Salt Bath Equipment

Design Considerations. Basic design considerations for salt bath cleaning systems (see the article "Salt Bath Equipment" in Heat Treating, Volume 4 of the ASM Handbook) are similar to those of heat treatment salt bath furnaces. However, the actual process equipment is unique. Two main distinctions between heat treatment/heat transfer salts and cleaning salts are that the cleaning salts are chemically active and the byproduct generation in cleaning baths is potentially much greater. Both of these factors must be taken into account when designing and engineering appropriate salt bath equipment.

Basic design considerations such as throughput, heat capacity, and part geometries are similar to those for heat treating baths. Because the baths are chemically active, the materials of construction must be carefully selected. Materials commonly used for fabricating heat treatment transfer/heat salt bath equipment are generally not suitable as cleaning salt baths because of chemical interactions with the cleaning salts.

Heating systems for molten salt baths may be either electric or gas fired. Due to the generation and settling of reaction byproducts and their insulating effects, most heating designs use internal or immersion heating devices, as opposed to external heaters. (Certain higher-temperature cleaning processes, however, may require external heating systems to achieve good heating system longevity. Care must be taken when using outside heating, to prevent localized "hot spots" where reaction byproducts may accumulate and retard heat transfer in the salt bath furnace.)

Electric immersion heaters may be either resistance elements, enclosed in a tube or bayonet, or electrode configurations that rely on the conductivity and resistance of the molten salt itself to convert electrical energy to heat. Due to their higher energy efficiencies and simplified electrical circuits, resistance immersion elements are more commonly employed with cleaning salt bath equipment than are electrode-type heating systems. Resistance heaters also offer easier and safer startup than electrode systems. Electrode systems require a molten pool of salt for electrical conduction. In a cold, solidified bath, this is formed by a "starting torch" or auxiliary resistance heater. Once an ample amount of salt has been melted, the auxiliary heater may be turned off and the main electrode system energized. Electrode systems also pose a potential safety hazard if a bath should partially "freeze over," forming an impermeable solid salt crust. The volume of a molten salt increases with increasing temperature, so if the electrode heating system is activated while the bath is crusted, the fluid or molten salt beneath the crust will attempt to expand against the crust. As the salt expands, its pressure increases until the crust ruptures. This sudden release of pressure may result in an eruption of the salt through the crust and possible injury of personnel and equipment.

Gas-fired immersion heating systems are very reliable and economical to use. Consisting of either an open-head or closed-head burner system, the ignited fuel mixture is drawn or forced through a burner tube immersed in the salt (Fig. 2).

Fig. 2 Cutaway view of a salt bath furnace incorporating an agitated molten salt bath and a sludge settling zone

Byproduct Collection and Removal. Provisions must also be made for the effective collection of reaction byproducts formed during cleaning operations. In addition, subsequent removal of these byproducts from the bath must be accomplished in a convenient, safe, and efficient manner. Most cleaning baths do not require routine chemical monitoring, but rather rely on the removal of reaction byproducts and additions of fresh process chemicals to maintain proper chemical balance and performance. If the byproduct collection system is ineffective, or the removal of the collected byproducts is inconvenient or unsafe, this necessary routine maintenance function will not be performed. This will result in overall process degradation and will eventually necessitate the complete disposal of the spent molten salt and recharge with fresh product.

Molten salt bath processes require properly designed and engineered equipment for their safe operation. In most installations, it is highly desirable to have the salt bath furnace and its associated process tanks (quench water, rinse water, sludge, or byproduct discharge zone) situated under a common hood system (Fig. 3, 4). The ventilated hood, outfitted with observation windows, internal lighting, exhaust system, and so on, protects the operator from accidental contact with the molten salt. It also captures and exhausts the steam generated during the quenching and/or rinsing of hot workloads.

Sludge dumping Salt bath Quench rinse zone furnace tank

Sludge dumping Salt bath Quench rinse zone furnace tank

Work travel

Fig. 3 Schematic of an enclosed molten salt bath cleaning line

Fig. 3 Schematic of an enclosed molten salt bath cleaning line

Fig. 4 Fused salt cleaning system that is completely enclosed by a hood to comply with Occupational Safety and Health Administration guidelines

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