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Analysis of Drying Kinetics and Moisture Distribution in Convective Textile
Fabric Drying
Article in Drying Technology · May 2006
DOI: 10.1080/07373930600611984
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Drying Technology, 24: 485–497, 2006
Copyright # 2006 Taylor & Francis Group, LLC
ISSN: 0737-3937 print/1532-2300 online
DOI: 10.1080/07373930600611984
Analysis of Drying Kinetics and Moisture Distribution in
Convective Textile Fabric Drying
Luiza Helena C. D. Sousa, Oswaldo. C. Motta Lima, and Nehemias C. Pereira
Chemical Engineering Department, State University of Maringa, Maringa-PR, Brazil
The drying process of crude cotton fabric is analyzed under two
main aspects: analysis of moisture distribution inside the textile
sheet, and analysis of certain operational convective drying process
variables. Experimental apparatus consisted of a drying chamber in
which samples of pure cotton textile were suspended inside the drying chamber and exposed to a convective hot air flow. The influence
of the operational variables on the drying process behavior was studied by two different ways with generalized drying curves. The behavior of moisture distribution profiles was compared to average
moisture content of the textile fabric verifying whether average
values were able to represent the textile moisture content during
the drying process.
Keywords Textile convective drying; Moisture content; Generalized drying curves; Moisture distribution
INTRODUCTION
Textiles, non-woven and other fibrous materials, are of
immense importance in today’s global economy. In
addition to their use in well-established consumer and
industrial markets, they have widespread use in novel,
non-traditional applications, such as in reinforced composites, geotextiles, personal absorption products, and biomedical materials, Sousa.[1]
Drying is a very broad subject and consists of an
extremely energy-intensive process. The theoretical analysis
and design of drying systems are complicated by a number
of factors. Simultaneous heat and mass transfer to and
from the surface and within the material, the hydrodynamics of particle motion inside the material, and the various mechanisms of moisture migration in a solid body
are some of the problems involved. Change of material
moisture content and temperature is usually controlled by
heat and mass transfer between the body surface, the surroundings, and the inside of the drying material.
Correspondence: Nehemias C. Pereira, Chemical Engineering
Department, State University of Maringa, Avenida Colombo
5790, Bl. D-90, 87020 900, Maringa-PR, Brazil; E-mail: nehemias@
deq.uem.br
This work presents a study of the drying process of
crude textile (cotton) fabric and two main aspects: analysis
of moisture distribution inside the textile sheet as a function of drying time and sample relative position due to
hot air convective flow, and a study of the influence of
several operational process variables—samples’ initial
moisture content, drying air velocity, and its temperature—on the convective drying process.
Heat and mass transfer in a porous medium is a process
that occurs in nature and in many engineering applications.
The movement of moisture in textile materials is of particular interest in the current study. Research for new experimental drying data will facilitate the development of
general models that may establish, within a good approach,
the kinetic behavior of textile drying, moisture and temperature profiles, drying rates, time of drying, and other
important data.
Concerned with the optimization of drying textile process, Parajia et al.[2] determined an optimum point of
humidity, for each following condition, thermal efficiency,
productivity, and operation costs. The authors affirm that
when the optimum point is the control of the humidity in
warm air dryers, it becomes universal.
According to Wolf and Bimbenet,[3] temperatures of
products are rarely measured during drying kinetics tests.
However, these data are extremely important for the
understanding of drying mechanisms and quality control,
mainly for term-sensitive materials.
Beckham et al.[4] studied magnetic resonance imaging
(MRI) and other techniques to investigate moisture transport
in textiles. MRI revealed that moisture distribution in carpets
is significantly influenced by details of vacuum extraction
procedures used to remove excess water. Pore structure and
diffusion of fluids within textiles were also studied.
Wicking rates of PET yarns were found to decrease with
increasing twist and tensile force, both of which resulted in
decreased capillary radii. MRI and light microscopy
revealed that fluids were primarily carried through yarn
interiors and not along surfaces.
485
486
SOUSA, LIMA, AND PEREIRA
Strumillo and Kudra[5] present curves related to the
second period of drying of textiles’ hygroscopic materials
and the behavior of these curves shows characteristics very
similar to those current research which corroborates the
experiments.
Ryan et al.[6] obtained experimental results on convective heat and mass transfer and fluid flow characteristics
of tissue and towel products using commercially realistic
structures. The comparison with literature data on wet,
pressed, dried, and rewetted sheets indicates significant differences in drying and permeability characteristics. This
fact confirms that the internal structure of the material
does indeed play a significant role in through-air-drying
and should be taken into account in modeling, optimization, and control of commercial systems.
Belhamri[7] developed a mathematical model to account
for experimental results regarding certain physical properties of a standard material. The internal profile of moisture
and penetration of the drying front during the falling rate
period are determined.
Generalized Drying and Drying Rate Curves
The proposal of generalized drying curves is very interesting because they compare the results of different experiments by reducing them to only one set, which may be
divided into two groups: generalized drying curves
(GDC), which relate a dimensionless moisture content to
a dimensionless drying time, and generalized=normalized
drying rate curves (NDRC), which relate normalized
(dimensionless) drying rates to sample moisture content.
Strumillo and Kudra[5] developed equations that represent the generalized drying curves. It is a simple mathematical description of the drying curve in which drying
rate related to moisture content in the second drying period
may be considered linear. The relative influence of temperature, velocity, and humidity of the drying agent on
the drying rate is the same both in the constant and falling
drying rate periods.
Motta Lima et al.[8] studied the application of the drying
curves generalization methodologies (GDC and NDRC) to
natural convection drying of short-fiber cellulose inside an
oven and to natural and forced convection ambient air drying of short-fiber cellulose. Very good results, especially for
the generalized drying curves, were obtained.
The authors successfully applied these generalization
methodologies to handmade kraft paper drying and proposed modified equations based on Page’s[6] drying curve
correlation with an improvement in its fitting performance.
Later on, this approach was successfully applied to
forced convection ambient air drying data on long-fiber
cellulose. The authors used the correlations=models of
Page[9] (Eq. (2)) and Hodges’[10] (Eq. (1)) to adjust GDC
and NDRC results, respectively.
Within this context and for generalized drying curves,
Motta Lima et al.[11] present a brief review of research on
a compilation for the drying of grains, fixed-bed drying
of cellulose pulp, and normalized drying rate curves, more
specifically for cellulose and paper.
Sousa[12] successfully applied this approach in drying on
knit and sarja textile fabric by forced convection with air at
ambient temperature.
The method of data generalization is based on the fact
that experimental drying curves for a given material
obtained under different conditions, but at the same value
of initial moisture content X0, form a single curve in a coordinate system.
Similarly, as for other methods of experimental data
generalization, this method requires an experimental verification for particular materials and process conditions in
drying.
The methodology of generalized drying rate curves discussed by Hodges[10] and Motta Lima et al.[13] correlated
a normalized drying rate curve (NDRC), defined as the
ratio between instantaneous rate and maximum drying rate
(assumed to be the constant drying rate, NC), (N=NC), to
the moisture content raw textile (X). Hodges[10] proposed
an equation to fit NDRC data, Eq. (1), with good results.
NDRC ¼ 1 exp½ðX =aÞðbÞ ð1Þ
where NDRC ¼ (N=NC).
Similarly to other methods of experimental data generalization, this method requires an experimental verification
for particular materials and process conditions in drying.
n
Y ¼ expðkAD tAD
Þ
ð2Þ
where Y is defined as the ratio (X=X0) and tAD is the dimensionless drying time, as the ratio (NCtAD=X0).
Moisture Distribution in Convective Drying Samples
The moisture content of the material in the course of
drying and especially the final moisture content are very
important parameters in drying technology. Under-drying
may result in the formation of mildew, bacterial growth,
agglomeration of particles, clogging of machinery, and
more negative effects. Over-drying may cause deterioration
of product quality and wastage of energy.[5]
Ghali et al.[14] developed a study for simulating the heat
and mass transfer in fabrics during the wicking process.
These authors verified that, unlike the temperature, the
variation of the fractional saturation is continuous along
the specimens (cotton and polypropylene fabric).
The structure of wet cotton fabrics consists of a fiber
matrix, liquid water, and gaseous phase of water vapor
and air. Predicting the heat and mass transfer in such a system is a difficult task, because the fiber swelling may affect
487
CONVECTIVE TEXTILE FABRIC DRYING
the porosity and the available void space for the liquid and
gaseous mixture transport inside the solid matrix.
Fabrics worn next to the skin have a direct influence on
thermal comfort. The no-evaporated sweat may be wicked
through a fabric that comes in contact with the skin and
alters its mechanical and thermal properties. Energy flow
is associated with wicking.
Sousa et al.[15] developed a study for simulating the
moisture distribution in knitt textile fabrics during the drying process. These authors verified that, unlike the moisture variation, fractional saturation is continuous for the
specimens.
In the experimental sarja fabric drying process, initial
moisture content varied 0.50 to 0.70 (d.b.). In both experimental drying processes, raw material underwent drying
temperatures between 50 and 70C, with air drying velocity
between 0.5 and 1.5 m=s. Moisture content of sheets (d.b.)
was determined by constant weight in an oven at 105 3C.
METHODOLOGY
Drying Rate Curves
Drying rate curves were built from all drying curves data
by curve derivation (finite differences) according to a procedure by Motta Lima et al.:[13]
For the construction of the drying curves, the samples
were weighted at regular intervals, 2 by 2 min, and their
moisture determined by their mass evaporation. Drying
rate curves were built starting from the derivation of the
respective drying curves by the method of differences
(DX=Dt) and adjusted so that the rate originally in the
points used in the construction of the first ones, may be
obtained:
Material
Figure 1 shows a sketch of the raw knitt fabric for the
determination of the drying curve in a convective drying.
Raw material is of approximately 220 g=m2 and yarn
Ne 8, produced by MR Knitt (Maring
a-PR, Brazil). It is
a flexible resistant type of industrial textile principally used
in sport shirts.
In the experimental drying process, raw knit fabric had
initial moisture of about 1.40 (d.b.).
The other raw material was woven sarja fabric (see
Fig. 2), approximately 420 g=m2, yarn No. 8, produced
by Textilpar (Apucarana-PR, Brazil). That is a resistant
type of industrial textile principally used for uniform and
jeans trousers. Samples were removed at the end of textile
production before they were washed with detergent
because the laundering was applied only superficially on
the yarn.
Drying Schematic Model
The hypothesis used in all textile fabric drying is based
on the simplification of the industrial cylindrical geometry
as a flat (rectangular) one, since the cylinder radius is much
bigger than the thickness of the raw textile.
drying rate at the point i
to calculate (DX=Dt)i between i 1 and i
to calculate (DX=Dt)iþ between i and i þ 1
(DX=Dt)i ¼ [(DX=Dt)i þ (DX=Dt)i þ ]=2
in X0: (DX=Dt)0 þ or (DX=Dt)1
in Xe: (DX=Dt)Xe
The measurement of air-drying temperatures was made
with a digital thermometer with a termocouple installed
inside the paper box near the raw textile fabric.
FIG. 1. Knit row material.
Drying Rate Curves Adjustment
Due to the behavior of the knit drying rate curves that
clearly showed the periods of constant and decreasing rate
of drying, an analysis has been made according to classic
studies on the two drying periods.
Constant Drying Rate Period
Dependence of constant rate on drying conditions (temperature and speed of the air) may be analyzed since the
evaporation of ‘‘free water’’ during this period is taken as
if it were pure water evaporating on a plane surface, the
presence of the solid being ignored.
Heat supplied by air is used in the evaporation of the
moisture, whereas the balance of energy in the solid for
convective drying described by Eq. (3).
FIG. 2. Woven row material.
hc ðTair TSL Þ ¼ kNC0
ð3Þ
488
SOUSA, LIMA, AND PEREIRA
When the drying rate for mass of dry solid (NC) is rearranged and expressed as a function of the coefficient of
heat transfer (hC), it produces Eqs. (4) and (5).
kNC
AðTair TSL Þ
ð4Þ
2hc
ðTair TSL Þ
Mss k
ð5Þ
hC ¼
NC ¼
According to Perry and Chilton,[16] when the dependence of NC on the thickness of material is incorporated
in Eq. (5), together with the external coefficient heat transfer, Eq. (6) is obtained if the physical properties of air are
constant.
NC ¼
ðH1 ÞðdSL vair ÞH2
ðTair Tvap Þ
qss kE
ð6Þ
where dSL ¼ (AS)1=2 is the characteristic dimension of the
samples.
Falling Rate Drying Period
Since the study was based on capillary flow, Perry and
Chilton[16] propose a proportional=linear reduction of the
drying rate (see Eq. (7)).
ND ¼ NC
ðX Xe Þ
ðXC Xe Þ
FIG. 3. Convective drying experimental apparatus.
Moisture Distribution in Convective Textile Fabric Drying
The procedure for construction of moisture distribution
curves was as follows: the samples were weighted at regular
interval, every 5 min; they were then cut by four pieces during the drying process and their moisture determined starting from the evaporation of their water mass (stove’s
method, 105C 3C, 24 h), see Figs. 4 and 5.
Air drying velocity was adjusted by a laboratory dryer.
A better analysis of moisture distribution in textile fabric
was obtained when the tests on convective drying were
developed with different position of samples. Each piece
ð7Þ
Research agrees with conclusions by Nissan and
Kaye[17] and Ratna Prabhu et al.,[18] (fabric) who also propose a linear reduction of constant rate for this period, with
satisfactory results.
Generalized Drying Rate Curves and Generalized
Drying Rate Curves Model
The proposed methodology of generalized drying rate
curves discussed by Motta Lima et al.[13] and Sousa[19] is
applied to the textile fabric drying results. Once again,
moisture content (X) is correlated to a GDC (Eq. (2))
and NDRC (Eq. (1)).
Experimental Apparatus
Figure 3 shows the experimental apparatus adapted by
Sousa[19] used for convective drying curves. It consists of
a metallic duct that controls the air with controlled velocity
by a damper connected to the duct and controls temperature by electrical resistance, as far as a closed box with a
perforated iron and steel wool plate, according to detail
A in Fig. 3. Tests are conducted in a resistant box or in
an adapted unclosed chamber on a perforated plate. The
agent (hot air) flowing through the overhanging textile
sample also removes the evaporated water.
FIG. 4. Scheme of horizontal moisture distribution.
FIG. 5. Scheme of vertical moisture distribution.
489
CONVECTIVE TEXTILE FABRIC DRYING
FIG. 6.
bution.
Scheme of average vertical and horizontal moisture distri-
was weighed and immediately placed in an oven for wet
weight. Air drying temperature was given by a digital thermometer installed inside the paper box near the textile fabric samples for precise adjustment.
The average moisture for each horizontal and vertical
samples tested were calculated from the punctual average
moisture of a given area. Results are shown in Figure 6.
The samples in laboratory experiments were 150 mm
wide and 150 mm long. The textile sample was then placed
vertically over the perforated plate inside the paper box
overhanging air-drying, with the two sides freely exposed.
Its weight loss was monitored at the end of each test. Air
drying temperature was 50 and 70C, and the velocity of
drying hot air close to the surface ranged from 1.0 to
1.5 m=s. This was adjusted with a portable digital anemometer. The conditions of ambient air were monitored by
a portable digital psychrometer.
FIG. 7.
Drying curves for the Sarja 3:1 (50C).
Figures 13 to 15 present the sarja fabric drying rate
curves initial moisture conditions. Figures 16 to 18 present
the knit fabric drying rate curves with only 1.40 (d.b.)
initial moisture conditions.
When the drying rate curves of sarja fabric are observed,
the constant drying rate period does not appear due to the
initial moisture content. This shows us that the woven
textile should have higher moisture content than 0.70
(d.b) so that the constant rate drying period can also be
evaluated.
In the case of the drying rate curves of knit fabric, drying constant rate periods appear, due to high values of
initial moisture.
Table 1 shows maximum results of sarja fabric drying
rate, whereas Table 2 shows knit fabric results.
RESULTS AND DISCUSSION
Drying Curves
Drying experiments with cotton fabric were done, with
three on initial moisture content. The first one, moisture
content around 0.50 (d.b.), did not present a constant drying rate period. Consequently, the samples’ moisture content was raised to rates around 0.60 and 0.70 (d.b), but a
constant drying rate period failed to occur. It was also
observed that for the larger rate of initial moisture content
there are two platform trends, also observed from temperature profiles. This indicates that the initial moisture content
of the material interferes in the drying rate and that the
constant rate period probably should happen for a higher
humidity value than that used in this research.
Figures 7 to 12 present the drying cotton fabric and drying rate curves for air temperatures of 50, 60, and 70C and
air velocity 0.5, 1.0, and 1.5 m=s. X1, X2, and X3 in the
graph legends correspond to the initial moisture data
0.50, 0.60, and 0.70 (d.b), respectively.
FIG. 8. Drying rate curves for the Sarja 3:1 (50C).
490
SOUSA, LIMA, AND PEREIRA
FIG. 9. Drying curves for the Sarja 3:1 (60C).
FIG. 11. Drying curves for the Sarja 3:1 (70C).
The behavior of the drying curves shows that the value
of the drying rate grows with an increase of speed and temperature of air drying, which agrees with the drying process
hypothesis. However, the initial moisture of drying does
not influence the drying rate of the decreasing drying
period.
The external heat transfer coefficient (hC) may still be
inferred from the literature, and is usually calculated as
in Eq. (9):
Constant Drying Rate Period
The adjustment of NC results for the convective drying
of knit fabric (R2 ¼ 0.9046) for 2400 kJ=kg is described
by Eq. (8), expressed in the MKS=SI system.
ð5:63 105 Þ ðdSL vair Þ0:42
NC ¼
ðTair Tvap Þ
ð2400Þ qss E
FIG. 10. Drying rate curves for the Sarja 3:1 (60C).
ð8Þ
Nu ¼ ðC1 ÞðReÞC2 ðPrÞ1=3
ð9Þ
where C1 ¼ 0.664 and C2 ¼ 0.5 for convection on a semiinfinite space and with Nu ¼ (hCdSL=k). Equation (10)
expresses hC as:
hc ¼ ðk=dSL Þ ð0:664Þ Re0:5 Pr1=3
ð10Þ
Table 3 presents the hC results by Eq. (4) (experimental)
and Eq. (8) (theoretical) determinated by dSL ¼ (0.200 0.005 m) and 2400 kJ=kg for the knit fabric sample convective drying.
FIG. 12. Drying rate curves for the Sarja 3:1 (70C).
CONVECTIVE TEXTILE FABRIC DRYING
491
FIG. 13. Drying rate curve, Sarja 3:1 60% (d.b).
FIG. 15. Drying rate curve, Sarja 3:1 70% (d.b).
Table 3 shows that hexp varied according to the speed of
the drying air. The hexp values presented, on average, a
variation of 20% in relation to hteo. These rates show that
the theoretical equation above does not describe well the
process. Such variation may be justified by the process of
mass transfer that occurs during drying, associated to the
process of evaporation of the water that occurs at the surface of fabric samples.
Values of Xc were knit fabric, starting from the intersection of the straight line of adjustment of the data of this
period (R2 > 0.995) with the values of the constant drying
rate and Xe, directly to the limit of the drying curves
(X!Xe when t ! 1).
Falling Rate Period
Table 4 shows the results of knit fabric convective
drying.
Adjusted Convective Drying Rate
Figures 16 and 17 show convective drying and drying
rate curves starting from procedures described in Sousa.[16]
It may be concluded that the study may be applied for the
cotton textile fabric.
FIG. 14. Drying rate curve, Sarja 3:1 50% (d.b).
FIG. 16. Convective drying, knit fabric.
492
SOUSA, LIMA, AND PEREIRA
FIG. 17. Convective drying rate, knit fabric.
Generalized Drying Curves and Generalized Drying
Rate Curves Model
Generalized sarja fabric drying curves and adjustable
model results are shown in Figs. 18 and 19 obtained from
the sarja convective experimental drying curves and convective experimental drying rate curves. Figures 20 and
21 present, respectively, the results of generalized knit fabric drying rate curves NDR and NDRC adjustable model.
Table 5 presents the sarja fabric experimental data
obtained from convective drying rate curves. Table 6
presents the parameters obtained from the knit fabric’s
FIG. 18. Generalized sarja fabric drying curves and GDC Page’s[9] model.
convective generalized drying. Table 7 presents the result
parameters of sarja fabric convective generalized drying.
In this case a small adjustment may still be made by
adopting a lineal adjustment for n as a function of dimensionless time, too. The rate period between 0 and 0.10 will
be better represented by the drying of textile fabric.
Results show a good performance for the two studied
situations: Page’s[9] model and Hodges’[10] model could be
successfully used to simulate these generalized drying and
drying rate curves.
The generalized drying rate curve results confirm that
the above is a very interesting tool for the study of convective drying processes. More specifically, the influence of the
three operational variables here studied (initial moisture
content, surface sample temperature, and air drying velocity) on textile drying rates may be analyzed by this
methodology and that Hodges’[10] equation has a good
performance.
New experiments with higher textile initial moistures
will be accomplished, especially on the dependence of constant drying rate period with the drying temperature, drying air speed, and the initial textile moisture. Hodges’[10]
model will be used for their analyses.
Moisture Distribution
The experimental results obtained for one initial moisture content and for two different positions from the knit
textile fabric sample, and with two different temperatures
and air-drying velocities showed the influence of these
parameters used in the drying process.
Results of average moisture content (vertical and horizontal positions) were predicted as a function of time for
knit cotton textile fabric analysed for several drying conditions, as shown in Table 8.
Moisture distribution within the knit textile fabric samples, as a function of each piece, is shown in Figs. 22 to 29.
Position 1 and 2 in Figs. 22 and 23 are more near to
air-drying flow. Changes in each position show that the
position of the most distant sample of the air-drying is
the one that evaporates more quickly.
In the case of a higher air-drying velocity in the horizontal position of the sample (cut at right angles to the flow air
direction), a more evident disturbance occurred at the
end of drying. This was probably caused by the drag of
moisture from the border nearest the air-drying flow to
the most outlying border.
In Figs. 24 and 25, this occurs in a more discrete way at
a higher temperature, since moisture evaporation impedes
that it migrates to the other border.
Increase of temperature and air-drying velocity accelerates the drying process, although they do not influence the
final result. Therefore, a considerable economy of energy
occurs if the process uses a hihger air velocity of drying
with a lower temperature.
493
CONVECTIVE TEXTILE FABRIC DRYING
TABLE 1
Drying rate sarja fabric results
Tair
(C)
vair
(m=s)
X1
(%)
NC1 max
(1=min)
Xe1
(%)
X2
(%)
NC2 max
(1=min)
Xe2
(%)
X3
(%)
NC3 max
(1=min)
Xe3
(%)
0.5
0.5
0.5
1.0
1.0
1.0
1.5
1.5
1.5
50
50
50
50
50
50
50
50
50
0.029
0.038
0.049
0.040
0.049
0.063
0.041
0.059
0.069
2.0
1.0
0.5
2.2
1.4
0.6
2.3
1.5
1.1
60
60
60
60
60
60
60
60
60
0.030
0.038
0.047
0.040
0.043
0.065
0.048
0.064
0.072
2.2
1.2
0.6
2.5
1.5
0.6
2.3
1.6
1.0
70
70
70
70
70
70
70
70
70
0.029
0.039
0.045
0.039
0.053
0.061
0.044
0.057
0.081
2.1
1.1
0.5
2.4
1.3
0.8
2.3
1.6
0.9
50
60
70
50
60
70
50
60
70
TABLE 2
Knit fabric convective drying NC (kg=m2min) results
(X0 ¼ 1.40 (d.b.))
Tair (C)
vair (m=s)
NC0 (kg=m2min)
0.5
1.0
1.5
0.5
1.0
1.5
0.5
1.0
1.5
0.00081
0.00093
0.00099
0.00109
0.00114
0.00140
0.00155
0.00153
0.00171
50
60
70
TABLE 3
Heat transfer coefficients (hC) results of knit
fabric convective drying
vair
(m=s)
0.5
1.0
1.5
Tair ¼ 50C
Tair ¼ 60C
Tair ¼ 70C
hteo
hexp
hteo
hexp
hteo
hexp
11.14
13.63
16.70
11.04
14.24
14.23
11.68
13.68
15.75
11.99
11.57
15.13
11.71
12.72
15.81
13.52
12.28
14.53
It may be observed that the drying process dividing the
material in the same parts occurs similar by to the normal
drying process, or, rather, moisture decreases as time passes.
In some cases, local and overall moisture regains were
available by position and time. If the moisture content is
higher than the hygroscopic moisture content, water
vapour near the surface is saturated.
Figures 26 and 27 show that the difference between final
moisture contents was relatively small. It may be concluded
that air-drying velocity did not show any quantitative influence on knit textile fabric final drying period. Figures 28
and 29 show that the behavior of the textile fabric drying
is similar to conductive drying. This fact occurs due to high
temperature and a lower air-drying velocity, since the sample first receives heat in its central area.
Results of the moisture content profiles of the knit
textile fabric for air-drying temperatures and air-drying
velocity were compatible and results did not present great
discrepancies.
TABLE 4
Results of knit fabric convective drying
Tf (C)
vair (m=s)
Xe (d.b.)
Xc (d.b.)
NC (min1)
50
0.5
0.03
0.91
0.08
1.0
0.03
0.85
0.09
60
1.5
0.03
0.79
0.09
0.5
0.01
0.54
0.11
1.0
0.03
0.49
0.11
70
1.5
0.02
0.32
0.14
0.5
0.01
0.78
0.15
1.0
0.01
0.78
0.15
1.5
0.01
0.66
0.16
FIG. 19. Generalized sarja fabric drying rate and NDRC Hodges’[10]
model.
494
SOUSA, LIMA, AND PEREIRA
TABLE 6
Parameters knit fabric convective generalized drying
Knit fabric
Parameters
R
F
a
b
2
GDC
NDRC
0.9924
230
2.23 0.04
1.45 0.03
0.9608
400
0.29 0.009
1.75 0.10
TABLE 7
Parameters sarja fabric convective generalized drying
Sarja fabric
[10]
FIG. 20. Generalized knit fabric drying rate curve Hodges’
model.
Parameters
R
F
a
b
2
GDC
NDRC
0.9936
329
2.88 0.57
1.20 0.04
0.9429
150
0.195 0.007
1.50 0.09
Analysis of results with knit samples shows a more
coherent behavior in air temperatures and air velocities
studied than with knit cotton textile fabric. This fact reinforces our conclusion that the moisture content variations
in the sample occur evenly in the process of convective drying of the knit.
FIG. 21. Generalized knit fabric drying rate curve, Motta Lima[8] model.
TABLE 5
Adjust sarja fabric convective drying rate curves
vair (m=s)
0.5
1.0
1.5
Tf (C)
Tvap (C)
Tair (C)
NC (1=min)
50
60
70
50
60
70
50
60
70
37.0
—
38.0
25.0
—
38.0
28.0
—
39.0
27.3
21.5
20.7
21.7
24.6
25.1
25.6
26.1
26.8
0.08
0.11
0.15
0.09
0.11
0.15
0.11
0.14
0.16
CONCLUSIONS
A reduction of drying times and higher vaporization
rates with an increase of the drying temperature was verified, which confirms that the heat mass and transfer controls the drying.
Hodges’[10] equation adjusts satisfactorily the experimental data, taking into account the analyzed effects for
the fabric sample.
The behavior of experimental drying rate curves shows
clearly the two classic drying rate periods and justifies the
choice of modeling these curves from the separate analysis
of the constant and falling drying rate periods. The effect
of air speed variation was shown to be more important
during the constant drying rate period (external
resistances), slightly interfering with the decreasing rate
period (internal resistances).
The adjustment of the constant drying rate period by
energy balance equations originating from solid was quite
appropriate, whereas the convection heat transfer
coefficient between the fabric and the drying air was verified too.
In the falling drying rate period, the choice of a capillary
flow behavior (lineal reduction of the drying rate function
495
CONVECTIVE TEXTILE FABRIC DRYING
TABLE 8
Convective drying, knit textile average moisture content
Average moisture content (b.s.)
Time (min)
0
5
10
15
20
50C and 0.5 (m=s)
50C and 1.5 (m=s)
70C and 0.5 (m=s)
70C and 1.5 (m=s)
1.40
1.15 0.01
0.47 0.01
0.26 0.01
0.04 0.01
1.40
0.94 0.01
0.24 0.01
0.11 0.01
0.04 0.01
1.40
0.76 0.01
0.16 0.02
0.04 0.01
0.04 0.01
1.40
0.54 0.01
0.07 0.01
0.04 0.01
0.03 0.01
FIG. 22. Knit fabric moisture horizontal distribution, 50C, 0.5 m=s.
FIG. 24. Knit fabric moisture horizontal distribution, 70C, 0.5 m=s.
FIG. 23. Knit fabric moisture horizontal distribution, 50C, 1.5 m=s.
FIG. 25. Knit fabric moisture horizontal distribution, 70C, 1.5 m=s.
496
SOUSA, LIMA, AND PEREIRA
FIG. 26. Knit fabric moisture vertical distribution, 50C, 0.5 m=s.
FIG. 28. Knit fabric moisture vertical distribution, 70C, 0.5 m=s.
FIG. 27. Knit fabric moisture vertical distribution, 50C, 1.5 m=s.
FIG. 29. Knit fabric moisture vertical distribution, 70C, 1.5 m=s.
of the moisture equilibrium) showed a satisfactory adjustment for this period.
Results obtained for the heat transfer coefficients were
more reliable when starting from the experimental procedure, due to the differences presented in relation to the
theoretical values. Such differences may be related to the
mass transfer process that occurs during drying, associated
to the evaporation of water in the fabric samples surface,
since the theoretical equation applied was developed for
processes in which only the heat transfer occurs.
The generalized drying rate curve results confirm that
this proposal is a very interesting tool for the study of convective drying processes. More specifically, it has been
shown that the influence of the operational variables above
(initial moisture content, temperature, and air drying velocity) on textile drying kinetics could be analyzed by this
methodology.
The profiles obtained indicate that moisture variations
in the samples in convective drying process take place uniformly in position and in time.
Current research contributes to improve the knowledge
on the processes of heat and mass transfer in fabric drying
due to the shortage of data in textile literature, and to
provide alternatives for a better understanding of this step
in the design, simulation, and optimization of industrial
dryers.
Future research will develop a representative model to
predict experimental results and to study the effect of
CONVECTIVE TEXTILE FABRIC DRYING
different samples area. This will facilitate the understanding of drying process and the use of the applied methodology for other textile materials.
NOMENCLATURE
a, b
Parameters of Eq. (1)
As
Superficial samples surface (L2)
b.s.
Dry basis
b.u.
Wet basis
d.b.
Dry basis (kgWATER=kgDRY SOLID)
dSL
Characteristic dimension (L)
E
Energy activation (Eq. (8)) (L2=T2)
F
F statistics, ratio between the mean of the
square of the predicted values and the mean
of the square of estimated residuals
GDC
Generalized drying curves
hC
Heat transfer coefficient, (M=hT3)
k
Mass transfer coefficient (M=L2 T)
K
Eq. (2) parameter
MSS
Wet solid mass (M)
N
Drying rate (1=T)
NC
Constant drying rate (1=L2T)
0
NC
Constant drying rate=area unit of wet solid
(M=L2T)
ND
Decreasing drying rate (1=T)
NDR
Normalized drying rate
NDRC
Normalized drying rate curves
Nu
Nusselt number
Pr
Prandlt’s number
R2
Correlation coefficient
Re
Reynolds’s number
t
Drying time (T)
Tair
Drying air temperature (drying temperature)
(h)
Tbu
Wet bulb temperature (h)
TSL
Solid temperature (h)
Tvap
Vaporization temperature (h)
Vair
Drying air velocity (L=T)
X
Moisture content (d.b.)
X0
Initial moisture content (d.b.)
X1, X2, X3 Initial moisture content (d.b.)
XC
Critical moisture content (d.b.)
Xe
Equilibrium moisture content (d.b.)
Greek Letters
k
Latent heat vaporization (1=L2T2)
qss
Specific mass, dry sample (M=L3)
497
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