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Experimental study on heat transfer coefficient in a rotary tube dryer.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 312–315
Published 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 rotary
tube dryer
Jing Wu,1,2 * Xuanyou Li,2 Hongyao Wang,2 Yongchun Shi2 and Benyin Chai1
1
2
Shandong University, No. 73 Jingshi Road, Jinan 250061, PR China
Shandong 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 conventional rotary dryer by installing small tubes inside the
rotary chamber. The small tubes, which are connected
to a steam source to provide heat for drying, rotate
with the dryer chamber. In this kind of dryer, heat for
water evaporation is transferred indirectly. Comparing
with the direct heating drying, indirect drying has many
advantages, such as high quality of final product, high
energy efficiency, low pollution to the environment,
etc.[1,2]
The heat transfer process in a rotary tube dryer is
much more complicated than that in a conventional
direct heating one. Heat transfer takes place simultaneously between the surfaces of the tubes, material
and gas. In spite of a few industrial applications, until
now, no proper design and calculation method has been
developed. Moreover, few reports on the mechanism
study could be found in the literature. Determination
of heat transfer coefficient is the first step towards the
understanding of the mechanism of indirect heating drying. The aim of this work is, therefore, to develop
an experimental method for the determination of this
parameter and to find out the crucial factors affecting it.
EXPERIMENTAL SYSTEM AND MEASURING
METHOD
The experimental facility is similar to an industrial
installation.[3] The heat for water evaporation is, however, supplied by electricity rather than the usually
employed drying medium, steam. Figure 1 shows the
schematic diagram of the experimental system built in
*Correspondence to: Jing Wu, Shandong University, No. 73 Jingshi
Road, Jinan 250061, PR China. E-mail: wujingsd@163.com
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
the authors’ lab. It consists of a drying chamber, 24
stainless steel tubes, 1 driving motor, 3 thermocouples and locations, 1 electromagnetism velometer to
adjust and measure the rotary speed of the motor, an
amperemeter, 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 with
a diameter of 500 mm. It rotates with the supporting
wheel, which was driven by a motor. The chamber’s
rotation speed could be changed by adjusting the speed
of the motor. The tubes inside the chamber were
substituted with a set of stainless pipes and measuring
pipes. A series of thermocouples were fixed downstream
the measuring pipes to determine the temperature of
particles or moisture carrying gas. The configuration of
a typical measuring type is shown in Fig. 2. It is similar
to the stainless tubes except that one section is replaced
by a copper shell with electrical insulation joints so
that the surface temperature can be measured with an
attached thermocouple. The electrical connection for
the stainless steel parts was made through an internally
placed heating wire insulated by ceramic rings.
Purified terephthalic acid (PTA) with 0.1% moisture
content was used as the wet material. Because of
the good dispersion of the particles by the tubes, the
experimental results were repeatable.
The preliminary tests showed that, in a cross-section,
the material particles move circularly inside a crescent
area, up to a quite high rotary speed (Fig. 3). This movement makes the particles in a stable fluidization state.
Consequently, the properties of the particles are uniform
within the drying chamber, and the gas temperature is
not much away from that of the particles. The heat transfer from the tube surfaces to the wet materials can be
determined by Eqn (1).[4] This transfer coefficient is a
reflection of heat transferred by solids contact, as well
Asia-Pacific Journal of Chemical Engineering
EXPERIMENTAL STUDY ON HEAT TRANSFER COEFFICIENT
Figure 1. Schematic diagram of the experimental rotary tube drying system. This figure is
available in colour online at www.apjChemEng.com.
Figure 2. Configuration of a typical measuring pipe. This figure is available
in colour online at www.apjChemEng.com.
where h is the heat transfer coefficient (W/m2 · ◦ C), I
the electric current (A), U the voltage (V), A the valid
surface area (m2 ), TW the surface temperature (◦ C) and
Tm the particle temperature (◦ C).
The integral heat transfer coefficient in the rotary tube
dryer was calculated by Eqn (2).
2π
hdθ
(2)
have = 0 2π
dθ
0
where have is the integral heat transfer coefficient
(W/m2 · ◦ C), and θ the angle (rad).
To measure the particles or gas temperature, two
special stainless steel tubes were made to fix the
thermocouples, which were assembled downstream the
measuring pipes, as shown in Fig. 4.
A snapshot of the movement of
particles. This figure is available in colour online at
www.apjChemEng.com.
Figure 3.
as the convective heat transfer to the gas.
h=
IU
A(Tw − Tm )
RESULTS AND DISCUSSION
(1)
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
It was found that different position of a tube gives
different heat transfer coefficient. Experimental results
Asia-Pac. J. Chem. Eng. 2011; 6: 312–315
DOI: 10.1002/apj
313
J. WU et al.
Asia-Pacific Journal of Chemical Engineering
Figure 4. Temperature measuring system.
also showed that the filling ratio, the position of heating
tubes and the rotary speed are the crucial factors
that affect the heat transfer coefficient, as discussed
subsequently.
Heat transfer coefficient at different position
The observation showed that the velocity and fraction
of particles vary significantly in the dryer. As a result,
the heat transfer coefficient between a tube surface and
the material or the moisture carrying gas periodically
varies with the position of the tube (Fig. 5). At the
bottom of the chamber (0◦ ), the tube can make more
contacts with the particles, thus heat transfer coefficient
is expected to be high. The number of particles the
tube met during this part of movement is relatively fixed
depending on the filling ratio because the particles were
not that dynamic. As the dryer rotates, the particles
the tube can meet increase in a given time because
of the falling of particles. More contacts with particles
would increase the heat transfer coefficient. Meanwhile,
the contribution of convective heat transfer is increased
also. The highest heat transfer coefficient appears at the
position of about 90◦ . From about 180◦ to the position
of 270◦ , the fraction of particle is almost zero, heat
exchange takes place only between the tube surface
and moisture carrying gas. It can be seen that the
variation of the heat transfer coefficient is not large
compared to the averaged value. This indicates that the
major heat transfer for this type of dryer is between
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Heat Transfer Coefficient (W/m2.°C)
314
170
160
150
140
130
120
110
100
90
80
70
60
50
−50
Near the axis
Far from the axis
0
50
100
150
200
250
300
350
400
Position (°)
Heat transfer coefficient at different position,
rotating velocity = 40 r/min, filling ratio = 20%, gas
flowrate = 40 l/min. This figure is available in colour online
at www.apjChemEng.com.
Figure 5.
the tubes and the moisture carrying gas. The latter
would transfer the heat to the wet solids for moisture
evaporation.
It is interesting to note that the level of heat transfer
coefficient 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 wall
of the drying chamber. These two reasons can explain
the significantly smaller heat transfer coefficient near
the axis.
Asia-Pac. J. Chem. Eng. 2011; 6: 312–315
DOI: 10.1002/apj
EXPERIMENTAL STUDY ON HEAT TRANSFER COEFFICIENT
150
150
140
140
Heat transfer coefficient (W/m2.°C)
Heat transfer coefficient (W/m2.°C)
Asia-Pacific Journal of Chemical Engineering
130
120
Near the axis
Far from the axis
110
100
90
80
70
60
50
5
10
15
20 25 30 35 40
Rotating Speed (r/min)
45
50
55
Figure 6.
Integral heat transfer coefficient changes
with rotating speed, filling ratio = 20%, gas flowrate
= 40 l/min. This figure is available in colour online at
www.apjChemEng.com.
130
120
Near the axis
Far from the axis
110
100
90
80
70
60
10
15
20
25
Filling ratio (%)
Figure 7. Integral heat transfer coefficient varies with
material filling ratio, rotating velocity = 40 r/min, Gas
flowrate = 40 l/min. This figure is available in colour online
at www.apjChemEng.com.
Influence of rotating speed
It is expected that increasing rotating speed can increase
the relative velocity between the tube and the particles,
and also the particles agitation speed inside the carrying
gas. Therefore, the increase in heat transfer is expected
to increase with the rotating speed as shown in Fig. 6.
This trend reached a limit at RPM around 40. The
decrease in heat transfer with the rotating speed at
RPM >40 was the result of the change of particles
moving pattern. The high RPM leads to a higher value
of centrifugal force than that of gravitational one, part of
the smaller sized particles would move with the drying
chamber wall without forming the particles cascades.
Thus, the convective heat transfer as well as the contact
heat transfer would all be reduced.
CONCLUSIONS
A unique experimental system for determining heat
transfer coefficient and investigating heat transfer mechanism in a rotary tube dryer was designed, built and
tested. It was found that heat transfer coefficient varies
with its radial and circumferential position. The integral heat transfer coefficient increases with the increase
in rotating speed until a critical value, 40 RPM in
this study. Further increase would bring down the heat
transfer coefficient due to the change of particle flow.
The integral heat transfer coefficient increases with the
increase in material filling ratio until a certain value,
about 20% in this study. Later, the increase in heat transfer coefficient is insignificant because of the change of
the particles fluidization state.
Influence of filling ratio
The ratio of filling material to the valid volume of chamber significantly should affect the velocity and fraction distributions of particle phase following the above
understanding of the heat transfer mechanism. The
experimental results confirm this expectation (Fig. 7).
However, it was observed that there exists such a certain value beyond which the increase in heat transfer
coefficient is insignificant. For the conditions investigated in this study, this critical value is about 20%. This
is because too much particles inside the drying chamber
would limit their fluidization or hinder the convective
heat transfer between the gas and the particles.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
REFERENCES
[1] J. Guomiao. Drying Equipment in Design of Chemical
Engineering Equipment, Chemical Publishing Company:
Beijing, 2003; pp.298–301 (in Chinese).
[2] Z. Xu, Y. Hongshan, Y. Yongfei. Technical Calculation on
HDPE 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 Factors
on 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, 7th
Edn, McGraw-Hill: New York, 1998.
Asia-Pac. J. Chem. Eng. 2011; 6: 312–315
DOI: 10.1002/apj
315
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