EFFECTS OF HEAT TRANSFER IN A HORIZONTAL ROTATING CYLINDER OF THE CONTACT DRYER UDC 536.24 : 66.047

FACTA UNIVERSITATIS
Series: Mechanical Engineering Vol. 5, No 1, 2007, pp. 47 - 61
EFFECTS OF HEAT TRANSFER IN A HORIZONTAL
ROTATING CYLINDER OF THE CONTACT DRYER
UDC 536.24 : 66.047
Slavica Prvulovi?1, Dragiša Tolma?2,
Miroslav Lambi?2, Ljiljana Radovanovi?2
1University of Belgrade, Technical Faculty, Vojske Jugoslavije 12, 19210 Bor, Serbia
2University of Novi Sad, Technical Faculty ''Mihajlo Pupin,' Djure Djakovic bb,
23000 Zrenjanin, Serbia
E-mail: ljiljap@tf.zr.ac.yu
Abstract. The paper presents a part of the experimental and theoretical research project
related to engineering and the application of the contact drying method. It also gives tests of
the system parameters of the cylinder dryer with the layer of the dried material, on the
surface of the rotating cylinder of the contact dryer in the exploitation conditions. On the
basis of the tests, the heat transfer coefficient, heat transfer model, and other process-relevant
parameters are determined.
Key words: Heat Transfer Model, Contact Dryer
1. INTRODUCTION
The drying on the contact cylinder dryers is a common diffused technological process
in chemical, food-processing and textile industry. These are the devices for drying colloi-
dal solutions, suspensions, viscous fluids, textile materials and the like.
Relatively simple construction and low specific energy consumption make these de-
vices very attractive for applications in the mentioned industries. They also assume only
few parameters that can be estimated. Their design needs data about the experimental
plants. Both the cylinder dryers and the improvements of the drying technique aim at the
efficiency and thrift of these plants, i.e. the reduction of the power and investing costs.
2. THE EXPERIMENTING APPARATUS
The tests are done on the industrial plant of the cylinder dryer, with cylinder diameter
d2=1220 mm and length L=3048 mm, that is heated inside by steam vapor. The scheme

Received April 10, 2007

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48
S. PRVULOVI?, D. TOLMA?, M. LAMBI?, LJ. RADOVANOVI?
of the industrial plant is in Fig. 1. When the cylinder is heated and the constant working
pressure of p=4bar is released, the necessary experimental measurements in the stationary
conditions are done.
The stationary conditions imply the stationeries during a great number of rotations
(when nonstationariness that appears in every cylinder rotation separately is excluded).
The tests are done under the following conditions:

Fig. 1 Technological Scheme of the Cylinder Dryer Plant and the Experimental
Apparatus; 1-cylinder; 2-bringing cylinders; 3-scattering cylinder; 4-knife;
5-pipeline for the wet material transporting; 6-the worm conveyer; 7-the reservoir
for the suspension preparing; 8-the screw eccentric pump; 9-steam pipeline;
10-the scheme of the measuring places
1) The environment temperature in the closed room, the dry thermometer temperature
t=180C
2) The atmospheric pressure
pa=1 bar
3) The water vapor pressure
pp=4 bar
4) The water vapor temperature
tp=1400C
5) The number of cylinder rotations
n=7.5 min-1
6) The thickness of the dried material moisture
?2=0.25 mm
8) The cylinder surface
A=11,5m2
7) The dried material moisture
- at the beginning of the drying it was
w1=65%
- at the ending of the drying it was
w2=5%
9) Water vapor consumption
mp=220 (kg/h)

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Effects of Heat Transfer in a Horizontal Rotating Cylinder of the Contact Dryer
49
Table 1 gives physical characteristics of the used fluids and material:
Table 1 Physical Characteristics of the Used Fluids and Material at 20 oC
Density Raising level density Viscosity Specific heat
Media
(kg/m3)
(kg/m3)
(m Pa s)
(kJ/kgK)
Water 998,2
-
1,01
4,187
Starch 1,164
550
-
-
35% mass solution of starch and water
1,055
-
8,330
3,15
The results of temperature measuring with dried materials layer on cylinder surface
are given in Table 2, and the results of measuring of air convection speed in direct vicin-
ity of cylinder, are given in Table 3. The measuring was performed in the plane of cylin-
der cross-section according to the scheme of experimental points given in Figure 1.
Table 2 Results of Temperature Measurement with
Dried Material Layer on the Cylinder Surface t(0C)
Number of
Distance from cylinder; x=(mm)
measurements points
0 10 20 30 40
1. 89,0
43,5
39,8
37,2
35,0
2. 98,0
40,0
38,0
37,0
35,5
3. 96,0
41,0
39,0
33,0
34,0
4. 81,0
42,0
40,0
31,0
30,0
5. 82,0
35,0
31,0
30,0
29,0
6. 85,0
36,0
28,5
38,0
27,0
7. 83,0
30,0
27,3
26,0
25,0
8. 86,0
38,0
35,0
32,0
30,0
Mean value t (0C) 85,0
37,8
34,2
31,6 30,2
Table 3 The results of Measuring Air Speed with the Layer of
Dried Material on the Cylinder Surface v (m/s)
Number of
Distance from cylinder; x=(mm)
measurements points
0 10 20
30 40
1. 2.
1. 0,48
0,32
0,28 0,26
0,24
2. 0,48
0,34
0,30 0,28
0,26
3. 0,48
0,38
0,34 0,30
0,28
4. 0,48
0,46
0,38 0,32
0,28
5. 0,48
0,36
0,32 0,28
0,24
6. 0,48
0,45
0,34 0,32
0,28
7. 0,48
0,44
0,36 0,34
0,32
8. 0,48
0,40
0,34 0,30
0,26
Mean value v (m/s)
0,48
0,40
0,33
0,30
0,27

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50
S. PRVULOVI?, D. TOLMA?, M. LAMBI?, LJ. RADOVANOVI?
3. METHOD DEFINING COEFFICIENT OF HEAT TRANSFER
The relevant parameter for defining of energetic balance of cylinder dryer presents the
coefficient of heat transfer from condensing vapor at cylinder interior onto the surround-
ing air.
Total heat flux from vapor onto surrounding air can be given in the following form:

q = h (t - t ) [W/m2]
(1)
u
t
p
v
For big cylinder diameters in relation to envelope thickness, and according the litera-
ture [15], we can, with great accuracy, use the term for the coefficient of heat transfer as
for flat wall in the equation (2).
So, for example, for cylinder diameter d2=1220mm and cylinder wall thickness
?1=35mm, if we define the heat transfer coefficient for flat wall, the mistake is 1.66% in
relation to the variable of the heat transfer coefficient for cylinder body. Because of that a
simpler form for total coefficient of heat transfer, given by the relation (2) will be applied.
When in the cylinder surface the level of the drying material is raised, the coefficient
of heat transfer is defined according to the following equation:
1

h =
[W/m2K] (2)
t
1
?
?
1
1
2
+
+
+
h
k
k
h
1
1
2v
2m
Influential parameter on the mechanism of heat transfer is the coefficient of heat trans-
fer (hm). The value of Nussele's number is defined out of the equation.
h d

m
2
o
N
B
=
= Re

(3)
u
kv
On the basis of the grouping influential parameters that are of the greatest influence on
the coefficient of heat transfer, the results of experimental and theoretical research are
correlated by the form of the equation of Nussele's type [1] :
c
k
? d G ?

v
2
h
=
B
(4)

m
?
?
d
? µ
2
?
Constants B and C are defined by the method of the least squares.
4. RESULTS OF RESEARCH AND DISCUSSION
By applying the correlation theory to the experimental results of measuring, we have
got empirical equations of change of temperature t, in the function of x, distance, from the
elevation of cylinder surface at every measuring point.
The temperature curve in the plane of the central cross-section of cylinder with the
presentation of standard deviations is given in Figure 2.

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Effects of Heat Transfer in a Horizontal Rotating Cylinder of the Contact Dryer
51

Fig. 2 The Curve of Temperature in the Plane of Central Cross –
Section of Cylinder with the Presentation of Standard Deviations
The empiric equation dependence of mean temperature and distance x, from the ele-
vation of cylinder surface has the following form:

2
t = 80- 3,80? x + 0,07 ? x (5)
sr
By applying the correlation theories to the experimental results of measuring we have
got empirical equation of speed v, in the distance function x, from the elevation of cylin-
der surface at every measuring point.
The speed curve of convection in the plane of central cross – section of cylinder with
the presentation of standard aberrations, is presented in Fig. 3.

Fig. 3 The curve of Air Convection in the Cylinder Cross-sectional Area
with the Presentation of Standard Aberrations

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52
S. PRVULOVI?, D. TOLMA?, M. LAMBI?, LJ. RADOVANOVI?
Empirical dependence of mean speed of air v, in direct surroundings of cylinder and
distance x, from the elevation of cylinder surface, has the following form:

-4 2
v = 0,48 - 0,009? x +10 x (6)
sr
The research implies that the following parameters are defined, namely: the coefficient
of heat transfer from the drying material on surrounding air (h2m) and total coefficient of
heat transfer (ht).
The results of experimental and theoretical research are correlated with correlated
equation of Nussele's type, equations (7, 8).
There is also fixed temperature gradient and temperature flux, Figs. (4, 5 and 6).
By applying the correlation theory to the results of experimental and theoretical re-
search projects, we have defined the empirical equation of heat flux dependence and tem-
perature gradient presented with the expression (10).
Figs. 4, and 5 give the results of defining temperature gradient and temperature flux.
It can be noticed that local values of temperature gradient and heat flux have variables
along the cylinder size. These variables of given values are formed due to varied air convec-
tion in cylinder vicinity, i.e. air mixing and thermo siphon effect of the hood for conducting
away heat air and evaporated fluid. So, at certain places (on cylinder surface parts) with a
greater air convection speed, higher temperature gradients also appear, Fig. 4.
On the lower"forehead” part of cylinder are higher temperature gradients, taking into
account that the air in the room in which the cylinder performs the first conditioning
(cooling) of the cylinder"forehead side”.

Fig. 4 Change of Temperature Gradient Near Cylinder Surface (d2= 1.220 mm, tm=85oC)
Greater variables originate in the lower part (zone) of cylinder, Fig. 5. The dominant
effect on the heat flux extent in the given cylinder zone has flux that originates by humid-
ity evaporation from drying material surface. In the given cylinder zone in drying process
(the first drying zone) due to the literature [2], intensive humidity evaporation out of dry-
ing material appears. Heat flux that originates with evaporating humidity, convection and

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Effects of Heat Transfer in a Horizontal Rotating Cylinder of the Contact Dryer
53
radiation, presents total heat flux, Fig. 5. There is also fixed temperature gradient and heat
flux, Fig.7. So, e.g. the mean value of heat flux in the upper part of cylinder is about
11.000 (W/m2), while on the lateral side (on the place of the position of the knife for re-
moving dried material) it is 8.000 (W/m2), Fig. 5.

Fig. 5 Heat Flux Change along the Cylinder Size (d2=1220 mm, tm = 850C)
When there is a layer of drying material, total heat flux contains part of flux equal to
the produce of heat conductivity of humid material and temperature gradient and flux part
equal to the produce of material flux of humidity and specifically humidity enthalpy, i.e.
the flux originating with evaporating humidity. Heat flux originating with evaporating
humidity by its intensity is a relevant factor in total heat flux when there is drying material
on surface.
On the basis of local heat fluxes values, Fig. 5, heat flux has a variable along cylinder
size. In the second drying period, and especially in the end of drying, temperature gradi-
ent has rising tendency, Fig. 4. Due to research results [2], during drying process at cylin-
der dryers, humidity remains near the end of drying are taken away at temperature rising
on material surface; because of that, there are also higher variables of temperature gradi-
ent at the end of drying.
Mean variables of temperature gradient along cylinder size, Fig. 4, are: at the cylinder
lower part 57?103(0C/m); on the cylinder lateral sides they are 55?103(0C/m) and on the top
side of cylinder they are 50?103(0C/m).
Fig. 6 gives the results of defining temperature gradient in distance function from
cylinder surface.
Mean variables of temperature gradient are:
? in the zone at cylinder surface 55,41?103(0C/m);
? at the distance of about 5 mm from cylinder surface 6,65?103(0C/m)
? at the distance of about 40 mm the cylinder surface 0,0115?103(0C/m) etc.
In view of that the change of temperature gradient is the highest at the very cylinder
surface, it is in accordance with [3], and [4].

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54
S. PRVULOVI?, D. TOLMA?, M. LAMBI?, LJ. RADOVANOVI?

Fig. 6 Mean Variable of Temperature Gradient in Distance Function from the
Elevation of Cylinder Surface (d2=1.220 mm, tm=85 0C)
During acting of heating surface of drying material on surroundings in view of heat
emitting, temperatures at the surface of the drying material layer are higher than the tem-
peratures at a greater distance of the surface layer of drying material, so that temperature
gradients are higher also at the surface layer of drying material, Fig. 6.
We can see two zones of air layer. The first one is in direct vicinity of cylinder surface
at the distance of 10 mm. The second zone is distant more than 10 mm, Fig. 6. In the zone
of the nearer cylinder surface of temperature gradient is steeper, Fig. 6, it is in accordance
with [3] and [5]. These points out higher temperature gradient nearer to the layer surface
of drying material.

Fig. 7 Dependence of Change Heat Flux, and Temperature Gradient
with Cylinder (d2=1220, tm=85oC)
Because of poor heat conductivity of air, layers more distant of the cylinder surface,
have lower temperature and, along with it, lower temperature gradients than air layer di-
rectly near the layer surface of drying material.

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Effects of Heat Transfer in a Horizontal Rotating Cylinder of the Contact Dryer
55
Table 4 gives the results of defining heat transfer coefficient by convection (hm), heat
transfer coefficient through radiation (hr), heat transfer coefficient through evaporating
humidity (hh) and combined heat transfer coefficient (h2m).
Figs. 8, 9 and 10 give research results correlated by the relation of Nussele's and Rey-
nolds's number.
Table 4 Combined heat transfer coefficient (h2m), heat transfer coefficient through
convection (hm), heat transfer coefficient through radiation (hr) and heat
transfer coefficient by evaporating humidity (hh)
Heat transfer
Heat transfer coeff.
Combined
Number of Heat transfer coefficient coefficient through through evaporating coefficient of
measuring through convection
radiation
humidity
heat transfer
place
hm (W/m2K)
hr (W/m2K)
hh (W/m2K)
h2m (W/m2K)
1. 2.
3.
4. 5.
4 15,0
7,2
475 497
5 15,8
7,2
335 358
6 17,0
7,1
189 214
7 17,8
7,2
128 153
8 14,7
7,3
87 109
1 16,9
7,1
41 65
Mean
15,8 7,2 210
233
value
It is noticed that heat transfer coefficient by convection from drying material layer on
(to) air is a variable along cylinder size.
Mean value of heat transfer coefficient is 15,8 (W/m2K). Maximal value of heat trans-
fer coefficient is in lower part zone of cylinder, and it is 17,8 (W/m2K). To greater values
of Reynolds's number, Fig. 10, suits as well higher temperature gradient, Fig. 4, and ac-
cording to it as well greater values of heat transfer through convection and Nussele's
number, Figs. 8 and 9.

Fig. 8 Dependence of Change of Nussele's and Raynolds's Number
with Cylinder (d2=1220mm, v=0.35m/s, tm=85oC)

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56
S. PRVULOVI?, D. TOLMA?, M. LAMBI?, LJ. RADOVANOVI?

Fig. 9 Nussele's Number Change Along Cylinder Size (d2=1220mm, v=0.35m/s, tm=85oC)
On the basis of Reynolds's number value according to Fig. 10, Re=29.950; what is less
than Rek=5 105, [6], [7], Convection in direct vicinity of cylinder is laminated.

Fig. 10 Change of Reynolds's Number along Cylinder Size (d2=1220mm, v=0.35m/s)
Changeable speed of air convection in direct vicinity of cylinder effects on change of
temperature gradient. Fig. 4 and heat flux, Fig. 5, and what is in accordance with [8], [9].
So local values of temperature gradient and heat flux have variables along cylinder size.
Applying the correlation equation (3) and (4) to the results of experimental and theo-
retical researches, Fig. 7, we get the following empirical equation:

0.691
N = 0.569? Re

(7)
u
This is Nussele's type equation and it, at the same time, correlates the results of ex-
perimental and theoretical researches. Taking into account equation (4), we can define
heat transfer coefficient through convection from drying material surface coefficient into
surrounding air with the help of the following relation:

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