Enamel heating

If only battery-grade sulfuric acid is available, it must be concentrated before use to a specific gravity of 1.833 by heating it in an enameled, heat resistant glass or porcelain pot until dense, white fumes start to appear. See paragraph 0103 for details. 

Fig. 19.14. Polarized infrared absorption spectrum ofthe carbonate ion that has replaced hydroxyl ions in the apatite lattice. (A) A 100- i longitudinal section of enamel heated at 1100°C in air for 2 hr. (B) A 50-/t longitudinal section of enamel heated at 900 C in CO2 for 30 min. (Elliott, 1965.)...

Tube Furnace. A type of furnace, particularly for vitreous enamelling, heated by tubes of heat-resisting metal in which gas is burned. 

For temperatures up to 100°, a water bath or steam bath is generally employed. The simplest form is a beaker or an enamelled iron vessel mounted on a suitable stand water is placed in the vessel, which is heated by means of a flame. This arrangement may be used for non-inflammable liquids or for refluxing liquids of low boiling point. Since numerous liquids of low boiling point are highly inflammable, the presence of a naked flame will introduce considerable risk of fire. For such liquids a steam bath or an electrically-heated water bath, provided with a constant-level device, must be used. If the laboratory is equipped with a... 

Fluoridation of potable water suppHes for the prevention of dental caries is one of the principal uses for sodium fluoride (see Water, municipal WATER treatment). Use rate for this appHcation is on the order of 0.7 to 1.0 mg/L of water as fluoride or 1.5 to 2.2 mg/L as NaF (2). NaF is also appHed topically to teeth as a 2% solution (see Dentifrices). Other uses are as a flux for deoxidiziag (degassiag) rimmed steel (qv), and ia the resmelting of aluminum. NaF is also used ia the manufacture of vitreous enamels, ia pickling stainless steel, ia wood preservation compounds, caseia glues, ia the manufacture of coated papers, ia heat-treating salts, and as a component of laundry sours. 

Types of internal enamel for food containers include oleoresins, vinyl, acryflc, phenoHc, and epoxy—phenoHc. Historically can lacquers were based on oleoresinous products. PhenoHc resins have limited flexibiHty and high bake requirements, but are used on three-piece cans where flexibiHty is not required. Vinyl coatings are based on copolymers of vinyl chloride and vinyl acetate dissolved in ketonic solvents. These can be blended with alkyd, epoxy, and phenoHc resins to enhance performance. FlexibiHty allows them to be used for caps and closures as weU as drawn cans. Their principal disadvantage is high sensitivity to heat and retorting processes this restricts their appHcation to cans which are hot filled, and to beer and beverage products. 

Polymers based on trimellitic anhydride are widely used in premium electromagnetic wire enamels requiring high temperature performance. Several types of trimellitic anhydride-derived polymers are used as wire enamels poly(amide—imide)s (133), poly(ester—imide)s (134), and poly(amide—imide— ester)s (135). Excellent performance characteristics are imparted by trimellitic anhydride-based polymers for wire enamel requirements of flexibiUty, snap, burnout, scrap resistance, heat shock, and dielectric strength. 

Enameling meets decorative as weU as protective requirements. Ceramic enamels are mainly based on alkaH borosiHcate glasses. The part to be enameled is dipped into or sprayed with a sHp, ie, a water suspension of glass fragments called frit. The sHp coating is dried and fused in an enameling furnace under careful heat control (see Enamels, porcelain or vitreous). 

BaCl2 is used in heat treating baths because of the eutectic mixtures it readily forms with other chlorides. The melting points of some eutectic mixtures are BaCl2 KCl, 672—680°C BaCl2 N Cl, 39 mol % BaCl2) 654°C 631°C. BaCl2 is so used to set up porcelain enamels for sheet... 

Porcelain enamels meet a variety of performance characteristics required for different appHcations. The common characteristics of all enamels include good adherence to the substrate and good thermal expansion fit to the metal. Specific properties depend on usage for example, acid and alkaH resistance, hot water resistance, abrasion resistance, thermal shock resistance, high gloss, high reflectance, specific color, heat resistance, and cleanabihty. 

Thermal shock failures using water result from the water vapor entering the enamel layer through small, submicroscopic cracks formed at the instant of shock. The water condenses in the cracks and in the bubbles of the enamel traversed by the cracks. On subsequent heating, the vapor from the entrapped water expands to cause spalling of the enamel layer. Other quenchant Hquids, such as toluene, oils, and other organic Hquids, also cause fine, almost invisible cracks, but thermal shock failures do not result with these quenchants on subsequent heating (39). 

In counterflow movement, heat from the outgoing sohds is transferred directly to cold incoming solids, reducing heat losses and fuel requirements. Continuous conveyor ovens are employed also for diy-ing refractoiy shapes and for drying and baking enameled pieces. In many of these latter, the parts are suspended from overhead chain conveyors. 

In enamelled tanks with protection electrodes of low current output, fittings [e.g., heating surfaces (cathodic components)] must be electrically isolated from the tank and the ground. Figure 20-2 shows such a bushing. Smaller cathodic components which take up only negligible protection current (e.g., temperature probes) do not need to be insulated. 

Fig. 20-2 Insulated installation of heating pipes in enamelled containers with electrical separation of container and ground.

Fig. 20-10 Potential-time curves for a copper tube heating element in an enamelled container with brass screws. Curve 1 potential in the voltage cone of the brass screws. Curve 2 potential outside the voltage cone. Tapwater with X (20°C) = 100 nS cm- 50°C.

The size of the balancing resistors must usually be decided empirically. A resistance of 500 Q. is usually sufficient for heating elements of up to 2.5 m in surface area. With larger surface areas, it must be raised to 1000 Q. because otherwise cathodic protection at defects in the enamel is endangered. 

The action of different balancing resistors can be seen in Fig. 20-11. The enamelled water heater had an electrically isolated heating element of stainless steel with a 2.5 m surface area. With R = 600 Q, the tank can no longer be cathodically protected at / = 800 D, on the other hand, all is well [13]. The potential of the... 

Fig. 20-11 Potential-time curves of an enamelled container with built-in stainless steel heat exchanger as a function of equalizing resistance, R. Curve 1 container potential in the region of the heat exchanger. Curve 2 heat exchanger potential in the voltage cone of defects in the enamelling. Curve 3 heat exchanger potential outside the voltage cone of the defects.

The first commercial applications of polypyromellitimides were as wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML, Du Pont). In film form (Kapton) many of the outstanding properties of the polymer may be more fully utilised. These include excellent electrical properties, solvent resistance, flame resistance, outstanding abrasion resistance and exceptional heat resistance. After 1000 hours exposure to air at 300°C the polymer retained 90% of its tensile strength. 

Einbrennemaille, /. baking (or stoving) enamel, einbrennen, v.t. burn in heat up bake, stove (enamels) anneal (colors) cauterize brand sulfur, match (casks). 

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