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What is the maximum amount of steel that can be run through a kettle?

If all questions about material handling, pickling, and other logistics are set aside, this question becomes one answered by thermodynamics. The essential question is how much heat loss a furnace can sustain and still hold to the galvanizing temperature. A paper written by Richard L. Amistadi and Charles S. Young entitled Galvanizing Kettle Production Capacity Calculations seeks to do just this. The following article is a summary of that paper and answers the question: Just how much steel can I theoretically galvanize? The paper uses two separate methods to calculate a kettles theoretical production limit; a method based on heat transfer criterion and a method based on zinc capacity. The smaller number produced by the two methods is chosen as the kettles theoretical production limit.

Heat Transfer Criterion

The heat transfer criterion of the galvanizing kettle is the first of two factors limiting theoretical production. Terms describing the heat required to maintain the temperature of the bath, bring the incoming work up to the galvanizing temperature, and melt new zinc additions, form a heat balance equation. The equation is used in conjunction with assumptions about the heating rate of a galvanizing kettle to determine the amount of steel that can be run through a kettle in a given amount of time. The essential idea of the equation is this: the rate at which heat is lost due to adding steel and zinc to the kettle (along with other thermodynamic losses), must be equal to the rate heat can be added to the kettle by the furnace. The purpose of the paper was to reduce the process into one easy-to-use equation that galvanizers can use simply by knowing the dimensions of their kettle. A number of assumptions must be made to generalize the equation to cover all hot-dip galvanizing kettles. The assumptions made were:

  • The molten zinc bath acts as a black body with an emissivity of one
  • The bath can be considered as a horizontal plate
  • Concrete is the conductor which limits the loss due to conduction
  • 10% zinc pickup during galvanizing
  • Heat can be put through the kettle walls at a rate of 10,000 BTU per square foot per hour
  • A galvanizing temperature of 850 F
  • An ambient temperature of 70 F

All of the assumptions listed above were chosen to be slightly on the conservative side which leads to an overall conservative estimate of the theoretical production limit. Using these assumptions, thermodynamic calculations for heat loss, and the essential equation of [Heat Input = Heat Required], the equation for theoretical maximum production rate is derived. In the equation below, P is the theoretical production limit in lbs per hour, D is the depth of the kettle, L is the length of the kettle, and W is the width of the kettle (all in feet). The other numbers in the equation are simply constants that come from the thermodynamic properties of a zinc kettle. The values of length, width, and depth, are all that is needed to produce an estimate with this method. As an example, lets calculate the theoretical production rate of a kettle that is 30 x 5 x 6 (L x W x D). The theoretical production of the kettle would be:

1

Zinc Capacity

When steel is dipped into the bath of molten zinc, the lower temperature of the steel causes the bath temperature to drop more than 15 F and the galvanizing reaction and coating will be negatively affected. To ensure this does not happen, a galvanizer must make sure there is an adequate amount of zinc in the bath so the temperature does not fluctuate out of this range upon dipping. Based on the heat content of each of these metals, and the fact there are about four temperature cycles per hour, 15 pounds of zinc will be needed for each pound of steel dipped per hour. Assuming the density of zinc in the kettle is 410 pounds per cubic foot, the zinc capacity becomes:

2

Where P is the theoretical production limit in lbs per hour, D is the depth of the kettle, L is the length of the kettle, and W is the width of the kettle (all in feet). Again, the constants in this equation will not change and are derived from the heat capacities of zinc and steel. Only the kettles dimensions are needed to produce an estimate.

Using the same example kettle as before, the theoretical production capacity of the kettle based on zinc capacity is:

3

Conclusion

For our theoretical kettle, we have now obtained a limiting factor of production through two separate means. As both of these methods calculate an upper limit based on different requirements, the lesser of the two predicted production rates is chosen as the theoretical production capacity of the galvanizing kettle. In the case of a 30 x 5 x 6 kettle, the production capacity is limited by heat transfer criterion at 17,654 pounds of production per hour. In summary, the theoretical production of a hot-dip galvanizing kettle can be estimated by the kettles dimensions and the following two equations:

4

The maximum safe production is the smaller of the two calculated amounts.


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