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. In some cases auxiliary heaters will be omitted if nonutilized electrical energy maintains oven temperatures at 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
