Evaluation of Textile Bobbins Drying Processes: Experimental and Modelling Studies

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Evaluation of Textile Bobbins Drying Processes:Experimental and Modelling StudiesJ. RIBEIRO a & J.M.P. VENTURA ba Departamento de Tecnologias de Combustao Instituto de Tecnologias Energoticas InstitutoNacional de Engenharia e Tecnologia Industrial Azinhaga dos Lameiros, 1699, Lisboa Codex -PORTUGALb Departamento de Engenharia Hecanica Instituto Superior Tecnico Avenida Rovisco Pais,1096, Lisboa Codex - PORTUGALPublished online: 07 May 2007.

To cite this article: J. RIBEIRO & J.M.P. VENTURA (1995) Evaluation of Textile Bobbins Drying Processes: Experimental andModelling Studies, Drying Technology: An International Journal, 13:1-2, 239-265

To link to this article: http://dx.doi.org/10.1080/07373939508916952

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DRYING TECHNOLOGY, 13(lk2), 239-265 (1995)

EVALUATION OF TEXTILE BOBBINS DRYING PROCESSES: EXPERIMENTAL AND MODELLING STUDIES

(') Departamento de Tecnologias de Combustao Instituto de Tecnolooiae Eneraeticas -~ ~.~

Institute Nacional de Engenharia e Tecnologia Industrial Azinhaga doa Lameiros, 1699 Lisboa Codex - PORTUGAL

(") Departamento de Engenharia Mecanica Instituto Superior Tecnico

Avenida Rovieco Pais 1096 Lisboa Codex - PORTUGAL

Key words and Phrases: drying; textile bobbins, evaporation front, modelling

ABSTRACT

Drying of textile bobbins was carried out on two kinds of laboratory-scale dryers: in a cross circulation drying rig. atmospheric pressure heating air is used, while in the ehrough circulation drying apparatus, the heating air is pressurized before the drying process takes place.

In both cases, a bobbin was fitted with seven thermocouples, placed in a plane normal to the bobbin axis but at different radial distances, in order to provide temperature data during the drying process.

In cross circulation drying, plots of temperature against time indicated the presence of two moving fronts, one starting from inside the bobbin, another from the outside, while in through circulation drying, similar plots showed the presence of an evaporation front, moving from inside the bobbin to the outside.

In both cares, numerical models were developed to simulate the drying processes.

Copyright 0 1995 by Marcel Dckker, Inc.

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240 RlBElRO AND VENTURA

INTRODUCTION

Textile industry represents in Portugal about 17 per cent of

the manufacturing industry production and 28 per sent of the

industrial manpower. Considering its main components - Spinning, Weaving, Dyeing and Finishing - the firat two use essentially

electrical energy, while the last two sectors consume mainly

thermal energy. Drying is a very important operation in these

component-, and its application in the dyeing sector has been

less studied than in the finishing sector. This was therefore one

of the motivations for the present study.

Drying procees of textile bobbins is normally carried out in

two steps, centrifugal extraction of water followed by heat input

that gives riee t o evaporation of moisture. Mechanical drying

processes, are more economical than thermal ones; however, they

are not sufficient to reach deeirable drying levels. Hence,

mechanical drying must be usually followed by thermal drying.

Thermal processes used in the textile industry, particularly

in the wool industry, consist of passing a hot air stream over

the surface of the material to be dried. Air flow transfers heat

to the material by forced convection and, at the same time,

carries away evaporated water. The process continues until

equilibrium is attained, depending upon drying air temperature

and humidity.

Drying rat*! depends upon several factors, which primarily

include the material structure, air temperature, air humidity.

air flow turbulence, material thickness and exposed surface area.

Data on textile bobbins drying are insufficient. Nissan et

al. (1959) did some work with woollen flannel of closed

structure. The material was wound as cylindrical bobbins and was

dried in a wind tunnel in which hot air flowed along the axis of

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TEXnLE BOBBINS DRYING PROCESSES 24 1

the cylinder. Heat transfer was observed to take place solely

from the outer surface. A more recent study using cans of scoured

wool, was conducted by Walker (1969). This work aimed at

determining the drying rate curve and the temperature

distribution.

In the present work, experiments were carried out using

Cheviot wool bobbins (500 g net weight) with an approximate

conical form. Internal diameter was varied from 0.035 to 0.Of.m.

External diameter was kept between 0.13 and 0.16~1. hi bobbins

were 0.15 m long and were supported by a polyethylene structure

with holes.

In industrial drying processes, textile wool bobbins are

mounted top-to-top, forming a long cylinder. In the work reported

in the first part of the present paper (cross circulation

drying), only one bobbin was used with bottom and top covered by

stainless steel plates. The textile bobbin was fitted with 7

copper/constantan thermocouples, equally spaced, both in the

radial end azymuthal directions and equidistant from both the

lower and upper edges, as shown in Pig. 1. Preliminary

experiments showed that temperature differences along the bobbin

length were not important.

This drying technique presents serious limications concerning

total drying time. Even with 90 'C air temperature and 5 mls air

velocity it was not possible to reduce the drying time below 7

hours. However, commercial equipment exists where drying times

are much smaller, using pressurized air which is forced to move

through the bobbin from inside to outside. From this knowledge,

it was decided to study this drying technology, in particular the

influence of air temperature and flow rate on drying time.

Therefore, a new experimental rig was built, using

pressurized air, where five bobbins were placed top-to-top; the

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RlBElRO AND VENTURA

FIGURE 1 - Sketch of bobbin with thermocouples in position

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TEXTILE BOBBINS DRYING PROCESSES 243

second bobbin was fitted with thermocouples in a similar way to

the only bobbin ueed in cross circulation drying. In thia way, a

set of experiments using through circulation drying was

performed.

CROSS CIRCULATION

Experimental Set-Up

Hot air, at the required temperature and velociry, w a s

directed to the bobbin located axially inside a 20 cm i.d. duct.

Figure 2 shows a sketch of the experimental set up.

An electronic scale. Sartorius U3600, 0.19 resolution,

allowed continuous weighing of the bobbin. Temperature and weight

data were monitored and acquired by a datalogger, Fluke Helios I,

which sent them to a file on a microcomputer.

Air velocity was monitored by an orifice plate, 9 1 mm ID,

calibrated according BS 1042: Part 1, ( 1 9 6 4 ) . located on the air

duct. Air was electrically heated and its temperature controlled

by s PID regulator, Eurotherm 810. Values of air temperature, air

humidity and temperatures inside the bobbin, were fed at

previously defined time intervals to the datalogger, to be

recorded in the data file.

Modelling

To simulate the drying process, a mathematical model was

developed. The assumptions embedded in the model are described

below.

Due to the bobbin geometry, the heat equation was written in

cylindrical coordinates.

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FIGURE 2 - Schematic diagram of the wind tunnel.(Words in small type denote measured properties)

where:

T temperature

t time

c t thermal diffusivity

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TEXnLE BOBBINS DRYING PROCESSES 245

r radial coordinate [ml

h, latent heat of evaporation [~.kg'' I

cp specific heat at conatant preasure [3'kg-"~-']

X local moiature content [-I

Evaporation was considered to happen in two mobile

evaporation fronts, which are kept at the wet bulb temperature of

drying air. These fronts travel towards each other until they

meet and disappear. Therefore, in the interface between the inner

wet none and the two outer dry zones, equality of temperatures

must be satisfied.

T , = T 2 = T , (2)

Where:

T I temperature in the dry zone

T~ temperature in the wet zone

T, temperature in the evaporation front

and the balance of energy equation may be written, taking in

account the latent heat of evaporation

where:

kl thermal conductivity of material in dry zone [W.m-l.sC-' 1

k2 thermal conductivity of materiel in wet zone [w.m-'.'c-l I

01 liquid bulk deneity (mass of liquid pee unit

volume of bobbin) [kg-m-l]

L evaporation front coordinate [ml

Following Fig. 3, 7 types of points may be identified along the

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RlBElRO AND VENTURA

/ rfi " e;zone \/ ::;; 1, Dry zone

P7 , P6 P5 P4 P3 P2 P1

FIGURE 3 - Schematic diagram shoving the location of the points for drying simulation

radius, corresponding to:

1 - boundary between exterior dry rone and drying air 2 - point in exterior dry rone 3 - boundary between exterior dry zone and wet zone (exterior

front)

4 - wet zone 5 - boundary betveen interior dry zone and wet zone (interior

front)

6 - point in the interior dry zone 7 - boundary between interior dry zone and drying air.

It ahould be noted that the distance between points varies in

time. In the beginning it is zero between points 1 and 3 on one

hand and points 5 and 7 on the other hand: it is maximum between

points 3 and 5. By the end of the run these conditione are the

opposite, as the wet rone decreases at the expense of the two dry

zones.

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The general equation presented above may be decomposed into 3

equations, two corresponding to the dry zones and one to the wet

zone.

where:

"1 thermal diffuaivity in dry zone

u2 thermal diffusivity in wet =one

re outer radius of the bobbin

ri inner radius of the bobbin

rLe position of the outer interface

r position of the inner interface

when the above equations are discretized, the last term in

the right-hand side exists only in the cells containing the

evaporation fronts.

Initial and boundary conditions were as follows:

TI = 1 5 - C

T2 - - 1 S'C

r ~ i = ri

rLe = re

Air was considered to be at 20% and 60% RH, before

being heated (to 60. 70, 80 or 9 0 ° C ) .

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Xh (moisture content in wet zone) =50a

X. (moisture content in exterior dry none) =lo\

Xi (moisture content in interior dry zone) =10P

k <I = -

I' c,, was obtained from a weighted average of the

properties of air, water and dry fibres.

Hi and He, heat transfer coefficients in the boundaries of

the bobbin, vere calculated through the correlation Dittue-

Boelter, reported by Holman (19761.

Results and Discussion

Experimental Results

In these experiments, temperatures of 70, 80 and 90 OC and

air velocities ranging from 2 to 4 mls were used. From the raw

data, profiles of temperature and moisture content against time

were obtained, e typical rseult being shown in Fig. 4.

Moieture content wae obtained weighing the bobbin along the

time and is expressed in percentage of dry weight.

After the first stage corresponding to the heating of the

bobbin (Region AB), a levelling of temperature is observed for

a11 the interior points (Region BC). The value at which the

levelling in temperature occurs, and its duration, depend on the

position of the thermocouple and correspond to the attainment of

a state of equilibrium between the m o u n t of heat transferred to

the material and the heat lost by evaporation. The constant value

of the temperature at this stage ie referred to as 'paeudo-wet-

bulb temperature- (Nisean et el., 1959). According t o Toei

(19831, the difference between this temperature end the wet bulb

temperature is 3 to 5'C if the thickness of the material is belov

5 mm.

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Time (min)

FIGURE 4 - Variation of temperature and moiature content with time. (The curve labelled MC refers to the bulk moisture content of the bobbin. Curves labelled Ti refer to temperaturee inside the bobbin (T2 to T61 and on outside surface (TI) and inside surface (T7)I

For each location on the interior, the end of this constant

period corresponds to the bobbin becoming dry: hence there is no

more water to evaporate and as a result the equilibrium can no

longer be maintained and the temperature rises again. However,

part of the heat which ia arriving is still transferred to the

interior of the bobbin where evaporation is still taking place,

slowing down the rate at which the temperature rises (Region CD).

When the bobbin is totally dry, the rising of temperature becomes

stronger (Region DE) and the experimental test run ie completed.

Evaoorafion. The points C (end of the constant

temperature r c n e ) for each thermocouple location can be

aseociated to an evaporation front which is the boundary between

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TABLE 1 - Drying time in duct experiments (min)

two zones. name1.y the wet, liquid moi~ture diffusion zone, on the

interior, and the dry, vapour diffusion zone, on the exterior. AS

air convection drying takes place on both sides of the bobbin.

two evaporation fronts develop progressing towards each other

until they meet (Point D). This point is used to define the end

of drying.

Table I ahows drying timea (in minutes) for each thermocouple

location, for different air temperatures and velocitiee.

Figure 5 was drawn from the values in Table 1; it shows the

locecion of the evaporation fronta with time for each different

condition of the inlet air.

The slight "on-symmetry observed in the curves in Fig. 5 can

be explained by the geometry of the bobbin, with different heat

transfer areas cn the ourside and inside.

From Fig. 5, drying times for the whole bobbin can be

obtained. and these are shown in Table 2.

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TEXTILE BOBBINS DRYING PROCESSES

0 I00 ZOO 300 400 500 600 700 BOO

Time (mi")

FlCUilE 5 - Evaporation front disappearance

TABLE 2 - Evaporation front disappearance

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This Table also shows the final moisture content o f the

bobbin, when the drying criterion is the disappearance'of the

evaporation fronts. However, wool is considered to have an

equilibrium moisture content of 18.25% on dry basis, for standard

temperature and humidity conditions (i. e. 20 OC and 65% RH).

This means that beyond 18.25% it is not necessary to continue the

drying process, implying that the above criterion will in general

cause overdrying of the bobbins.

Modelling Results

Figure 6 presents results of a typical run showing evolution

of temperatures and moisture content with time.

The general behaviour is comparable to the experimental

results and similar comments could be produced.

for ease of comparison, Fig. 7 presents simultaneous plots of

experimental and modelling results.

In order to emphasize the differences, two extreme cases

concerning air temperature and velocity were chosen.

Table 3 preeente the theoretical results (and their percent

variation) compared to che experimental ones.

The most significant difference is related to the constant

temperature regiona. while in the experimental results, those

regions correspond to the so called pseudo-humid temperatures,

which depend on the point position, in the modelling results they

are the same for all points and correspond to the wet bulb

temperature of the drying air.

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0 'O u 1:: 0 100 200 300 400 500 600

Time (min)

FIGURE 6 - Temperatures and moisture content evolution. Modelling values. (The curve labelled nC refers t o the bulk moisture content of the bobbin. Curves labelled T i refer to remperarures inside the bobbin (T2 to T6) and on outside surface (Tl) and inside surface (T7))

THROUGH CIRCULATION

Experimental Set-up

A new experimental rig was built, using pressurized air,

where five bobbins were placed top-to-top; the second bobbin was

fitted with thermocouples. Figure 8 ehows a aketch of the

experimental rig.

Air was farced to move through the bobbins from inside to

outside. The air was electrically heated, its temperature being

set by a PID controller; air velocity was controlled by two

valves and monitored by a rotameter.

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I 0.00 0 100 200 300 400 500 600

Time (mi")

FIGURE 7a - Temperature and moisture content evolution inside the bobbin

Drying air conditione: Temperature 90 'C. Velocity 4 mls. Symbols: Experimental results. Lines: Modelling results

With this experimental set-up, it was not possible to measure

directly the evolution of the bobbin's mass and therefore to

follow its overall moisture content. Thie has been evaluated from

the knowledge of air moisture variation between dryer input and

output.

Measurements in industry have indicated air flow rates of

about 150 litreo per minute and per bobbin. Our compressor could

provide up to 900 litres per minute at a pressure between 4.5 and

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0 200 400 600 800

Time (min)

0 T I . T2 O T3 T4

FIGURE 7b - Temperature and moisture content evolution inside the - . - - . . .

Drying air conditions: Temperature 1 0 'C. Velocity 2 mls. Symbols: Experimental results. Lines: Modelling results

6.5 bar. Before entering the drying rig, air paeeed through a

pressure reducer where its pressure was stabilized in 4 or 2 bar.

according to the desired flow rate.

Modelling

In pressurized drying, air goes from the inside t o the

outside o f the bobbin, its moisture increasing at constant

enthalpy until becoming saturated.

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256 RIBEIRO AND VENTURA

TABLE 3 - Comparison between theoretical and experimental drying time

Figure 9 shows the evolution of the drying air in the

psychrometric diagram.

From point A air is heated until point 8 and the process

inside the bobbin is represented by the evolution 8-C.

Experimental Results

In this experiments there is only one drying front which

progresses from inside to outside. Figure 10 presents a typical

result. The end of drying w a s considered to be when the

temperature of the most exterior point reached half of the total

temperature rise.

The difference between the imposed 90'c and the measured

values of about 82'C is attributed to the distance between the

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FIGURE 8 - Schematic diagram of the pressurized rig

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t Absolute Humidity

FIGURE 9 - Evolution of the drying air in the psychrometric diagram

Time (mnn]

FIGURE 10 - Temperature and air exhaust humidity along the time. Presnurized drying. (The curve labelled RH refers to the relative humidity of the exhaust air. Curves labelled Ti refer to temperatures inside the bobbin (T2 to T6) end on outside surface (TI) and inside surface (T7) )

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- 90%. 750 llmin

90°C. 500 llrnin --.-- 80'C. 750 llmin

80°C. 500 Vmin

. . . . , . . . . 70°C. 750 llmin

--A'--- 70%. 500 Vmin

0 5 0 100 150 200 250 300

Time (min]

FIGURE 11- Evaporation front disappearance. Pressurized experiments

thermocouple which fixed the controller setting point and the

instrumented bohbin, implying some cooling of the air between the

two stations.

Each line in Fig. 1 1 , which represents approximately the

evolution of the evaporation front [Nissan, Kaye and Bell, 1 9 5 9 1 .

was obtained from a set of data like the one in Fig. 1 0 .

Drying times for the bobbin were obtained from Fig. 1 1 and

are shown in Table I .

Modelling Results

Figure 12 presents a typical reault from the simulation of

the present experimental situation

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RIBElRO AND VENTURA

TABLE 4 Drying times on pressurized drying

0 20 4 0 6 0 80 100

Time (min)

FIGURE 12 - simlllated results from the pre3surired drying process

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TABLE 5 - Comparison between simulated and experimental results

Drying times

Table 5 contains e summary of the results for the different

conditions which were simulated, a e well as the corresponding

experimental results.

CONCLUSIONS

Crose Circulation Drying

The modelling results show a reasonable correspondance to the

experimental ones. For each point there is a heating period, a

constant temperature level and two further periods of temperature

rise: in the steeper, last one, there is not any heat provided

for water vaporization, and therefore it corresponds to pure

heating of the material.

In the modelling reeults, we can see, in the beginning of the

process, a sudden drop in moisture content, which is not observed

in the experimental results. This is attributed to the fact that,

in the model, we are conaidering evaporation to occur at the wet

bulb temperature; therefore, the rise in the interface

temperature will be smaller and a larger amount of heat will be

used to evaporation.

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262 RlBEIRO AND VENTURA

The sudden drop in moisture content near the end of the

proceee results from considering that the material is dry when

the wet zone thickneaa goes below 1 m.

The final rise in temperature in the model is much steeper

than in the experiments, because we consider the transition

between drying and pure heating of the material a discontinuous

one. This does not happen in reality and therefore the

experimental results show a smoother transition between the two

regions.

The difference in temperature level, between experimental and

modelling results, for the constant temperature zone (specially

noticeable for measuring positions 3, 1 and 5) is attributed to

the difference between wet bulb temperature (used in the model)

and peeudo-wet bulb temperature.

Through Circulation Drying

Comparing Fig. 1 2 wrth Fig. 10, a sharp traneitian in

temperature for each thermocouple is observed in the modelling

results, when the region around the thermocouple becomes dry.

Exporimentally, this is never observed, because the evaporation

front possesses a finite thisknesa, albeit small.

A difference of the same kind may be seen in the evolution of

humidity: experimental resulcs are always characterized by

smoother transitions between humidity levels.

In contrast with the results of cross circulation, where

experimental and modelled drying times are very similar, the

results of through circulation show differences of about 60 per

cent. This is due to the fact that, in the model, air is

considered saturated until the bobbin is completely dry. not

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being recognized the failing rate drying period, while in the

experiments this is only true for the first part of the run.

Comparing the two drying techniques, 'through circulation'

shows shorter drying times (between 2 and 3 times shorrer,

depending on the air flow rate). However, an extra energy input

is needed to pressurize the air, and therefore the choice between

both processes is not clear, depending on the operating

conditions (type and amount of material to be processed).

NOTATION

constant pressure specific heat [~.kg-'.~-Ij

exterior convection heat transfer coefficient [W.m-'.'c-']

interior convection heat transfer coefficient [w.~-~.'c-']

vaporization latent heat [~.kg-' ]

specific enthalpy of the drying air in state A [kJ kg-']

specific enthalpy of the drying air in state B [kJ kg-']

specific enthalpy of the drying air in state C [kJ kg-']

dry zone thermal conductivity [H.m-l ..C-~l

wet zone thermal conductivity [w.m-l

evaporaticn front coordinate I ~ I exterior evaporation front advance lml

interior evaporation front advance [ml

bulk moisture content of the bobbin [ - I

flow rate [litres/min]

air exhaust relative humidity [ - I radius of bobbin at the point considered I ~ I outer radius of bobbin ["'I inner radius of bobbin [ml

position cf the outer interface [ml

position cf the inner interface [ml

temperature l K 1 temperature in the dry zone [KI

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2 64 RlBElRO AND VENTURA

temperature in the wet zone

temperature of the drying air in state A

temperature of the drying air in state B

temperature of the drying air in state C

temperatures in the bobbin

temperature in the evaporation front

time

air velocity

moisture content of the material

moisture content in exterior dry rone

moisture content in wet zone

moisture content in interior dry rone

absolute humidity of the drying air in state A

absolute humidity of the 'drying air in state B

absolute humidity of the drying air in state C

Greek symbols

r t thermal diffusivity [m2.s-'1

'ti thermal diffusivity in the dry zone [m2.e-'1

"2 chermal diffusivity in the wet zone [m2.s-'1

liquid bulk density (mass of liquid per unit

volume of bobbin) 1kg.m-'1

REFERENCES

1 Nissan, A.H.; W. A. Kaye; T. V. Bell (1959) Mechanism of

Drying Thick Porous Bodies During the Falling-Rate Period. I

The Pseuda-Wet-Bulb Temperature. AICHE J., 5, 103-110.

2 walker, B:v: (1969) The Drying Characteristics of Scoured

Wool. N.Z.J.Sci.. 12, 139-164.

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3 British standard 1042i Part 1: (1964) Methods for the

Measurement of Fluid Flow in Pipes. Part 1, Orifice Plates,

Nozzles and Venturi Tubes. British Standard Institution.

London.

4 Holman. J.P.[1976], Heat Transfer, McGraw-Hill, New York

5 Toei, R. [1983], Drying Mechanism of Capillary Porous Bodies

- in - Advances in Drying, vol. 2, 269-297, Hemisphere

Publishing Corporation. Washington.

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Documents

Evaluation of Textile Bobbins Drying Processes: Experimental and Modelling Studies