ARTICLE pubs.acs.org/IECR
Experimental Investigation of a Novel Polymeric Heat Exchanger Using Modified Polypropylene Hollow Fibers Yuchun Qin,†,‡ Baoan Li,†,‡,* and Shichang Wang† †
Chemical Engineering Research Center, School of Chemical Engineering and Technology and ‡State Key Laboratory of Chemical Engineering, and Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China ABSTRACT: Plastic heat exchangers have attracted more and more attention because of their superior resistance to chemicals and fouling. However, the thermal conductivity of plastic materials is much lower than that of metal, which limits the wider application of plastic heat exchangers. In this study, polypropylene-based hollow fibers as a heat-conducting medium for heat exchangers was developed by melt-mixing polypropylene with graphite particles and maleated polypropylene (PP-g-MA). Results show that the addition of graphite fairly improved the crystalline, thermal stability and conductivity of the polypropylene resin and further improved the heat transfer efficiency of polypropylene-based hollow fiber heat exchangers. The overall heat transfer coefficient of 15.0 wt % graphite modified polypropylene hollow fiber heat exchangers reached 1228.7 W/(m2 3 K), which is 5 times higher than that of pure PP-based hollow fiber heat exchangers, and the overall conductance per unit volume reached 1.1 106 W/(m3 3 K). Further, the heat transfer efficiency increases fairly with the increase of the fluid flow rate, especially with the flow rate of the tubeside. The optimized operation mode is that the hot water flows on the tube-side and the cold water flows on the shell-side.
’ INTRODUCTION The most common problems of metal heat exchangers are corrosion and fouling, especially in a caustic environment. In recent years, scientists and engineers have studied pure polymer,14 polymer-based,5,6 and other material heat exchangers7,8 to find solutions to solve the problems of metal heat exchangers. As known, the advantages of polymers over metals as heat exchanger materials include excellent resistance to corrosion and fouling, ease of molding, manufacturing, and maintenance, etc. As the density of polymers is about 45 times lower than that of metals, polymer-based heat exchangers can greatly reduce costs of construction, transportation, and installation.9,10 In recent years, polymer-based heat exchangers have been successfully applied in the fields of seawater and brackish water desalination,11,12 air conditioning,13 material separation and purification,14 and heat recovery,15 etc. But the quite low thermal conductivity of normal polymers, which is about 100300 times lower than that of metals, limits the wide application of polymerbased heat exchangers. A thinning tube/flat sheet wall was employed to improve the heat transfer performance of polymerbased heat exchangers.13 Song,16 Sirkar and Zarkadas,5 etc. studied polymeric hollow fiber heat exchanger (PHFHE) devices, and achieved very good results. The overall heat-transfer coefficients for the waterwater, ethanolwater, and streamwater systems reached 6471314, 414642, and 2000 W/(m2 3 K), respectively. Meanwhile, many researchers filled polymers with graphite,17 carbon nanotubes,1820 and other fillers21,22 to improve the thermal conductivity and other properties of polymers. However, we have not found any reports regarding the polymer-based hollow fiber heat exchangers prepared by the promoted materials. The aim of the present study is to prepare a kind of efficient heat exchangers using graphite-modified polypropylene hollow fibers as the heat transfer medium. r 2011 American Chemical Society
’ EXPERIMENTAL DETAILS Preparation of Graphite Modified Polypropylene Hollow Fibers. Polypropylene (PP) with a density of 0.91 g/cm3 and
melt index of 5.8 g/10 min and graphite (G) with density of 2.1 g/cm3 and average particle size of 15 μm were used here. PP/G composites were prepared by melt-mixing PP with G using maleated PP (PP-g-MA) as interface modifiers.23 The meltmixing process was performed using a twin-screw extruder as a thermo-kinetic mixer. Furthermore, hollow fibers were fabricated by a single-screw extruder. Heat exchanger modules were made by assembling the hollow fibers with polypropylene random copolymer (PPR) shells. Materials of Heat Exchangers. Four different types of PPbased hollow fibers were selected to prepare heat exchangers as follows: Module 1 (M1) was prepared by pure PP hollow fibers. Module 2 (M2) was manufactured using PP/G (5%)/PP-g-MA (8%) hollow fibers. M3 was fabricated with PP/G (10%)/ PP-g-MA (10%), and M4 with PP/G (15%)/PP-g-MA (12%) hollow fibers. The hollow fibers possess very thin tube wall thickness about 70 μm and the other details of these heat exchangers are presented in Table 1. The two ends of the hollow fiber surfaces were attached to the PPR shells with epoxy resin glue and no baffles were used. The injection molding headers were fixed to the shells with stainless steel bolts and nuts. A module photograph and the tube layout were shown in Figure 1a,b, respectively. Apparatus and Procedure. Two different experimental processes were employed in this study. First, hot water as the feed Received: September 11, 2011 Accepted: December 20, 2011 Revised: December 12, 2011 Published: December 20, 2011 882
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Table 1. Geometrical Characteristics of PP-Based Hollow Fiber Heat Exchangersa module
N
do (μm)
Db (μm)
L (cm)
Ds (cm)
α (m2/m3)
1
296
734.36
955.63
28.5
3.2
849.10
2 3
292 289
733.22 735.99
954.31 956.85
28.0 28.7
3.2 3.2
836.33 830.86
4
293
734.50
955.92
29.0
3.2
840.66
a
N, do, and L are the effective number, the outside diameter, and the effective length of the hollow fibers, respectively. Db is the distance between the tubes, Ds is the inside diameter of the shells, and α is the surface area to volume ratio based on total volume.
Figure 1. (a) Photograph of the PP heat exchanger module; (b) layout of the PP-based hollow fibers in the shell of the heat exchanger module.
solution circulated through the shell-side of heat exchanger module by a diaphragm pump, and was kept at a constant temperature using a thermostatic bath. Tap water, used as the coolant, flowed through the tube-side of the module. Second, as shown in Figure 2, the hot water was circulated through the tube-side, and the coolant flowed through the shell-side of the heat exchanger module. The flow rates of the system were 165 L/min on the shellside and 110 L/min on the tube-side. Inlet temperatures of feed and coolant were kept at 85 and 18 °C, respectively. Heat insulation materials were used to reduce heat loss of the whole system and a chiller (model, AE1600; cooling capacity of 1.6 kW; recirculation pump flow rate of 612 L/min; tank volume of 12 L) was used to provide cooling water during the experiments. The leakage test of the heat exchanger module was carried out before the heat transfer measurements. During the test, the inlet of the shell-side of module was connected to the tap water, and the outlet was sealed. Water pressure was gradually increased to 0.2 MPa, and then the system was kept steady for 0.5 h. The leakage was determined through the observation of water seepage of the tube-side. In each experimental run, the device was kept running for about 1.5 h to achieve a steady state and the heat balance was verified by eqs 110 before data recording. The inlet and outlet temperatures of hot water and of cooling water were measured by thermocouples (PT100) and shown on the temperature data logging devices with the absolute uncertainty of (0.1 °C. The flow rates of the liquid system were obtained from the flow meters with a relative uncertainty of (1.5%.
Figure 2. Experimental setup for heat-transfer measurements of polypropylene-based hollow fiber heat exchangers.
where, ΔTlm is the logarithmic mean bulk temperature difference between the shell-side and tube-side; Ai is the surface area of hollow fibers based on the inside diameter, and di is the inside diameter of the hollow fiber. The calculation of hi and ho were given by Sirkar and Zarkadas for the PP hollow fiber heat exchangers;5 we extended the calculations for the PP-based hollow fiber heat exchangers in this study.
’ DATA REDUCTION Calculation of Overall Heat Transfer Coefficients. The
1 Ai ΔTlm 1 1 ¼ ¼ þ U hi Uw Q
ð5Þ
1 di di do ¼ þ ln Uw do ho 2λw di
ð6Þ
following equations (eq) were used to calculate the overall heat transfer coefficient (U). Q c ¼ mc Cpc ΔTc
hi ¼
ð1Þ
where Q c is the heat transfer rate, mc is the mass velocity, Cpc is specific heat, and ΔTc is the temperature difference between inlet and outlet of cold water, respectively. Q ¼ UAΔTlm ΔTlm
ðTt, in Ts, out Þ ðTt, out Ts, in Þ ¼ Tt, in Ts, out ln Tt, out Ts, in
Ai ¼ NΠdi L
NuT3 kf i di
NuT3 ¼
ð2Þ
ð7Þ
ð48=11Þ þ Nuw , 1 þ ð59=220ÞNuw
Nuw ¼
Uw d i kfi
ð8Þ
where hi and ho is heat transfer coefficient of inside and outside tubes, NuT3 is the Nusselt number calculated under the T3 boundary condition (the third kind boundary condition), which implies a convective boundary condition at the inside tube surface and a constant temperature for the medium surrounding the tube.5 Nuw is Nusselt number of tube wall, λw, (0.19 W/(m 3 k)) is the thermal conductivity of pure PP hollow fibers; kfi is the
ð3Þ
ð4Þ 883
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thermal conductivity of fluid inside tubes. 1 ki 1 1 ¼ ¼ þ Nuov NuT3 Nuw Ui d i 1
ð9Þ
59 48 48 Nuov Nuw 2 þ 2Nuov Nuw Nuov ¼ 0 220 11 11
Figure 3. SEM images of (a) polypropylene/graphite (5%) composites and (b) polypropylene/graphite (5%)/PP-g-MA (8%) composites.
ð10Þ where Nuov is the overall Nusselt number. Equation 10 yields two roots and the positive one is the desired wall Nusselt number. Determination of Thermal Boundary Layer Thickness. Thermal boundary layer thickness24 was calculated using eqs 11 and 12. δt ¼ Prn δ
Number of transfer units (NTU) was calculated using eq 20. NTU ¼
CUV ov ¼ αi Ui ¼ αo Uo α¼
ð12Þ
duF μ
ð13Þ
λ¼
ð15Þ
kf ¼ 0:0097Tav 2 þ 2:1662Tav þ 559:2
ð16Þ
Cc ¼ mi Cpi
ð19Þ
λ2 =λ1 1 , λ2 =λ1 þ A
ϕ2 ð1 ϕm Þ ϕm 2
ð24Þ
Morphology of Composite Study by SEM. By comparing Figure 3 panels a and b, it can be seen that, the morphology of the hollow fiber cross-section in Figure 3b is smoother than that in Figure 3a due to the addition of PP-g-MA modifier. The slices on the hollow fiber cross-section in Figure 3b are also smaller than those in Figure 3a. As noted above, the combination of graphite and polypropylene (PP) was improved by the addition of the modifier. Dispersion of Composite Study by XRD. Samples were pressed into films with the thickness of approximately 800 μm. The sample films were scanned in 2θ ranges from 0 to 50° at a rate of 1° /min. Measurements were recorded at every 0.02°. Figure 4a shows the XRD spectrum of the alpha (α) crystalline form of pure isotactic polypropylene (iPP) with peaks located at 2θ = 14°, 17°, 18.5°, 21° and 22°, which is consistent with the references.27,28 Figure 4b shows the XRD spectrum of the graphite23 (G) with a sharp narrow peak at 2θ = 26.6° and a broad peak at 2θ = 10°. Figure 4c shows the XRD spectrum of
where, Qactual is the actual rate of heat transfer and Qmax is the possible maximum heat transfer rate, Thi is the inlet temperature of hot water and Tci is the inlet temperature of cooling water, Uo is based on the outside surface area (Ao) of hollow fibers and Cmin is the minimum heat capacity rate (Cmin = Cc when Cc < Ch; Cmin = Ch when Ch < Cc). Ch and Cc were calculated using eqs 18 and 19. ð18Þ
B¼
’ RESULTS AND DISCUSSION
ð17Þ
Ch ¼ mo Cpo
1 þ ABϕ , 1 Bψϕ
where, λ, λ1, and λ2 are the thermal conductivities of the composites, the matrix, and the filler, respectively, and j is the volume fraction of the fillers. A, and jm are constant of particles size and shape. A = 3 and jm = 0.637 were used in this paper.
Where, Tav is the averages of the inlet and outlet of the tube-side or shell-side liquid temperatures. The EffectivenessNTU Method. The heat exchanger efficiency (ε) for modules was calculated using eq 17. Qactual Ui Ai ΔTlm Uo Ao ΔTlm ¼ ¼ Qmax Cmin ðTh1 Tc1 Þ Cmin ðTh1 Tc1 Þ
ð22Þ
ψ¼1 þ
ð14Þ
μ ¼ 0:4607 ln Tav þ 2:3669
ε¼
4Ndi Ds 2
Nielsen-Lewis model:26
where, Re is Reynolds number, u is the linear velocity of fluid, and F is density of fluid; F (g/cm3 ), μ (g/(cm 3 s)), and kf (mW/(m 3 K)) for hot water and cooling water were determined from the following published correlations.16 F ¼ 108 Tav 3 6106 Tav 2 3106 Tav þ 1:003
ð21Þ
where α is the surface area to volume ratio and Ds is the shell inside diameter. Estimation on Thermal Conductivities. Russell model:25 " # ϕ2=3 þ λ1 =λ2 ð1 ϕ2=3 Þ λ ¼ λ1 2=3 ð23Þ ϕ þ λ1 =λ2 ð1 ϕ2=3 Þ ϕ
where, Cp (the specific heat of the water) is 4.18 J/(g °C), μ is the viscosity of fluid, and kf is thermal conductivity of fluid. Re ¼
ð20Þ
Determination of Overall Conductance per Unit Volume (CUV)
ð11Þ
where, δt is the thickness of thermal boundary layer, δ is the thickness of velocity boundary layer, Pr is Prandtl number, and n is the exponent. CP μ Pr ¼ kf
UA Uo A o ¼ Cmin Cmin
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Figure 4. X-ray diffraction patterns of (a) pure polypropylene, (b) graphite, (c) polypropylene/graphite (5%) composites, and (d) polypropylene/ graphite (5%) /PP-g-MA (8%) composites.
Table 2. Thermal Property of PP and Its Compositesa melting composite (composition in wt %)
crystallization
temperature temperature Ts Tmax char (°C) (°C) (%) (°C) (°C)
PP
174
126
422.0 461
0.21
PP/G(5%)
174
127
426.3 478
4.74
PP/G(5%)/PP-g-MAH(8%)
174
130
426.1 474
7.11
Char, the remaining fraction of non-volatile material left at 600 °C; Ts, the temperature of samples starts to degrade instantaneously; Tmax, the temperature of the maximum rate of samples degradation attains. a
PP/G composites containing 5% G with no interfacial modifier. The broad peak at 2θ = 10° almost disappears, which indicates that PP occupy the space between aggregate sheets of G to some extent.23 The peak at 2θ = 26.6° in Figure 4c indicates a lower intensity than that in Figure 4b. Further, the peaks representing the α crystalline form of PP still exist. Figure 4d shows the XRD spectrum of PP/G composites containing 5% G and 8% modifier of PP-g-MA. The peak at 2θ = 26.6° of G represents a lower intensity than that in Figure 4c, which indicates that the dispersion of G in the resin is improved by the modifier. However, the peaks of PP become higher, which suggests that the presence of G has promoted the crystallization of the PP resin. Thermal Properties of PP and Its Composites. The thermal gravimetric analysis instrument (TGA) was used to study the degradation processes of PP and PP/G composites. Crystallization properties of composites were studied by differential
Figure 5. TGA plots of (a) pure polypropylene, (b) polypropylene/ graphite (5%) composites, and (c) polypropylene/graphite (5%)/ PP-g-MAH (8%) composites.
scanning calorimeter (DSC) in nitrogen at a heating rate of 10 °C/min. The data were presented in Table 2. As shown in Table 2 and Figure 5 that Ts is increased by about 4 °C as the addition of graphite, and Tmax is increased by about 17 °C, which clearly indicates that the thermal stability of PP is improved by adding the graphite and modifier. Measurements of thermal conductivities (λ) were performed by utilizing the thermal conductance tester (model: TC3020) with hot wire technique. Samples with thickness of 0.4 mm and diameter of 2 cm were prepared by the mold compression process (molding temperature, 185 °C; pressure, 4 MPa, air-cooling). 885
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Figure 6. Comparison of models and experimental thermal conductivity values of PP-based composites as functions of graphite (G) amounts in volume % (j) at 40 °C. (G, j corresponding to amount in weight%, 5%15%; thermal conductivity of pure PP = 0.19 W/(m 3 K); G = 197 W/(m 3 K)).
Figure 8. (a) Effect of hot fluid linear velocity of shell-side on overall heat transfer coefficient at an inlet temperature of 85 °C, (tube-side: tap water, 18 °C of inlet temperature, 0.35 m/s of linear velocity). (b) Effect of cold fluid linear velocity of tube-side on overall heat transfer coefficient at an inlet temperature of 18 °C, (shell-side: hot water, 85 °C of inlet temperature, 0.35 m/s of linear velocity).
embraced well by PP. When the addition of G reaches to 20.0 wt %, the tensile strength and elongation at the break of the hollow fiber decrease to 13.1 MPa and 155.8%, respectively. Heat Transfer Performance. The influence of fluid flow rate the on overall heat transfer coefficient (U) was studied. Plateau16 values of U for the four heat exchanger modules were obtained when the fluid velocities increased to certain values. More details on heat transfer of the modules were given in Figure 8 and Figure 9. Figure 8a gives the results of U versus linear velocity of feed (hot water) on the shell-side. The U values of the four modules were significantly improved by increasing the linear velocity of hot water. The U values of Module 3 (M3) and Module 4 (M4) increased faster than those of M1 and M2 at the velocity lower than 0.5 m/s. However, the growth trend of the four U curves became very slow after 0.5 m/s. A plateau of U value (UP) of M4 around 708 W/(m2 3 K) was achieved which is more than two times higher than 302.38 W/(m2 3 K) of M1with pure PP hollow fibers. UP of M2 and M3 were 352.11 W/(m2 3 K) and 648.66 W/(m2 3 K), respectively. In Figure 8b, a highest Up value of M4 around 1141W/(m2 3 K) was obtained at the tube-side velocity of 0.55 m/s, which is almost four times higher than 275 W/(m2 3 K) of M1. In Figure 9a the growth trend of the four curves is similar to that of Figure 8b. The U value reached a plateau after the tube-side linear velocity reaching around 0.5 m/s. Up values of M1M4 were 596.3, 607.1, 734.1, and 1228.7 W/(m2 3 K), respectively.
Figure 7. (a) Tensile strength and (b) elongation at break of hollow fibers as a function of graphite amount in weight %.
The experimental results were compared with the existing Russell and Nielsen-Lewis models and shown in Figure 6. Figure 6 indicates that the thermal conductivity of PP composites is improved by the addition of graphite fillers (λ increases to 0.348 W/(m 3 K) from 0.19 W/(m 3 K)). It is seen that the Nielsen-Lewis model (eq 24) with A = 3 and jm = 0.637 predicts the thermal conductivities of the composites well in the whole range of graphite volume content (2.23%7.11%), the Russell model (eq 23) agrees well with the experimental results when the G content is less than 4.5%. Tensile Properties of the Hollow Fibers. Tensile strength and elongation at break of hollow fibers were measured using an electronic tensile testing machine under the elongation speed of 50 mm/min and the room temperature of 25 °C. As shown in Figure 7, when the graphite (G) addition is less than 5.0 wt %, the tensile strength of hollow fibers just increases a little and no obvious changes are observed for elongation at break. In this case, G is well dispersed in the PP matrix. Also, the tensile strength increases with the degree of crystalline as indicated by XRD that the presence of G has promoted the crystallization of PP. However, the two parameters decrease severely when the G content is higher than 5.0 wt %. The reason is that excessive G cannot be dispersed well in the PP matrix, and cannot be 886
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Figure 10. (a) Heat exchanger effectiveness factor (ε) and (b) its growth rate (rε) of hollow fiber heat exchangers as a function of graphite content in wt %.
Figure 9. (a) Effect of hot fluid linear velocity of tube-side on overall heat transfer coefficient with an inlet temperature of 85 °C, (shell-side: tap water, 18 °C of inlet temperature, 0.35 m/s of linear velocity). (b) Effect of cold fluid linear velocity of shell-side on overall heat transfer coefficient with an inlet temperature of 18 °C, (tube-side: hot water, 85 °C of inlet temperature, 0.35 m/s of linear velocity.
Figure 11. (a) Number of heat transfer (NTU) and (b) its growth rate (rNTU) of hollow fiber heat exchangers as a function of graphite content in wt %.
In Figure 9b, the U values of the four modules increase with the increase of shell-side cooling water linear velocity. However, the Up values of M1M4 in Figure 9b were less than those in Figure 8a (e.g., Up values of M4 were around 468.2 W/(m2 3 K) and 708 W/(m2 3 K) in Figure 9b and Figure 8a, respectively). Figures 8 and 9 indicate that heat transfer efficiency can be optimized by fairly increasing the fluid velocity. The reason is that both tube-side and shell-side exist in a thermal boundary layer (normally called the temperature polarization). The increase of the velocity reduces the thickness of the velocity boundary layer (δ) and leads to a thinner thickness of the thermal boundary layer (δt) (the relationship of δ and δt was presented in eq 11), consequently, lowering the thermal resistance of the thermal boundary layer, and therefore enhancing the heat transfer of the heat exchangers. However, when the velocity increases to a certain value, the thermal boundary layer gets much thinner and has a slight influence on heat transfer, and the U value reaches a plateau (e.g., as the curves shown in Figure 8a). Figures 8 and 9 also reveal that the optimized operating mode is one in which the hot water but not the cold water flows on the tube-side. The reason is that the viscosity (μ) of hot water (85 °C) is much lower than that of the cold water (18 °C); meanwhile, there is a little difference of specific heat (Cp) and thermal conductivity (kf). So, the hot water with a much smaller Prandtl number (Pr, calculated by the eq 12) accordingly led to a thinner thermal boundary layer thickness than did the cold water,
consequently causing a better heat transfer performance of hollow fiber heat exchangers. Heat transfer of the shell-side is mainly convection and existing smaller thermal resistance than tube-side. Fluid of the shellside passing around and between the flexible hollow fibers with very low flow rate induced the vibration of the hollow fiber tubes.29 The vibration increased the turbulence of the shell-side fluid and caused the convective heat transfer of shell-side.30 In a comparison of Figures 8 and 9, it is found that the U values of the modules prepared by graphite modified PP hollow fibers were higher than that of pure PP-based hollow fiber heat exchangers under the same operation conditions (the U values: M4 > M3 > M2 > M1). In other words, the heat transfer performance was improved by the addition of graphite. The heat transfer performance parameters experimental data were achieved under conditions of hot water flowed on the tubeside at an inlet temperature of 85 °C and at the velocity of 0.55 m/s (shell-side, tap water, 18 °C of inlet temperature, 0.35 m/s of linear velocity). The values of the ε, NTU and CUV shown in Figure 10, Figure 11, and Figure 12 were calculated by eqs 17, 20, and 21, respectively. Figure 10, Figure 11, and Figure 12 indicate that the performance parameters of ε, NTU, and CUV of heat exchangers with various graphite modified PP hollow fibers are much higher than that with pure PP hollow fiber heat exchangers. When the graphite 887
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Figure 13. Comparison of PP-based hollow fiber heat exchangers with conventional metal heat exchangers (three-quarter in. tubes in a 1 in. triangular pitch and a 30° layout,5,16 U data were from ref 31, surface area to volume ratio based on total volume (α) = 105).
Figure 12. (a) Overall heat conductance per unit volume (CUV) and (b) its growth rate (rCUV) of hollow fiber heat exchangers as a function of graphite.
HTN shell-and-tube heat exchangers by Liu, Davidson, and Mantell, and the costs were estimated.32 Lower costs of our heat exchangers could be obtained as a result of cheaper raw materials being used (PP: $1.4/kg; PP-g-MA: $ 1.8/kg; graphite: $ 3.1/kg) in this paper. Comparison with Metal Heat Exchangers. Figure 13 gives a comparison of the performance of the four modules with metal heat exchangers by means of the overall conductance per unit volume (CUV).5,16 The measurements were carried out under conditions of hot water flowing on the tube-side at an inlet temperature of 85 °C and at the linear velocity of 0.55 m/s (shell-side, tap water, 18 °C of inlet temperature, 0.35 m/s of linear velocity). The CUV values were achieved by eqs 21 and 22. As shown in Figure 13, M4 presents the CUV value of 1.1 106 W/(m3 3 K) which is almost 1.5 times higher than that of conventional metal shell-and-tube heat exchangers. As mentioned above, the heat exchangers with thin tube wall, small tube diameter, and graphite modified hollow fibers possess higher heat transfer performance than some metal heat exchangers.
Table 3. Comparison of the PP-Based Hollow Fiber Heat Exchangers with Reported Plastic Heat Exchangers for Clean “WaterWater” or “Hot BrineWater” System U, W/(m2 3 K)
heat exchanger type
CUV, W/(m3 3 K)
5678
6 105
1100
6 105
1314
1.8 106
PEEK plate heat exchanger
900
1.03 106
PP-based hollow fiber (M4)
1228.7
1.1 106
HEPP3 HEPEEK2e
2076 1929
2.9 106 1.0 106
Membrana 041938e
1100
3.5 106
2109
1.5 106
platea b
HTN shell-and-tube PP hollow fiber
c d
e
e
PES a
Plates were 0.4 mm thick, spaced 2 mm apart. U data were from ref 31. “Waterwater system”. b From ref 32, HTN = high temperature nylon. U values were estimated by Zarkadas and Sirkar. “Waterwater system”.5 c From ref 5. “Waterwater system”. d From ref 33. U values were estimated by Zarkadas and Sirkar.5 “Waterwater system”. e From ref 16, “Hot brinewater” system, HEPP3 and HEPEEK2 is one kind of PP-based and PEEK based hollow fiber heat exchangers, the other two are with tubes obtained from coated porous hollow fiber. PEEK, polyetheretherketone; PES, polyethersulfone; M4, heat exchanger module with 15 wt % graphite modified PP hollow fibers.
content reached 15%, the CUV value of M4 even reached 1.1 106 W/(m3 3 K) which is 3 times higher than that of M1with pure PP hollow fibers and the rCUV increased to 233.7%. The heat transfer performance of PP-based hollow fiber heat exchangers was effectively improved by the addition of graphite. Comparison with Other Plastic Heat Exchangers. Comparison of the PP-based hollow fiber heat exchangers with some reported plastic heat exchangers, based on the maximum values of overall heat transfer coefficient (U) and overall conductance per unit volume (CUV) were presented in Table 3. Table 3 shows that our PP-base hollow fiber heat exchangers have better heat transfer performance than some existing plastic exchangers. Meanwhile, the U and CUV values of our heat exchangers are less than some values provided in Song’ paper, as dense hollow fibers with lower tube wall thermal resistance obtained from coated porous hollow fibers were used in their study. Unreinforced High temperature nylon (HTN, $7.9/kg32) tubes and 35% glass reinforced HTN manifolds were used to prepare
’ CONCLUSIONS In this study, novel PP-based hollow fibers and their heat exchangers were characterized and developed. Graphite (G) as a thermal filler and maleated PP (PP-g-MA) as an interface modifier were employed to modify the pure PP hollow fibers. The experimental data indicated that the addition of G improved the crystalline, thermal stability and conductivity of the PP resin. The heat transfer performances of the novel heat exchangers were assessed using a waterwater system. The overall heat transfer coefficient even reached to 1228.7 W/(m2 3 K) which is almost 5 times higher than that of pure PP hollow fiber heat exchangers, and the overall heat conductance per unit volume (CUV) achieved 1.1 106 W/(m3 3 K) which is 1.5 times higher than typical conventional metal shell-and-tube heat exchangers. The optimized CUV data are higher than those of almost reported plastic heat exchangers for liquid-to-liquid application. The heat transfer efficiency can be optimized by fairly increased the fluid linear velocity, especially the linear velocity of the tube-side. And the optimized operation mode is that the hot water flows on the tube-side and the cold water flows on the shellside. With great thermal transfer performance, excellent corrosion, fouling resistance, and higher surface area to volume ratio, the graphite modified PP hollow fiber heat exchangers are promising in industrial application. 888
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’ AUTHOR INFORMATION
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*Tel.: +86 22 2740 7854. Fax: +86 22 2740 4496. E-mail: baoan.li@ gmail.com.
’ ACKNOWLEDGMENT The authors gratefully acknowledge the National Key Technology R&D Program of China (Grant No. 2006BAB03A06), Key and Technology R&D Program of Tianjin (08ZCKFSH02200, 07QTPTSH06700) and The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. ’ NOMENCLATURE A = Heat transfer area, m2 CUV = overall heat conductance per unit volume, W/(m3 3 K) NTU = number of heat transfer unit, dimensionless do = outside fiber diameter, m Ds = shell inside diameter, m U = overall heat transfer coefficient, W/(m2 3 K) Q = rate of heat transfer, W T = temperature, K ΔT = temperature difference, K u = velocity, ms1 V = volume, m3 N = number of effective fibers Pr = Prandtl number, dimensionless L = the effective length of the hollow fibers, m Re = Reynolds number, dimensionless Cp = specific heat of fluid, J kg1K1 m = mass flow rate, kg s1 kf = thermal conductivity of fluid, W m1 K1 Greek Letters
α = surface area to volume ratio based on total volume, m2 m3 ε = heat exchanger effectiveness factor F = density of fluid, kg m3 μ = dynamic viscosity, kg m1s1 δ = the thickness of velocity boundary layer δt = the thickness of thermal boundary layer Subscripts
M = module h = hot water c = cooling water i = inside o = outside r = the growth rate max = maximum min = minimum s = shell-side t = tube-side av = average p = plateau
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