Batch Furnaces with Liquid Baths

Heating solids by immersion in liquid baths happens by convection. For viscous liquids (liquid salts and liquid metal), motion is so minor that conduction is the primary heating mode. Conduction qansfers heat to the load pieces so much more rapidly than from flame to bath liquid that the conduction resistance between liquid and solid surface often can be ignored. Soak time from the solid surface to solid core might be a consideration in salt baths or liquid metal baths if the load pieces are of very heavy cross section. 

Weight of the load includes any containers, hooks, and conveyors that might be immersed in the bath. 

In addition to the heat to be imparted to the total load during immersion (right side of eq. 3.8 and 3.9), heat input is needed to make up for loss from an uncovered bath surface by radiation and convection. Emissivity (e) of a salt bath is approximately 0.9. Lead baths are purposely covered with lead oxide (e = 0.63) and with charcoal (estimated mean e = 0.7) to reduce radiation and convection heat loss and to minimize oxidation. 

Molten zinc for galvanizing (surface oxide emissivity 0.1) is contained in open-topped, rectangular steel tanks or kettles, with walls of 1 to 2 boiler plate or firebox steel. Test data on the tank shown in figure 3.28 (reference 49) showed that the container wall temperature was more uniform with four type H flames than with 18 type E flames (fig. 6.2), but such comparisons are highly dependent on burner spacing, burner size, and distance from container to wall. 

Galvanizing gurus Larry Lewis and Jim Bowers recommend 14 tons of molten zinc in the tank for each ton of load to be galvanized per hour. Others recommend as high as 20 1. Because dross settles to the bottom of the kettle, the kettle should be deep enough that articles to be galvanized will be at least 1 ft (305 mm) above the kettle bottom. For the same reason, heat should be applied no closer to the outside bottom of the tank sidewall than 1 ft or preferably 1.7 ft (0.5 m). 

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