Determination of Convective Heat Transfer Coefficients in the Direct Flame Heating Zone of a Continuous Hot-dip Galvanizing Unit

Metallurgy is among the most energy intensive industries both in Russia and around the world. Heat treatment of cold–rolled steel is the most important process for applying a corrosion-resistant coating. The preferred use of continuous hot-dip galvanizing is due to a high output and full automation of this process. The hot-dip galvanizing unit’s main energy-consuming part is the direct flame heating zone; the heat transfer enhancement in it should become the main line of efforts taken to improve the energy efficiency of the cold-rolled steel heating process. Currently, the cold-rolled steel heating process is adjusted solely depending on the preset temperature of the metal at the exit from the direct flame heating zone. This process arrangement has a certain drawback: when a change occurs in the unit’s output, the heat transfer processes become less efficient, which entails an increase in heat losses and specific fuel consumption for heating of cold-rolled steel. The change in the unit’s output may be caused by a change in the type and speed of the semi-finished product to be processed. The solutions for heat transfer enhancement in the direct flame heating zone should be elaborated on the basis of a comprehensive thermal model that would take into account all heat transfer processes in the furnace that occur between the metal, gases, and masonry. This thermal model makes it possible to calculate the temperatures of the metal processed, the atmosphere and the furnace masonry along the entire length of the direct flame heating zone, depending on the type of product, and the gas and air flowrates in the furnace zones. Based on the developed and verified thermal model of the direct flame heating zone of the continuous hot-dip galvanizing unit, the existing thermal modes can be optimized in terms of fuel utilization efficiency. The article describes how the furnace space of a continuous hot-dip galvanizing unit set is modeled with the aim to determine the convective heat transfer coefficients during the interaction of gas flow with the rolling strip.