Handbook of Industrial Drying

83 downloads 0 Views 251KB Size Report
textile s and there is a danger of sco rching or burning the product. The thermal rad iation heat-transfer rate, qrad , emitted by the IR emitter is equal to A«s T 4, ...
34

Drying of Textile Products Wallace W. Carr, H. Stephen Lee, and Hyunyoung Ok

CONTENTS 34.1 34.2

Introduction ......................................................................................................................................... Classification of Drying Steps for Textile Products ............................................................................. 34.2.1 Predrying and Final Drying .................................................................................................... 34.2.2 Mechanical Dewatering ........................................................................................................... 34.2.3 Thermal Dryers ....................................................................................................................... 34.3 Properties of Textiles............................................................................................................................ 34.4 Typical Dryers for Textiles .................................................................................................................. 34.4.1 Mechanical Dewatering ........................................................................................................... 34.4.1.1 Squeezing................................................................................................................. 34.4.1.2 Vacuum Extraction ................................................................................................. 34.4.1.3 Centrifuge ................................................................................................................ 34.4.2 Conduction Dryers .................................................................................................................. 34.4.2.1 Hot-Cylinder Drying ............................................................................................... 34.4.3 Convection Dryers .................................................................................................................. 34.4.3.1 Circulating-Air Dryers ............................................................................................ 34.4.3.2 Through-Air Dryers ................................................................................................ 34.4.3.3 Impingement Dryers................................................................................................ 34.4.4 Infrared Dryers........................................................................................................................ 34.4.5 Radio Frequency and Microwave Dryers ............................................................................... 34.5 Conclusion ........................................................................................................................................... References ......................................................................................................................................................

34.1 INTRODUCTION The word ‘‘textiles’’ comes from the Latin textilis, meaning ‘‘woven’’; but in textile science, textile is defined as any product made from fibers. Thus textiles refers not only to woven fabrics but also to nonwoven fabrics, knitted fabrics, tufted fabrics such as carpets and bedspreads, and specially constructed fabrics [1]. Figure 34.1 depicts the major segments and linkages of the textile industry, from fibers to products. The textile mill portion of the textile complex includes many chemical wet processes such as slashing, dyeing, printing, latex bonding, and finishing. In many of these processes, drying is required to remove the excess moisture in the porous materials to produce the desired product. For example, the typical steps used to produce latex-backed tufted carpet are shown schematically in Figure 34.2.

ß 2006 by Taylor & Francis Group, LLC.

781 782 782 783 783 783 785 785 785 785 786 787 787 787 787 788 788 789 789 791 791

As there are many textile products ranging from yarns to carpets which have to be processed, many different drying processes are used by the textile industry. To complicate things further, various processes are used for the same product. Typical drying systems used by the textile industry for drying fabrics and tufted carpets are discussed in this chapter. The term drying is commonly used to describe the process of thermally removing the volatile substances from a product [2]. In textiles, the term is more generally used to mean the dewatering of a product. Mechanical dewatering is generally much less expensive than thermal drying. Thus, as much water as possible is usually removed mechanically. Approximately 25% of the energy used in wet processing is consumed in drying [3]. In addition to energy cost, drying is a common bottleneck in the wet processes. Thus, improvement of the drying rate and

Fiber production

Yarn production

Fabric production

Fabric production

• Nonwoven

• Weaving • Nonwoven • Knitting

Apparel production

Engineeredproducts production

Carpet and rug production

Home-furnishing production

FIGURE 34.1 The textile industry.

energy efficiency of the drying system and selection of the proper type of dryers are of great importance to the textile industry. In this chapter, moisture regain (%), defined as the weight of water in the textile to the dry weight of Yarn

Tufting

Batch dyeing Continuous dyeing and drying Continuous drying

Latex backing

Secondary backing

Drying and curing

FIGURE 34.2 Process diagram of a typical carpet mill.

ß 2006 by Taylor & Francis Group, LLC.

textile, is used to indicate the amount of moisture contained in the textile.

34.2 CLASSIFICATION OF DRYING STEPS FOR TEXTILE PRODUCTS The drying or dewatering processes can be classified either as predrying and final drying or as mechanical and thermal drying. Predrying is accomplished using either mechanical or thermal processes whereas final drying is achieved using thermal processes. Predrying is the lowering of the moisture regain to some predetermined level. This is followed by the final-drying step in which the moisture regain is lowered to the desired level. For example, fabrics that are heat set in the drying process will be predried by some means to a moisture regain of approximately 30%. They will be transported through a convection oven using a tenter frame that fixes the width of the fabric, giving it a dimensional stability. Mechanical dewatering is usually used to predry fabrics when feasible because it requires much less energy expenditure per mass of water removed (Table 34.1) [4]. However, the level to which the moisture regain can be lowered by mechanical methods is usually too high, and thermal processes are required to obtain the final moisture regain.

34.2.1 PREDRYING

AND

FINAL DRYING

Drying is sometimes separated into two steps: (1) predrying and (2) drying. This may be done for either economical or technical reasons. Predrying may be

TABLE 34.1 Typical Energy Requirements for Common Textile Drying Equipments Equipment Type

Mechanical dewatering Squeeze roll Vacuum extractor Thermal drying Steam can Convection dryer Radiant predryer

Energy Requirement (kJ/kg)a

58 700 4700 7000 9300

a

Kilojoules per kilogram of water removed. Source: From Georgia Institute of Technology, Report No. ORO5099-T1, U.S. Department of Energy, 1978, p. 15. With permission; Badin, J.S. and Lowitt, H.E., Report No. DOE/RL/01830-T56, U.S. Department of Energy, January 1988. With permission.

carried out by methods that are efficient and require low energy input. This will be followed by final drying either because the moisture regain that can be obtained with the predrying method is limited or because better product quality can be obtained by final drying using some other method. Sometimes predrying is used for technical reasons. For example, in continuous dyeing, fabric saturated with dye solution must be dried to fix the dye. The moisture regain must be lowered uniformly to about 30% to prevent dye migration. Predrying is often accomplished using gas infrared (IR) systems and final drying is achieved using either steam cans or convection ovens.

34.2.2 MECHANICAL DEWATERING Several methods are available for mechanically reducing the amount of water in textile materials. The most common techniques are mangling or squeezing, vacuum extraction, and centrifuging. The technique selected is influenced by the type of material to be processed and on whether a batch or continuous process is to be used. Many delicate fabrics are damaged by squeezing between rollers or spinning in a centrifuge. Since vacuum slots cause less damage, they are often used with delicate fabrics. Centrifuging is normally limited to batch processes. Mangling followed by vacuum extraction is often used for continuously processing fabrics and carpets. Mangling removes free water very efficiently, and vacuum extraction lowers the moisture regain to levels unachievable with mangling alone. The moisture regain achievable with mechanical methods depends significantly on the type of process used and on the type of textile being dried. For

ß 2006 by Taylor & Francis Group, LLC.

example, the reduction in moisture regain obtainable with squeezing rollers varies with both the fiber type and the textile construction. For structures composed of hydrophilic fibers such as cotton, the moisture regain can typically be reduced from about 150% to 60–100%. On the other hand, for structures composed of hydrophobic fibers such as polyester, significantly lower moisture regain can be obtained.

34.2.3 THERMAL DRYERS Thermal dryers are sometimes used for predrying, but are almost always used for final drying because of the limitations of mechanical dryers. After mechanical predrying, much of the remaining water is chemically bonded to the fiber and must be evaporated. This is accomplished using several types of thermal system such as heated cans, convection ovens, and radio frequency (RF) dryers. Recently, microwave dryers have been designed for drying textiles and carpet tiles [5].

34.3 PROPERTIES OF TEXTILES The physical, thermal, and sorptive properties of textiles are very important in calculating drying-energy usage, modeling the thermal drying processes, and determining the dryer operation conditions. The physical and thermal properties of several fibers are given in Table 34.2 [6]. The equilibrium moisture regain vs. relative humidity of several fibers is given for absorption and desorption in Table 34.3 [7]. As illustrated in TABLE 34.2 Physical and Thermal Properties of Fibers Material

Natural Fiber Cotton Cotton bats Wool Wool bats Synthetic Fibers Nylon 6, 66 Polyethylene Poly(ethylene terephthalate) Polypropylene Polyacrylonitrile Others Carbon fiber

Thermal Thermal Density Specific (g/cm3) Heat Conductivity Diffusivity (J/kg/K) (W/m/K) (cm2/s)

1.52 0.08 1.34 0.5

1250 1300 1340 500

0.07 0.06 — 0.054

0.000368 0.00577 — 0.00216

1.14 0.97 1.37

1419 1855 1103

0.25 0.24 0.14

0.001545 0.001334 0.000926

0.93 1.18

1789 1286

0.12 —

0.000721 —

05–500

0.10–3.97

1.8–2.1

710

Source: From Warner, S.B., in Fiber Science, Englewood Cliffs, NJ: Prentice-Hall, 1995. With permission.

ß 2006 by Taylor & Francis Group, LLC.

TABLE 34.3 Moisture Regain of Fibers for Moisture Absorption and Desorption at 21˚C Fiber

Cellulose acetate Cotton Kevlar aramid Nomex aramid Nylon 66 Poly(ethylene terephthalate) Silk Viscose rayon Wool

Relative Humidity (%) 10

20

30

40

50

60

70

80

90

95

a

0.87/2.70 2.08/4.38 1.73/2.69 2.34/3.88 0.78/1.77 0.07/0.14 3.25/6.20 4.87/8.28 3.58/8.13

1.67/4.47 3.72/5.67 1.98/3.01 2.66/4.48 1.49/2.36 0.12/0.20 5.20/7.70 7.03/10.62 6.67/10.80

2.73/5.40 4.86/6.06 2.58/3.96 3.27/5.01 2.17/2.69 0.28/0.29 6.76/9.72 8.76/12.42 8.28/12.60

3.53/6.50 5.86/7.61 3.58/5.38 4.38/5.68 2.70/3.33 0.34/0.36 7.91/11.71 10.45/13.65 10.02/14.78

4.27/7.81 6.95/8.36 3.82/5.55 4.70/5.76 2.70/3.87 0.39/0.43 8.11/11.51 12.20/14.67 12.39/16.08

5.71/8.83 7.90/9.71 4.12/6.15 5.00/6.03 3.77/4.40 0.43/0.47 10.67/12.26 14.39/16.13 13.62/17.71

6.67/10.59 9.45/11.72 4.68/6.73 5.48/6.27 4.45/5.30 0.50/0.55 11.96/14.85 16.22/18.32 15.33/19.33

7.96/13.54 11.04/13.90 5.51/6.97 6.15/6.75 5.01/5.53 0.53/0.55 13.54/17.36 18.36/20.57 17.26/20.20

9.12/14.01 12.74/14.12 5.49/7.14 6.45/6.80 5.47/6.02 0.53/0.55 15.74/20.97 20.65/25.00 19.34/21.09

0.52 /0.80 1.40/2.28 0.88/1.45 1.15/2.66 0.44/0.70 0.02/0.04 1.14/3.45 2.38/3.96 2.14/3.68

a First number is for moisture absorption and second number is for moisture desorption. Source: From Fuzek, J.F., Ind. Eng. Chem. Prod. Res. Dev., 24, 140–144, 1985. With permission.

0.6

14

Moisture regain (%)

Moisture regain (%)

16 12 10 8 6 4 2 0

0

20

40 60 80 Relative humidity (%)

100

0.5 0.4 0.3 0.2 0.1 0

0

(A) Cotton

20

40 60 Relative humidity

80

100

(B) Poly(ethylene terephthalate)

FIGURE 34.3 Moisture regain vs. relative humidity for absorption and desorption of cotton and poly(ethylene terephthalate) fibers. (From Fuzek, J.F., Ind. Eng. Chem. Prod. Res. Dev., 24, 140–144, 1985. With permission.)

Figure 34.3, comparison of the equilibrium moisture regains for absorption and desorption shows that the hysteresis [8] is large for hydrophilic fibers, such as cotton, and low for hydrophobic fibers, such as polyester.

34.4 TYPICAL DRYERS FOR TEXTILES The typical dryers used for textiles and carpets are summarized in Table 34.4. These dryers are briefly described below.

TABLE 34.4 Typical Dryers Used in Textile and Carpet Industry Application Yarn preparation and weaving 1. Slashing Fabric finishing 1. Preparation

2. Dyeing and printing Dyeing

34.4.1.1 Squeezing Printing 3. Finishing

34.4.1.2 Vacuum Extraction

2. Application of secondary backing

The next step in dewatering textiles and carpets is usually vacuum extraction as shown schematically in

ß 2006 by Taylor & Francis Group, LLC.

Squeezing Steam cans

Predrying

Squeezing/vacuum extraction/steam cans Steam cans or convection oven

Predrying

Drying

The first step in the dewatering process is usually squeezing or mangling, shown schematically in Figure 34.4. It is accomplished by passing the textiles between a pair of rollers and is by far the least expensive method for dewatering fabrics. The energy requirements of water removed are only approximately 58 kJ/kg. However, squeezing can reduce the moisture regain only by 60 to 100%, depending on the properties of the fibers and the fabric construction.

Drying Predrying

Drying Floor covering (tufted carpet) 1. Drying after dyeing

Type

Predrying Drying

Drying

34.4.1 MECHANICAL DEWATERING The most common procedure for mechanically dewatering in continuous processes is the squeezing using nip rollers followed by vacuum extraction. In processing batch materials, centrifuging is commonly used.

Drying Step

Predrying Drying Drying

Squeezing/vacuum extraction/steam cans Infrared oven Steam cans or convection oven Convection oven Squeezing/vacuum extraction/steam cans Steam cans or convection oven

Squeezing/vacuum extraction Convection Oven Convection Oven

Figure 34.5. With this method, water is extracted from the textile in open width as it passes over a slotted or perforated box in which vacuum is maintained by a pump. The final moisture regain depends on operating parameters, such as initial moisture regain, vacuum pressure, and production speed, and is also highly dependent on textile construction and fiber hydrophilicity. The energy requirements (700 kJ/kg of water removed) are high compared with squeezing, but much lower than for thermal methods. The typical moisture regain achievable for fabrics made from several types of fibers is shown in Table 34.5 [9]. It is about 30 to 50% for tufted nylon carpets, depending on the vacuum system and carpet construction.

(a) Fabric

⫹ ⫹



Squeeze rolls



(b) ⫹ Fabric Water

Roll

34.4.1.3 Centrifuge Roll

Centrifuging involves rotating the textile at high speed to remove water, and is normally limited to batch processing of materials such as skeins of yarns, packages of yarns, and small rugs. The moisture regain of various kinds of fibers after centrifuging is shown in Table 34.6.



FIGURE 34.4 Dewatering using squeeze rolls.

Suction slot

Fabric

Water separator

Exhaust

Vacuum pump

Air bleed Valve

Atmospheric pressure Fabric

Low pressure

FIGURE 34.5 Schematic of a vacuum extraction.

ß 2006 by Taylor & Francis Group, LLC.

TABLE 34.5 Moisture Regain after Vacuum Extraction Fabric

⫹ ⫹

Vacuum (Hg Vac)

Moisture Regain (%)

15’’ 10’’ 15’’ 15’’ 17’’ 17’’

10–15 25–30 25–30 35–40 50–55 55–65

100% Polyester 100% Polyester 80% Polyester/20% cotton 65% Polyester/35% cotton 100% Cotton 50% Rayon/50% cotton

Source: From Ostervold, J.A., America’s Textiles International, 11, 16j–16l, 1982. With permission.

34.4.2 CONDUCTION DRYERS Conduction drying involves placing the surface of the material in direct physical contact with a heated surface. An advantage of conduction dryers is that heattransfer rates achievable are usually higher than for convection drying. A disadvantage is that the direct contact with the solid surface may cause damage to the textile. 34.4.2.1 Hot-Cylinder Drying Conduction drying is usually carried out using hot cylinders, which are rotating metallic cans that are heated using steam or special heat-transfer liquids. When steam is used, the drying system is often referred to as steam cans, which are shown schematically in Figure 34.6. The energy requirements removed are typically around 4700 kJ/kg, which is usually lower than that of other thermal dryers. Steam cans are used in predrying and final drying of fabrics. They are often used to predry fabrics that

TABLE 34.6 Moisture Regain after Centrifuging Material Acetate Cotton Nylon Silk Viscose rayon a

Moisture Regain (%)a 31 48 16 52 103

Moisture regain after rotating for 5 minutes using a centrifuged force 1000 times the gravitational field. Source: From Morton, W.E. and Hearle, J.W.S., Physical Properties of Textile Fibers, 3rd ed., Manchester, UK: The Textile Institute, 1993. With permission.

ß 2006 by Taylor & Francis Group, LLC.

⫹ ⫹ Steam cans

⫹ Fabric



FIGURE 34.6 Schematic of a steam can dryer.

must be dimensionally stable after final drying. The steam cans are used to reduce the moisture regain to about 30%, and convection ovens with tenter frames are used for final drying. In some cases where contact with the hot surface does not negatively affect fabric properties, all of the thermal drying is achieved with the steam cans. Although the direct contact of the fabric with the hot surface gives high heat-transfer rates, it can cause problems. One problem is that the pressure between the can and the fabric can distort the surface of delicate fabrics. Another problem occurs when a fabric containing materials such as dyes, print paste, and adhesives touches the hot surface. Sometimes materials from the fabric will transfer to the hot surface, and then subsequently transfer to regions of the fabric, causing unwanted effects.

34.4.3 CONVECTION DRYERS Convection dryers are the most common type of dryers used for drying textiles and carpets. The drying medium is usually hot air though steam can be used if the temperature can be raised sufficiently high without damaging the textile [10,11]. The manner in which air is applied to the product greatly affects the heattransfer and drying rates that are achievable in convection ovens. There are three types of convection dryers: (1) circulating-air, (2) through-air, and (3) impingement. 34.4.3.1 Circulating-Air Dryers In circulating-air dryers, drying is achieved by transporting the textile through hot air that circulates through the oven. Products that have varying sizes and weights such as towels and washcloths are looped over hangers that slowly pass through the oven. Drying rates are usually low, and residence times in this type dryer (sometimes referred to as loop dryers) are usually in the order of 40 to 45 min.

34.4.3.2 Through-Air Dryers Through-air (flow-through) dryers have been widely used to remove moisture from various textile products for over 50 years. The principle of through-air drying is to evaporate moisture by forcing the air to flow through the porous material. The basic concept is illustrated in Figure 34.7. As through-air drying greatly increases the contact surface area between the hot air and the wet surface, it provides high overall drying rate. There are two types of through-air dryers often used in the textile industry: (1) suction or perforated drum drying systems and (2) convection ovens with tenter frames for fixing the transverse dimension of the product while it is transported through the oven. Suction or perforated drum dryers are often used to dry fabrics, particularly nonwovens, and sometimes unbacked tufted carpets. These dryers usually consist of two or more perforated drums mounted horizontally in a compartment (Figure 34.8). Several twodrum compartments are typically linked together to form a complete machine. Fans draw air from the interior of the drums producing suction on the surface area in contact with the material. This suction holds the material to the surface of the drum permitting hot air to pass through the material being dried. A portion of the drum has no suction, which permits the material to transfer to the next drum without interference. The second type of through-air dryer commonly used to dry fabrics and unbacked tufted carpets is shown schematically in Figure 34.9. It is a convection oven with a tenter frame that controls the transverse dimension of the product while it transports the fabric through the dryer. Fans blow hot air in the chamber on one side of the fabric creating a pressure drop across the fabric, causing hot air to flow through the fabric. Some of the warm moist air exiting the fabric is usually reheated and recycled through the fabric. In the through-air drying process, most of the energy is required for heating air and blowing it through the wet material. For a given blower configuration, the velocity of the air flowing through the material depends on the air permeability of the

Fabric Hot-air in

To fan

FIGURE 34.8 Schematic of a suction drum dryer.

material. Air permeability is a measure of how easily air can flow through a unit area of porous material at a given pressure drop. As the air permeability of the material increases, the airflow rate increases resulting in a higher drying rate. In this case, through-air drying is a good option; however, if air permeability of the material is too low, the electrical energy for driving the fan to obtain a required airflow rate would be too high. In this case, impingement drying is often used instead of through-air drying. 34.4.3.3 Impingement Dryers Convection impingement drying systems are often used to dry textiles with low air permeability, for example, latex compounds used to back tufted carpets. This technology utilizes columns of high velocity hot air directed at the product surface via nozzles mounted above and below the product, as shown schematically in Figure 34.10. Due to the complex fabrication and high air-handling costs of impingement dryers, they are preferably used for thick fabrics and fabrics with backings such as latex. Typically, the impingement air hits perpendicularly or near perpendicularly onto one or both of the product surfaces. If air impinges onto both surfaces, a

Fan Exhaust Air in Gas burner

Through-air in Recycle

1 Fabric

FIGURE 34.7 Schematic of a through-air drying system.

ß 2006 by Taylor & Francis Group, LLC.

2

3

4

5

Fabric

FIGURE 34.9 Industrial through-air dryer.

6

tenter frame is often used to control the transverse dimension and to transport the product through the dryer. If air impinges onto only one product surface, the product is typically supported on a mesh screen, conveyor, or roller. The designing of impingement dryers may involve selecting nozzle configuration and geometry, determining velocity and temperature of impingement air, nozzle-target spacing, calculating drying rate, estimating air-recycle ratio, etc. The details of designs of impingement dryers are well described in another chapter of this handbook.

34.4.4 INFRARED DRYERS Thermal-radiation emitters, referred to as IR emitters, are used primarily to predry textile and carpet products. Thermal radiation is a mode of heat transfer characterized by energy transport in the form of electromagnetic waves. It is the energy emitted by the body solely by virtue of its temperature. Although high-temperature emitters emit thermal radiation in the IR, visible, and ultraviolet regions of the electromagnetic spectrum, almost all of the radiations are in the IR region (Figure 34.11). Thus the dryers using the thermal-radiation emitters are referred to as IR heaters. IR ovens are typically used to predry materials that are finally dried in tenter-frame convection ovens. They are also used to augment the existing ovens where additional production is needed and space is limited. IR ovens are not normally used to accomplish final drying because IR energy is absorbed by dry textiles and there is a danger of scorching or burning the product. The thermal radiation heat-transfer rate, qrad, emitted by the IR emitter is equal to A«s T 4, where A is the emitter surface area, « is the emissivity of the emitter, s is the Stefan–Boltzmann constant (5.6710 8 W/m2/K4), and T is the temperature of the emitter. As the energy emitted varies with the fourth power of temperature, the heating power

Impingement-air in

Fabric

Impingement-air in

FIGURE 34.10 Schematic of a double-impingement drying system.

ß 2006 by Taylor & Francis Group, LLC.

densities of IR emitters vary greatly with emitter temperature. IR systems with high-temperature emitters are available that can provide much higher energy densities than conduction and convection dryers. The depth of IR penetration into the textile depends on the wavelength. The shorter the wavelength, the deeper is the penetration. However, even at short wavelengths, the maximum depth is typically no greater than about 1.5 mm. Thus, IR drying is generally used for thin materials such as fabrics in dyeing, finishing, or coating processes. Two types of IR emitters (electric and gas-fired) are used in IR dryers for textiles. Both types of IR emitters can be used to produce medium-wavelength (2 to 4 mm) and long-wavelength (4 to 10 mm) IR radiation; however, only the electric IR emitter can generate the short-wavelength (0.76 to 2 mm) IR radiation. For the electric IR drying systems, 100% of the input energy is utilized to heat the emitters and up to 86% is converted to IR radiation. For the gas IR drying system, 25 to 50% of the input energy typically goes into the flue gas and is exhausted. Thus, the gas IR drying systems usually produce less radiation per unit of input energy. However, cost per unit of input energy for electricity can be 3 to 5 times that of gas. Based on the efficiency and the unit price of energy, the energy cost for drying must be calculated for each application. IR drying is accomplished using various setups. An IR emitter suspended over the textile or mounted vertically with the textile moving parallel to it transfers heat that may be adequate in some cases. However, placing the IR emitter in a properly designed enclosure (Figure 34.12) enables it to be more energy efficient, create better temperature uniformity across the web, put less load on the plant air-conditioning system, and be safer for personnel [12]. A low-velocity air impingement between the IR emitters is useful in removing the moisture-laden air from the surface of the textile.

34.4.5 RADIO FREQUENCY

AND

MICROWAVE DRYERS

Although RF and microwave dryers have been used by the textile industry, the market penetration of RF dryers is much greater. The cost of the equipment has been a barrier to the use of both types of dryers, and uniformity of treatment has been a problem associated with microwave drying. As RF and microwave systems are operating on electric power, the unit of energy cost is usually higher than for conventional dryers. However, the energy efficiency of RF and microwave systems are normally much higher than for conventional system, which tends to offset the higher unit of energy cost.

Thermal radiation Visible Gamma rays 10–5

10–4

X-rays

Ultraviolet

10–3

10–2

Infrared

10–1 1 l (μm)

101

Radio waves 102

103

104

FIGURE 34.11 The electromagnetic spectrum.

RF and microwave dryers are based on a thermal effect known as dielectric heating. When a dielectric, i.e., a material which is an electrical insulator, is placed in an alternating electric field, successive distortion of the molecules causes heating. Although there are several heat dissipation mechanisms, only two are of major importance to heating textiles. One is ionic conduction, where ions are accelerated by the electric field and collide with other molecules, giving up kinetic energy. The other is dipolar rotation, where dipoles or induced dipoles in the molecules making up the material tend to align with the electric field. When the field alternates, the dipoles tend to rotate to follow the electric field. The rotating dipoles interact with the surrounding material and the intermolecular friction results in heat being given off [13]. The frequencies for RF heating are between 1 and 100 MHz while those for microwave heating are between 300 MHz and 300 GHz. However, Industrial, Scientific, and Medical (ISM) bands for industrial heating have been established by international agreement. These are 13.56, 27.12, and 40.68 MHz for RF heating and 896, 915, and 2450 MHz for microwave heating [14].

Since 1978, RF dryers have been used by the textile industry primarily to dry bulky textile materials such as hanks, muffs, tops, and yarn packages. Four types of systems are currently being manufactured: (1) batch units; (2) continuous belts; (3) combination RF and air/vacuum system where water is vaporized and removed at a temperature lower than 1008C; and (4) air, RF assisted (ARFA). When compared with conventional dryers, RF dryers usually have the advantage of providing a more uniform final drying and reduced chance of overdrying. The energy requirements of RF dryers depend on process parameters such as fiber type, substrate, and process: for example, 0.53 kWh/kg of water removed for drying 100% polyester packages and 4.91 kWh/kg of water removed for drying loose-stock cashmere [15]. Until recently, the use of microwave systems for drying textiles has been extremely limited. The major limitation of microwave heating has been a problem with uniformity of treatment. Nonuniform treatment can lead to hot spots, resulting in overheating in some areas and underheating in others. Recent research has led to a new approach to microwave drying of textiles. The use of waveguides to serpentine the microwave Blower

Exhaust duct

Heaters retract Panel heater

Panel heater

Fabric Reflector or additional panel heaters

FIGURE 34.12 Schematic of an infrared oven.

ß 2006 by Taylor & Francis Group, LLC.

Panel heater

energy back and forth across the material being treated gives the needed improvements in uniformity. Microwave drying systems are now being tested for drying a range of textile products including tubular knits, sheets of yarns, terry towels, and carpet tiles [5,16].

34.5 CONCLUSION As there are many textile products that are to be processed, many different drying systems are used by the textile industry. To complicate things further, various processes are used for the same product. Typical drying systems used by the textile industry for drying fabrics and tufted carpets are briefly discussed in this chapter. No attempt is made to give full design details. Also the mechanical, electrical, or control aspects of these dryers are not discussed. The reader can refer to other chapters of this handbook for more details and additional references.

REFERENCES 1. ML Joseph. Introductory Textile Science, 3rd ed. New York: Holt, Rinehart, and Winston, 1986, p. 1. 2. AS Mujumdar. Handbook of Industrial Drying, 2nd ed, New York: Marcel Dekker, 1995, p. 1. 3. Georgia Institute of Technology. Energy conservation in the Textile Industry: Phase II. Report No. ORO5099-T1. U.S. Department of Energy, 1978, p. 15

ß 2006 by Taylor & Francis Group, LLC.

4. JS Badin and HE Lowitt. The U.S. textile industry: an energy perspective. Report No. DOE/RL/01830-T56. U.S. Department of Energy, January 1988. 5. MC Thiry, The magic of microwaves Text. Chem. Colorist & Am. Dyestuff Rep. 32:2–4, 2000. 6. SB Warner. Fiber Science. Englewood Cliffs, NJ: Prentice-Hall, 1995. 7. JF Fuzek, Absorption and desorption of water by some common fibers Ind. Eng. Chem. Prod. Res. Dev. 24:140– 144, 1985. 8. WE Morton and JWS Hearle. Physical Properties of Textile Fibers, 3rd ed. Manchester, UK: The Textile Institute, 1993. 9. JA Ostervold, Vacuum extraction comes of age. America’s Textiles International 11:161–16l, 1982. 10. DR O’Dell. The Drying Behavior of Carpet Tiles in a Medium of Superheated Steam. MS Thesis, Georgia Institute of Technology, Atlanta, GA, 1994. 11. DR O’Dell and WW Carr, Effect of humidity on the drying rates of carpet titles Text. Res. J. 66:366–376, 1996. 12. T VonDenend, Effective use of infrared heating for textile coating and laminating applications J Coated Fabrics 23:131–149, 1993. 13. M Orfeuil. Electric Process Heating: Technologies/ Equipment/Applications. Columbus, OH: Battelle Press, 1987, p. 519. 14. DE Clark, WH Sutton, and DA Lewis. Microwaves: Theory and Application in Materials Processing IV. Ceramic Transactions, Vol. 80. Westerville, OH: The American Ceramic Society, 1997, p. 41. 15. The Industrial Electrotechnology Laboratory, CADDET Energy Efficiency, March 1996. 16. INJ Departments, INJ. Summer:4–6, 2001.