Tank furnace

The float glass manufacturing process was developed by PHkington Brothers Ltd. of England in 1959. It starts with a large continuous tank furnace... 

The "marble-melt" process consists of producing I -inch l2.5-cenlimeter) marbles by a separate tank furnace, The marbles are then fed lo a bushing unit, which is heated by electrical resistance. From this point, the process is identical to the direct-melt prnccss. 

The product is called water glass, because when solid, it actually is a glass but unlike lime-soda glass (ordinary window glass), it is soluble in water. The process is carried out in large tank furnaces similar to window glass furnaces. The materials are introduced in at intervals, but the products may be drawn off continuously if desired. A mixture of salt cake and coal may replace a portion of the soda ash. As the melt leaves the furnace, a stream of cold water shatters it into fragments. These are dissolved with superheated steam in tall, narrow steel cylinders with false bottoms,14 and the product liquor is clarified.15 Sodium silicates are sold as solutions that vary from the most viscous, 69°Be, to the thinner solu-... 

Chemical and processing towers, condensers. Furnace parts such as retorts and low stressed parts subject to temperatures up to 800 "C. Type 430 nitric-acid storage tanks, furnace parts, fan scrolls. Type 430F-pump shafts, instrument parts, valve parts Products requiring high yield point and resistance to shock... 

FIG. 78. Longitudinal cross section through a melting tank furnace showing flow with strong free convection Trier, 1963). 

The rate of flow in normal tank furnaces is of the order of meters per hour, with a maximum which can exceed lOmph at the surface. In front of the temperature maximum, the rate of surface flow in the longitudinal direction decreases by the elTect of natural convection, increasing again beyond the maximum. The maximum thus acts as a thermal barrier which may be made more effective and stabilized by gas bubbling or by electric boosting in particular with high-pull tanks (with a rapid throughput flow). 

Surface currents can be followed with the use of ceramic, graphite or quartz floaters this is more difficult with currents in the deeper layers where special submerged probes or isotope tracing are employed. Complete data on the flow rate distribution in a glass tank furnace are difficult to obtain by direct measurement so that... 

The use of dimensionless parameters permits direct comparison of melting furnaces of various dimensions and outputs. The concentration distribution measured by means of the indicator provides evidence on the behaviour of the tank furnace during both intentional and undesirable changes in charge composition. 

The intensity of volatilization is not identical in all the tank furnace zones. More intensive volatilization can be expected to occur at the front of the melting zone because volatilization is promoted by the higher concentration of volatile substances in the primary melt, as well as by the carry-over of fine particles of the volatile batch components which are readily converted to vapours. According to the analysis performed by Ldffler (1958) for a sheet glass tank furnace, vapours may constitute even more than 90% of the material lost during melting, only the rest being dust carry-over. 

A recuperative tank furnace of lower output for container glass production is... 

Tank furnaces in the fields of technical and domestic glass production are smaller, having a melting area of the order of tens to several m. ... 

The thermal efficiency of glass melting furnaces is relatively low, in particular that of pot furnaces. Values of 20—35% are reported for tank furnaces with classical heating (see below). Efforts to raise thermal efficiency led to experiments with shaft and rotary furnaces, with fluidized bed melting furnaces, etc. Only electric boosting and all electric glass furnaces have so far found wider practical application. 

Crystal glasses were traditionally melted in pot furnaces, but nowadays small continuous tank furnaces are used. Lead glasses are conveniently melted in Unit-Melter furnaces, lead-free glasses in all-electric furnaces with a daily output of several tons (cf. Fig. 102). Machine forming is being gradually introduced even for these types of glass. 

Gunther R., Glasschmetzwannendfen, Frankfurt, 1954. Glass Tank Furnaces, Soc. Glass Technology, Sheffield, 1958. 

The glass melting process always involves a fairly intensive melt flow it must be taken into account when melting in pots or tanks, but is of special significance in the case of continuous tank furnaces where it is a prerequisite of correct furnace function. Melt flow has the positive effect of accelerating the mass and heat transfer. Increased corrosion of refractories and possible carry-over of unmelted batch into the refining and working zone are its undesirable consequences. 

Flow in continuous tank furnaces is considered to be composed of two principal types (l) throughput flow, i.e. movement of glass from the batch charging end to the working end, and (2) circulating or secondary flow. Both types have the character of laminar flow, since Re < 1. The former is based on forced convection, the latter on free (natural) convection. ... 

The glass tank furnace is essentially a continuous-flow reactor where it is desirable to know not only the kinetics of the main chemical and physical processes, but also the characteristics of mass transfer through the reactor. 

In practice the type of flow can be determined by introducing into the system entry a suitable indicator at a certain time either instantaneously (pulse signal) or permanently (step signal) and by measuring its concentration at the exit as a function of time. Radioactive substances or small amounts of ZnO, CeOj, etc. are used for this purpose in glass tank furnaces. 

A number of studies published in recent years made use of the response characteristics. On the basis of measurements on 13 tank furnaces with outputs of 2 to 90 tons/day (heated in various ways) Smreek (1973) found that a distinct temperature maximum, e.g. one boosted by electric heating, prolongs significantly the minimum residence time which amounted on average to 0.12 of the mean residence time. The proportion of stagnant zones, situated mostly at the working end and at the tank bottom, amounted to up to a quarter of the tank capacity. 

FIG. 83. The response curve for a pulse change of concentration for net transfer flow in a tank furnace (Cooper, 1959). 

The use of mechanical stirrers (forced convection) is a special case of intensive homogenization. In practice, the method has been used for many years in the manufacture of optical glasses and has more recently also been introduced into tank furnaces for other glasses. Quantitative evaluation of the mixing effect has so far only been carried out on models and in laboratory melting (e.g. Cable and Hakim, 1972). 

FIG. 88. The change in radius of an Si02 particle during passage through a tank furnace along the fastest path (Goldberg, 1972). 

FIG. 89. Decrease of Si02 concentration at the centre of a spherical inhomogeneity in a tank furnace at lower (/) and a higher (2) pull the dashed line indicates the position of maximum temperature (Goldberg, 1972). 

FiG. 90. Response curve to pulse introduction of ZnO into a tank furnace Joosen, 1973). 

FIG. 96. A cross-fired tank furnace for container glass production (after Gunther, 1954, niedified). 

According to Kircher (1978), the concentrations of dust emissions in the melting of ordinary glasses in tank furnaces amount to 150 — 300mg of flue gases... 

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