INFRARED RADIANT HEATING F. M. TILLER AND H. J. GARBER University of Cincinnati, Cincinnati, Ohio
Thirdly, it is assumed that the HE b a k i n g of a u t o During the past three years there has been a intensity of radiation is everymobile finishes by the steady increase in the industrial utilization where uniform, a condition Ford Motor Company of infrared radiation produced by lownot frequently attained in in( 4 . 1 2 , 1 8 ) was t h e f i r s t temperature electrically excited filament dustrial units. widely publicized commercial lamps. Although most of the existing inThe quantity of radiant application of infrared radiant energy converted into heat deheating. Following this lead, stallations are used for the drying and bakpends on the surface reflection several producers of equiping of coatings and finishes, many other characteristics of the stock. ment adapted their manufacapplications have been made or indicated. turing facilities to the proIn the case of materials with As is the case in the history of many other large linear absorption coeffid u c t i o n of r e f l e c t o r l a m p industrial processes, theory has lagged becients for infrared radiation, assemblies. A side line a t such as metals, penetration b e &st, radiant heating appahind practice. ratus became a major item yond the surface in accord This paper presents a theoretical derivawith the exponential extincof manufacture as additional tion of temperature-time expressions for industrial applications were tion law is slight. Hence, the the radiation of thin metallic bodies. Exsurface reflection characterfound. Promptly recognizing perimental data are presented to establish istics primarily determine the the load-building possibilities of infrared radiation, power total energy absorbed. The the validity of the equations. Values of properties of the coating mautilities investigated (2) and absorptivities for different enameled surterial play a major role in deadvertised equipment for rafaces and convectional coefficients of heat diant heating. During the termining the behavior of an transfer for vertical panels are given. first stages of development, object subjected to radiation. Industrial equipment and factors i d u e n c paint producers were slow to If the coating is opaque to respond since many of their infrared, all heat generation ing design of radiant heat ovens, including finishes were not well adapted occurs near or a t the surface, efficiency of radiant heating, are discussed. to radiant baking. Recently, and the metal underneath has however, they, have devoted little to do with the radiation increasing efforts to producing suitable baking enamels. absorbed. On the other hand, if the coating is relatively Most of the present state of advancement is a result of the transparent to infrared, the properties of the metal determine the behavior of the radiated object. For transluscent persevering efforts of a few concerns who have maintained an coatings, the phenomena exhibited are dependent on the open attitude, admitting the limitations while citing the advantages of this type of heating. properties of both metal and coating. Depending upon the linear absorption coefficient and absorptivity of the film and the reflectivity of the undersurface, baking or heating of the Theory of Radiant Heating film may occur from the inside out, the outside in, or uniformly throughout the h. Primarily, industrial infrared units are employed for e l e vating the temperature of material objects as they are exThe temperature distribution throughout the solid depends posed to the radiant energy, either in a batch or continuous on the thickness and thermal conductivity of the material, conveyor-type oven. The information desired in radiant the heat generated near or a t the surface being transmitted heating is contained in an expression that predicts the variaby conduction to the interior of the metal. For relatively tion of temperature as a function of time and independent thick bodies the temperature is nonuniform throughout the parameters such as radiant intensity and the various physical solid during the transitory stages of exposure to radiation. properties of the stock. The rate of temperature rise depends Solution of the problem of nonuniform temperature distribuprimarily on the difference between the energy gain of the tion is a boundary type problem requiring a Fourier series stock by absorption of radiation and the loss of heat by reanalysis for rectangular bodies or equivalent harmonic radiation and convection. If the temperatures of the mateanalysis for objects with different configurations, giving rerial and its surroundings are of the same order of magnitude, sultant bulky solutions. For the thin metallic objects (11) loss of energy by reradiation is negligible. Most of the energy frequently met in industrial practice, both transient effects of losses, then, consist of heat transferred to the surrounding atnonuniform temperature distribution during the initial expomosphere. sure and end effects caused by absorption a t the edges may be neglected. Material 12 gage or thinner can be treated In the theoretical discussion that follows, certain deviasatisfactorily in the manner described in this paper, where tions from actual industrial conditions are made and should the temperature is considered to be uniform throughout the be noted. First, it is assumed that all heat effects are sensible solid. in nature, including heating of the stock and surrounding atmosphere. Processes involving evaporation are excluded. If a thin object with unit surface, specific heat e, and mass Secondly, this discussion is restricted to relatively thin stock m is exposed to a uniform field of radiant energy of intensity as represented in the industrial coating of sheet metal objects I (8), and if the absorbed energy is transformed entirely into ranging from 12 to 28 gage (0.1104 to 0.0174inch) in thickness. heat, the radiant energy thus converted into thermal energy 273
T
*
INDUSTRIAL AND ENGINEERING CHEMISTRY
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in time de is kAId8. Absorptivity A , as used in this discussion, refers to over-all absorption and hence is a function of the reflectivity, linear absorption coefficient, and thickness of both metal and fi.The absorbed energy is shared between sensible heat gained by the stock and heat lost by convection to the surrounding atmosphere. The sensible heat gained in time dB is crndT, and the heat lost by convection in this time is -h(T - To)d8. For stock with unit area having a uniform thickness L and density p, the mass m can be replaced by pL. A differential heat balance gives kAI&
cpLdT
+ h(T - To)&
(1)
Solution for the instantaneous rate of temperature rise gives the differential equation for the process, dT= _IcAI _ d6 cpL
- h(T - Ta) CPL
(3)
Solution for
This is the final equation relating temperature of the stock to time. I n terms of the other variables used, the expression can also be written as
After an article has been baked by radiant heat, it must be cooled to some convenient temperature, usually by natural convection, before it can be handled. While the problem of cooling by convection is old ( I I ) , it is in order to include the expression for calculating the time required for the stock to drop from T,o to some lower temperature To. Since there is no heat input, I = 0, and Equation 2 becomes:
dT de
(2)
A simple integral solution of this equation necessarily involves the assumption of the constancy of all variables with the exception of temperature T and time 8. The other quantities, A, I , c, p, L, and To,that are subject to variation in industrial practice may be kept constant during experimental work. The dependence of the convectional coefficient of heat transfer, h, on other quantities is a matter of speculation. Whether it varies with temperature and radiant heat intensity can be determined only by experiment. One of the objects of this investigation was to obtain approximate convectional heat transfer coefficients and observe their variation as a function of operating conditions. The experimental data indicate that h is essentially constant for a given radiant energy intensity over the temperature ranges investigated and may be used as such in engineering calculations. The radiated solid attains a maximum temperature when there is a balance between the radiant heat absorbed and heat lost by convection to the atmosphere-i. e., when the instantaneous rate of temperature rise vanishes (dT/dO = 0). Designating Tm as the maximum temperature attained, Equation 2 gives:
Vol. 34, No. 7
-L(T CPL
- To)
Initially, when 8 = 0 the material is a t Ta, which may or may not be Tm, and a t time 8 reaches temperature To. Integration between the limits indicated gives the expression for cooling: To = T ,
+ (T&-
Ta)e-hd/cpL
(10)
Since radiant heat is used for raising the temperature of the stock, any energy absorbed and then lost from the system by convection or otherwise represents nonutilieation of the electrical energy; and only that energy finally retained by the stock is useful. The ratio of sensible heat retained divided by the total radiant energy striking the stock is a measure of the utilization of the radiant energy and is here designated as the performance ratio of radiant heating, P . In terms of variables already used,
- To) P = CPL(T kI6 This expression may be rewritten in several alternative forms:
P = A(-)('
- ~ ~ ~ ~ c p L (11B) )
Tmgives: (4)
Equation 4 indicates that the maximum attainable temperature i s independent of c, p, and L except as they may affect h. The absorptivity is solely a function of the surface characteristics, provided the linear absorption coefficient of the coating and material underneath is high. The original differenbid Equation 2 can be rewritten in a more convenient form by combining Equations 2 and 4:
The magnitude of the performance ratio of radiant energy is a maximum st initial exposure to radiant energy when 8 = 0, and decreases as the time of exposure increases. Designating PO as the initial radiant energy performance ratio, Equation 11B yields at 8 = 0: (12)
The initial temperature at which the stock enters the oven is TO. After separating variables and substituting limits, Equation 5 becomes:
Integration yields:
From the definition of P given in Equation 11, it is evident that values for P in excess of unity will arise as long aa the sensible heat gain by the stock exceeds the total radiation incident to the stock-i. e., as long as the net cumulative convection loss from the stock is negative. Additional exposure beyond the point a t which cumulative convection losses become positive causes P to take on progressively smaller values below unity. These facts are illustrated graphically in Figures 9 and 10.
July, 1942
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
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jobs. Various shapes and surfaces were proposed, but none gave entirely satisfactory results. Lenses have been used with success to obtain better control of the dispersion of the radiation. Botb gold and aluminum have been used for reflector surfaces; the use of gold (4) was patented. For some time gold was considered superior to aluminum, silver, and other materials because of its high infrared reflectivity and chemical resistance. However, gold reflectors in service have shown a tendency to deteriorate by alloying with the base metal undercoating and making it essential to replace reflector surfaces periodically. At present gold is falling into disfavor and vaporized aluminum is coming into use. It is currently believed FIGURE 1. TYPICAL OPEN-TYPE REFLECTORS that vaporized aluminum has reflection Front, Eejt to r b h t , Fostoria Pressed Steel Corporation's QR and I R gold-plated infrared characteristics superior to those of gold over reflectors. Back.. Eeft gold-alated reflector, and Dayton . to right. Fostoria KB deea-bowl . the normal reflector life. Manufacturing Company's deep-bowl gold-plated reflector. Reflector contours have been parabolic, spherical, elliptical, and combinations of the three. The trend in design is away Industrial Equipment from the parabolic reflector which gives parallel beams and uneven distribution. At present empirically determined Many sources of infrared radiant energy are available. contours and combinations of parabolic ,spherical, and elliptiOpen hot-wire and strip electric resistors, gas radiators, and cal surfaces are used. Corrugated reflector surfaces are conlamps with relatively low temperature filaments (less than sidered to give a more uniform energy dispersion than smooth 2500' K.) are used for various types of industrial heating. surfaces. If radiation is directed angularly to the reflector Lamps have the advantage over the former three types of axis, cool spots between adjacent units in an installation are radiators in being more flexible and reducing fire hazards. eliminated and irregular-shaped objects are heated more uniSince low-temperature filament lamps were used in this informly. vestigation, the remarks will be confined to this type of enThe depth of the reflectors varies from a few inches upergy source. Details and data concerning the characteristics ward. A greater percentage of the energy is controllable in of commercial drying lamps were assembled by Haynes and deep bowl reflectors since less of the radiation escapes without Oetting (8). being reflected from the bowl surface. Commercial reflector LAMPS. Radiant heating lamps consist of gas-filled bulbs containing either tungsten or carbon filaments designed to diameters vary from 7 to 12 inches. The smaller perimetera permit close packing with resultant higher heat densities operate at 2500' and 2200' K., respectively. Although carbon filament lamps have a higher initial efficiency, they while the larger perimeters supposedly give better distribushow considerable depreciation by blackening after approxition patters. Figure 1shows a few of the open reflector types mately 100 hours of operation. Tungsten filament lamps are commonly employed in radiant heating. considered to be more economical in the long run because of For heating small objects, a convergent beam is sometimes their higher over-all efficiency and longer lie. Indeed, some desirable so that high energy densities may be obtained. R e large installations of tungsten filament lamps have been in flectors with adjustable focal lengths are manufactured to service in excess of 10,000 hours and have required few redirect the beam. While they have certain advantages, it is placements. Replacements are frequently necessitated by mechanical breakage caused by handling, the lamps lasting almost indefinitely at the low filament temperature. Filaments are available in triangular, spiral, and horseshoe patterns. The triangular filament is highly concentrated and is considered to give the best energy distribution. Carbon lamps are available with power outputs up to 375 watts and tungsten lamps up to 1000 watts. Prior to 1940 the majority of installations used 250-watt lamps. REFLECTINQ SURFACE.Proper application of the incandescent heating lamp requires the use of reflectors to direct the radiant beam. Reflector lamps with aluminum reflecting surfaces built into the bulb are now available. No auxiliary reflector is necessary with this type of lamp. The original parabolic design of reflectors was discarded early by some manufacturers after it w a ~ found that the uneven energy C o u ~ t e s y (7. , M . Hall Lamp Company distribution from such reflectors resulted h ; too high a percentage of faulty S s h i n g FIQURE2. SEALED LENS THERMALAMP U S E D I N EXPERIMENTAL WORK
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Vol. 34, No. 7
indications are that the ratio of maximum to minimum intensities may be of the order of five to one on a flat surface parallel t o openreflector lamp banks. The variation of radiant intensity with distance from a single lamp and reflector is peculiar to the unit, depending primarily on the design of the dispersion system. For the type of lamp used in the experimental work discussed here, the radiant intensity decreases almost linearly with the distance removed from the lens surface a t the rate of 0.2 watt per square inch per inch up to a distance of 25 inches, for single lamps. The intensity decrease is a measure of the amount of spreading that occurs. For lamps assembled in a bank, overlapping of the spreadings takes place, Because of this overlapping, the average radiant intensity normal to the bank remains essentially constant, irrespective of the distance from the lamps.
Courtesy, C. M . Hall Lamp Company
I n this investigation two t o eight 1000-watt sealed Thermalamps, as illustrated in Figure 2, were used. Each lamp contained a 1000-watt General Electric G40 bulb with a bioost base and a triangular-tungsten filament. The rim was la-inches in diameter, and the length from the back of the unit to the front of the lens was 16 inches. The sealing lens protected the goldplated reflector surface from fumes and dust, and thus ensured constant reflector characteristics. The only maintenance necessary was cleaning of the outside lens surface with thinner or dilute caustic. The intensity of radiant energy t o which the test panels were exposed was varied by adjusting the voltage across the filaments. Variations of intensity ranging from about 3 to 9 watts per square inch were employed. In this connection it was assumed that the t pe of radiation produced did not change appreciably with the sgift in wattage output and that the absorptivity of the surfaces of the anels remained essentially constant (8), irrespective of the wave gngths produced by the voltages and color temperatures used. Radiant energy intensity measurements were made with a s ecial calibrated instrument consisting of a thermocouple encfosed in an evacuated chamber. The precision of the instrument was within the order of 10 per cent. A simple rapid-reading accurate instrument for determining radiant energy densities is much t o be desired. The test panels were 12-, 16-, 20-, 24-, and 28-gage cold-rolled steel squares, either 6 or 10 inches on the side. In order to obtain surfaceswith different absorptivities, a set of samples was sprayed with thin coats of white, red, blue, and black polymerizing paints of the urea-formaldehyde ty e furnished. by Aul? & Wiborg Cor oration. The paint was iaked under infrared light, and the res&ing dried panels were used in the investigation. In this manner small effects such as evaporation of thinners were avoided. An iron-constantan thermocou le was welded to each test panel before the surface was enamegd. Durin exposure t o radiant energy these thermocouples were connectej to a Leeds & Northrup Micromax controller that balanced every second and thus gave the complete time-temperature test curves desired. In each experimental run A , I , and L were held constant. The density and specific heat of the stock varied but slightly, and average values were used in calculations. Since the coating material was thin (approximately 0.001 inch), the specific heat and density of the stock were taken as that of the under-material. In order that the thermocouples should exert negligible influence on the behavior of the test panels, a relatively large panel area was exposed to radiation, and the relative heat capacity of the thermocouples was kept small. The air temperature, T,, is usually constant in continuous commercial installations; however, serious errors may be introduced and misinterpretations of experimental data may result if the air temperature is permitted to vary during a test run in a typical batch-type experimental apparatus, Variations in air temperature were avoided by placing draft shields around the test oven and by operating the experimental apparatus for a sufficient length of time t o ensure
FIGURE 3. LATEST DESIGNOF SEALED LENSTHERMALAMP
doubtful that uniformity can be obtained in more than one position of the filament. Elliptical reflectors may also be used to focus the energy onto a narrow band. Experimental work on lamps with elliptical reflectors focused to produce a region with a high energy density for soldering and similar applications is in progress. Convergent lenses, though not offered commercially at present, would also adequately serve the purpose of producing unusually high radiant intensities. Deposition of fumes and dust on reflector surfaces reduces their efficiency and causes annoying maintenance problems. A soft thin plating of metals such as gold and aluminum does not stand up well under frequent cleaning. Further, the removal of bulbs during cleaning causes an undue number of broken filaments. In July, 1940, a new type unit with a sealed lens as illustrated in Figure 2 was announced. This unit has been superseded by the improved lamp assembly exhibited in Figure 3. The lens gives a more uniform dispersion of the radiation and also controls the energy emitted radially from the lamp. With this unit no change in the reflector surface occurs since fume deposition is obviated. I n addition, fire hazard is largely eliminated and, when desired, objectionable visible light glare is removed by the use of green glass in the lens. Some absorption of energy occurs in the glass even though its infrared transmitting ability is high. This absorption converts some of the radiation into sensible heat in the lens itself. Elimination of the visible light glare reduces the effective radiant intensity, since visible light also has capacity for producing heat. The average radiant energy intensities obtainable normal to the lamps depend upon the spacing and the wattages o€ the bulbs used and the distance from the lamps. No industrial arrangement of lamps in a bank has given a perfectly uniform energy density over a surface normal to the lamps. A specification of maximum intensity required without stating the minimum allowable intensity is indefinite and can be misleading. In order to obtain fairly uniform results in an oven, it is necessary to carry out an intensity survey throughout the installation and make adjustments in accord with the uniformity required. Very few accurate data are available concerning the actual intensity distribution obtained. However,
Experimental Equipment and Details
July, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE 4. TIME-TEMPERATURE CURVES FOR VARIOUS INTENSITIES ON %GAGE COLD-ROLLED STEEL P A I N T E D BLACK
constancy of conditions before the material was introduced into the field of radiation. When these precautions were observed, no difficulties were encountered in maintainin a relatively constant air temperature. The heat capacity ofthe experimental unit used compared to the total energy radiated is important in this connection. If the heat capacitv of the unit is small, air temperature variations occur as a dire& consequence. During the test period, air circulation was limited to natural convection as governed by the experimental conditions. Thus, in those runs made with higher heat densities, the air Circulation was greater than those made with relatively low heat densities. When the desired equilibrium had been established, the test panels were placed in the oven and held in a vertical position with small clamps. Different results are obtained, depending upon the placement of the panels with respect to the radiation source. Vertically held panels behave differently from those placed horizontally. In the data reported, all panels were supported vertically 8 inches from the lamps. With the thin panels used, the same results were obtained when the material was radiated on one side with a definite intensity or radiated on both sides with half the intensity. Most of the test pieces were radiated on both sides, but data given here include some one-side radiation tests.
Data and Results The directly measurable quantities involved in the timetemperature relations are specific heat c, radiant intensity I , density p, thickness L, and temperatures T and T.. The derivable quantities are absorptivity A and convectional coefficient of heat transfer h. The maximum temperature attainable by the stock, T,, can be observed directly or may be derived from the data if an experimental run is interrupted before the equilibrium thermal state indicated in Equation 3 is reached. Although Equation 4 interrelates T,, T., A , I , and h, the experimental results indicate that for a given material h is a function of I alone when natural circulation prdvails; hence Equation 4 involves essentially T , and I .
911
FIGURE 5. RECTIFICATION OF EXPERIMENTAL DATAIN ACCORDWITH EQUATION 16 Values of Tm calculated by Equation 4 agree with the experimentally observed values in all cases. For determining K = h/cpL and calculating the value of h, both differential Equation 5 and integral Elquation 7 are useful. The rectification of the experimental data in accord with Equation 5 involves measuring the tangents to the temperature-time curves, a relatively inaccurate and tedious process, and the plotting of T us. dT/dO. Equation 5 can be rewritten as (14)
The intercept of the line resulting from the T us. dT/dO plot is T,, and the slope is -1/K. Several alternative methods make use of Equation 7. If, in Equation 7, differences in temperatures, AT, for equal increments of time, A@, are taken, AT = -(Tm - To)[e-K(OSAO) e--KO] (15)
-
Dividing Equation 15 by ( T m - TO)as determined from Equation 7 and then solving for T,
If K is constant, this is the equation of a straight line for T us. AT, with intercept Tm and slope q = -l/(l - e - K A s ) . If the slope, q, is determined from a plot of T us. AT as obtained from experimental data,
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of heat transfer for this series of runs varies from 6 to 9 B. t. u./(hr.)(sq. ft.)(' F.), increasing as the radiant energy intensity is boosted. &though extrapolation of heat transfer data is untrustworthy, the curve extended indicates a value of h around 3 or 4 B. t. u./(hr.)(sq. ft.)(O F.) a t zero radiation intensity, a value approximating free convection coefficients. The conyectional coefficient of heat transfer varied in the same manner with radiant intensity in all the other tests made on black, blue, red, and white panels. Drafts were not sufficiently controlled, however, in many of the runs so that it is not possible to draw quantitative conclusions regarding the variation of h with I. The distribution of data obtained for h us. I as illustrated in the composite plot for black panels of various gages in Figure 6 and other data for the remaining colors, mentioned but not included here in I, WATTS/SQ. IN. chart form, indicate that for thin stock his independent of the thickness of the stock. For a given set of conditions the exFIGURE6. CONVECTIONAL COEFFICIENT OF HEATTRANSFER perimentally observed maximum temperature attained was us. INTENSITY the same, irrespective of the thickness of the stock, for thin materials. According to Equation 4 this can occur only when h is not influenced by L. This method for determining K by differences is superior to The absorptivity, A , may be calculated from the experimental data in accord with Equation 4 as the slope of a plot the method of tangents since the differences may be read directly from the experimental data. of ( T , - T,)us. M/h, when consistent units are employed. Figure 7 gives composite data for three of the four colors and Figure 4 is a graph of a few of the experimental temperacoatings considered. The absorptivities are 0.86, 0.62, 0.59, ture-time data for 24gage (0.0262-inch) steel, black-coated panels; Figure 5 shows these experimental data plotted in acand 0.48 * 10 per cent, respectively, for black, blue, red, and cord with the method described-i. e., T vs. AT for 0.25white. minute time increments. The straight lines obtained indiI n a recent bulletin (1) the infrared absorption characteristics of paint pigments ground in similar vehicles were investicate that K , and hence h, is constant for a given radiant ingated. The order of decreasing absorptivity was found to be tensity I. Other methods for determining K and Tminvolve determining ratios of successive AT values, plotting In AT us. carbon black, iron blue, chrome green, chrome yellow, and B or ln(dT/d5) us. 8. When applied to the data obtained, all white, The chrome yellow had but slightly higher absorptivity than white. However, as small an addition as 0.1 per methods gave the same results. The data presented were cent iron blue to the yellow gave a noticeable increase in heatrectified in accord with the method illustrated in Figure 5, the other methods being used for check purposes. The ing rate. After addition of 10 per cent blue, little further increase in absorptivity was noted. Reds were, in general, values of T , and h as determined in Figure 5 were then inserted in Equation 7, and values of T calculated. These more absorptive than yellows but less so than dark greens. A small amount of black added to white greatly increased the calculated values are plotted on Figure 4 along with the experimental data to illustrate the precision of the rectification. The excellent agreement between the observed and calculated data in Figure 4 substantiates the correctness of the form of Equations 7 and 8 to describe temperature-time relations re1.6 sulting during radiant heating. The values of h may be calculated directly from Equation 17 if slope p is known. Figure 6 gives the values of h plotted 1.4 against I resulting from the data illustrated in Figure 4 and other runs. Points for 24gage A, shown on Figure 6, correspond to the data in Figure 4. The convectional coefficient 1.2
s+
a 1.0
500r
W 0
5I 0.8 LT
u. 0 CT
w 0.6 a
0.4
0.2
'0
0 100
200
300
400
500
kvh
FIGURE7. ABSORPTIVITY CURVES
600
700
AIR TEMPERATURE, DEGREES F:
FIGURE8. EFFECTOF AIR TEMPERATURE ON PERFORMANCE RATIOOF RADIANT HEATING
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July, 1942
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14
I.2
1 . 0 0.8
0.6 0.4 0.2
-30
b
-5
I I
TIME, MINUTES
I 1.5
I
2
0 TIME, MINUTES
RELATIONS us. TIME FIGURE 9. ENERGY
FIGURE10. INTERRELATION OF PERFORMANCE RATIO,TIMHI, TEMPERATTJREI, AND INTENSITY
rate of temperature rise. For incomplete hiding, which frequently exists, the metal surface characteristics were found to be important.
perature in order to obtain a uniform product, or the ingensity should be raised. Maximum utilization of electrical energy is obtained when the air temperature is higher than the stock temperature. In Figure 8 the effect of T. on the performance ratio is strikingly illustrated. Not only are higher efficiencies obtained with high air temperature, but also higher rates of temperature rise. As long as the air temperature exceeds the temperature attained by the stock, the absorbed radiation is completely utilized as sensible heat retained by the stock. The intercepts on the abscissa in Figure 8 represent the minimum air temperatures that can be employed to attain the given stock temperatures under the specified conditions. If infrared lamps are the only energy source, heating of the air arises from nonutilization of relatively expensive electrical energy. Auxiliary air heaters energized by a cheaper means such as gas or oil may be incorporated in the design. The advantages of a combination radiant heat and convectional oven are apparent. The first infrared radiant heating ovens consisted of pipe supports for holding the lamps and were devoid of draft shields or insulation. It was thought that the radiation passed from the lamp to the stock and that all the radiation striking the work was utilized, so that little or no energy losses occurred. Experience modified these original notions, and the present trend is toward well insulated ovens. The proper quantity of insulation depends upon an economical balance between the installation cost and the resultant energy saving. For good results the inside surfaces of the tunnel should be fbished and maintained with a highly reflective
Industrial Utilization of Infrared Radiation For economic baking of industrial finishes with infrared the over-all absorptivity must not be too low. Properties of both h i s h and undersurface determine the absorptivity. Combinations consisting of transparent coatings and highly reflective materials possess low over-all absorptivities, and attempts to employ infrared radiant heating directly to such pairs will meet with little success. If the advantages of infr% red radiation to attain high heating rates are to be wed successfully, close attention must be paid to obtatning finishes with both high absorptivities and linear absorption c@@cients. Further, if it is impossible to obtain coatings meeting these requirements, the material to be finished must possess either an inherently high absorptivity or its surface must be pretreated to yield a satisfactory combination. In starting a radiant heat oven, the lamps deliver their full working intensity almost immediately. However, the air temperature, T., does not reach its equilibrium value until some time later. Therefore, in the initial period of operation in accord with Equations 2 and 4, both the rate of temperature rise and the maximum temperature attainable by the stock will be lower than corresponding values after the oven has operated for some time. During the preliminary period the work should be run through the oven more slowly or auxiliary heaters should be used to increase the air tem-
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Evidently Ta will increase as I is boosted in any industrial oven. Because of the lack of information Figure 10 was constructed with Taarbitrarily selected at 150' F., irrespective of I . Other data used in the construction of the chart are based upon experimental data, the convectional heat transfer coefficients and absorptivity having been taken from Figures 6 and 7. It is to be noted that because T, probably increases as I increases, the efficienciesfor high radiant intensities are somewhat lower than they would be for actual data in industrial installations. The function of this plot is to emphasize the desirability of employing combinations of high energy densities and high air temperatures in radiant heating ovens. According to Equation 5 the rate of temperature rise of thin stock should be more rapid than that of thicker stock. Further, Equation 4 implies that for a given radiant intensity the maximum temperature attained by the material should be the same for all thicknesses. Figure 11 gives a graphical picture of the influence of the thickness on the heating rate as calculated from Equation 10, using the experimentally observed absorptivity and convectional coefficients of heat transfer. BO 0
2
4
6 8 TIME, MINUTES
IO
12
14
THICKNESS ON HEATINQ FIQURE11. EFFECTOF STOCK RATE
material to decrease the quantity of radiation absorbed by the oven itself. The effect of using highly reflective oven walls is to increase the effective radiant intensity and the uniformity of irradiation. Natural circulation of air has been frequently used for removal of vapors. While this is satisfactory in open ovens, some positive means for vapor removal must be supplied in a completely closed oven. If air stratification is undesirable, a downdraft may be used to keep the air temperature more uniform. Excessive air currents increase the coefficient of heat transfer, and if the stock temperature is above Tal the over-all efficiency of the unit is decreased. The performance ratio of radiant heating for a specific material decreases as the time increases, in accord with Equation 13. This decrease is to be expected qualitatively. As Figures 4 and 9 show, most of the sensible heating and the greater portion of the full temperature rise occurs during the first few moments of exposure. During this time convectional losses of heat to the air are low or may even be negative if the air temperature is high. As more time passes, however, the additional temperature rise is relatively small, but the cumulative convectional heat losses continue to increase. The energy input W B increases linearly as time passes so that as the exposure time is increased, the ratio of the energy retained to the radiant energy input decreases. Figure 10 shows the variation of temperature and performance ratio with time for materials of the same thickness and absorptivity radiated a t different intensities. From an operational standpoint Figure 10 points to the use of as high a heat density as permissible, for this condition not only makes for rapid attainment of high temperature and high capacity for an installation, but also results in more efficient use of the radiant energy. A plot of equal performance ratio curves on the temperature vs. time data for a given material radiated with various intensities is valuable for ascertaining suitable industrial operating conditions. I n order to construct such a chart giving these data, it is necessary to know how Ta is influenced by I in the oven. I n the experimental work performed, the air temperature was controlled at a constant value independent of the intensity. Further, at the present writing no industrial data giving the T. vs. I variation are availabIe.
Design of Radiant Heat Ovens Radiant heat ovens are well adapted for multiple heatdensity operation. If low initial temperature rise is desirable for producing wrinkle finishes or for driving off thinners slowly to prevent pinholing, a low-intensity section may be
Courtesz/, C. M . Hall Lamp Company
RADIANT HEATOVEN FIGURE 12. PREFABRICATED This oven contains 48 1-kilowatt lamps, each equipped with a lens. The walls are prefabricated with the lens clamped in place. The reflectors are removable for bulb replacement and have an eye to indicate whether or not the lamp is burning. The oven is roughly 7-8 feet long and 4-5 feet high.
July, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
conveniently employed. This preliminary heating zone can be followed by a greater heat-density section in which high temperatures are obtained. Other combinations of sectionalized design are evident. A combination of views obtained from radiant heat practice indicate that the oven of the near future will consist of radiant heat lamps augmented by auxiliary heaters. Ovens will be completely enclosed and properly insulated. I n some cases auxiliary heaters will be omitted if nonutilized electrical energy maintains oven temperatures a t a sufficiently elevated value. Uniformity of radiant intensity will be demanded, and lenses will probably answer this demand. For convenience, ovens may be prefabricated with the lenses built as an integral part of the wall. In this manner reflectors could be mounted outside the w e n proper where their surfaces would be cooled by air currents. In an enclosed oven of this type there is no need to filter out visible light for elimination of glare. If the visible light is not filtered, less heat is generated in the lens, and the radiant intensity is increased.
Comparison of Conveyor and Batch Operations With the nonuniform intensities frequently existing in commercial units, conveyorized operation is essential for minimizing hot spots. With intensity variations of five to one, a high thermal conductivity of the metal may not be sufficient for smoothing out the nonuniform heating. At. present few installations are in operation that give a sufficiently uniform field for successful high-quality batch operation. More uniform intensities are definitely needed.
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fields of application such as drying, evaporation, and activation should be investigated.
Acknowledgment The authors wish to express their appreciation to P. H. Goodell of the C. M. Hall Lamp Company for his kind cooperation. Reflectors for photographs were furnished by Burton Zook of the Fostoria Pressed Steel Corporation and J. G. Braden of the Dayton Manufacturing Company.
Nomenclature A
= absorptivity of stock, or ratio of radiant energy absorbed and
converted to heat to incident radiant energy, dimensionlees = specific heat of stock, B. t. u./(lb.)(” F.) h = convectional coefficientof heat transfer during radiant heating B. t. u./(hr.)(sq. ft.)(O F.) h, = convectional coefficient of heat transfer during cooling, B. t. u./ (hr.) (sq. ft.) (” F.) Z = intensity of radiant energy incident to surface of stock, watts/ sq. ft. k = factor to oonvert electrical energy to thermal heat, 3.41B. t. u./ (watt) (hr.) K = ratio h/cpL, l/h. L = thickness of stock, f t . m = mass of stock per unit area, Ib. sq./ft. P = performance ratio of radiant heating, or ratio of sensible heat retained by stock to radiant energy incident to surface, dimensionless q = slope of T 88. AT curve, dimensionless T = variable temperature of stock, O F. To = initial temperature of stock, O F. T. temperature of air, O F. T , = temperature attained by stock upon cooling, F. initial temperature of stock a t start of cooling, O F. Too T,,, = maximum temperature attainable by stock, F. = logarithmic mean temperature difference, O F. (T, = density of stock, Ib./cu. f t . p 0 = time, hr. c
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The Future Today radiant heating is in competition with hot air heating, Radiant heating has the advantage over convection ovens in that very high energy transfer rates are obtained, which result in more compact installations. Many claims have been made with reference to relative economies of the two, but only an actual cost balance over installation, maintenance, energy cost, replacement, etc., will tell which is better. Obviously, the conclusions will have regional aspects. The use of heat densities in excess of 2.5 watts per square inch began in 1940 with the introduction of 1000-watt lamps. Radiation with high energy densities is desirable for operation a t maximum efficiency and high capacity. At present 10 watts per square inch is the maximum practical heat density, and this can be obtained only with thin materials by radiating both sides simultaneously. In the future, new fields will unfold as higher heat densities are obtained. It is possible that the present peak intensity will be increased. Many of the first installations were based on guesswork or on laboratory test data obtained under conditions not comparable to those encountered in commercial practice. Numerous modifications became necessary after installation to make jobs more satisfactory. However, with the establishment of adequate testing facilities and customer laboratories in the last two years, better designed radiant heat ovens have been installed. Accurate and dependable methods for determining absorptivities and intensities are needed. Industrial data for coefficients of heat transfer and oven air temperatures are of importance and should be obtained if oven design is to be placed on a more sound engineering basis. As yet, theory has played but a small part in the engineering aspects of radiant heating problems. In the theoretical and experimental phases of industrial radiant heating, it would be desirable to obtain time-temperature solutions for thick and irregularly shaped objects. Other
Bibliography (1) Beakes, H. L., “Heat Absorption of Chemical Pigments Using the Hall Infra-Red Lamp”, Louisville, Kentucky Color and Chemical Co., 1941. (2) Faulkner, J. H.. et al., “Study of Radiant Energy Heating”, Chicago Utilities Research Comm., 1940. (3) Goodall, P. H., Am. Inst. Elec. Engrs., Tech. Paper 40-156 (1940). (4) Groven, F. J. (to Ford Motor Co.), U. 5. Patents 1,998,616 (April 23,1935): 2,057,776(Oct. 20,1936): 2,186,067(Jan.9,
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19411). -_, .
(5) Haynes. Howard, Products Finishing, 5, No. 1, 50-2, 54-6, 5860, 62, 64-9 (1940) (6) Haynes, Howard, and Bennett, H. J., Chem. & Met. Em., 47, 1068 94--, 0). ~ . . 11 _ (7) Haynes, Howard, and Bennett, H. J., Produd Eng., 11, 307 (1940). (8) Haynea, Howard, and Oetting, R.L., General Electric Co., Nela Park Eng. Dept., Bull. LD-16 (1941). (9) Ickis, L. S., Jr., and Haynes, H., Cen. Elec. Rev.,42, 145 (1939). (10) Klinkenstein, G., Metal Finishing, 39,398-403 (1941). (11) McAdams, W.H., “Heat Transmission”, pp. 27-8, New York, McGraw-Hill Book Co., 1933. (12) McCloud, J. L.,IND. Ewo. C ~ n x .33,225-30 , (1941). (13) McCloud, J. L., S. A. E. Journal, 42, 1314T (1938). \--
Synthetic Organic Compounds as Potential Insecticides-Correction In the article printed under the above title in the April, 1942, issue of INDUSTRIAL AND ENIQIN~ERINQ CxmMIsTRY, an error occurs on page 500. Line 10 of the second column should read as follows: 4,6-DMtro-o-cresol Methyl Ether. 4,bDinitro-o-cresol has been, eto. L. E. SMITE
