INDUSTRIAL AND ENGINEERING
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surface but drops to about 1600” a t the bottom, with a probable average of 2300” F. Then, for example, if the flame has 90 per cent emissivity but is 100” lower in temperature than the reference standard of 2900” F., the heat transfer is 0.90 X [(2800 1.00 X [(2900
+ 460)4 - (2300 + 460)‘] = o.71 o1 710J, + 460)4 - (2300 + 460)4]
and the heat transfer (or equivalent emissivity) of the entire system (flame plus roof) is the value to be read from the solidline curve of Figure 3 for 71 per cent flame emissivity, or 92 per cent. ABSORPTION OF R ~ D I A T EHEAT D BY GLASS. The distribution of intensity of radiation from a black body, over the heat spectrum, is shown by the Planck curve in Figure 9. The radiation from luminous flames is of similar nature. Konluminous flames, however, radiate only in the carbon dioxide and water bands, which are shown section-lined in Figure 9. I n the lower part of the figure is also shown the percentage of radiated heat of various wave lengths transmitted through one foot of glass; based on data by Gehlhoff (1) for transmission through cold glass. (On that account, as well as on account of the small thickness of the specimens from which the data were obtained, this curve must be regarded as illustrative only, and subject t o great uncertainty as to the numerical values.) By planimetering the areas under the curve, it was found
CHEMISTRY
Vol. 25, No. 8
that 14 per cent of the radiation from a luminous flame would penetrate through the glass to a depth of one foot, while only 5.5 per cent of the radiation from a nonluminous flame would pass through one foot of glass. As far as radiation from the flame alone is concerned, therefore, the luminous flame is about two and a half times as effective as a nonluminous flame of the same temperature in causing penetration to this depth. As regards the effect of the roof or cap, that part of the radiation from a nonluminous flame nThich is reflected from the roof is also in the carbon dioxide and water bands and, therefore, is a l m e t entirely absorbed in the top layers of the glass. That part which is absorbed and reradiated from the cap is full-spectrum radiation, but, since the cap has a lower temperature than the flame, the peak of the Planck curve (Figure 9) lies farther to the right; hence, the penetration of heat radiated from the roof is less than that of radiation from a luminous flame. LITERATURE CITED (1) Gehlhoff. G.. J. SOC.Glass Tech.. a. 150 (Dec.. 1928). (2) Heilrnan; R.’H., Trans. A m . So;. M e & . Engrs., Fuel? Steam Power, 51, 287-302 (1929). (3) Sohack, A., 2. tech. Physik, 6, 530 (1925). (4) Sherman, R. A., paper presented before the meeting of the Bm. Soc. hlech. Engrs., Pittsburgh, Pa., Feb. 17, 1933. RECEIVEDApril 6, 1933.
Methods of Melting Glass S. R. SCHOLES, !Yew York State College of Ceramics, Alfred University, Alfred, N. Y.
T
HE meIting process is a step of paramount importance in glass manuf a c t u r e . The g l a s s melting tank, a reverberatory furnace, is the most e f f i c i e n t d e v i c e thus far developed for the purpose. It is e s p e c i a l l y suited for l a r g e - s c a l e production or for s u p p l y i n g automatic machines.
;Clodern furnaces, classified as to pot and tank furnaces are described in regard to design, efjkiencies, application lo ware made, merits, and defects. Recent deceloPmerlts and imProcements are discussed. Temperatures and their measuremen! are treated f r o m the standpoints of effectit,eness and control. The melting mechanism is discussed with a description of the behavior of the batch ingredients and their fusion into th? clear homogeneous silicate solution-the ideal glass.
TANKS FOR MELTINC GLASS Two types of tanks are in use in America: one having a single chamber, alternately filled and emptied, and the continuous tank, in which the glass remains a t a constant level, replenished by raw material fed into the melting chamber a t the same rate that glass passes into the working chamber and is drawn out for production of ware. The day tank is a simple, rather inefficient unit, of no great importance in the industry. Continuous tanks are made in considerable number and variety. For the production of all sorts of hollowware and tableware, by hand and by machine, the tank takes a somenrhat standardized form. The melting chamber averages 18 feet in width, with a length up t o twice its width. Greater length seems useless because of the difficulty in massing forming-machinery enough a t the working end of the tank to draw it up to capacity. The glass passes from the melting chamber through an orifice or channel called the “throat,” through a bridge wall, across an air space through another bridge wall into a separate compartment, the working chamber. The throat may be located a t the bottom level, or its
channel may be a c t u a l l y depressed below the bottom of the tank. The usual depth of glass in the tank is 42 inches, Deeper and longer tanks are in u s e f o r m e l t i n g w i n d o w glass, where complete freedom from b u b b l e s i s d e s i r a b l e . T h e b r i d g e wall and throat are omitted, and the g l a s s is s k i m m e d by passing under a sort of (‘boom” c o n s i s t i n g of f ire-clay “floaters” extending across the t a n k . T h e s e are canoe-shaped affairs, n o t c h e d into each other a t the ends and held in position by the slow stream of glass. They extend some 15 inches below the surface and serve to hold back unmelted material. Depths of 5 feet and lengths of 150 feet are not unusual in these tanks. JF70rkingends are especially adapted to the type of drawing machines employed. The regenerators run parallel to the tank, on both sides, the length of the melting chamber. Each air regenerator is 4 to 5 feet wide, and about 6 feet high, stacked with checker brick in such a manner that the free cross-sectional area for the passage of gases is about 40 per cent of the total. Regenerators for the producer gas are somewhat narrower than those for air, built in battery with the latter, and in all respects similar. Flues called “uptakes” rise from the regenerator crown a t 6-foot intervals, and these are connected by horizonkal necks t o port openings in the jamb walls of the furnace. The ports are ordinarily 24 inches high by 30 inches. Through these ports, streams of flame pass across the bath
August, 1933
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INDUSTRIAL AND ENGINEERING CHEMISTRY
of molten glass, spreading laterally and vertically t o occupy a large portion of the combustion space, and extending entirely across the tank. Reversal of the valves a t 20- to 30minute intervals maintains the regenerators at a high temperature for the preheating of fuel gas and air. Heat economy and high flame temperatures are sometimes provided for by recuperation instead of regeneration. I n the recuperator, the spent fire gases leaving the tank pass through a number of small flues, whose thin conducting walls transfer heat to the incoming air, which passes in countercurrent fashion through channels interwoven with the hot gas flues. Recuperators are suitable for small units but are seldom applied to large continuous tanks. The jamb walls. carried on steel members outside the block walls, rise to a height of 2 feet above the glass level or "metalline." The crown, resting on the jamb walls, arches across the tank, rising in the center to 5 feet above the glass. There is accordingly formed a combustion space that is high enough to permit the flames to expand altogether too far vertically. The arched crown is simple, inexpensive, anti lasting but very ineffective in holding the flame to its work of delivering heat to the bath. Consequently, improvements are being sought in the adaptation to glass tanks of the flat, suspended crown, already successful in boiler furnaces. The combustion space may then be adjusted according to need, rather than accepted because of the exigencies of masonry construction; and with a shallower combustion space, forcing the fire into closer proximity t o the bath, better heat transfer must be expected. The efficiency of glass melting in tanks hardly ever exceeds 15 per cent, and this figure can be reached only by drawing the tanks to the limit of their capacity for making clear glass. The remaining 86 per cent of the heat generated by the fuel is lost, partly in stack gases but mainly by radiation from the huge areas of exposed surfaces-bottom, side walls, crown, regenerators, and uptakes. And yet the melting operation remains one of the lesser items of operating cost, for, with good producer operation and coal a t a reasonable price, a ton of glass can be delivered a t the working end of the tank for a fuel cost of two dollars. Therefore the problems of fuel economy are not regarded as urgent, although the cost of heat, in the aggregate, is enormous. Tanks for glass melting are, in general, much more economical than pots, both in fuel and refractories. They are essential for continuous machine production. While the quality of glass melted in them is surprisingly good, it is inferior to pot glass in freedom from bubbles, although it is often more homogeneous and less apt to suffer from clay contamination. The amazing development of automatic machinery for glassworking in the past two decades has stimulated improvement in tank design. Refractories have been bettered to meet the requirements of greater tonnage and higher temperatures. Ports have been redesigned to secure the combustion of more fuel per unit area. Interesting facts have been learned about the effectiveness of narrow and shallow tanks for small units. Serious experiments have been made with electrical melting in the attempt to overcome the obstacles of high cost of electrical heat and the untoward effects of the reducing atmosphere created by the arc. TEMPERATURES FOR GLASSMELTING Glass melting is of necessity carried on a t very high temperatures. Molten glass is a viscous liquid a t best, and, in order to hasten the rise of bubbles through it, the melt must
87 1
be a t the maximum heat the walls of the container can endure. For closed pots, 1400" C. is the practical limit. Open pots, having no crown to be considered, may be somewhat hotter. In tanks, temperatures have been consistently stepped up as tonnage demands grew, until 1600" C. is now quite ordinary, with occasional instances of the more refractory glasses requiring even higher ranges. The working temperatures are much lower, varying with the nature of the glass and the character of the ware made. Nelting temperatures are controlled by reference t o pyrometers. The two principal types are the thermocouple potentiometer and some form of optical instrument. Platinum-rhodium thermocouples are enclosed in protection tubes of mullite porcelain and installed vertically in the crowns of tank furnaces, or horizontally in the pillars between the arches of pot furnaces. Holes for this purpose are provided in the silica crown brick or the fire-clay pillar blocks, extending within an inch or so of the inside. Thus the thermocouple is not exposed to the full heat of the furnace. There is a corresponding lag between the indicated temperature read from the galvanometer and the true temperature of the furnace interior. To ascertain the amount of this lag and to check the performance of the thermocouple as a guide to the furnace man, frequent readings on the furnace interior must be made with an optical or a radiation pyrometer. Even though absolute measurements may not be obtained by these combined methods, they serve to establish a certain uniformity of firing, which is the important consideration. Excessive heat for a few hours may result in rapid disintegration of tank walls or in the actual collapse of pots. On the other hand, too low temperatures may cause great loss by the production of "seedy" glass, from which the bubbles have not risen. Therefore, the modern factory is generous in provision for instruments, and plant electricians are as well acquainted with pyrometers as n-ith motors.
MECHANISJI OF MELTISG The glass pot, particularly the closed pot, is not a t full heat when it receives its charge of raw material and cullet (waste glass, broken, or bad ware). Rut in the tank, whatever the method of filling, the charge arrives directly upon the molten bath, exposed to the full heat of the furnace. However great the resulting difference in speed of melting, the essential phenomena are the same. The alkali members of the batch begin almost immediately t o fuse. There ensues reaction with sand, and the alkali silicates, forming eutectics liquid as low as 800" C., are produced. Lime and other bases begin to find their complement of silica and enter solution or perhaps double silicate formation with the alkali silicates. The excess of silica now begins to dissolve, as the melt becomes hotter and viscosity lessens. Meanwhile, gases are liberated from carbonates and hydrates, nitrates and sulfates. The mass is violently agitated by these escaping gases, and this is an aid to complete mixing. The cullet entering with the charge has also played its part (a quiet one) in helping to dissolve the less fusible ingredients. Complete fusion and solution of the materials having been effected, there remains the more difficult and time-consuming process of plaining, or freeing the melt from bubbles. Large bubbles rise much more rapidly than small ones because of their greater buoyancy compared with their resistance to movement. Large bubbles, by coalescing with each other and with smaller ones, tend to sweep the melt clear. Hence it is desirable that the fusion process, which evolves gases from the raw materials, should proceed a t a rapid rate, once started. This demands rapid transfer
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of heat to tlie charge, wlticli operates to reduce the viscosity of the melt a.s it forms and thus to accelcratc further the escape of bubblcs. Hence the recognition by glassniakers of tile advantage in working for rapidity of nrelting. For, if melts are made slowly a t moderate temperatures, a stage is reached where the large bubbles have escaped, but the glass is full of fine bubbles, or seeds, which go out very slowly, in spite of the subsequent application of higher temperaturc. Definite limitation upon bigh temperature is placed hy the increased attack of the glass upim the container. Clay in a finely divided form is quite readily incorporated in a glass melt. Clay in a solid form, as in a tank block or a pot wall, is also soluble in molten glass only at a reduced rate. As the softening temperature of the clay is approached, the solubility rate increases enormously. It may be estimated that tank blocks a foot thick, which are corroded to dangerous thinness in 12 months by glass at an operating t c m p e r a k e of 1400” C., will be equally worn away in G months if the operating temperature is 15W” C. Moreover, we are coming to realize that glass rctains dissolved gases. Prolonged treatment at high temperatures, following the copious development of gas during the initial fusion, may defeat its own object by causing the release of these dissolved gases in very small bubbles. Chemical means for promoting the plaining or fining of glass include tire design of compositions to reduce viscosity and the incorporation of batch iiigredients whicli will generate gases in relatively large volume a t late stages in the melt. In the adjusting of compositions to secure greater fluidity, due attention must be paid to the character of the glass at working temperatures, to its physical properties in the finished state, and to its chemical resist.ance. Borax has come into favor in recent years because the presence of as little as one per cent of boric oxide in soda-lime glasses has a powerful influence on their plaining rate and even enhances their desirable properties. Salt cake, which is not decomposed by silica except at very high tempera-
Vol. 25, No. 8
tures, is likcaise a favored reagelit. Salt cake has the further desirable effect of forming a floating layer of molten sulfate on thc bath, which prevents the formation of a siliceous scum that would blanket tlle bath against radiant heat. Interesting experiments with ammonium sulfate have been carried on in England, and this may prove to be a valuable addition to batches. In batches which are to be melted in pots, niter and arsenic are included. The niter, since it nielt,s vefv readily, accelerates the start of the fusion process and also acts as an oxidizer tolrard tlie arsenic. The resulting arsenic pentoxide probably remains in the glass as an alkali arsenate, possessing a positive decomposition pressure. The oxygen n-hich it eolves, in later stages of the melt, entering tlie small residual bubbles, enlargcs them so that their escape is facilitated. The older glassmakers, operating at temperatures below those in use today, resorted to various expedients for agitat,ing pots of seedy glass. Sometimes they thrust in pole made of green wood and stirred the liquid; a block of water-soaked wood, impaled on an iron rod, or a potato similarly applied, immersed in the glass, would also generate volumes of steam bubbles which swept out the stubborn seeds. Echoes of these practices survive in what is now called “blocking.” Modern optical glassmakers prefer ammonium nitrate, wrapped in wet paper and thrust down toward the bottom of the pot. I n Germany, lumps of vitreous arsenic are sometimes tlirown into a pot of glass. Owing to its high density, the arsenic sinks into the glass. Then it vaporizes rapidly, producing the desired purging effect. Packages of powdered arsenic in wet paper are also used in the blocking operation. The aims of the melting process, by whatever means attained, are to produce glass that is free from visible bubbles, striae, or s t o n e e t h a t is, a clear and homogeneous liquid. To establish practices that will reach this ideal more consistently is one of the great problems of glass technology. R ~ c e r v eMaroh ~ 28. 1983.
(Refer to July h u e , pp. 742-764, for additional nympaaium papera.)
OARLAND, CALIP.,PLANTOF HAZE~ATLAS GLASSCOXPANY
