al.[18]studied the half helical baffle heat exchanger,which adopts the bending method where the baffle plate is divided into inclined middle section and perpendicular end sections,to eliminate the leakage at the joints.Du et al.[19]studied a sextant helical baffle scheme,which arranges six quadrant baffles in one cycle.However,it is not a good solution,since the circumferential overlap along the radius direction is not properly designed so that the overlap is small at the center where the gap is large,but it is great at the peripheral where the gap is small.Farhad et al.[20]simulated several quadrant helical baffle heat exchangers with different axial overlap but identical incline angle.The results show that when the baffle axial overlap increases or the helical pitch decreases,both the shell side heat transfer coefficient and pressure drop per unit length increases,but the comprehensive index decreases;and the end-to-end scheme with zero axial overlap is the best among the studied schemes.It verified the postulation that the axial overlap scheme is not a good design[21],since the outer notch of a X-notch in an axial overlap scheme opens a direct(not reversed)shortcut leakage to the downstream channel.

Fig.1.Connection configurations of helical baffles.

The quadrant helical baffle scheme is more suitable for the square tube arrangement,while for the more popular,compact and efficient equilateral triangular tube layout,the quadrant helical baffles cannot match the natural interval of the tube layout well so that the half of the holes at one of the straight edges of the baffle plate cannot be avoid.Nevertheless,the fact that the quadrant helical baffles are unmatched with the equilateral triangular tube layout had not been realized until Chen[21]proposed the trisection helical baffle heat exchanger structure.Chen et al.[22,23]then conducted further modification with the trisection circumferential overlap helical baffles that accommodate one row of tubes in the overlapped area to dampen the leakage flow as shown in Fig.1(d),then the leakage at the joints of adjacent baffles can be inhibited,and performance of the helical baffle heat exchangers can be improved.The experimental results by Chen et al.[22]with heat conduction oil and glycol aqueous solution,Dong et al.[23]with water-water,and Chen et al.[24]with the oil/water-water,verified that the performance of the trisection helical baffle schemes are much better than that of the segmental ones.The results were improved not only with reduced pressure drop but also with increased values of both heat transfer coefficient and comprehensive index,in contrast to those who obtained improved comprehensive index but reduced heat transfer coefficient,such as Ref.[25].Dong et al.[26]and Chen et al.[27]also performed numerical simulation research on the trisection overlap helical baffle heat exchangers with the flow fields and thermal distributions demonstrated not only on the meridian and transverse slices,but also on the hexagonal and spiral slices for the deep observation.The simulation results show that the secondary vortex flow is formed in the shell side helical channels of the heat exchanger,which can not only strengthen the mixing of fluids at different positions beneficial to the heat transfer enhancement,but also prevent fouling and scaling.Dong et al.[28]also obtained a conclusion by numerical simulation that the performance of the trisection circumferential overlap helical baffle heat exchanger are much better than those of the continuous helical baffle heat exchanger.The possible reason is that by restricting the shortcut leakage with the circumferential overlap helical baffles,the structure of the non-continuous helical baffles has also the advantages of enhancing heat transfer by cutting off the boundary layer and exchanging micelle fluid positions,just analog to the performance of the offset fins which is much better than that of the straight fins or even the corrugated fins in a plate-fin heat exchanger.

Chen and Wu[29]suggested adopting the method of inclined laser beam cutting in ellipse orbits for the manufacture of helical baffles,to remediate the complexity in manufacturing the inclined helical baffles for drilling the tube holes and turning the outer surface of the curved contour edge of the inclined baffles.It is especially valuable for saving the preparing time and tooling cost with small batch multi-task applications.Tightening helical baffles at the circumferential overlapped area can solidify the structure and reduce the number of the fasten rod assemblies in real application.However,some measures are needed since the adjacent baffles of the helical heat exchanger are tilt to different stereo direction.There are two ways to solve the problem,one is to use wedged washers for compensating both sides of the baffle at the rod holes,and the other is to fold the baffle ears[30].The folding ear scheme can not only facilitate the fastening the helical baffles,but also can further inhibit the negative effect of the shortcut leakage.By such correction,the more convenient perpendicular sleeve tubes can be used to span and fasten the helical baffles of the tube bundle.In addition,for both compensation methods with wedged washers and folding baffle ears,the helical pitch can be calculated conveniently by knowing the geometric parameters.

From the existing applications it shows that the heat transfer coefficient of the large helical baffle heat exchangers might be worse than that of the segmental baffle ones[25].The main reason is that the greater the diameter of the heat exchanger is,the greater the gaps between adjacent baffles for shortcut leakage flow are,especially with greater incline angle.Also the improper design concept of axial overlap baffle configuration is prevalent,which leads to the higher heat transfer coefficient but lower comprehensive index.This situation will be changed if the sextant circumferential overlap helical baffle heat exchangers are adopted with their features of restricting leakage flow and fitting equilateral triangular tube layout.Due to the increased number of baffle plates,the helical line formed by the sextant helical baffle scheme is closer to the continuous helix,which makes the fluid flow smooth,and its heat transfer performance enhanced at minimized pressure drop penalty.In addition,with the reduced size of the baffle plate,the chord length of the sextant helical baffle plate is similar to that of the radius,and the manufacture difficulty is reduced for the large-sized helical baffle heat exchangers.

2.Methods

2.1.Physical models

The physical models established in the simulation are the tube-andshell heat exchangers with sextant and trisection circumferential overlapped helical baffles and segmental ones.The heat transfer fluids are water to water.The inside diameter of the shell of studied heat exchangers is Φ150 mm and the projection diameter and the thickness of the baffle are respectively Φ147 mm and 3 mm.The diameter of the inlet and outlet nozzles is Φ50 mm for both tube and shell sides.The diameter and length of heat exchange tube are Φ10 mm and 1500 mm respectively;the tube thickness is simplified as zero and the tube pitch(distance between adjacent tube centers)is 16 mm.The total numbers of tubes and assemblies of rods and sleeve tubes are 55 and 6.Two comparison groups were set for the sextant schemes and their trisection counterparts of the identical helical pitch and the identical incline angle.The schemes in the two groups are denoted as X-20°,T-24.1°,T-20°and X-25°,T-29.7°,T-25°,where X and T represent respectively sextant and trisection schemes and the digit number followed is the inclined angle.The main structural parameters of the six schemes are shown in Table 1,and the physical models of schemes X-25°,and the left halves of T-29.7°and T-25°are shown in Fig.2.

Table 1 The main structural parameters of the heat exchangers

2.2.Mathematical model

Numerical simulation of flow and heat transfer in helical baffle heat exchanger and segmental baffle heat exchanger complies with the basic conservation laws of mass,momentum and energy.The RNG k-ε model based on anisotropy of turbulent viscosity was adopted for the shell side channel of helical baffle heat exchanger[14,26-28].The general governing equations are expressed as Eq.(1)[26-28].

where,ρ is fluid density(kg·m-3);U is velocity vector;Φ represents common variables,such as u,v,w,T,k or ε;ΓΦis generalized diffusion coefficient;and SΦis generalized source term.

2.2.1.Boundary conditions

The inlet of mass flow(or velocity inlet)is set at both the tube side and the shell side,the pressure,temperature and turbulence conditions at the fluid inlet are set,and the control of mass flow(or inlet velocity)is a variable parameter.Pressure outlet is adopted for both the tube side and the shell side,and the outlet pressure is set to 0.

2.2.2.Wall condition

The wall surface of heat exchanger tube and baffle plate is set to the coupled heat exchanger surface without penetration and slip.Other wall surfaces are set as non-permeable,non-slip adiabatic wall surfaces.In numerical simulation,the heat exchanger material is carbon steel.The fluid media on both tube and shell sides are water;and the physical properties change with temperature.The density,specific heat and conduction coefficient are fitted by forth power equations,and the dynamic viscosity is fitted by exponential equation.

2.3.Model verification

A sextant circumferential overlapped helical baffle heat exchanger with incline angle 20°(scheme X-20°)for grid independence verification was selected.The grid systems were established with the mesh numbers respectively 2.36 million,3.31 million,4.20 million,5.12 and 5.97 million.The conditions were set as the shell side inlet temperature of 50°C and the flow rate of 3.5 kg∙s-1,the tube side inlet temperature of 65°C and the flow rate of 4.0 kg∙s-1.The numerical simulation results of the average heat transfer coefficient and pressure drop of shell side of the five sets of the grid systems are shown in Fig.3(a).In the last two grid systems,the deviations of heat transfer coefficient and pressure drop were less than 1%.Considering the calculation efficiency and accuracy,the grid size system with number of 5.12 million was selected for all the schemes.

To verify the numerical simulation method,the simulation results are compared with the experimental results of a similar geometry trisection helical baffle heat exchanger with tube number of 35 and shell inner diameter of 125 mm in reference[22].Fig.3(b)shows that the simulation results are comparable with the experimental results,the average/maximum deviations between the simulation values and experiment values are 9.82%/12.07%,and(-7.28%)/(-17.14%)respectively for heat transfer coefficient and pressure drop of shell side,which are within reasonable limits,and thus the numerical models are deemed as credible.

Fig.2.Models of heat exchangers.

Fig.3.Model validation.

3.Results and Discussions

3.1.Flow field analysis

With numerical simulation data analysis and visualization processing,the flow fields were investigated to compare the shell side flow and heat transfer characteristics.For better demonstration of shell side flow field visualization analysis of the helical heat exchanger,four layers of concentric hexagon slices were constructed in two helical cycles at the middle section of the heat exchanger.For further comparative study of sextant scheme with trisection ones,the group 2 were selected including sextant scheme X-25°,trisection schemes with identical pitch(T-29.7°)and with identical angle(T-25°).

The fluid flow pattern due to the guidance of the helical baffles is important to influence the performance of the helical baffle heat exchanger.Fig.4(a)shows the flow field distribution on meridian slice of scheme X-25°,the secondary vortex flow can be observed as the centrifugal flow near the starting line and the centripetal flow near the end line of a baffle chamber are generated,and the latter is generated by the differential pressure in the radial direction.

To remediate the limitation of the main helical flow and leakage flow not being shown in the meridian slice,Fig.4(b)-(d)shows the flow field distribution especially the main helical flow and leakage flow patterns on four unfold layers of concentric hexagon slices with integration of pressure nephograms and velocity vectors.There are many similarities between the sextant scheme and the trisection ones,due to the common structure of circumferential overlap helical baffles.Firstly,the pressure,velocity vector and the leakage flow at the joints of the baffles are distributed periodically under the guidance of the helical baffles.Secondly,on the slices of the outermost layer H1,almost no leakage flow phenomenon is observed due to very small gaps and the overlapping cohesion between baffles.And starting from the second layer,the closer to the central axis is,the greater the gap is between adjacent baffles and more obvious is the leakage flow situation.Finally,in general the leakage flow is only a small fraction in contrast to the main helical plug flow in the sextant or trisection schemes.Since the number of baffles in the sextant scheme is about twice the trisection one,the flow pattern is closer to the ideal helical flow,and the leakage gap is smaller and the heat transfer enhancement features are more obvious of cutting off the boundary layers and exchanging micelle fluid positions.The helical pitch of the sextant scheme with identical incline angle is much greater than that of the trisection one,or in other words,the incline angle of the sextant scheme could be smaller for the identical helical pitch than that of the trisection scheme.

3.1.1.Comparison of shell side performances of sextant and trisection schemes

Fig.5 shows the main performance curves of six heat exchanger schemes.The shell side heat transfer coefficient ho,shell side pressure drop Δpo,and comprehensive indexall rise with the increase of the shell side flow rate.From Fig.5(a)it can be seen that the shell side heat transfer coefficient of scheme T-20°is 3.01%higher and that of scheme T-24.1°is 4.19%lower than that of the scheme X-20°in average;and that the shell side heat transfer coefficient of scheme T-25°is 2.09% higher and that of scheme T-29.7° is 4.34% lower than that of the scheme X-25°in average.Fig.5(b)shows that the shell side pressure drop of scheme T-20° is 24.55% higher and that of scheme T-24.1°is 2.22% lower than that of the scheme X-20° in average;and that the shell side of pressure drop of scheme T-25°is 21.46%higher and that of scheme T-29.7°is 3.07%lower than that of the scheme X-25°in average.Fig.5(c)shows that the average values of the comprehensive indexof schemes T-20°and T-24.1°are 4.26%and 3.47%lower than that of the scheme X-20°;and that the average values of the comprehensive indexof schemes T-25° and T-29.7° are 4.32%and 3.34%lower than that of the scheme X-25°.

In the same group,the trisection scheme with identical incline angle has slightly higher hobut much greater Δpothan those of the sextant scheme;and the trisection scheme with identical helical pitch has considerably lower hobut slightly lower Δpothan those of the sextant scheme.For the comprehensive indexboth trisection schemes are considerably lower than that of the sextant scheme in each group.The trisection schemes T20°and T25°have correspondingly slightly lowerthan the schemes T24.1°and T29.65°.

The results of both sextant and trisection schemes with identical helical pitch demonstrate similar values and trends of both shell side heat transfer coefficient and pressure drop,which verifies that the performance of the helical heat exchanger are mainly determined by the helical pitch rather than the incline angle.Still the comprehensive indexes of sextant schemes are better than those of the trisection counterpart schemes.Considering the reduced manufacture difficulty with smaller helical baffles,the sextant circumferential overlap helical baffle heat exchangers could have a strong significance to large-and-mid-sized heat exchanger applications.

Fig.4.Flow field diagrams of pressure nephograms and velocity vectors on meridian slice and unfolded multilayer concentric hexagon slices.

3.1.2.Comparison of shell side performances of sextant and segmental schemes

The performance of five sextant schemes characterized as X15°,X20°,X25°,X30°and DX30°are compared with those of the segmental scheme S100,where DX represents dual-thread helical baffle scheme,while S100 is a segmental scheme with baffle pitch of 100 mm.Fig.6(a-d)shows that both the shell side heat transfer coefficient hoand the pressure drop Δporaise with the increase of the shell side flow rate and decline with increasing of the baffle incline angle.Fig.6(e,f)shows that the shell side comprehensive indexraises with the increase of shell side flow and decrease of baffle incline angle,and the relative increment of the sextant helical schemes to the segmental one is in the range of 15.46%to 23.60%.By comparing the performance curves of the single-thread and dual-thread schemes X-30°and DX-30°with identical baffle incline angle,the dual-thread scheme demonstrates higherto that of the single-thread one.For large helical baffle heat exchangers,dual-thread or even triple or four thread helix scheme is recommended according to the need of tube bundle strength.This is so since the increase of shell diameter may increase the baffle pitch or the supporting span,which is conducive to the vibration of tube bundle.All the helical schemes have better comprehensive indexthan that of the segmental scheme S-100,while the sextant X-30°scheme shall be excluded due to its lower shell side heat transfer coefficient than that of the S-100.Also the helical scheme X15°should be excluded since its pressure drops is higher than that of the segmental one.Thus within the studied scope the optimal scheme is DX30°from the viewpoint of higher comprehensive indexwith no less heat transfer coefficient and no higher pressure drop than those of the segmental one.The average values of shell side heat transfer coefficient hoand comprehensive indexof sextant scheme DX30°are respectively 7.22%and 23.56% higher than those of segmental scheme S100.Also the sextant helical scheme X25°or X20°could be an alternative choice.

Fig.5.Performance comparison of six helical baffle schemes of two groups.

4.Conclusions

1)The shell side flow fields of both sextant and trisection schemes are presented on both meridian and unfolded multilayer concentric hexagon slices with integration of the pressure nephograms and velocity vectors.The distribution verifies that the better performance of the sextant scheme is the results of the leakage flow between adjacent baffles is restricted with minimized gaps with the increased helical baffle number,apart from the anti-shortcut structure of the circumferential overlapped helical baffles.

2)The research was focused on two groups of schemes,each includes a sextant scheme and two trisection ones with identical helical pitch and identical incline angle.The comprehensive indexes of trisection scheme with identical incline angle are much lower than the other schemes.It reveals that the performance of the helical heat exchanger is mainly determined by the helical pitch rather than the incline angle.The shell side heat transfer coefficients and comprehensive indexes of the sextant schemes are all better than those of the trisection ones.

Fig.6.Comparison of performances of five helical baffle schemes with segmental one.

3)The performance comparison was also conducted on five sextant helical schemes and a segmental one.The dual-thread helix sextant scheme DX30° is the optimal one within the studied scope from the viewpoints of higher comprehensive performance with not less heat transfer coefficient and not higher pressure drop than those of the segmental one.Also the sextant helical scheme X25° or X20°could be an alternative choice.