Comprehensive Evaluation of the Performances of Heat Exchangers with Aluminum and Copper Finned Tubes

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Introduction
Te continuous improvement in people's living standard has led to increased demand for comfortable living environments.Tis has increased reliance on air conditioning, and thus the air conditioning industry is developing at an accelerating rate.China's building energy consumption accounts for approximately 20% of its total energy consumption, and air conditioning energy consumption accounts for 40% of its building energy consumption.With such high energy consumption by air conditioning systems, reducing this consumption is necessary for the country to progress with sustainable development.Tis is also one of the key issues to study for building energy efciency [1,2].Te core component of an air conditioning system is the fnned-tube heat exchanger, in which the tubes are usually made of copper and the fns consist of aluminum.Currently, China has a copper supply shortage, copper prices are increasing, and the need for air conditioning is still great.Terefore, to reduce the copper and energy consumptions of air conditioning, it is necessary to study new materials for fnned-tube heat exchangers.Aluminum is cheaper and less dense, so using aluminum instead of copper for the tube can reduce the cost of such heat exchangers.However, the thermal conductivity of aluminum is lower than that of copper, so the heat transfer performance of an aluminum fnned tube is lower than that of a copper fnned tube.Tis makes it necessary to consider the applicability of each from two perspectives: heat transfer and cost.
As a common heat transfer device, the fnned-tube heat exchanger is widely used in the chemical, refrigeration, air conditioning, aerospace, and marine felds [3][4][5][6][7][8][9].Its heat transfer performance directly afects the operating efciency of an air conditioning system.Nemati et al. [10] found that using circular fnned tubes reduced the fnned-tube pressure drop by 31%, reduced the fn weight by 23%, and reduced the total air-side heat transfer by 14%, thus maximizing the total heat transfer efciency and minimizing the pressure drop.Nuntaphana et al. [11] conducted an experimental study on the air-side performance of corrugated fnned heat exchangers based on the tube diameter, rib spacing, transverse tube pitch, and tube arrangement.Tey showed that for an inline arrangement, the pressure drop increases with increasing tube diameter, the heat transfer coefcient decreases with increasing tube diameter, and an increase in fn height also causes a signifcant increase in pressure drop but a decrease in heat transfer coefcient.Fourar et al. [12] numerically studied the performance of eccentric annular fnned-tube heat exchangers under natural convection conditions and showed that the eccentricity efect is stronger for small-diameter fnned tubes with materials that have high thermal conductivity.Liu et al. [13] conducted a numerical study on open-hole corrugated fnned-tube heat exchangers in air conditioners and found that simultaneously optimizing fn spacing and corrugation parameters improved the heat transfer coefcient by 23.4% and opening holes at the top of corrugated fns improved the heat transfer coefcient by 6%.Hu et al. [14] studied the infuence of fn material on the performance of heat exchanger under dehumidifcation condition, and the results showed that increasing the air velocity and relative humidity could enhance the heat transfer of heat exchanger, copper fn heat exchanger had the best performance, and aluminum fn with hydrophilic layer had the worst heat transfer performance.Jabbour et al. [15] analyzed the efect of polymer flling on the overall performance of fnned-tube heat exchangers; numerical results show that the low electrical conductivity of this polymeric material can be compensated by optimizing the geometrical parameters to achieve properties similar to those of the metal compact HX.Kadhim et al. [16] conducted an experimental study on the heat transfer characteristics of nanofuid enhanced fnned-tube heat exchangers and the results showed that the use of nanofuid enhanced the heat transfer characteristics of the fuid by improving the thermal conductivity and density of the fuid.Anish et al. [17] performed calculations on the heat transfer of a multifnned-tube horizontal-shell heat exchanger using erythritol (with a melting point of 117 °C) as the phase change material and thermosol-55 as the heat transfer fuid and found that the fn structural parameters had a signifcant infuence.
To strengthen the heat transfer of fnned tubes, researchers are putting much efort into searching for new materials to replace copper tubes, which have been widely used in the air conditioning industry.Because aluminum is less dense and cheaper, it has in recent years been highly valued as a replacement for copper in fnned-tube heat exchangers in the air conditioning industry.Tis paper numerically analyzes the heat transfer of a heat exchanger with aluminum fns and aluminum tubes and compares its performance with that of an identically structured heat exchanger with aluminum fns and copper tubes.Finally, the two heat exchangers are compared in terms of cost, which provides theoretical reference for application of aluminum fnned-tube heat exchangers in the air conditioning industry.

Model Building
2.1.Physical Model.Te physical model studied in this paper is a heat exchanger with vertical base tubes going through closely spaced fns [18].Tis model is used for two exchangers: one with aluminum fns and aluminum tubes and the other with aluminum fns and copper tubes.Te model has a total of 32 base tubes in a forked row arrangement, as shown in Figure 1.
Te model parameters of the fnned-tube heat exchanger are shown in Table 1.
Because the geometric model has symmetry, just one of the symmetric units is selected as the object of study, which greatly reduces the amount of computation required for the simulation.Figure 2 shows the selected calculation domain.Te fow domain of air is a 3D model and the model structure is such that a single row of tubes can be used to study the air fow in the x-direction, while the y-direction is the distance of half of the centerline of two adjacent rows of tubes, and the zdirection is the distance between two adjacent upper and lower fns [19].

Mathematical Model.
FLUENT 2021R1 was used for numerical simulation, assuming that the fow and heat transfer in the computational domain was stable; the contact thermal resistance between the base tubes and fns was negligible; the radiation heat transfer was negligible; the fuid was incompressible and uniform air; and the simulation solution control equations were the continuity equation, momentum conservation equation, and energy conservation equation [20].
Te fow intensity of heat exchangers is generally expressed by Reynolds number (Re).In this example, Re was signifcantly higher than 1010, which belongs to turbulence.Among all the Reynolds average Navier-Stokes (RANS) models of simulated heat exchangers, RNG k-ε turbulence model has better stability and accuracy, and the calculation speed is relatively fast.Terefore, this model was also used for calculation in this example.Te second-order windward format and SIMPIEC algorithm were used for each equation [21].Te boundary conditions were set as follows: the fuid was air with inlet velocity (velocity-inlet) set as 1-5 m/s, the inlet temperature was uniformly set as 293 K, the air outlet was set as free fow (outfow), the temperature of the inner surface of the base tube was set constant as 313 K without a slip wall (wall), the boundary between the air and material contact surface was set as the fuid-solid coupling boundary (coupled), and both sides of the fns and the upper and lower sides were symmetric boundaries [22].To ensure the solution accuracy, the calculation was considered converged when the residuals of the continuum and momentum equations reached 10 −5 and the residuals of the energy equation reached 10 −8 .When a 16-core machine was used, the number of iteration steps was 2100, and the calculation time was 3 hours, the solution converged.
Te physical quantities involved in the numerical simulation were as follows: where Re is the Reynolds number, Nu is the Nusselt number, D 0 is the outer diameter of the base tube, h is the convective heat transfer coefcient, λ and u are the thermal conductivity and kinematic viscosity of air, respectively, and u m is the velocity at the minimum cross-sectional fow area.

Grid-Dependence Analysis and Model Validation.
Te mesh was divided using ANSYS MESH software.Te mesh at base tubes and fns was encrypted using the local control method (Figure 3), and the boundary layer mesh was generated [23].
Te mesh irrelevance was verifed before the formal solution was reached using FLUENT 2021R1.A mesh model with 1.1 million grid cells was fnally selected for meshing, as shown in Figure 4.
To verify the reliability of the simulation model, a fnnedtube heat exchanger model was established with the same structural parameters as in the literature [24].Te solution method used in this paper was used to calculate the Nusselt number (Nu) of the fn surface, and the resultant values were compared with those of the literature [24].Te results are shown in Figure 5.
Te Nusselt number (Nu) varies with the Reynolds number (Re) in both the experimental and simulation results.Te diference between this simulation and the literature experimental data is not large, and Nu in both results increases with increasing the Reynolds number.Te error in Nu does not exceed 8%, which is small and within the acceptable range.Terefore, the model and algorithm of this paper are verifed.International Journal of Chemical Engineering

Results and Discussions
3.1.Fin Surface Temperature Distribution.Te bottom fn surface temperatures of the copper tube-aluminum fn and aluminum tube-aluminum fn heat exchangers were compared and analyzed for a head-on wind speed of 3 m/s.Te fn surface temperature distribution is shown in Figure 6.
It is seen from Figure 6 that the fn surface temperature distribution is highest near the base tubes.Air passes over each tube from the left side under convective heat transfer, and the surface temperature gradually increases along the fn length.Te air fows into the fn channel through the left side.Te temperature distribution on the fn surface is lowest in the inlet area, and the temperature gradient is larger.Te air and fn tube continuously carry out convective heat transfer.Te fn surface temperature gradient decreases, and the temperature distribution tends to be stable.Comparing Figure 6(a) with Figure 6(b) reveals that, owing to the high thermal conductivity of the copper tube, the temperature distribution near the tube is wide and high, and the temperature of the copper tube near each base tube is slightly higher than that of the aluminum tube.Overall, the fn surface temperature with the aluminum tubes is lower than that with the copper tubes.Meanwhile, the temperature gradient along the length of each aluminum fn is lower with the aluminum tubes than with the copper tubes, and the fn temperature range is wider with the copper tubes because the thermal conductivity of aluminum is lower than that of copper.

Heat Transfer Coefcient Analysis.
As shown in Figures 7  and 8, respectively, the heat transfer and heat transfer coefcient change with inlet wind speed for both the coppertube and aluminum-tube heat exchangers.
Under the same inlet air speed, the heat transfer and heat transfer coefcient of the copper tube-aluminum fn exchanger are higher than those of the aluminum tubealuminum fn exchanger, which is because the copper tube has good thermal conductivity and can transfer more heat through the same heat transfer area.When the wind speed exceeds 5 m/s, the heat exchange gradually decreases with the increase of the wind speed.When the wind speed exceeds 8 m/s, the heat exchange of copper and aluminum tubes basically does not change.Te heat transfer and heat transfer coefcient of the aluminum-tube heat exchanger are 4%-12% and 7%-9% lower than those of the copper-tube exchanger, respectively, under diferent wind speeds.Because the heat transfer area of each base tube is small, the heat transfer of the aluminum-tube heat exchanger is not signifcantly lower than that of the copper-tube exchanger.

Fin Efciency.
Te fn efciency is the actual measure of the performance of each heat exchanger.Figure 9 shows the fn efciency of the copper-tube and aluminum-tube heat exchangers with changing inlet air velocity.
Te fn efciency decreases with increasing inlet air speed for both exchangers, and the reduction trend gradually fattens with increasing inlet air speed.At the same inlet air speed, the fn efciency of the copper-tube exchanger is only slightly higher than that of the aluminum-tube exchanger because of the higher thermal conductivity of copper, so its air-side heat transfer coefcient is higher.Tis indicates that copper has better heat transfer characteristics.Overall, the diference in fn efciency between the diferent materials under the same wind speeds is not signifcant, indicating that changing the base tube material does not have a signifcant impact on the fn efciency of the fnned-tube heat exchanger.

Cost Analysis.
Most tubes in fnned-tube heat exchangers are made of copper because of its good thermal conductivity, but the shortage of copper and its rising prices make it necessary for the air conditioning industry to consider the material cost of a fnned-tube heat exchanger that uses this material.According to the Changjiang metal network, the average price of copper is about 3.8 times that of aluminum, as can be seen in Figure 10.
Te properties and costs of the two materials are listed in Table 2, which shows that copper has better heat transfer but is more expensive.

Simulation Results for the Heat Exchanger with Copper
Tubes and Aluminum Fins.In the simulation, the copper tube diameter was 5 mm, the transverse tube spacing was 18.5 mm, the longitudinal tube spacing was 11 mm, the fn spacing was 1.8 mm, and the fn thickness was 0.1 mm.Te heat transfer fuid was water-air with wall boundary conditions (wall).Te air inlet temperature was 293 K, and the wall temperature was set to 333 K.
Te thermal conductivity of copper is λ � 387.6 W (m•K), and the unit price of copper is 69.47 yuan per kg.Te entire model had a total of 32 heat exchanger tubes, each with length l � 0.72 m and external surface area A2 � 0.21887 m 2 .Terefore, the total surface area of the heat exchanger including all base tubes was F � 32 × 0.21887 � 0.00 704 m 2 .Te cost of a heat exchanger with copper base tubes and aluminum fns is calculated as follows: Te internal surface area of each base tube is ( Te volume of each tube is ( Te mass of each tube is Te total cost of all the copper tubes in the heat exchanger is International Journal of Chemical Engineering Te mass of each tube is Te total cost of all aluminum tubes in the heat exchanger is 3.4.3.Heat Transfer Strength.Te heat transfer strength of the fnned-tube heat exchanger is defned as the ratio of the total heat exchange to the total mass of the exchanger: heat transfer strength � total finned tube heat transfer total finned tube mass . ( A greater value of this ratio means the exchanger achieves a higher amount of heat transfer per unit mass of the exchanger, and thus requires less cost.As seen from the calculation, the heat transfer strength of the aluminum tube is greater than that of the copper tube.Tis means an aluminum-tube heat exchanger with the same structural parameters can exchange more heat per unit mass of the exchanger, so an aluminum tube can reduce the cost of constructing an exchanger.Figure 11 compares the heat transfer strengths of the two types of fnned tubes.
Te heat transfer strengths of both the aluminum and copper tubes with aluminum fns increase with increasing inlet wind speed.Under the same wind speed, the heat transfer strength of the aluminum tubes is 67% higher than that of the copper tubes, which indicates that the aluminum tubes require less mass to achieve the same amount of heat transfer, thus reducing the cost of construction.

Comprehensive Analysis.
Using an aluminum fnned tube is initially intended to reduce the cost of heat exchanger materials.Te previous analysis of the performances of both fnned tubes can be combined with analysis of the material cost per unit heat exchange of each material.Te material cost of each fnned-tube heat exchanger is calculated with the following formula: material cost � amount of material × unit price of materials, material cost per unit heat transfer � finned tube material costs finned tube heat transfer . ( As seen from the previous section, the ratio of the unit price of copper to that of aluminum is 3.8.Figure 12 compares the material cost per unit heat exchange of a fnned-tube heat exchanger with copper tubes with that of an identically structured exchanger with aluminum tubes.
Te quality, material cost, and material cost per unit heat exchange of the aluminum-tube heat exchanger are lower than those of the copper-tube exchanger with the same structural parameters.Te quality of the aluminum-tube exchanger is about 30% of that of the copper-tube exchanger, while the material costs per unit heat exchange for the aluminum-tube exchanger are 9%-10% of those of the copper-tube exchanger.It can be seen that aluminum tubes can greatly reduce the construction cost.
Tables 3 and 4 show the performance parameters and material costs of the aluminum-tube and copper-tube heat exchangers according to the above analysis for surface wind speeds of 1-5 m/s.
Te calculations show that the construction cost of a copper-tube fnned heat exchanger is 113.2 yuan, while that of an aluminum-tube heat exchanger with the same structure is 9.01 yuan.Te cost of an aluminum-tube fnned 8 International Journal of Chemical Engineering heat exchanger is very low because of the lower density and lower cost of aluminum.For the same structural parameters, the aluminum-tube heat exchanger costs only 8% as much as the original copper-tube exchanger.Tis reduction of 92% shows that the aluminum tube can greatly reduce the cost of constructing a fnned-tube heat exchanger.When the preservation coating is provided on the two pipe materials, the heat transfer efciency of the pipe will not be afected.Te coating used is a special thermal conductive coating, which is diferent from the general coating; the special thermal conductive coating contains thermal conductive molecules which will not afect the heat transfer efciency of the tube fn heat exchanger.After the heat exchanger anticorrosion, a smooth anticorrosive coating will be formed on the surface of the heat exchange tube, which has obvious scale inhibition efect, and the fow rate of the medium fowing through it is fast, which improves the heat transfer efect.Te comparison of the heat transfer coefcient of coated and uncoated tubes over time is shown in Figure 13.
In the long run, the heat exchanger after anticorrosion has good heat exchange efect and is safe and stable [25].Heat transfer strength refects the heat transfer capacity per kilogram of a heat exchanger, with a greater value indicating that under the same heat transfer conditions, lower heat exchange corresponds to lower cost.Comparing Tables 3 and 4 reveals that the heat transfer strength of the  International Journal of Chemical Engineering aluminum-tube heat exchanger is greater, showing that this exchanger can still provide a required amount of heat transfer at a lower cost.Te material cost per unit heat exchange refects the ratio of the material cost of the heat exchanger to its heat exchange, with a larger value indicating a higher cost under the same heat exchange conditions.It can be seen that the material cost per unit heat exchange of the copper tube is much higher than that of the aluminum tube under diferent head-on wind speeds, indicating that the cost of an aluminum-tube heat exchanger is less than that of a copper-tube heat exchanger.Overall, compared with the copper-tube fnned heat exchange, the aluminum-tube exchanger has a heat transfer that is 7%-9% lower at a cost that is 92% lower, greatly reducing the cost of construction.Meanwhile, the heat transfer strength of the aluminum-tube exchanger is 67% higher than that of the copper-tube exchanger.With the shortage of copper resources today and rise in copper prices, a comprehensive examination shows the feasibility of aluminum-tube heat exchangers in the air conditioning industry.

Conclusion
Tis study numerically simulated a heat exchanger with aluminum fns and aluminum tubes and an exchanger with aluminum fns and copper tubes.Te two were compared in terms of heat transfer performance and cost, and the following are the main conclusions: (1) Te higher thermal conductivity of copper gives a fnned-tube heat exchanger better heat transfer performance than that of a heat exchanger with aluminum tubes.Under diferent inlet air speeds, the heat transfer of an aluminum-tube heat exchanger is 4%-12% lower than that of a copper-tube exchanger with the same structure, and the heat transfer coefcient of the former is 7%-9% lower.
(2) Because aluminum is less dense and cheaper than copper, the cost of a heat exchanger with aluminum fns and aluminum tubes is only 8% of the cost of one with aluminum fns and copper tubes.(3) Te heat transfer strength of an aluminum-tube heat exchanger is 67% higher than that of a copper-tube exchanger, which can reduce the cost of construction, while the unit heat transfer cost of a copper tube is higher, which also reduces the construction cost of an aluminum-tube heat exchanger.

Figure 1 :
Figure 1: Local model of the fnned-tube heat exchanger.

Figure 2 :
Figure 2: Simplifed model of the computational domain.

Figure 11 :
Figure 11: Comparison of heat transfer intensity of two fnned tubes at diferent wind speeds.

Figure 12 :
Figure 12: Cost comparison of aluminum fnned tubes and copper fnned tubes.

Figure 13 :
Figure 13: Relationship between heat fow and preservation coating.

Table 2 :
Physical parameters of the materials.

Table 3 :
Heat transfer parameters of the copper-tube fnned heat exchanger.

Table 4 :
Heat transfer parameters of the aluminum-tube fnned heat exchanger.