The glass bottles on the market are divided into four categories according to their color: colorless, light cyan, emerald green and brown. Regarding the color of glass, people are more concerned about its product properties and the chemical reaction phenomena in the production process; and for the physical phenomena related to the color of glass in the production process, such as the relationship between the color of glass and heat transfer and the difference in heat transfer process to the production process Problems such as the impact of the lack of necessary research. Here, based on the production practice experience and some test data, we will make a shallow discussion on the above issues for the reference of our colleagues.
1, glass color and "heat transfer difference" Different color glass for near ultraviolet, visible light and near-infrared light with different absorption capacity. The light transmittance curves of several color glasses are shown in the figure. The transmittance curves of several color glasses can be seen from the figure. The brown glass has a strong absorption ability for visible light at a wavelength near 400 nm, and exhibits a maximum at 400 nm. The emerald green glass has a strong selective absorption capability for visible light at wavelengths around 440 nm and 630 nm, and exhibits a maximum at 440 nm and 630 nm. The light blue glass absorbs weak light and has a minimum at 500 nm. For the absorption of light in the near-ultraviolet region, brown glass is strong, emerald glass is moderate, and pale cyan glass is weak. For the absorption of light in the near-infrared region, there is a certain difference between the three color glasses, but this difference has gradually narrowed. When the color quality is the same and the color concentration is different, the absorption ability of light and the change of color concentration are in a positive proportion relationship. In the glass production process, light radiation and heat transfer are two indispensable physical phenomena. For glass of different colors, the ability to absorb radiation is stronger, that is, the stronger the ability to absorb high-temperature radiant heat, the more heat absorbed by the glass surface, and the less heat transmitted through the vitreous body in the form of radiation. This difference in heat absorption capacity due to the difference in the color of the glass and the difference in the heat transfer capacity are what we call the “heat transfer differenceâ€. In the glass production process, when the glass color changes or the glass color concentration changes, the conditions of the melting, molding, annealing and other processes will change significantly. Only by understanding the "heat transfer difference" of different color glass, and taking corresponding countermeasures due to the technological changes brought about by the "heat transfer difference", can the production process be effectively controlled.
2, glass color and melting process in the glass furnace, there are radiation, convection, conduction three forms of heat transfer. The change in the color of the glass has a crucial influence on the heat transfer form and heat transfer efficiency. As far as the melting process is concerned, the effect of changes in glass shade on the process conditions is much more pronounced and much more severe than the effect of glass composition changes. There is a big difference in the temperature distribution of different colors of glass in the furnace. For comparison, a 48m2 fuel horseshoe flame furnace is used as a reference furnace, the furnace body is fully insulated, no bubble device is installed, the melting pool is 1200mm deep, and the temperature distribution monitoring of different colors of glass is performed by using a plumb line as a plumb line. At the same melting temperature, there are obvious differences between the liquid surface temperature and the bottom temperature of different colors of glass. From the liquid surface temperature point of view, there is a corresponding relationship with the “difference in heat transfer†of the glass color. The brown glass has the highest heat absorption capacity and the highest liquid surface temperature; the emerald green glass takes the second place, and the light cyan glass takes second place. From the bottom of the pool temperature, the problem becomes a little more complicated; the situation of light cyan glass is easy to understand because it has a poor ability to absorb radiated light. The amount of heat that is transmitted to the bottom of the pool through the vitreous radiant system is high, so the bottom temperature Higher; emerald green glass is also easy to understand, it has a stronger ability to absorb radiated light, and transmits less heat through the vitreous body to the bottom of the pool by radiation, so the bottom temperature is lower. However, the situation of brown glass is somewhat difficult to understand. Its ability to absorb radiated light is extremely strong. Why is the temperature at the bottom of the pool higher than that of emerald green glass? What causes this phenomenon of "abnormal heat transfer?" This may be the case: we can divide the vitreous body in the material pool into several liquid layers. Since the light transmission ability of the brown glass is weak, the amount of heat transferred from the upper liquid layer to the lower liquid layer by radiation is less, so each The temperature difference between layers is large, and there should be a large temperature gradient along the depth of the pool. However, due to the fact that the brown glass has a very good heat absorption capacity, the upper glass liquid absorbs heat, and the temperature rises and the volume expands, generating a thrust toward the periphery in the horizontal direction. This thrust is transmitted to the lower liquid layer through the change of the wall of the pool. Convection forces are formed. The lower glass liquid is gradually compressed by the convection force, absorbs heat during the rise, and expands in volume, generating thrust to the surroundings, which in turn enhances convection. The strong heat absorption capacity of brown glass leads to a strong convection in the melting pool. The enhancement of convective heat transfer compensates for the lack of radiation heat transfer. This is the reason why the brown glass bottom temperature is higher. In general, under the same process conditions and temperature regime, melting glass brown glass can obtain better glass uniformity and higher melting rate than glass with the same basic composition and different color. The reason for this is precisely the strong convection caused by the strong endothermic ability of brown glass. Of course, the existence of a bubbling device is avoided here. The intervention of the bubbling device will change the heat transfer conditions, and the function of the bubbling device is precisely to enhance convection. When melting green glass, installing bubbling devices is an effective measure to increase the bottom temperature, glass uniformity and melting efficiency. When the same color liquid is to be replaced in the same melting furnace, the process elements of the melting section, working section, and feed channel must be adjusted accordingly to adapt to changes in process conditions caused by the "heat transfer difference" of the glass color. .
3. Glass color and forming process In the glass forming stage, the heat transfer between the inside of the preform and between the blank and the mold is mainly conducted by means of radiation and conduction. In this temperature range, radiant heat is mainly transmitted in the form of visible light and near-infrared light. At this time, there are considerable differences in the heat absorption capacity and heat transfer capacity of different color glass, and this “heat transfer difference†will inevitably exert an important influence on the heat transfer process. We can think of the prototype and the bottle as a whole combination of several glass layers. During the entire molding process, the temperature of the glass layer in the middle is the highest, and the temperature of the inner and outer layers contacting the process gas and the mold is the lowest, and the heat is constantly transferred from the high temperature layer to the low temperature layer in a radiation and conduction manner. The color of the glass is different, and the speed of heat transfer by radiation is different. The order of heat transfer speed is: colorless, light cyan, emerald green, and brown. Herein, the concept of "skin hardening speed" is introduced, which is one of the important factors for the left and right mold cooling strength and bottle forming speed. For different shades of glass, the faster the speed of heat transfer, the slower the hardening of the surface layer, and the slower the rate of heat transfer, the faster the surface hardening. During the molding process, the billet undergoes process steps such as air blowing, inverted blowing, reheating, extension, and positive blowing. Heat is constantly transferred from the inside of the glass body to the surface layer of the glass, and heat is transferred to the space around the mold through the glass surface layer. The mold and process gas absorb heat and gradually cool the glass body. When the glass surface hardens and is sufficient to maintain the shape of the glass body, the bottle is molded. In fact, when the bottle comes out of the mold, the bottle body temperature of the glass of different colors is quite different. The colorless glass has a faster heat transfer rate and more heat release. The temperature of the bottle body is lower when the mold is released. The brown glass has a slower heat transfer rate and emits less heat; the bottle body temperature is higher when the mold is ejected; light blue and emerald green Glass is between them. When a colorless glass bottle is produced, a high cooling strength is required, otherwise the bottle is easily deformed; when a brown glass bottle is produced, a lower cooling strength is required, otherwise the bottle is prone to cold spots and surface microcracking. There are two main ways to adjust the cooling intensity, namely mechanical adjustment and process adjustment. Mechanical adjustments include adjusting the amount of process gas, adjusting the amount of cooling air for the mold, and adjusting the operation timing of the mechanism; the process adjustment includes adjusting the speed of the machine, adjusting the temperature of the dropping material, and adjusting the shape and weight of the preform.
4, glass color and annealing process In the glass annealing stage, radiant heat is mainly transmitted in the form of near-infrared. At the annealing temperature, the radiant heat transfer capacity of brown glass and emerald green glass is basically the same, while the radiant heat transfer capacity of light cyan glass and colorless glass is higher. Under the same equipment conditions, there are differences in bottle body temperature when the bottles of different colors enter the annealing kiln. The brown bottle temperature is higher, the emerald green bottle is next, and the light blue bottle is secondarily. Therefore, in the heating of the heat-preservation belt, the externally-compensated heat required for different color bottles is different. When using an electrically heated annealing kiln, differences in the power consumption of bottles of different colors can be found. Of course, this will not have a significant effect on the elimination of stress in the glass. In slow cooling belts, colorless and light blue bottles release heat faster, while brown and emerald bottles release heat more slowly. Therefore, when the slow cooling zone enters the quench zone, bottle bodies of different colors have slightly different bottle temperatures. In the subsequent cooling process, thermal radiation gradually enters the far-infrared region. There is no significant difference in the heat release rate of different color glass, but the previous temperature difference of the bottle body will have a certain impact on the cooling process. When the glass color changes, due to the requirements of the cold-side spraying process, in order to obtain the appropriate kiln door outlet temperature, the cooling zone temperature system needs to be adjusted accordingly.
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