Devise of a W serpentine shape tube heat exchanger in a hard chromium electroplating process☆

Surasit Tanthadiloke ,Paisan Kittisupakorn ,*,Pannee Boriboonsri,Iqbal M.Mujtaba

1 Department of Chemical Engineering,Faculty of Engineering,Chulalongkorn University,Bangkok 10330,Thailand

2 School of Engineering,University of Bradford,West Yorkshire BD7 1DP,UK

Keywords:W serpentine shape Hard chromium electroplating Mathematical modeling Simulation Heat exchanger

ABSTRACT In a hard chromium electroplating process,a heat exchanger is employed to remove the heat produced from the high current intensity in an electroplating bath.Normally,a conventional U shape heat exchanger is installed in the bath,but it provides low heat removal.Thus,this study designs a novel W serpentine shape heat exchanger with identical heat transfer area to the conventional one for increasing heat removal performance.The performance of the heat exchange is tested with various flow velocities in a cross-section in range of 1.6 to 2.4 m·s-1.Mathematical models of this process have been formulated in order to simulate and evaluate the heat exchanger performance.The results show that the developed models give a good prediction of the plating solution and cooling water temperature,and the novel heat exchanger provides better results at any flow velocity.In addition,the W serpentine shape heat exchanger has been implemented in a real hard chromium electroplating plant.Actual data collected have shown that the new design gives higher heat removal performance compared with the U shape heat exchanger with identical heat transfer area;it removes more heat out of the process than the conventional one of about 23%.

1.Introduction

A hard chromium electroplating process is one kind of chromium electroplating.This process is usually applied for protecting the surface of base materials against a harmful environment,extending the maintenance time and increasing the material properties i.e.,corrosive resistance,wear resistance and shear stress[1,2].In the hard chromium electroplating process,the workpieces such as pistons,rollers,gaskets,vehicle molds and electrical parts,etc.are coated with the chromium metal from 2.5 to 500 μm in thickness[3].Generally,the performance of this plating and the probability of defect occurrence on coated products are depended on the operating conditions during the plating period such as the concentration of plating solution,current density,power voltage and temperature[4,5].To provide the best quality of the coated products,the optimal range for the hard chromium electroplating is in range of(50±3)°C[5,6].Allunitoperations in the hard chromiumelectroplating process are shown in Fig.1.This process comprises of an electroplating bath with an immersed tube heatexchangerthatis also connected to a cooling tower.The cooling water is a media to deliver the heat from the bath and to coolthe plating solution through the heatexchanger.When the cooling water temperature is high after flowing through the electroplating bath,the cooling tower takes away the heat and supplies the low temperature cooling water to the bath again.

However,the main factor affected on the product quality is the temperature of the plating solution during the operation.Due to the fact that the plating solution temperature is continually raised by the heat produced from the high current load during the plating time,then the accumulated heat of plating solution can cause the defects on the surface of products.When this problem takes place,the defected products are recoating again[7].Since this high temperature problem is normally found in the hard chromium electroplating plant,an effective heat exchanger is needed to keep the plating solution temperature in the optimal range around 47 to 53°C along the plating period[8].

In general,a conventionalUshape tube heatexchanger is installed at the wall of the electroplating bath.This pipe is made of the titanium to protect against the corrosion from the chromic acid.Normally,the U shape tube heat exchanger in the bath is placed in parallel with the direction ofheat flow which directly affects to heattransfer coefficient[9].Then,it leads to the thermal resistance film that reduces the heat transfer between two fluids,at the outer surface of the tube[10].In order to improve the heat transfer coefficient of the tube heat exchanger,piping patterns or shape ofheatexchangers reported by recentliteratures such as a spiralcorrugated tube[11,12],a curved tube[13,14],an inserted triangle coil tube[15,16]and a vibrating tube[17,18]have been studied.To evaluate the performance of the heat exchanger,mathematical models of the process and heat exchangers are also developed.

Fig.1.Hard chromium electroplating process with the cooling system.

The objective of this work is to devise a novel heat exchanger for the hard chromium electroplating process to improve the heat transfer coefficient.In addition,the mathematical models of the hard chromium electroplating process and the heat exchangers have been developed and validated with the actual data.The developed models have been used to study the temperature profile and the heat removal performance of the conventional U shape and the novel design heat exchangers.Finally,the novel design heat exchanger has been implemented in the bath and its performance has been evaluated.

2.Methods

2.1.Process overview

A hard chromium electroplating process in this study consists of an electroplating bath(1.7-m diameter and 4.5-m height),a tube heat exchanger,two reservoir tanks of cooling water and a cooling tower as shown in Fig.1.In this process,the height of plating solution is 4 m from the bottom of the bath and the objects to be plated are connected with the rectifier and are submerged into the plating solution.The conventional U shape tube heat exchanger(2.54-cm diameter and 30 m in total length)with cooling water,as the medium is installed inside the bath for removing any heat generated from the electrical current.Then,the high temperature cooling water from all electroplating baths collected at the first water reservoir tank are fed into the cooling tower in order to cool down its temperature to about 34°C.After that,the cooling water is recirculated to the electroplating bath again.Some amounts of cooling water are loss from the drag out,wind and evaporation atthe reservoir tanks and the cooling tower.In orderto collect the data from the real plant,a data logger connected with Type K thermocouples is used forthis purpose.Three thermocouples are placed at 1 m,2 m and 3.5 m from the plating solution surface in order to observe the temperatures in the bath during 8 h of the operation,and the plating bath temperatures can be collected and used to validate the mathematical models of the bath.

2.2.Devise of the W serpentine shape tube heat exchanger

Fig.2.Illustration of the heat exchangers inside the hard chromium electroplating bath:(a)U shape heat exchanger and(b)W serpentine shape heat exchanger.

The novel heatexchangerofthis work is devised with a pattern ofW serpentine shape(Fig.2(b)).This new design attempts to prevent the thermal resistance film formulation by introducing inclined and curve shapes that give the unparalleled flow direction of the cooling water and the plating solution.Thus,the W shape heat exchanger has less the thermal resistance film formulation at the outer tube surface resulting in more heat transfer rate than the original one.In addition,the curve design of the W shape induces the secondary flow of the cooling water inside the tube[19,20]that enhances the heat transfer rate between the plating solution and the cooling water[21,22].Titanium is chosen as the piping material for this novel heat exchanger.In this study,the W serpentine shape heat exchangers with 1.27 and 2.54 cm in diameter,which have the identical heat transfer area as to the U shape heat exchanger,are designed to compare the performance.Furthermore,the heat exchangers are tested with various flow velocities in a cross-section at 1.6,2.0 and 2.4 m·s-1for evaluating its performance.In orderto obtain the same flow velocity in each heatexchanger,the volumetric water flow rates at 1.55×10-4,1.94×10-4and 2.32× 10-4m3·s-1are used for the 1.27 cm diameter heat exchanger and the volumetric water flow rates at 7.12×10-4,8.9×10-4and 10.68× 10-4m3·s-1are used for the 2.54-cm diameter heat exchanger.

2.3.Assumptions

To derive the equations for the hard chromium electroplating process,the following assumptions are made as follows,

·The variation of the plating solution concentration is negligible.

·The physical and chemical properties of the plating solution such as density or heat capacity are constant along the plating period.

·The electroplating bath is well-mixed.

·The heatgenerated fromelectroplating currentload is remarkably significant compared to heat released from chemical reactions.

·Heat loss of the cooling water in the heat exchanger outside the bath to the surrounding is negligible.

2.4.Mathematical modeling of the hard chromium electroplating process

Mathematical models of the electroplating bath and cooling water system in this study(Fig.3)are based on the principle of mass and energy conservation[23,24].

2.4.1.The electroplating bath

Mass conservation equation

Energy conservation equation

The equation ofenergy conservation in Eq.(2)containsthe heatproduction owing to the power currentload,the heat loss at the upper surface of the plating solution to the surrounding and the term related to heat transfer between the electroplating bath and the heat exchanger.The Qlossterm related on the power current load and plating solution temperature is calculated from Eq.(3).

In Eq.(3),the function of f(IV)can be obtained by the least squares method with the relation ofthe actualdata between the plating solution temperature and the current load.

2.4.2.The tube heat exchanger

The energy conservation equation of the tube heat exchanger that immersed in the electroplating bath is applied using a lumped model[25],while a constant volumetric water flow rate is considered on the mass balance.

Mass conservation equation

Energy conservation equation

Fig.3.Schematic diagram of the hard chromium electroplating process with mass and energy balance.

where Uorefers to the overall heat transfer coefficient;ΔTlmexpresses to the logarithmic mean temperature and can be computed by Eq.(6).The rightterm of energy balance in Eq.(5)is constituted of the different of heat flow between outlet and inlet tube heat exchanger and the heat exchanged with the electroplating bath and heat exchanger.

2.4.3.The cooling system

The cooling system composes two water reservoir tanks and a cooling tower.The water flowing from the tube heat exchanger to a tank 1 is delivered to the cooling tower in order to reduce its temperature.The cooling water flows from the cooling tower to a tank 2 before entering to the electroplating bath.Eqs.(7)to(9)provide the mass conservation equations for both tanks and the cooling tower,respectively.

Mass conservation equations

The water makeup of the cooling tower in Eq.(9)consists of the summation of blowdown,drift loss and evaporation loss[26].Blowdown discards a portion of the concentrated circulating water due to the evaporation process in order to lower the system solid concentration.Drift loss is entrained water that carried out from the cooling tower by the wind.

Energy conservation equations

The three terms in the energy conservation equation of the cooling tower as shown in Eq.(12)demonstrate the difference of heat flow between outlet and inlet of the cooling tower with the last two terms showing the heat transferred by convection and evaporation,respectively.Term of hAscan be obtained by the least squares method with the water temperature profile at the cooling tower.The Tavgin the above equation is calculated by

The overall heat transfer coefficient of tube heat exchanger involves two convective and one conductive resistance,while the fouling is negligible.In case of both U shape and W shape tube heat exchangers,the overall heat transfer coefficients are calculated by optimization based on actual data.The optimization problem can be formulated as follows:

Subject to process models are from Eqs.(1)to(13).

The physical properties,geometric characteristics and operating conditions used in this work are summarized in Tables 1 to 3.In the model validation,the simulation results are validated with the actual data collected from a real plant and demonstrated in Fig.4.

Table 1 Physical properties[27,28]

Table 2 Simulation system geometric characteristics

Table 3 Operating conditions for simulation

Since the workpieces require a high thickness of hard chromium coating,the current load of electroplating is supplied at a high rate.The high current load can lead to the increase in the plating solution temperature if the heat removed out of the solution by the heat exchanger is lower than that of the heat generated from the current load.Fig.4 shows a good agreement between simulation results and the actual data with the coefficient of determination(R2)of more than 90%so the mathematical models of this process can be used to predict the temperature profile of the process.

Fig.4.Comparison results between simulation and actual data of plating solution temperature and water temperature at inlet and outlet of the electroplating bath.

Fig.5.The temperature profile of plating solution,water inlet and outlet at the electroplating bath for 1.27-cm diameter of W serpentine shape tube heat exchanger at various water velocities.

3.Results&Discussion

3.1.Simulation results

Figs.5 and 6 demonstrate the temperature profile of the plating solution,water inlet and outlet at the electroplating bath for the W serpentine shape with diameters of 1.27 and 2.54 cm comparing to the conventional U shape tube heat exchanger.To test the heat exchangers,the cooling water flow velocities of 1.6,2.0 and 2.4 m·s-1are introduced in this study.These results show thatthe plating solution temperature profiles of both sizes of the W serpentine shape are lower than that of the U shape heat exchanger.Since,the new design has the combination of the inclined and curved shapes which makes an unparalleled flow direction between the cooling water and the plating solution inside the bath.This prevents the formulation of thermal resistance film at the outer tube surface that resulting in enhances the heat transfer rate[10].

Fig.6.The temperature profile of plating solution,water inlet and outlet at the electroplating bath for 2.54-cm diameter of W serpentine shape tube heat exchanger at various water velocities.

Fig.7.The implementation of W serpentine shape tube heat exchanger.

Furthermore,the curve shape of this devised heat exchanger increases the heat transfer rate between the plating solution and the cooling water from the secondary flow of the cooling water inside the tube[20–22].In Fig.5,when currentload isapplied to the electroplating bath during the 8 h of operation,the 1.27-cm diameter of the W shape heat exchanger with 2.4 m·s-1of flow velocity can adequately remove the heat that generated from the bath and the temperature of the plating solution can be maintained in the optimalrange of(50±3)°C.However,in the case of 1.6 m·s-1of flow velocity,the temperature difference between plating solution and water at the outlet is small.As a consequence,the heat removal is insufficient to maintain the plating solution temperature at the optimal range;the temperature of the bath is more than 53°C.Nevertheless,the result of the W serpentine shape with 2.54-cm diameter in Fig.6 shows that the cooling water at all velocities can remove the generated heat from the process and the plating solution temperature can be kept at the optimal range along the electroplating period;the maximum temperature of the plating solution is 52.6,52.3 and 52.2 °C at 1.6,2.0 and 2.4 m·s-1of flow velocities,respectively.With these results,thus the W serpentine shape with 2.54-cm diameter is selected for implementation at the real plant.

Fig.8.Comparison of temperature profile of plating solution,water inlet and outlet at the electroplating bath with the cooling water flow velocity at 2.0 m·s-1.

3.2.Implementation results of the W serpentine shape tube heat exchanger

According to the simulation study,the Wserpentine shape tube heat exchanger requires the 2.54-cm diameter and 30 m in total length.However,in practical and due to the limitations of the free space for installation inside the real electroplating bath,this heat exchanger is divided into four pieces.Each piece has a diameter of 2.54 cm and a length of 7.5 m(Fig.7),which is conveniently implemented in the real bath.Each heat exchanger is immersed in half of plating solution depth because a large amount of heat from the hard chromium electroplating process releases at this position.The end of both sides is connected with a polyvinylchloride(PVC)tube to supply and recirculate the cooling water.

Fig.8 demonstrates the actualdata after the implementation ofthe W serpentine shape tube heat exchanger and the U shape tube heat exchanger.The result indicates that the W serpentine shape tube heat exchanger can keep the plating solution temperature during the plating period lower than the original one.The plating solution temperature can be kept in the range of 50–53 °C during the plating period,and the water outlet temperature is about 39–42 °C.With the collected data,the novel W serpentine shape tube heat exchanger removes more heat out of the process than the conventional heat exchanger around 23%.

4.Conclusions

To control the temperature of the hard chromium electroplating bath,an effective heat exchanger is needed to remove heat occurred during the electroplating process.In this work,heat removals of two tube heat exchangers have been compared.In addition,mathematical models of the hard chromium electroplating process have been formulated to predict the dynamics temperature of the electroplating bath and evaluate the heat exchanger performance.Unknown parameters in the developed models are determined based on the actual plant data.The simulation results show that the developed models can give a good accurate prediction of the plating solution temperature with the coefficientofdetermination(R2)ofmore than 90%.The heatremoval performances of the 1.27-and 2.54-cm diameter of the W serpentine shape tube heat exchangers with the flow velocity in a cross-section at 1.6,2.0 and 2.4 m·s-1are compared.The simulation results indicate that the W serpentine shape with 2.54-cm diameter is applicable to maintain the plating solution temperature at the desired range at any flow velocity.Furthermore,four W serpentine shape tube heat exchangers with identical heat transfer area to the U shape tube heat exchanger(2.54-cm diameter and 30 m in total length)are implemented in the real electroplating bath.The plating solution temperature after the implementation can be kept at the range.Moreover,the novel design provides higher heat removal than the conventional U shape tube heat exchanger around 23%.

Nomenclature

Ahtheat transfer area of a tube heat exchanger,m2

Aocross section area of a tube heat exchanger,m2

Asurcontacted area of the surface of plating solution with surroundings,m2

Assurface area between water droplet and air,m2

Cppspecific heat capacity of plating solution,kJ·kg-1·°C-1

Cpwspecific heat capacity of water,kJ·kg-1·°C-1

Fbwater blowdown rate of a cooling tower,m3·s-1

Fciinlet volumetric water flow rate of a cooling tower,m3·s-1

Fcooutlet volumetric water flow rate of a cooling tower,m3·s-1

Fdwater drift loss of a cooling water,m3·s-1

Fewater evaporation rate in a cooling tower,m3·s-1

Fmwater makeup to a cooling tower,m3·s-1

Fotvolumetric flow rate of cooling water from other electroplating baths to a cooling tower,m3·s-1

Fovvolumetric flow rate of cooling water from a water reservoir tank 2 over flows to a water reservoir tank 1,m3·s-1

Fwvolumetric flow rate of cooling water,m3·s-1

fTheat exchanger correction factor

h heat transfer coefficient of convection between water and air,W·m-2·°C-1

I electric current,A

Ltlength of a tube heat exchanger,m

Qlossheat loss from an upper surface of the plating solution to surroundings,W

Tairair temperature,°C

Tavgaverage different water temperature at a cooling tower,°C

Tciwater temperature at a water reservoir tank 1,°C

Tcooutlet water temperature in a cooling tower,°C

Tmwater makeup temperature,°C

Totoutlet water temperature from other electroplating baths,°C

Tpplating solution temperature,°C

Tp,actualplating solution temperature which collected from a real plant,°C

Tp,simulationplating solution temperature from simulation,°C

Tsimulationtemperature results from the simulation,°C

Twiinlet cooling water temperature,°C

Twi,actualinlet cooling water temperature which collected from a real plant,°C

Twooutlet cooling water temperature,°C

Two,actualoutlet cooling water temperature which collected from a real plant,°C

TT2water temperature at a water reservoir tank 2,°C

ΔTlmlogarithmic mean temperature at a tube heat exchanger,°C

tftotal operation time,s

Uooverall heat transfer coefficient,kW·m-2·°C-1

V electrical voltage,V

Vcvolume of a cooling tower,m3

Vpvolume of an electroplating bath,m3

VT1volume of a water reservoir tank 1,m3

VT2volume of a water reservoir tank 2,m3

Vtvolume of a tube heat exchanger,m3

v flow velocity in a cross-section of a tube heat exchanger,m·s-1

λelatent heat of vaporization of water,kJ·kg-1

ρpdensity of plating solution,kg·m-3

ρwdensity of water,kg·m-3

μwdynamic viscosity of water,kg·m-1·s-1