Reference cited in this section

6. Richard F. Lynch, Hot-Dip Galvanizing Alloys, J. Met., Vol 39 (No. 8), Aug 1987, p 39-41 Batch Galvanizing Equipment

Because the galvanizing kettle is the most important piece of equipment used in galvanizing, its selection should be based on the careful evaluation of several major variables, such as size, shape, wall thickness, tank material, source of heat, and auxiliary equipment requirements.

Size and Shape. Although the size and shape of the galvanizing kettle are governed primarily by the parts to be galvanized in it, other factors must also be considered. The kettle must be large enough to contain an adequate thermal mass; that is, it must possess sufficient heat capacity in the molten zinc to compensate for the loss of heat encountered when cold workpieces are immersed in the tank. The minimum and maximum operating temperatures that must be maintained depend on production requirements; usually, the weight of zinc in the tank should be equivalent to 15 to 20 times the weight of parts that are to be galvanized in 1 h. In many production installations, the weight ratio of zinc to workpieces is more likely to approach 40 to 1.

Although the shape of the kettle must accommodate the workpieces that are to be immersed, it should also be designed to expose a minimum of bath surface. If the size of a kettle is to be increased to accommodate a particular part, the depth of the kettle rather than its length or width should be increased to minimize the exposed surface area of the bath. A minimum surface area conserves heat and produces less surface oxide than a larger area. Kettles of complicated shape should be avoided because they are susceptible to damage by severe thermal stresses. Simple, rectangular kettles are most widely used.

Wall Thickness. Theoretically, the selection of wall thickness of a galvanizing kettle should be governed by:

• The rate of corrosive attack by liquid zinc

• The hydrostatic load imposed against the kettle walls by the volume of the zinc bath

• The strength of the kettle wall material at the operating temperature of the bath

• The support afforded the kettle walls by the surrounding brickwork or by other reinforcing elements

Because the variables are so numerous and complex, accurate calculation of a required wall thickness is not practical, and selection is based entirely on empirical data. Depending on the size of the kettle and its reinforcing elements, wall

thickness usually varies from 20 to 50 mm (^ to 2 in.).

Kettle Material. Aside from strength, the principal requirement of a galvanizing tank material is the ability to resist the corrosive attack of molten zinc. The most widely used material is boiler plate of firebox quality with low silicon. The chemical composition of this steel ensures a minimum rate of attack by molten zinc; also, the good welding and bending characteristics of this material are essential features in kettle fabrication. The chemical composition of the welding rods used in kettle fabrication should also be of low carbon and low silicon. Composition of the rod is very critical; obtaining the advice of welding experts is strongly recommended.

If a flux layer is to be maintained on the bath surface, a collar of firebrick or other suitable ceramic material should surround and abut the top 150 or 180 mm (6 or 7 in.) of the tank to retard heat transfer in this area and thus reduce attack by the flux on the steel kettle wall. A similar brick or insulated area of 150 to 205 mm (6 to 8 in.) should exist at the bottom of the kettle to reduce dross attack.

Source of Heat. Galvanizing kettles can be suitably heated by combustion of oil or gas, by electrical resistors, or by electromagnetic induction. The source of heat is of minor importance provided the heating installation satisfies the following requirements:

• High efficiency factor

• Good adjustability and control to maintain an even temperature

• Ability to maintain the minimum temperature required on the outside walls of the kettle

• Uniform heating along the outer walls, without hot or cold spots

Failure to satisfy all these requirements severely curtails the life of the kettle and may result in unexpected kettle failure. Due to energy costs, electric heat and induction heat are not widely used for job shop kettles.

Temperature Controls. When a new galvanizing tank is installed, a complete temperature survey should be made of the molten zinc bath. Based on this survey, control thermocouples may be located in the bath to maintain temperature uniformity and control.

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