Effects of Heat Transfer on Convection Dryer with Pneumatic Transport of Material

Effects of Heat Transfer on Convection
Dryer with Pneumatic Transport of
Material
Dragiša Tolma?

Professor
University of Novi Sad
Experimental and theoretic research was conducted and the results were
Techical Faculty “Mihajlo Pupin”, Zrenjanin
implemented in a real industrial environment on convection dryer with

Slavica Prvulovi?
pneumatic transport of material. The numeric values are given for
optimum parameters of drying, energetic characteristics and balances as
Professor
University of Belgrade
well as the models of heat transfer. Accomplishment of the heat transfer in
Technical Faculty, Bor
these systems is based on the principle of direct contact of dried material

and warm air. Then, an intensive transfer of heat and mass is
Ljiljana Radovanovi?
accomplished. This work presents the results of research which can be
Assistant
useful in designing and construction of such dryers in food industry. It
University of Novi Sad
Techical Faculty “Mihajlo Pupin”, Zrenjanin
refers to the technological and technical characteristics of the dryer,
energetic balances and coefficients of heat transfer.

Keywords: heat transfer, convection drying, pneumatic transport, numeric
data.


1. INTRODUCTION
pneumatic dryer, Fig. 1. Drying agents are heated with

the gas burner (1). Drying is performed in the direct
Application of the convection pneumatic dryers is
contact of warm gases with the moist material. The
represented, especially in food industry in plants for
principle of direct drying is represented here. The
industrial processing of grains (wet milling processing
drying material is corn bran.
of wheat and corn). Generally, such dryers can be used
for drying of meal-like and fine-kernel materials.
Simple construction and relatively low consumption of
energy have enabled successful application of such
dryers in the above stated industrial branches. The
construction of the convection dryer enables
simultaneous pneumatic transport of wet material and
its drying.
In these dryers, a continuous drying of loose materials
is being made, the concentration being (0.05 – 2.00) kg of
material per 1 kg of air. Average particle size of the
drying material can be (0.05 – 2.00) mm. The circulation
speed of the heated agent of drying (air or gas) in the
dryer is (10 – 30) m/s. The initial moisture of the material
dried can be about w1 = 40 %, and the remaining moisture
after drying is usually about w2 = 15 %. The specific
consumption of energy is usually about to 4500 kJ/kg of
evaporable water. Efficiency of such dryers is evaluated
according to the thermal degree of utilization which is
about of 75 %, depending of the drying system (indirect
or direct drying). The quantity of evaporated moisture in
the dryer pneumatic pipe is approximately 350 kg/m3h,
according to literature [1-3]. The drying time in these

dryers is very short, only several seconds, therefore they
1 – Gas burner; 2 – Rotary feed of moist material; 3 – Dryer
can be used for drying of the materials susceptible to high
pipe; 4 – Dryer head; 5 – Cyclone; 6 – Centrifugal fan; 7 –
Helical conveyor for bringing of moist material; 8 – Helical
temperatures in a short drying period of time.
conveyor; 9 – Pipe of pneumatic transportation; 10 – Fan

Figure 1. Scheme of convection pneumatic dryer
2. DESCRIPTION OF EXPERIMENTAL PLANT

Dosing of moist material to the dryer is performed
Experimental research is made in the convection
through the rotation dozer (2) with the capacity of m1 =
9925 kg/h, through the auger conveying system, as
Received: February 2008, Accepted: May 2008
given in the scheme of experimental equipment in
Correspondence to: Dragiša Tolma?
Figure 1. Auger conveyor (7) has a role of mixing the
Technical Faculty “Mihajlo Pupin”,
moist material. In such a way, a homogenous moist
?ure ?akovi?a bb, 23000 Zrenjanin, Serbia
material is obtained at the dryer inlet. In the Table 1, the
E-mail: dragisat@gmail.com
characteristics of convection pneumatic dryer are given.
© Faculty of Mechanical Engineering, Belgrade. All rights reserved
FME Transactions (2008) 36, 45-49 45

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Table 1. Characteristics of convection pneumatic dryer
Total heat quantity:
No. Position
Name of equipment and characteristics
.
.
.
.
U
Q
= Qw + QS+ Qg [kJh-1], (3)
Gas burner type: Saacke SG, heat power
1 1
Q =
3.40 MW
.
2
3
Dryer pipe, diameter d = 625 mm, height 21 m
Q
= ?
??
U
B Hd
T [kJh-1]. (4)
Cyclone separator, diameter Dc = 1350 mm,
3 5 height cylindrical part of the cyclone is 1920
Quantity of drying air:
mm, conical part of cyclone is 3350 mm
?
Centrifugal fan,
3/h,
Q
4 6
V = 26000 mn
p = 3500 Pa,
U
V =
[m 3h-1]. (5)
N = 75 kW
L
n
H
?
5 7 Rotary
feed,
N = 18.5 kW, n = 660 min-1
Specific consumption of energy:
6 4
Dryer
head

U
Q
Moist material is transported via hot air – the drying
q =
[kJkg-1]. (6)
W
agent through the dryer pneumatic pipe (3), it passes
through the dryer head (4) and goes to the cyclone
Thermal degree of utilization:
separators (5) for separation of dried material, and the
.
.
hot gases are expelled by a ventilator (6), into the
?
Q ? Q
t
t
atmosphere. The dried material is transported from the
1
2
U
g
?T =
=
. (7)
.
cyclone via auger conveyors (8), and through a separate
1
t
Q
line of pneumatic transport (9), up to the material
U
warehouse department. During drying, the determined
Total heat power of drying:
fuel – gas consumption is B = 293 m3/h.
Table 2 contains average values of the results of
QU = hu ? A ? ?tsr [W]. (8)
measuring the air temperature – the drying agent and
Total coefficient of the heat transfer:
moisture of dried material. Experimental measuring is
being made in the approximate stationary conditions of
hu = QU/(A ? ?tsr) [Wm-2K-1]. (9)
the dryer operation. The stationary conditions mean the
Heat for drying, i.e. its convective part consists of the
stationary conditions during a longer period of the dryer
heat for water evaporation (Q
operation and a greater number of measurings (where
w) and heat for heating of the
drying material (Q
non-stationary conditions of the process are excluded
s), meaning without heat losses (Qg):
during the realistic conditions of the dryer operation).
co
Q nv = w
Q + S
Q [kJh-1]. (10)
Table 2. Average values of the results of measuring the
drying temperature and the material moisture
During convective drying the following equation of
the heat transfer is applied as well:
Measuring place, according to 1-1 2-2 3-3 4-4 5-5
the Figure 1
.
Q
= h ? A? ?t [W]. (11)
Temperature of the hot air,
conv
c
sr
425 342 222 155 110
t [ºC]
Coefficient of the heat transfer through convection
Moisture of the dried
30 22 16 14 12
(hc) is a relevant parameter of Nusselt criteria of heat
materijal, w [%]
transfer, according to Fyhr [7], Tolmac [8]:

3. DETERMINATION METHOD OF THE ENERGETIC
c
h ? d
=
. (12)
BALANCE AND COEFFICIENT OF HEAT
Nu
k
TRANSFER

Based on the analysis of influential parameters of
In the drying process, the total invested energy is spent
heat transfer, the following equation is acquired of the
on: water evaporation, heating of drying material and
Nusselt type:
heat losses. Energetic balances show appropriate
relations between the total invested energy, utilized
a
k
? d ?G ?
=
. (13)
energy and heat losses during the drying process. The
c
h
K ?
?
d
? µ ?
energetic balances can be useful when showing the
dryer condition diagnosis.
Based on it, the research results can be shown with
The difference of enthalpy, according to Holman [4],
the help of correlation equation of the Nusselt type:
Liu [5], Tolmac [6].
= ( )a
N
K Re
. (14)
?
-3
H = H1 ? H 2 = cp ( 1
t ? t2 ) [kJmn ]. (1)
Reynolds number is determined by the following
Quantity of evaporated water:
equation:
?
100 ? 1
w ?
d ? G
W = 1
m ?1?
? [kgh-1]. (2)
Re =
. (15)
100 ?
?
2
w ?
µ
46 ? VOL. 36, No 1, 2008
FME Transactions

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The constant (K) and the exponent (a) are being
capacity is 1640 kg/h, and the air temperature at the dryer
determined by the method of the least difference squares.
inlet is 425 ºC. According to the literature, Indarto [13],

the mass air flow is 0.289 kg/sm2, the drying capacity is
4. THE RESULTS OF EXPERIMENTAL RESEARCH
1152 kg/h, at the drying temperature of 90 °C.
OF ENERGETIC BALANCE HEAT TRANSFER
COEFFICIENTS AND DISCUSSION
Table 4. Total coefficient of heat transfer (hu)

Volume
Total heat
Experimental research on the convection pneumatic
Total
Middle log.
of pipe Drying
transfer
dryer, Fig. 1. was aimed at determining the energetic
quantity heat
difference of
drying surface,
coefficient,
(heat power),
temperature,
balance, specific consumption of energy, thermal degree
place, V
A, [m2]
h
Q
k
?t
u
of utilization and other relevant parameters of drying,
U [kW]
[m3]
sr [ºC]
[Wm-2K-1]
according to the literature, Prvulovic [9]. The results of
2210 6.44
41.20 157 342
the energetic balance are given in the Table 3.

Table 3. Energy balance of convection pneumatic dryer
According to the research, Prvulovic [11], on the
convection pneumatic dryer, the value of the total
Energy drying
Sign and
Energy value
No.
coefficient of heat transfer in the process of drying corn
parameter
measure unit
parameter
starch is 308 W/m2K, and in drying of potato starch the
Air temperature at the
1
coefficient of heat transfer is 320 W/m2K. The
inlet of dryer
t1 [ºC]
425
coefficient of heat transfer under the dynamic
Quantity of evaporable
2
conditions of the dryer operation (non-equal dosing of
water
W [kgh-1] 2030
material to be dried, oscillations in the initial moisture
3
Total heat quantity
QU [kJh-1] 7956000
content, temperature of drying, heat flux, etc.) depends
4
Drying heat power
Q
on the greater number of different values which
U [kW]
2210
5 Energy
specific
use
characterize the heat transfer. The objective of this part
q [kJkg -1] 3919
of research is to determine the character of heat transfer
6
Quantity of drying air
V
3
L [mn h-1] 19452
in such complex dynamic model, considering that the
Specific quantity of
7
[kgm-2h-1] 49
heat transfer comprises a phenomenon of heat transfer
evaporable water
by convection, conduction and radiation. Based on the
Specific quantity of
8
[kgm-3h-1] 315
results of research, the value of the coefficient of heat
evaporable water
transfer by convection has been determined, Table 5.
Air temperature at the
9
Table 5. Coefficient of heat transfer by convection (
outlet of the dryer
t2 [ºC]
110
hc)
Thermal degree of
Heat
Heat
10
?
Coeff. of
utilization
T [%]
74
Heat power power power of
Mean log.
Surface
convection

for water for mat.
heat
difference
drying,
heat
Based on the research, the total heat force of drying
evaporation, heating, transfer by
of temp., transfer,
convection, A [m2] ?
hc
of
Qw [kW]
QS
tsr [ºC]
Q = 2210 kW is acquired as well as the specific
[Wm-2K-1]
[kW] Q
consumption of energy
conv [kW]
q = 3919 kJ/kg of evaporable
1 2 3 4 5 6
water. According to the literature He? [10], Prvulovic
[11], a specific consumption of energy in convection
1502 67 1569 41.20 157 242
drying amounts (3850 – 5040) kJ/kg, of evaporable

water. According to the data from literature, Islam [12],
Values of the Reynold’s and Nusselt’s number are
specific consumption of energy amounts q = (4642 –
given in the Table 6.
5283) kJ/kg, of evaporable water.
According to the research results, the criteria
On the basis of the results of energetic balance and
equation of the Nusselt type has the following form:
results of the drying parameters measuring, according to
Nu = – 6.385 + 0.011 ? Re. (16)
the literature, Indarto [13], the total coefficient of the
heat transfer during convection drying is h
Based on the (16), the results of the experimental
u = 342
W/m2K, Table 4. On the basis of the research results,
and theoretic researches are correlated by the relation
the mass air flow amounts 0.169 kg/sm2, the drying
between the Nusselt’s number (Nu) and Reynold’s

Table 6. Reynold’s number and Nusselt’s number
Mass speed
Dryer pipe
Dynamic viscosity
Coefficient of convection Thermal air
Reynold’s
Nusselt’s
stream air, G
diameter, d
of air, µ x 10-6
heat transfer,
conductive,
number,
hc
k
number,
[kgs-1m-2]
[m]
[kgs-1m-1]
Re x 10-2
[Wm-2K-1]
[Wm-1K-1]
Nu
1 2 3
4
5
6 7
0.169 0.625 34.05
31
242
5.34 28.32
0.169 0.625 31.09
34
242
4.86 31.12
0.169 0.625 26.73
39
242
4.07 37.16
0.169 0.625 23.97
44
242
3.60 42.01
0.169 0.625 22.38
47
242
3.27 46.25
0.169 0.625 27.64
39
242
4.23 36.97
FME Transactions
VOL. 36, No 1, 2008 ? 47

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number (Re). Based on it, by increasing of the
convection pneumatic dryer in food industry. Based on
Reynold’s number due to the increase of hot air
the analysis of energetic balance, the heat force of
circulation – the drying agent, the Nusselt’s number is
drying has been determined QU = 2210 kW, specific
increased. The coefficient of the heat transfer by
consumption of energy q = 3918 kJ/kg of evaporable
convection (hc) is increased then. On the basis of the
water, as well as the thermal degree of utilization ?T =
experimental research, the following relation is acquired
0.74. Energy balance of the dryer can serve to evaluate
for the coefficient of heat transfer by convection:
power condition of the dryer as well as to review the
possibility of rational consumption of energy. A
k
significant share of the energy during drying is
c
h = (?6.385 + 0.011? Re) ?
. (17)
d
forwarded to transfer of heat to the material, necessary
Table 7 contains the values of the coefficient of the
for evaporation of moisture and heat for the breaking of
heat transfer (
connection forces of moisture with the basis of the
hc) for the various values of the Reynold’s
number. By applying the correlation theory, the method
material to be dried. Specific consumption of energy
of the least difference squares on the results of
and quality of dried material are basic data which
experimental and theoretic research, we acquire the
characterize the results of drying on the convection
phenomenology equations of the dependence of the heat
dryer. By following and control of these parameters in
transfer coefficient (
the drying process, the optimum consumption of energy
hc) and the Reynold’s number (Re).
is provided as well as the quality of dried material.
Table 7. Heat transfer coefficient (hc), for different values of
On the basis of the results of research of energetic
Reynold’s number and diameters of the dryer pipe d =
balance and the results of measuring the temperature of
0.625 m
the drying agent, the total coefficient of the heat transfer
The dryer Coeff. of
is determined in the convection dryer in the amount of
Reynold’s
Air
Thermal air
pipe
convection
number, temperature, conductivity,
hu = 342 W/m2K, and the coefficient of the heat transfer
diameter, heat transfer,
Re
t [°C]
k [Wm-1K-1]
by convection hc = 242 W/m2K. The effects of the heat
d [m]
hc [Wm-2K-1]
losses during drying are expressed through the separate
1 2 3 4 5
value hu – hc = 100 W/m2K, the so called coefficient of
3000 400 5.21 0.625 222
the heat transfer for the heat losses together with the
3500 300 4.60 0.625 230
outlet air and the heat transfer by conduction and
radiation through the dryer pipe. In such way, the
4000 200 3.93 0.625 236
effects of the heat transfer are determined as well as the
4500 150 3.56 0.625 245
basic parameters of the heat transfer.
5000 110 3.21 0.625 250
The acquired results of research are based on the
5500 100 3.10 0.625 257
experimental data from the industrial dryer. Based on that,

the results of research have a value of use, i.e. they are
The empirical equation of dependence of the heat
useful to the designers, manufacturers and beneficiaries of
transfer coefficient (h
these and similar drying systems as well as for the
c) and the Reynold’s number (Re)
for the diameter of the dryer pipe d = 0.625 m, is given
educational purposes. The results of research can also be
by the following relation:
used for: determination of dependence and parameters of
the heat transfer during convection drying, as well as in
hc = 179.8 + 0.014 ? Re. (18)
designing and development of convection dryers.
On the basis of the results of research, Table 4. the
total coefficient of the heat transfer
REFERENCES
hu = 342 W/m2K.
The coefficient of the heat transfer by convection hc =
[1] Likov, A.: Drying Theory, Energy, Moscow, 1988.
242 W/m2K is given in Table 5. The largest quantity of
[2] Prvulovic, S., Tolmac, D. and Lambic, M.:
heat during drying is consumed for heating of the
Determination of energetic characteristics of
material to be dried and water evaporation. Coefficient
convection drying place on pneumatic
of heat transfer by convection hc = 242 W/m2K in the
transportation material, Journal of Process
complex conditions of the dryer operation depends on
Technique, Vol. 1. 70-74, 2001.
the various values which characterize the heat transfer.
[3] Tolmac, D., Prvulovic, S. and Lambic, M.:
These values are the heat flux, the area of drying, the
Mathematical Model of the Heat Transfer for
temperature differences, etc. In order to determine the
Contact Dryer, FME Transactions, Vol. 35, No. 1,
effects of the heat transfer during convection drying, the
pp. 15-22, 2007.
topic of heat losses is reviewed as well. On the basis of
that, as a separate value, the coefficient of the heat
[4] Holman, J.P.: Heat Transfer, McGraw-Hill, New
transfer has been determined
York, 1981.
hu – hc = 100 W/m2K,
which shows the share of the heat losses through the air
[5] Liu, Q. and Bakker-Arkema, F.W.: Capacity
outflow from the dryer and the losses due to conduction
Estimation of High-Temperature Grain Dryers – A
and radiation through the dryer pipe.
Simplified Calculation Method, Agricultural

Engineering International: the CIGR Ejournal, Vol.
5. CONCLUSION
I, pp. 1-17, 1999.

[6] Tolmac, D.: Contribution theory and drying
This work presents the experimental and theoretic
practice, University of Novi Sad, Technical Faculty
research of relevant parameters of drying on the
“Mihajlo Pupin”, Zrenjanin, 1997 (in Serbian).
48 ? VOL. 36, No 1, 2008
FME Transactions

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[7] Fyhr, C. and Rasmunson, A.: Mathematical Model
content of the wet material at the outlet of
of a Pneumatic Conveying Dryer, AIChE Journal,
w2
dryer [%]
Vol. 43, No. 11, pp. 2889-2902, 1997.
?tm
mean logarithm difference of temperature [°C]
[8] Tolmac, D. and Lambic, M.: Heat transfer through
Q
heat quantity [kJ/h]
rotating roll of contact dryer, International
B
fuel gas consumption [m3/h]
Communications in Heat and Mass Transfer, Vol.
Hd
lower gas heat power [kJ/m3]
24, No. 4, pp. 569-573, 1997.
A
drying surface [m2]
[9]
V
Prvulovic, S.: Modeling the Mechanism for Heat
k
volume of dryer pneumatic pipe [m3]
Q
Transfer at the Convective Drying and Establishing
w
heat for water evaporation [kJ/h]
Q
of Numerical Readers, PhD thesis, Technical
S
heat for heating of the drying material [kJ/h]
Faculty “Mihajlo Pupin” University of Novi Sad,
Qg
heat losses [kJ/h]
Zrenjanin, 2004 (in Serbian).
?T
thermal degree of utilization [%]

[10] Heß, D.: Comparison of processing Economics of
Different Starch Dryers, Journal of Strach/Starke,

Vol. 36, pp. 369-373, 1984.
?????? ??????? ??????? ??
[11] Prvulovic, S., Tolmac, D. and Lambic, M.:
???????????? ?????? ??
Convection drying in the food industry, Agricultural
??????????? ???????????
Engineering International: the CIGR Ejournal, Vol.
??????????
IX, Invited Overview No. 9. pp. 1-12, 2007.

[12]
??????? ??????, ??????? ?????????, ??????
Islam, M.T., Marks, B.P. and Bakker-Arkema,
F.W.: Optimization of Commercial Ear-Corn
???????????
Dryers, Agricultural Engineering International: the

CIGR Ejournal, Vol. VI, Manuscript FP 04 0071-
? ???? ?? ????????? ?????????? ??????????? ??????
16, 2004.
?? ??????????? ??????????? ??????????, ???? ??
??????? ?? ?????? ?????????? ? ?????????????
[13] Indarto, A., Halim, Y. and Partoputro, P.:
??????????
?
????????????
??????????.
Pneumatic Drying of Solid Particle: Experimental
??????????????? ?????? ? ?????????? ?? ????????
and Model Comparison, Experimental Heat
?? ????????????? ?????????? ??????????? ?????? ?
Transfer, Vol. 20, No. 4, pp. 277-287, 2007.
??????? ??????????.
??????? ?? ??? ???????????????? ? ??????????
NOMENCLATURE
??????????? ??????? ?? ??????? ? ??????? ??????
Nu
Nusselt’s number
???????????? ??????. ????????? ?? ?????????? ?
Re
Reynold’s number
?????? ?????????? ???????????? ?????? ?
d
the dryer pipe diameter [mm]
??????????????? ????????. ?? ?????? ?????????
k
thermal air conductivity [W/mK]
??????????, ???????? ?? ?????????? ?????????
G
mass speed stream air [kg/sm2]
???????.
µ
dynamic viscosity warm air [kg/sm]
???????? ? ?????????????? ???? ?????? ????
h
?? ?? ?????????? ?????? ?????????? ???????????
u
coefficient of heat transfer [W/m2K]
h
???????. ? ?????? ???? ???? ??? ?? ??????????
c
coefficient of convection heat transfer [W/m2K]
H
enthalpy [kJ/kg]
???????
????????
????
????????
?????
t
??????????????? ??????. ???? ?? ??????????
1
air temperature at the inlet of dryer [°C]
t
???????????? ??????? ???????, ???????? ?????
2
air temperature at the outlet of dryer [°C]
C
3
??????, ???????? ?????? ??????????? ? ??????
p
specific air heat [kJ/mn K]
W
quantity of evaporated water [kg/h]
??????? ???????, ??? ????? ???????? ??????? ?
m
???????? ???? ??????. ?? ?????? ????????????????
1
quantity of moist material [kg/h]
content of the wet material at the inlet of
?????? ???????? ?? ???????????? ????????? ???????
w1
dryer [%]
??????? ? ???? ?????? ???????.














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VOL. 36, No 1, 2008 ? 49

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