Handbook of Industrial Drying

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18.4 Industrial Infrared Drying Applications . .... Drying is the most common and most energy-consum- ... pensions and slurry of surplus activated sludge) have.
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Infrared Drying Cristina Ratti and Arun S. Mujumdar

CONTENTS 18.1 18.2

Introduction ......................................................................................................................................... Basic Principles .................................................................................................................................... 18.2.1 Theory ..................................................................................................................................... 18.2.2 Radiation Properties of Materials ........................................................................................... 18.3 Steady Infrared Drying ........................................................................................................................ 18.3.1 Modeling the Process .............................................................................................................. 18.3.2 Advantages and Limitations ................................................................................................... 18.4 Industrial Infrared Drying Applications .............................................................................................. 18.4.1 Applications of Infrared Radiation in Industry ...................................................................... 18.4.2 Industrial Infrared Dryers ....................................................................................................... 18.4.2.1 Infrared Sections ..................................................................................................... 18.4.2.2 Recent Developments in Design of Infrared Dryers ............................................... 18.4.3 Costs........................................................................................................................................ 18.5 Conclusions .......................................................................................................................................... Acknowledgment............................................................................................................................................ Nomenclature ................................................................................................................................................. References ......................................................................................................................................................

18.1 INTRODUCTION Drying is the most common and most energy-consuming industrial operation. With literally hundreds of variants actually used in drying of particulate solids, pastes, continuous sheets, slurries or solutions, it provides the most diversity among chemical engineering unit operations. One of the increasingly popular, but not yet common, methods of supplying heat to the product for drying is infrared (IR) radiation. Although this type of heat transmission was used incidentally in the past accompanying other types of heat transfer during dehydration, IR dryers are now designed to utilize radiant heat as the primary source (Williams-Gardner, 1971). The most common current applications of IR drying are in dehydration of coated films and webs and to correct moisture profiles in drying of paper and board. Theoretical work and laboratory-scale experimental results on IR drying of paints, coatings, adhesives, ink, paper, board, textiles, etc. can be found in the literature (e.g., Navarri et al., 1992; Kuang et al., 1992; Therien et al., 1991; Cote et al., 1990). On the other hand,

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423 424 424 425 427 427 430 431 431 431 431 434 434 436 436 436 437

reports on IR drying applied to other products like foodstuffs, wood or sand are not very common as yet. Most published data on IR drying of foods comes from (the former) USSR, the United States, and the East European countries (Hallstro¨m et al., 1988). Ginzburg (1969) described IR drying of grains, flour, vegetables, pasta, meat, fish, etc. and showed that IR drying can be successfully applied to foodstuffs. There are many current industrial applications of drying agricultural produce by IR. Sandu (1986) pointed out as advantages of IR drying in foods, the versatility of IR heating, simplicity of the required equipment, easy accommodation of the IR heating with convective, conductive, and microwave heating, fast transient response, and also significant energy savings. Experimental and theoretical works on IR drying, of opaque and semitransparent materials (silica sand, brick, brown coal, graphite suspensions and slurry of surplus activated sludge) have been performed by Hasatani et al. (1983, 1988). The purpose of this chapter is to give a general review of IR drying with special reference to industrial applications. A detailed description of this process as applied to paper drying, can be found elsewhere

in this handbook (chapter by K.T. Ojala and M.J. Lampinen). Note that many direct as well as indirect dryers can be modified to accommodate IR heaters. Indeed, combined convective and IR dryers have been shown to be very attractive. Also, IR heating can be coupled effectively with vacuum operation to permit removal of evaporated moisture. IR heating may be applied continuously or intermittently (in space or time) to save energy and often to improve product quality.

18.2 BASIC PRINCIPLES 18.2.1 THEORY Transmission of electromagnetic radiation does not need a medium for its propagation. The wavelength spectrum of the radiation depends on the nature and temperature of the heat source. Every body emits radiation due to its temperature level, which is called ‘‘thermal radiation’’ because it generates heat. The wavelength range of thermal radiation is 0.1–100 mm within the spectrum. IR radiation falls in this category and is conventionally classified as (Sandu, 1986): near IR (0.75–3.00 mm), medium IR (3.00–25 mm), and far IR (25–100 mm). Thermal radiation incident upon a body may be absorbed and its energy converted into heat, reflected from the surface or transmitted through the material following the balance: rþaþt ¼1

(18:1)

where r is the reflectivity, a the absorptivity, and t the transmissivity. For monochromatic incident radiation, these properties are called ‘‘spectral’’ and when that radiation is polychromatic they are defined as ‘‘total’’ (Sandu, 1986). Materials may be classified based on their transmissivity, depending on the physical state of the body where the radiation impinges. A body that does not allow the radiation to be transmitted through it is called ‘‘opaque’’ and is characterized by t ¼ 0. Examples of these are most solids. On the other hand, liquids and some solids like rock salt or glass have a defined transmissivity so they are ‘‘transparent’’ to radiation. The reflection may be ‘‘regular’’ (also termed specular) or ‘‘diffuse,’’ which depends on the surface finish of the material. In the former case, the angle of incidence of the radiation is equal to the angle of reflection due to highly polished surface or a smooth surface. When the surface has roughnesses larger than the wavelength, radiation is reflected diffusely in all directions. Generally solid bodies absorb all of the radiation in a very narrow layer near the surface. This is a very important consideration in modeling the heat transfer process, since mathematically this concept

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transforms a term within the energy balance into a boundary condition. An ideal body that absorbs all of the incident energy without reflecting or transmitting is called a ‘‘black body,’’ for which a ¼ 1. The total amount of radiation emitted by a body per unit area and time is called ‘‘total emissive power E’’ (Kreith, 1965) and depends on the temperature and the surface characteristics of the body. This energy is emitted from a surface in all directions and at all wavelengths. A black body is also defined as the one that emits the maximum radiation per unit area. The emissive power of a black body, Eb, depends only on its temperature. The emissivity of a body, «, is then defined as the ratio of its total emissive power to that of a black body at the same temperature, « ¼ E/Eb. As was pointed out earlier the total emissive power has energy from all the wavelengths in the spectrum of the radiation. On the other hand, the monochromatic emissive power, El, is the radiant energy contained between wavelengths l and l þ dl (Welty et al., 1984). For a black body, this power is expressed by (Planck’s law of radiation) Eb,l ¼

2pc2 hl5  ch  1 exp klT

(18:2)

so the monochromatic emissivity of a body is defined as «l ¼ El/Ebl. Kirchhoff’s law states that under thermodynamic equilibrium (which requires all surfaces be at the same temperature), the monochromatic absorptivity and emissivity of a body are equal. Equation 18.2 has a maximum that is related to the temperature by the following expression (Wien’s displacement law): lmax T ¼ 2897:6 mK

(18:3)

Equation 18.2 may be integrated over all wavelengths to obtain the total emissive power for a black body (Stefan–Boltzmann law): Eb ¼

1 ð

Eb, l dl ¼ sT 4

(18:4)

0

where s is the Stefan–Boltzmann constant. A gray body is defined as one which has the same emissivity over the entire wavelength spectrum. Thus, Kirchhoff’s law may be applied to gray bodies independently of their temperature. Heat exchange by radiation between two black bodies at different temperatures may be obtained using the Stefan–Boltzmann law and is expressed by Qr ¼ Ai Fij s(Ti4  Tj4 )

(18:5)

where Fij is the shape or view factor between surfaces i and j. By definition, this geometrical factor takes into account the part of the total radiation emitted by the surface i that is intercepted by surface j. To calculate theoretically this factor is rather complicated but for most common geometries there are charts and formulas available in the literature (Welty et al., 1984; Kreith, 1965). A useful equation known, as the ‘‘reciprocity theorem,’’ relates the shape factors and the areas of both surfaces through the following equation: Ai Fij ¼ Aj Fji

(18:6)

If the exchange of energy by radiation is between N bodies, the shape factors must follow the relation: N X

Fij ¼ 1

(18:7)

j ¼1

In fact, very few bodies behave as black bodies so a more realistic assumption would be to treat those as gray bodies. The net radiation between two gray bodies is then given by the following equation: s(Ti4  Tj4 )  rj ri 1 þ þ «i Ai Ai Fij «j Aj

Qr ¼ 

(18:8)

Also, it must be noted that sometimes the electromagnetic radiation that impinge on a body may be attenuated inside the body by scattering along with absorption. Scattering takes into account that electromagnetic radiation may undergo a change in direction, which can result in a partial loss or gain of energy (Siegel and Howell, 1972, p. 420). Suppose that Il represents a spectral radiation impinging normally a layer of material where it is absorbed and scattered, so the intensity of monochromatic radiation is attenuated following the relationship (called Bouguer’s law, Siegel and Howell, 1972, p. 413): 2

ðz

3

Il (z) ¼ Il (0) exp4 Kl (z ) dz 

5

(18:9)

0

where Kl is the extinction coefficient, z the distance, and Il(0) is the radiation at the surface of the body. The extinction coefficient depends on temperature, pressure, composition, and the wavelength of the incident radiation. It may pointed out that Equation 18.9 is also termed as ‘‘Lambert’s law’’ or ‘‘Bougher–Lambert law,’’ and ‘‘Beer’s law’’ when the extinction coefficient is put in mass terms, but it must not be confused with the ‘‘Lambert’s cosine law.’’

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18.2.2 RADIATION PROPERTIES OF MATERIALS The design and modeling of any process always requires a profound knowledge of the materials. Specifically for IR drying, this fact may be the clue to accomplish a safe and efficient process, because radiation properties of both the radiator and the material to be dried must be matched in order to obtain most efficient results. Emissivity, absorptivity, reflectivity, and transmissivity are the key radiation properties. The relative magnitudes of a, r, and t depend not only on the material, its thickness, and its surface finish, but also on the wavelength of the radiation (Kreith, 1965). Nevertheless the emission of electromagnetic waves is a property of the material only. Electrical conductors (e.g., metals) generally show an increase in emissivity « with an increase of wavelength of the radiation. On the other hand, nonconductors such as asbestos, cork, wood, concrete, etc. show the opposite trend. The emissivity of many bodies also show directional properties but as the data available are scarce, a good approximation is to suppose an average value for «/«n ¼ 1.2 for polished metallic surfaces and 0.96 for nonmetallic surfaces (Kreith, 1965). For practical purposes, only a mean value of the emissivity or absorptivity over the direction is required. Sieber (1941) obtained experimental data on total emissivity of opaque materials depending on the temperature of the source. Many authors (Ginzburg, 1969; Kreith, 1965) have reproduced these results graphically (Figure 18.1). The behaviour of electrical conductors and nonconductors with temperature of the radiator can be approximately interpreted from the dependency of the monochromatic emissivity on wavelength and the relationship between temperature of the radiator and the wavelength. At radiation temperatures in the range from 227 to 6208C, the total reflectivity of polished pure silver is between 0.98 and 0.968 and for polished pure gold from 0.982 to 0.965. For polished aluminum, the reflectivity varies from 0.961 to 0.943 in a temperature range from 223 to 5778C (Welty et al., 1984). The high reflectivity of these materials is the reason why reflectors of radiation lamps are made of a thin layer of silver, and polished aluminum is used as a facing material for internal partitions in equipment for IR radiation (Ginzburg, 1969). It may be noted that for the construction of the equipment for IR drying and in selecting the reflectors for radiator lamps, opaque materials with high reflectivity are required. The material to be dried by IR requires a low reflectivity in order to minimize the power required to heat it, and depending on the specific drying process, a high or medium absorptivity. When drying

1 20

2

Absorptivity (%)

3 4

40

5 6

60

7 8

80 9 100 140

540

1539

3537

9531

Temperature (⬚F)

FIGURE 18.1 Absorptivity of some materials as a function of temperature (1: fireproof clay, white; 2: aluminium; 3: wood; 4: cork; 5: asbestos; 6: porcelain; 7: concrete; 8: graphite; 9: roofing silvers).

Spectral directional

paints or coatings, a high absorptivity of the material is usually better, but in drying thick moist materials such as foodstuffs, it is preferable to use a material with high transmissivity to avoid extremely intensive heating and thermal damage of the surface. It is important to point out that if the absorptivity of a material is low, its transmissivity is high, and vice versa. Properties like absorptivity and transmissivity of moist materials are not frequently encountered in the literature. In addition to the dependency with wavelength and thickness, they also depend on the water content. One of the most extensive reports on experimental data of these properties can be found in Ginzburg (1969). Mohsenin (1984) also presents a good compilation of data on radiation properties of agricultural and food products.

1.0

.5

0

2

4

FIGURE 18.2 Absorption spectrum of water.

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The variation of absorptivity or transmissivity of moist materials with wavelength is difficult to estimate without experimental data. For many materials, transmissivity is higher at lower wavelengths (Ginzburg, 1969). Foodstuffs, as an example, are complex mixtures of different large biochemical molecules and polymers, inorganic salts, and water (Sandu, 1986) and the absorption bands of each of these constituents are not the same. As an example, Figure 18.2 shows the IR absorption spectrum of liquid water. Generally, many fully wet materials have their minimum absorptivity at those wavelengths where water has its maximum transmissivity pointing out the important role that water plays in radiation absorption. As drying proceeds, the material that is dried suffers a change in its radiation properties, increasing its reflectivity, and consequently lowering its absorptivity at low water contents. It is then possible to change adequately the temperature of the emitter in order to improve the absorption of radiation during drying. The transmissivity decreases with an increase in layer thickness, whereas absorptivity increases. An approximate way of representing experimental transmissivity data as a function of thickness is presented by Ginzburg (1969). Table 18.1 and Table 18.2 show a compilation of experimental data from the literature (Kreith, 1965; Ginzburg, 1969; Sandu, 1986) about the variation of transmissivity of foodstuffs and other materials commonly dried, with thickness, water content, and wavelength. In order to show an example of how both properties of the emitter and the product to be dried should be matched during IR drying, Figure 18.3 shows the radiant energy peaks for quartz tungsten filament at 2500 and 1925 K together with the spectral absorptivity of potato with 74.5% of water content (Sandu, 1986) and 10-mm thickness. To avoid overheating of the surface and to allow the radiation to penetrate into the product it would be better to choose the heat source at 2500 K because its maximum is located in the wavelength where the absorptivity of the foodstuff is not very high. On the other hand, if the lower

6

8 10 Wavelength (µm)

12

14

TABLE 18.1 Transmissivity of Selected Foodstuffs Product

Spectral Peak, l (mm)

Tr (8C)

Bread made of wheat flour 400 — Dough made of wheat flour

Mirror lamp —

Tomato paste

Potatoes Potato starch

Beet Fruit gel (marmalade)

1.075 1.075 1.075 1.190 1.350 3.400 1.100 1.100 1.100 1.100 1.100 1.100 1.100 1.100 1.100

— —



— — —

heat source temperature is used, the surface may be damaged (scorched) due to intense surface heating. Mohsenin (1984) presents an interesting discussion on the approach to be followed in order to obtain a successful IR drying of foods and agricultural products.

Thickness (mm)

W (%)

2.00 2.50 3.50 5.00 1.00 2.75 5.00 9.00 0.50 0.50 0.50 0.50 0.50 0.50 2.00 8.00 1.00 2.00 8.00 2.00 6.00 2.00 7.00



44.0

60.0 70.0 85.0 85.0 85.0 85.0 80.5 80.5 11.8 11.8 11.8 85.5 85.5 30.0 30.0

t or tl (%) 1.04 0.97 0.46 0.00 5.70 1.76 0.39 0.03 95.0 91.4 55.8 46.7 38.5 30.8 50.0 16.0 13.0 5.00 0.23 40.0 18.0 18.0 2.50

18.3 STEADY INFRARED DRYING 18.3.1 MODELING THE PROCESS Only the energy balance equation must be modified to include IR in the calculation procedure for drying

TABLE 18.2 Transmissivity of Selected Materials Product Cigarette paper (raw)

Paper, thick (moist wall paper)

Wool cloth, dry

Sand Wood

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Spectral Peak or l (mm) 1.075 3.220 6.150 1.190 3.750 6.150 1.190 3.220 7.750 1.190 6.150 1.190 6.150

Thickness (mm)





— 3.00 3.00 1.50 1.50

t or tl (%) 73.0 48.0 20.0 18.0 10.0 7.0 14.3 12.3 2.0 4.8 2.0 4.3 0.7

Energy peak and distribution (equal input basis)

α

0.75 2500 K

0.50 1925 K

0.25

1

2

3 Microns

4

5

6

0

FIGURE 18.3 Energy peak and distribution (equal input basis) of quartz tungsten filaments together with the absorption bands of a slice (10 mm) of potato with 74.5% water content ( –––).

Initial condition: t ¼ 0, 8z

whereas the mass balance equation remains the same as that for conventional drying: @W Dr @ 2 W ¼ 2 Z @z2 @t

T ¼ To

(18:13)

Boundary conditions: t ¼ t, z ¼ 0 (center) (18:10)

@T ¼0 @z

(18:14)

The water flux is given in terms of (Crapiste et al., 1988) t ¼ t, z ¼ Z (surface) rs,o Xo @W Dr nw ¼  Z @z

(18:11)

As the driving force for mass transfer depends on temperature, the high temperatures achieved by the drying surface with IR heating enhance the drying rate. In order to simplify the modeling of IR drying, it is convenient to start with IR drying of a flat partially ‘‘transparent’’ particle in which the heat conduction is one-dimensional. In the special case when the internal to external heat transfer resistance ratio is much greater than 0.1, the energy balance is given by rm Cpsh

@T @ 2 T @qa ¼k 2 þ @z @t @z

@T ¼ hg A(Tg  T cz¼Z )  nw ADHs (18:15) c @z z¼Z

The above model is commonly called ‘‘semitransparent’’ (Hasatani et al., 1983). The variable q may be obtained by averaging Equation 18.9 over all the wavelengths. If the average net radiation that impinges on a surface is Qr (defined in Equation 18.8), qa is given by 2

ðz

3

qa ¼ Qr exp4 K dz 5

(18:16)

0

(18:12)

with the following boundary and initial conditions.

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kA

If the particle is ‘‘opaque’’ to radiation, the energy equation becomes

rm cpsh

@T @2T ¼k 2 @t @z

(18:17)

As was pointed out earlier in this chapter, an ‘‘opaque’’ solid absorbs radiation in a narrow zone near the surface rather than attenuating within its volume; so the second term on the right side of Equation 18.12 must appear in the boundary condition. Thus the boundary condition given by Equation 18.15 transforms to kA

@T c ¼ hg A(Tg  T cz¼Z )  nw ADHs þ Qr @z z¼Z (18:18)

whereas the other boundary condition and the initial condition remain the same. This model is called ‘‘opaque’’ (Hasatani et al., 1983). In the case that only an average particle temperature is required or that the internal to external heat transfer resistance ratio is smaller than 0.1 (which means that the temperature profiles inside the particle can be considered to be almost uniform), the following simplification of Equation 18.17 is permissible: rm Cp

    @T 1 @T 1 @T ¼k ¼ k c @t Dz @z Dz @z z¼Z

(18:19)

Inserting Equation 18.18 into the above equation, the energy equation becomes mCp

dT ¼ hg A(Tg  T )  nw ADHs þ Qr dt

(18:20)

The model equations to represent the behavior of a dryer are an extension of the above explanation for IR drying of a single particle. In paper, paint, or textile drying, continuous dryers are commonly used. The equations describing continuous IR dryers can be found in the literature (Kuang et al., 1992; Cote et al., 1990). Nevertheless, a batch dryer may also be used for deep bed drying of foodstuffs because such materials have a solid structure from the beginning of the process (Heldman and Singh, 1981). The modeling equations for a batch dryer with circulation of air through the bed can be developed as follows. Food particles experience shrinkage during drying, but since shrinkage is not appreciatively affected by air or particle temperature (Ratti, 1994) it may be assumed that IR drying does not affect shrinkage differently from conventional drying. The key assumptions employed in this model are (1) One-dimensional transport of heat and mass (2) Uniform velocity distribution in the dryer (plug flow of drying air)

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(3) Adiabatic system (well insulated) (4) Conduction heat transfer between particles in the bed and contact diffusion are negligible (5) Shrinking particles (6) The air is completely transparent to radiation. Although in the drying process the air contains water vapor that absorbs IR radiation, this may be neglected because the amount of water vapor is small relative to the air. As an example, at a dry bulb temperature of 608C and 40% of relative humidity, the absolute humidity is 0.053 kg water/kg dry air. To avoid the problem of shrinkage, a moving coordinate system that follows the movement of the shrinking particles may be used, taking as basis a differential control volume that contains always the same amount of dry mass as in the initial time. This coordinate system is expressed as (Ratti, 1991) d(z=Lo ) ¼

rs,o (1  «lo ) dL rs (1  «l )

(18:21)

Then the resulting equations that represent batch fixed-bed IR drying are Mass balance in the gas phase:   @Y nw av (1  «l ) 1 Gs rs (1  «l ) @Y ¼  @t L r a «l SLo ra «l rs,o (1  «lo ) @L (18:22) Mass balance in the solid:   @X nw av ¼ rs @t L

(18:23)

Energy balance in the solid: 

@Ts @t

 ¼ L

av [hg (Tg  Ts ) rs (1 þ X )Cpsh  nw DHs þ Q0r ]

(18:24)

Energy balance in the gas phase:   @Tg hg av (1  «l ) ¼ (Tg  Ts ) ra «l Cpah @t L 

1 Gs rs (1  «l ) @Tg SLo ra «l rs,o (1  «lo ) @L

(18:25)

where Qr’ is defined from Equation 18.8 as Q0r ¼

s(Te4  Ts4 ) (1  «s ) 1 (1  «e ) þ þ «s Fse «e (Ae =As )

(18:26)

The initial profile of the four variables in the fixed-bed is stated as 8 X ¼ Xo > > < Ts ¼ Tso l¼0 > Y ¼ Ygo > : Tg ¼ Tgo

8 X ¼ Xo > > < Ts ¼ Tso l 6¼ 0 > Y ¼ Ysat (Tso ) > : Tg ¼ Tso

(18:27)

acoustic tiles (Dostie et al., 1989). Also, IR drying offers solution to problems that seemed to be unsolvable in the past such as those associated with the carrying of volatile organic compounds from solvent-based paints by the exhaust hot air in conventional convective dryers. A summary of the advantages of IR drying follows: 1. High efficiency to convert electrical energy into heat for electrical IR. 2. Radiation penetrates directly into the product without heating the surroundings. 3. Uniform heating of the product. 4. Easy to program and manipulate the heating cycle for different products and to be adapted to changing conditions. 5. Leveling of the moisture profiles in the product and low product deterioration. 6. Ease of control. 7. IR sources are inexpensive compared to dielectric and microwave sources; have a long service life and low maintenance. 8. Directional characteristics that allow to dry selected parts of large objects. 9. Occupies little space and may easily be adapted to previously installed conventional dryers. 10. Low-cost technology.

and the values of Tg and Yg at l ¼ 0 are set in Tgo and Ygo for constant inlet conditions. Some advances in the application of engineering to the modeling and design of dryers under radiation are made in the recent years. As an example, a complete model for a multiple-zone process for drying polymer– solvent coatings has been developed by Cairncross et al. (1995). Parrouffe (1992) has demonstrated on the basis of extensive experimental data that one may, within engineering accuracy, use analogy between heat and mass transfer to estimate the convective heat or mass transfer coefficients even in the presence of intense radiative heat flux on the evaporating surface. Appropriate corrections must be employed, however, for the high evaporative mass flux at the surface.

18.3.2 ADVANTAGES

AND

LIMITATIONS

Many authors have pointed out the advantages and disadvantages of using IR drying (van’t Land, 1991, p. 251; Hallstro¨m et al., 1988, p. 218; Nonhebel and Moss, 1971, p. 286; Dostie et al., 1989). In fact IR drying has many positive attributes, the main one being the reduction in drying time. Figure 18.4 shows the effect of IR drying compared to conventional convective drying of

On the other hand, the disadvantages are: 1. Scaling up of the heaters is not always straightforward. 2. Essentially surface dryers. Nevertheless, a great effort is done to improve this technology in

Curve heating mode

Water content (w) (kg/m2)

5

4

3

2

4

1

2 3

Curves 1, 5 0 0

30

60

90

120

150

Time (min)

FIGURE 18.4 Drying times according to the heating process employed. (From Dostie, M., Se´guin, J.-N., Maure, D., TonThat, Q.-A., and Chaˆtigny, R., in Drying’89, A.S. Mujumdar and M.A. Roques (Eds.), Hemisphere, New York, 1989. With permission.)

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order to adapt it to drying thick materials with successful results. 3. The testing of the equipment must be carried out in the plant to assure a successful design. 4. Potential fire hazards must be considered in design and operation.

products, braising meat and frying. An automatic IR system for selective heating of foods, which could have a big impact in the sterilization and pasteurisation processes, has been recently analyzed (Irudayaraj and Jun, 2000).

18.4.2 INDUSTRIAL INFRARED DRYERS 18.4 INDUSTRIAL INFRARED DRYING APPLICATIONS 18.4.1 APPLICATIONS IN INDUSTRY

OF INFRARED

RADIATION

IR heating is widely used in industry for surface drying or dehydration of thin sheets such as textiles, paper, films, paints, stove enamels, etc. Specifically in the automotive industry, the IR baking for painton-metal application is the most successful. Another sector where IR plays an important role is in pulp and paper industry. As an example, in Sweden this sector imposes higher demands on energy consumption than any other, and a relative new method that not only improves paper quality but also achieves energyefficient drying is to use electrically operated IR radiation heating (Hannervall et al., 1992). The disposal of hazardous waste is a less general IR application but, for instance, the IR drying of metal hydroxide sludge minimizes the cost impact of increasingly stringent disposal restrictions and when the metal content of sludge is of interest to recycles, it may be one way of changing a costly by-product into a profitable one (Davis and Wachter, 1992). Although IR drying of thick porous materials has not yet been fully developed, some researchers showed, mainly by means of experimental results, that one of the applications where long wave IR heating is most efficient is in dehydration of foods (Ginzburg, 1969; Kimura et al., 1992). Recently, IR radiation has been investigated for drying of cashew kernels (UmeshHebbar and Rastogi, 2001), herbs (Paakkonen et al., 1999), barley (Afzal et al., 1999), potato (Afzal and Abe, 1999; 1998; Masamura, 1998), shrimp (Fu and Lien, 1998), rough rice (Abe and Afzal, 1997), onion (Itoh, 1995), persimmons (Kim, 1993), and red pepper (Koh et al., 1990). In general, the previous articles deal with the application of IR heating to drying of different foods and the analysis of the characteristics and performance of these particular IR dryers. Japanese food industry uses this type of heating for drying of sea weed, curry sauce, carrots, and pumpkins among other things (Kimura et al., 1992). Other important IR applications in the food area (not specifically in drying), are cooking soybeans, cereal grains, cocoa beans and nuts, precooking rice, bacon, and barley grains for ‘‘ready to eat’’

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Although IR dryers may be batch or continuous type, the latter is the most common arrangement. IR ovens for dryers are usually designed and constructed from standard IR sections arranged and integrated to the conventional dryers in such a way that IR radiation is directly intercepted by the product to be dried. These sections are selected on the basis of the particular application. It is desirable to test the product on a laboratory-scale IR oven under simulated conditions and to design the large-scale unit on the basis of the experimental data obtained. To accomplish a reliable design, it is also necessary to know the efficiency of conversion from electric to IR energy of the radiators used in the plant (unless gas-fired IR heaters are used). The main data required are the intensity of radiation and the residence time (Nonhebel and Moss, 1971, p. 289) but although oven style and cross section are easily determined; on the other hand, the selection of the heat source, time–temperature cycle, and the power density requires oven design experience (Fostoria, Technical Bulletin, 1992). Air flow is required in IR ovens for two primary purposes: (a)

Air movement to cool and protect oven walls and terminals (b) Oven exhaust to remove smoke, moisture, solvents, hazardous vapours, etc. The decision of using natural or forced convection and the amount of air flow rate to meet the appropriate cooling effect must be adjusted to the specific application.

18.4.2.1 INFRARED SECTIONS IR sections basically consist of a heat source (called radiator or emitter), a reflector, source sockets, electrical connections, and a shell where the parts of each sections are built together (Figure 18.5). The main component is the radiator, which depending on the mode of heating, may be classified as: 1. Electrically heated radiators: In these radiators, the IR radiation is obtained by passing an electric current through a resistance, which raises its

Section shell

Secondary pan

Socket bracket and socket

Reflector Heat source

End cap

Air movement through section

Assembled section

FIGURE 18.5 Parts of an IR section. (Courtesy of Fostoria Industries, Inc., Fostoria, OH. With permission.)

temperature (Hallstro¨m, 1988, p. 217). The most common are: metal sheath radiant rods, quartz tube, and quartz lamp. A typical cross section of a tube emitter is sketched in Figure 18.6a. One of the most important characteristics of such emitters is the radiant efficiency, which may be defined as the percentage of radiant output from a heat source referred to the energy input. There is a positive relationship between this efficiency and the temperature of the radiator. Also, as was pointed out previously, there exists an inverse relationship between this temperature and its

peak energy wavelength. The peak wavelength can be controlled by changing the temperature of the source so if different types of emitters operate at the same temperature they will all have the same peak wavelength as well as other characteristics like penetration and color sensitivity. Figure 18.7 presents the relationship between voltage and temperature of the radiator together with the efficiency curve and Figure 18.8 shows graphically the heat up and cool down rate of response of the more common emitters which can be an important criterion in the

Filling

(a)

Filament Sheath

Air

Gas

Ceramic plate Grid Reflector (b)

FIGURE 18.6 (a) Sketch of an electric IR source. (b) Sketch of a gas-fired IR source.

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2. Gas-fired radiators: These radiators consist of a perforated plate (metal or refractory), which is heated by gas flames in one of the surfaces so the plate raises its temperature and emits radiant energy. The porosity of the plate determines the temperature of the other surface so as to ensure a safe process. Figure 18.6b shows a sketch of this type of radiator (van’t Land, 1991, p.250). The temperature of such a radiator is generally between 1500 to 17008C with wavelengths from 2.7 to 2.3 mm (van’t Land, 1991, p. 249). The radiant efficiency of such radiators is typically about 60%.

% Radiant efficiency 20

40

60

80

100 Quartz lamp design voltage

4,200

rtz

la

m

p

3,400

3,000

effic ienc y

Q ua

2,600

For practical purposes, choosing an emitter involves consideration of the following factors (Bischof, 1990):

Rad iant

Operating temperature of element (⬚F)

3,800

2,200

Quartz tube design voltage

1,800

Metal sheath rod design voltage

ube

t artz

1,400

Qu

th

a she tal Me rod

1,000 20

40

60

1. Absorption characteristics of the material that is heated 2. Power density of the radiating area ‘‘seen’’ by the product 3. Ratio of convected heat to radiant heat 4. Nature of the installation 5. Type of control required

80

One of the most successful emitters is the quartz lamp because it ensures high power densities, maximum heat efficiency, flexible design parameters, and ease of controllability. Also, this type of emitter is fitted with a gold reflector to direct the radiation toward the product to be heated. Various reflector systems are also used (Hallstro¨m, 1988, p. 217):

100

% Voltage

FIGURE 18.7 Radiant efficiency and relationship between voltage and temperature of various radiators. (Courtesy of Fostoria. With permission.)

selection of the proper source for a particular application. The main characteristics of the electric emitters are shown in Table 18.3.

Individual metallic/gold reflectors Individual gilt twin quartz tube Flat metallic/ceramic cassette reflectors

Heat up

Available radiant energy (%)

100

Cool down

Quartz IR lamp

80 Metal sheath

Quartz tube 60

40 Ceramic type heater

20

0 0

1

2

3

4

Time power on (min)

5

6

0

1

2

3

4

5

Time power off (min)

FIGURE 18.8 Heat up–cool down time cycles for IR sources. (Courtesy of Fostoria Industries, Inc., Fostoria, Ohio. With permission.)

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TABLE 18.3 Properties of Electric Radiators Type of Emitter Sheath material Filament Sheath diameter Filling material Maximum filament temperature Peak wavelength Voltage Wattage Radiant efficiency

Metal Sheath

Quartz Tube

Quartz Lamp

Stainless steel Nickel-Chrome 3/8’’ Insulating powder 18008F — 240 V 60 W per linear inch 50%

Translucent quartz Nickel-Chrome 3/8’’, 5/8’’, 7/8’’ Air 18008F 2.3 mm up to 600 V 30, 60 or 90 W p.l.i. 60%

Clear quartz Tungsten 3/8’’ Inert gas 40008F 1.15 mm ~ 600 V 100 W p.l.i. 86%

Figure 18.9 shows a sketch of such reflectors. The materials and the shape of the reflector determines its efficiency. Reflector materials must have high reflectivity, resist corrosion, heat and moisture, and be easily cleaned. They must also maintain the high reflectivity over a long period of time.

(a)

18.4.2.2 RECENT DEVELOPMENTS OF INFRARED DRYERS

IN

DESIGN

Although an IR dryer can be built using an existing convective dryer by putting in the appropriate number of radiators over the product to be dried so as to direct the radiation on it, this technology is under improvement and new combinations of dryers have appeared in the market. As examples of these new trends and applications, Figure 18.10 and Figure 18.11 show two industrial IR dryers, one high velocity hot air impingement oven with IR electric heaters mounted between adjacent nozzles (Figure 18.10, Glenro, Technical Bulletin, 1992) specially for drying adhesives and inks on papers, foam and composite web substances, etc. and the other is a gas-heated IR dryer for metal hydroxide sludge volume reduction (Figure 18.11, JWI, Technical Bulletin, 1992). Another promising technology is the combination of intermittent IR radiation with continuous convection heating (Dostie et al., 1989) for drying thick porous materials such as panels made of wood and of acoustic tiles.

18.4.3 COSTS (b)

(c)

FIGURE 18.9 Different types of reflectors: (a) individual reflector; (b) individual gilt twin quartz tube; and (c) flat metallic–ceramic cassette reflector. (From Hallstro¨m, B., Skjo¨ldebrand, C., and Tra¨ga˚rdh, C., Heat Transfer and Food Products, Elsevier Applied Science, London, 1988. With permission.)

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The capital costs per kilowatt installed depending on the different heating modes are presented in Table 18.4 (Dostie, 1992). Specifically for IR drying, the radiators are generally the main cost of a dryer. Table 18.4 also presents an approximate relationship between the costs of different types of emitters. A lamp radiator has a life of 2,000 to 3,000 h whereas a sheathed element from 5,000 to 10,000 h (Nonhebel and Moss, 1971, p. 290). The replacement of the radiator elements is the main maintenance item. The figures in this table should be taken as guidelines rather than precise. With changes in technology,

Recirculation and exhaust

Air supply

To rewind

From coater

Exhaust port Hot air impingement nozzle Infrared heater

exhaust

Water vapour

FIGURE 18.10 Impingement IR dryer. (Courtesy of Glenro, Inc., Paterson, New Jersey. With permission.)

Exhaust blower

Recirculation blower

Heated air

Proportional gas burner Stainless muffle chamber Sludge on endless stainless steel conveyor belt

FIGURE 18.11 IR dryer for treatment of sludge. (Courtesy of JWI, Inc., Holland, Michigan. With permission.)

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TABLE 18.4 Capital Costs for Different Modes of Heating and Types of Emitters (figures for Quebec, Canada, 1992) Mode of Heating

Capital Cost ($/kW)

Capital Cost (Equal Basis) $/kW Referred to the Highest

300 2000 500 — — —

— — — 0.49 1.00 0.93

Convection Radio frequency IR Electric lamp Electric sheathed filament Gas-fired emitter

these figures are likely to change with time. The costs are 1992 figures for Quebec.

18.5 CONCLUSIONS In view of their several advantages, it is likely that IR drying in combination with convection or vacuum will become increasingly popular. Intermittent (spatial or time-wise) supply of IR heating has the potential merit of saving energy, reduce air consumption, and enhance the quality of heat-sensitive products. Dryers for continuous sheets or large surfaces (e.g., planks), utilizing a combination of impinging jets and radiant heaters spaced between jets, have already proven to be industrially viable. Also, combination of radiant heating under vacuum operation is a technically a sound process for drying certain products. Much fundamental and industrial R & D needs to be carried out to exploit fully the potential of IR drying technologies.

ACKNOWLEDGMENTS The authors gratefully acknowledge the information provided by Glenro Inc., Fostoria Industries Inc. and JWI Inc. and for their permission to reproduce relevant figures in this chapter.

NOMENCLATURE A av Cpah Cpsh c Dr E F Gs h

area surface area/volume humid air specific heat humid solids specific heat speed of the light effective moisture diffusion coefficient total emissive power view factor dry air flow rate Planck constant

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hg I K K k Lo m nw Qr qa S T t W X Y z Z

heat transfer coefficient intensity of radiation extinction coefficient average extinction coefficient thermal conductivity initial bed height mass water mass flux heat exchange between bodies heat absorbed dryer transversal section temperature time dimensionless water content, X/Xo solid water content (dry basis) air absolute humidity distance particle half thickness

SUBSCRIPTS b e g i j n o r s sat l

black body emitter gas phase surface surface in a direction normal to the surface initial radiator solid at saturation monochromatic

GREEK SYMBOLS a DHs « «l k l

absorptivity heat of sorption emissivity bed porosity Boltzmann constant wavelength

lmax r ra rm rs s t

wavelength where E is maximum reflectivity air density solid density solid mass concentration Stefan–Boltzmann constant transmissivity

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