ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERINGAsia-Pac. J. Chem. Eng. 2011; 6: 312–315Published online 17 February 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI:10.1002/apj.423

Research and Development Note

Experimental study on heat transfer coefficient in a rotarytube dryer

Jing Wu,1,2* Xuanyou Li,2 Hongyao Wang,2 Yongchun Shi2 and Benyin Chai1

1Shandong University, No. 73 Jingshi Road, Jinan 250061, PR China2Shandong Tianli Drying Equipment Co. Ltd, 19 Keyuan Road, Jinan 250014, PR China

Received 29 September 2009; Accepted 24 November 2009

KEYWORDS: indirect heat transfer; rotary tube dryer; heat transfer coefficient; drying

INTRODUCTION

The rotary tube dryer is developed from the conven-tional rotary dryer by installing small tubes inside therotary chamber. The small tubes, which are connectedto a steam source to provide heat for drying, rotatewith the dryer chamber. In this kind of dryer, heat forwater evaporation is transferred indirectly. Comparingwith the direct heating drying, indirect drying has manyadvantages, such as high quality of final product, highenergy efficiency, low pollution to the environment,etc.[1,2]

The heat transfer process in a rotary tube dryer ismuch more complicated than that in a conventionaldirect heating one. Heat transfer takes place simulta-neously between the surfaces of the tubes, materialand gas. In spite of a few industrial applications, untilnow, no proper design and calculation method has beendeveloped. Moreover, few reports on the mechanismstudy could be found in the literature. Determinationof heat transfer coefficient is the first step towards theunderstanding of the mechanism of indirect heating dry-ing. The aim of this work is, therefore, to developan experimental method for the determination of thisparameter and to find out the crucial factors affecting it.

EXPERIMENTAL SYSTEM AND MEASURINGMETHOD

The experimental facility is similar to an industrialinstallation.[3] The heat for water evaporation is, how-ever, supplied by electricity rather than the usuallyemployed drying medium, steam. Figure 1 shows theschematic diagram of the experimental system built in

*Correspondence to: Jing Wu, Shandong University, No. 73 JingshiRoad, Jinan 250061, PR China. E-mail: wujingsd@163.com

the authors’ lab. It consists of a drying chamber, 24stainless steel tubes, 1 driving motor, 3 thermocou-ples and locations, 1 electromagnetism velometer toadjust and measure the rotary speed of the motor, anamperemeter, and a voltage meter, a voltage booster,and a gas flowmeter for regulating the inert gas (N2)flow.

The drying chamber was made of stainless steel witha diameter of 500 mm. It rotates with the supportingwheel, which was driven by a motor. The chamber’srotation speed could be changed by adjusting the speedof the motor. The tubes inside the chamber weresubstituted with a set of stainless pipes and measuringpipes. A series of thermocouples were fixed downstreamthe measuring pipes to determine the temperature ofparticles or moisture carrying gas. The configuration ofa typical measuring type is shown in Fig. 2. It is similarto the stainless tubes except that one section is replacedby a copper shell with electrical insulation joints sothat the surface temperature can be measured with anattached thermocouple. The electrical connection forthe stainless steel parts was made through an internallyplaced heating wire insulated by ceramic rings.

Purified terephthalic acid (PTA) with 0.1% moisturecontent was used as the wet material. Because ofthe good dispersion of the particles by the tubes, theexperimental results were repeatable.

The preliminary tests showed that, in a cross-section,the material particles move circularly inside a crescentarea, up to a quite high rotary speed (Fig. 3). This move-ment makes the particles in a stable fluidization state.Consequently, the properties of the particles are uniformwithin the drying chamber, and the gas temperature isnot much away from that of the particles. The heat trans-fer from the tube surfaces to the wet materials can bedetermined by Eqn (1).[4] This transfer coefficient is areflection of heat transferred by solids contact, as well

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Asia-Pacific Journal of Chemical Engineering EXPERIMENTAL STUDY ON HEAT TRANSFER COEFFICIENT 313

Figure 1. Schematic diagram of the experimental rotary tube drying system. This figure isavailable in colour online at www.apjChemEng.com.

Figure 2. Configuration of a typical measuring pipe. This figure is availablein colour online at www.apjChemEng.com.

Figure 3. A snapshot of the movement ofparticles. This figure is available in colour online atwww.apjChemEng.com.

as the convective heat transfer to the gas.

h = IU

A(Tw − Tm)(1)

where h is the heat transfer coefficient (W/m2 · ◦C), Ithe electric current (A), U the voltage (V), A the validsurface area (m2), TW the surface temperature (◦C) andTm the particle temperature (◦C).

The integral heat transfer coefficient in the rotary tubedryer was calculated by Eqn (2).

have =

∫ 2π

0hdθ

∫ 2π

0dθ

(2)

where have is the integral heat transfer coefficient(W/m2 · ◦C), and θ the angle (rad).

To measure the particles or gas temperature, twospecial stainless steel tubes were made to fix thethermocouples, which were assembled downstream themeasuring pipes, as shown in Fig. 4.

RESULTS AND DISCUSSION

It was found that different position of a tube givesdifferent heat transfer coefficient. Experimental results

2010 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2011; 6: 312–315DOI: 10.1002/apj

314 J. WU et al. Asia-Pacific Journal of Chemical Engineering

Figure 4. Temperature measuring system.

also showed that the filling ratio, the position of heatingtubes and the rotary speed are the crucial factorsthat affect the heat transfer coefficient, as discussedsubsequently.

Heat transfer coefficient at different position

The observation showed that the velocity and fractionof particles vary significantly in the dryer. As a result,the heat transfer coefficient between a tube surface andthe material or the moisture carrying gas periodicallyvaries with the position of the tube (Fig. 5). At thebottom of the chamber (0◦), the tube can make morecontacts with the particles, thus heat transfer coefficientis expected to be high. The number of particles thetube met during this part of movement is relatively fixeddepending on the filling ratio because the particles werenot that dynamic. As the dryer rotates, the particlesthe tube can meet increase in a given time becauseof the falling of particles. More contacts with particleswould increase the heat transfer coefficient. Meanwhile,the contribution of convective heat transfer is increasedalso. The highest heat transfer coefficient appears at theposition of about 90◦. From about 180◦ to the positionof 270◦, the fraction of particle is almost zero, heatexchange takes place only between the tube surfaceand moisture carrying gas. It can be seen that thevariation of the heat transfer coefficient is not largecompared to the averaged value. This indicates that themajor heat transfer for this type of dryer is between

−50 0 50 100 150 200 250 300 350 4005060708090

100110120130140150160170

Hea

t Tra

nsfe

r C

oeffi

cien

t (W

/m2 .

°C)

Position (°)

Near the axisFar from the axis

Figure 5. Heat transfer coefficient at different position,rotating velocity = 40 r/min, filling ratio = 20%, gasflowrate = 40 l/min. This figure is available in colour onlineat www.apjChemEng.com.

the tubes and the moisture carrying gas. The latterwould transfer the heat to the wet solids for moistureevaporation.

It is interesting to note that the level of heat transfercoefficient varies with the position of the heating tube.The one near the axis has a smaller tangential velocity.The particles are also flowing relatively towards the wallof the drying chamber. These two reasons can explainthe significantly smaller heat transfer coefficient nearthe axis.

2010 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2011; 6: 312–315DOI: 10.1002/apj

Asia-Pacific Journal of Chemical Engineering EXPERIMENTAL STUDY ON HEAT TRANSFER COEFFICIENT 315

5 10 50 5550

60

70

80

90

100

110

120

130

140

150

Near the axisFar from the axis

Hea

t tra

nsfe

r co

effic

ient

(W

/m2 .

°C)

Rotating Speed (r/min)

15 20 25 30 35 40 45

Figure 6. Integral heat transfer coefficient changeswith rotating speed, filling ratio = 20%, gas flowrate= 40 l/min. This figure is available in colour online atwww.apjChemEng.com.

Influence of rotating speed

It is expected that increasing rotating speed can increasethe relative velocity between the tube and the particles,and also the particles agitation speed inside the carryinggas. Therefore, the increase in heat transfer is expectedto increase with the rotating speed as shown in Fig. 6.This trend reached a limit at RPM around 40. Thedecrease in heat transfer with the rotating speed atRPM >40 was the result of the change of particlesmoving pattern. The high RPM leads to a higher valueof centrifugal force than that of gravitational one, part ofthe smaller sized particles would move with the dryingchamber wall without forming the particles cascades.Thus, the convective heat transfer as well as the contactheat transfer would all be reduced.

Influence of filling ratio

The ratio of filling material to the valid volume of cham-ber significantly should affect the velocity and frac-tion distributions of particle phase following the aboveunderstanding of the heat transfer mechanism. Theexperimental results confirm this expectation (Fig. 7).However, it was observed that there exists such a cer-tain value beyond which the increase in heat transfercoefficient is insignificant. For the conditions investi-gated in this study, this critical value is about 20%. Thisis because too much particles inside the drying chamberwould limit their fluidization or hinder the convectiveheat transfer between the gas and the particles.

10 15 20 2560

70

80

90

100

110

120

130

140

150

Near the axisFar from the axis

Hea

t tra

nsfe

r co

effic

ient

(W

/m2 .

°C)

Filling ratio (%)

Figure 7. Integral heat transfer coefficient varies withmaterial filling ratio, rotating velocity = 40 r/min, Gasflowrate = 40 l/min. This figure is available in colour onlineat www.apjChemEng.com.

CONCLUSIONS

A unique experimental system for determining heattransfer coefficient and investigating heat transfer mech-anism in a rotary tube dryer was designed, built andtested. It was found that heat transfer coefficient varieswith its radial and circumferential position. The inte-gral heat transfer coefficient increases with the increasein rotating speed until a critical value, 40 RPM inthis study. Further increase would bring down the heattransfer coefficient due to the change of particle flow.The integral heat transfer coefficient increases with theincrease in material filling ratio until a certain value,about 20% in this study. Later, the increase in heat trans-fer coefficient is insignificant because of the change ofthe particles fluidization state.

REFERENCES

[1] J. Guomiao. Drying Equipment in Design of ChemicalEngineering Equipment, Chemical Publishing Company:Beijing, 2003; pp.298–301 (in Chinese).

[2] Z. Xu, Y. Hongshan, Y. Yongfei. Technical Calculation onHDPE Rotary Steam Tube drye, vol. 1, Chemical Machinery:Gansu Province, Lanzhou City, 2000; pp.20–22 (in Chinese).

[3] G. Zengshan, Y. Kai. Analysis of the Effects of the Factorson PTA Drying, vol. 3, Henan Chemics: Henan Province,Zhengzhou City, 2000; pp.17–18 (in Chinese).

[4] R.H. Perry, D.W. Green. Chemical Engineers Handbook, 7thEdn, McGraw-Hill: New York, 1998.

2010 Curtin University of Technology and John Wiley & Sons, Ltd. Asia-Pac. J. Chem. Eng. 2011; 6: 312–315DOI: 10.1002/apj