Regular Article
High Temperature Zinc Vapor Confinement in Snout by Using Wind Curtain
2025 Volume 65 Issue 8 Pages 1172-1178
Details
2025 Volume 65 Issue 8 Pages 1172-1178
In a hot-dip galvanizing production line for high-aluminum zinc-aluminum-magnesium steel strip, the temperature of zinc pot may reach 600°C. The continuous zinc evaporation may cause serious pollution of zinc ash to the steel strip. In this paper, the confinement of zinc vapor by using a wind curtain is explored, in an attempt of preventing the zinc vapor flowing upward into the furnace. Numerical simulations are carried out to investigate the velocity, concentration, and temperature of zinc vapor in the snout of the hot-dip galvanizing production line. The impacts of the inflow velocity, position, and temperature of the curtain on the concentration of zinc vapor are also paid attention. The results show that the wind curtain can effectively prevent the upward flow of zinc vapor. Furthermore, the concentration of zinc vapor in above of the snout decreases with the increases of the inflow velocity and the height of the wind curtain, but it increases when the temperature of the wind curtain’s inflow is raised. The position of the wind curtain significantly influences the concentration of zinc vapor above the snout, which is the critical factor of confining the zinc vapor by using the wind curtain.
With the requirement of enhanced corrosion resistance for galvanized products, iron and steel enterprises in various countries have carried out technical competition for the development of novel hot-dip galvanizing technologies and processes.1,2) Compared to the traditional zinc pot at 460°C, the temperature of zinc pot in the high-aluminum zinc-aluminum-magnesium hot-dip galvanizing production line may reach up to 600°C, and the improvement of coating quality and corrosion resistance has been verified in practice.3) However, pollution of zinc ash becomes more serious due to the intensified zinc evaporation caused by high temperature.4) The molten zinc in the zinc pot evaporates on the bath surface and condenses into zinc ash on the walls of the snout. The zinc ash not only contaminates the snout walls but also falls to the steel strip easily due to the vibrations of the production line, which severely affects the quality of the steel strip.5)
In order to solve the problem, numerous investigations have been conducted to decrease the concentration of zinc vapor and clean the zinc ash in the snout. Bao Steel tried cleaning zinc ash in the snout using the slag scraping robot. Tan et al.6) suppressed the evaporation of zinc by forming an oxide layer on the bath surface, but zinc slag defects may be formed on the steel strip. Sciffer et al. investigated the turbulent flow in the snout using numerical simulations. The results showed that the concentration of zinc vapor could be decreased effectively with the external circulation system. Dong et al.7) analyzed the impacts of baffle plates and exhaust holes on zinc ash deposition in the snout. The result indicated that the combination of baffle plates and exhaust holes successfully prevented the zinc ash deposition. These investigations had indeed provided some understanding of the flow phenomena and the zinc ash removal methods in the snout. However, the majority of these investigations were focus on the zinc pot at 460°C, and the above measures were not effective in preventing the flow of zinc vapor in high-temperature zinc pot. Therefore, it’s urgent to explore a new technology that can prevent the upward flow of zinc vapor in the high-temperature zinc pot at 600°C to produce the steel strip with high corrosion resistance. So far, there are only limited studies on preventing the flow of zinc vapor in the high-temperature zinc pot.
The wind curtain is a technology that uses a confinement formed by airflow to prevent the diffusion of specific substances, and it is widely used in preventing smoking, toxic gases and heat insulation in buildings.8,9) In this paper, a curtain is established above the surface of the zinc bath in the snout to prevent the upward flow of zinc vapor. The confinement of zinc vapor by using the wind curtain is explored. Numerical simulations are carried out to analyze the flow of zinc vapor in the snout, and the impacts of the inflow velocity, position, and temperature of the curtain on the concentration of zinc vapor are studied. The purpose of the study is to provide a reference for the confinement of zinc vapor so as to control the zinc ash in the high temperature zinc-aluminum-magnesium galvanizing production line.
The remaining sections of the paper are organized in four sections. Section 1 is the equations of flow in the snout. The three-dimensional model of the flow configuration and numerical setup are listed in Section 2. The results and discussion are described in Section 3, followed by conclusions in Section 4.
The complex flows in the snout include the evaporation of zinc vapor on the bath surface, convection caused by temperature gradients, and the movement of steel strip. Multi-component governing equations are used to describe the flow in the snout. The zinc vapor in the mixed phase shares the same velocity and pressure as the nitrogen. The Einstein summation convention is adopted for simplicity, and the conservation laws of mass, momentum, energy, species transport, and equations of state are presented in Eqs. (1), (2), (3), (4), (5), respectively, as follows
| (1) |
| (2) |
| (3) |
| (4) |
| (5) |
Where ρm is the density of the mixed phase. u, p, and k are the velocity, pressure , and turbulent kinetic energy, respectively. μm and μt represent the dynamic viscosity and turbulent viscosity coefficient of the mixed phase, respectively. gi is the gravitational acceleration in the i direction. cp, T and qj represent the specific heat at constant pressure, temperature, and heat flux caused by temperature differences, respectively. Prt and ϕ are the Prandtl number and dissipation function, respectively. Ci is the concentration of specie i (i=1, 2). DM and Sct are the mass diffusion coefficient and turbulent Schmidt number, respectively. ρ1 and ρ2 represent the density of zinc vapor and nitrogen, respectively. R1, R2, M1, and M2 are the universal gas constant and molar mass of zinc vapor and nitrogen, respectively.
The density, dynamic viscosity, and thermal conductivity of the mixed phase are calculated by Eqs. (6), (7) and (8), respectively, as follows
| (6) |
| (7) |
| (8) |
Where μ1 and μ2 represent the dynamic viscosity of zinc vapor and nitrogen, respectively. λm is the thermal conductivity of the mixed phase. λ1 and λ2 represent the thermal conductivity of zinc vapor and nitrogen, respectively. The standard k-ε turbulence model is selected to account for the turbulent flows in the snout,10) and the standard wall function is adopted to capture the flow near the wall.
2.2. Model of Zinc EvaporationThe zinc evaporation rate at bath surface is related to the saturation vapor pressure of zinc and the partial pressure of zinc vapor above the bath. The zinc evaporation rate per unit area can be calculated as following:
| (9) |
Where JZn is the zinc evaporation rate per unit area, α is the evaporation coefficient, and its value is associated with the partial pressure of zinc vapor, zinc oxidation reactions, and the permeability of the oxide layer. Tbath and PZn represent the temperature of bath and the partial pressure of zinc vapor above the bath, respectively. PZnsat(Tbath) is the saturation vapor pressure of zinc, which is a function of temperature at the bath. The relationship between the saturation vapor pressure of zinc and temperature at the bath is given in Eq. (10).
| (10) |
The three-dimensional model of the snout is shown in Fig. 1. Figure 1(a) shows the snout in the production line. It consists of four zones, i.e., the bath surface, the steel strip, the snout walls, and the annealing furnace. The wind curtain is established above the bath surface, as shown in Fig. 1(b). The size of the wind curtain is 350 mm×20 mm, and the distances (a) between the centerline of the wind curtain and the bath surface are 250 mm, 500 mm, and 750 mm, respectively. The red lines in Fig. 1(b) represent the high-concentration zinc vapor, while the blue lines are the zinc vapor with low concentration, and the green lines represent the nitrogen. Nitrogen is injected into the snout through the wind curtain’s inlet and exits from the wind curtain’s outlet. The wind curtain is used to prevent the upward flow of high concentration of zinc vapor, so the concentration of zinc vapor above the wind curtain is much lower than that below. For convenience of description, the two models are denoted as the baseline and the wind curtain, respectively. The main geometrical parameters of the snout are summarized in Table 1.
| Geometrical Parameters | Value | Geometrical Parameters | Value |
|---|---|---|---|
| Width of steel strip (mm) | 1200 | Length of furnace (mm) | 3042 |
| Thickness of steel strip (mm) | 4 | Width of furnace (mm) | 1700 |
| Length of snout (mm) | 4040 | Height of furnace (mm) | 583 |
| Width of snout (mm) | 1700 | Angle of snout (°) | 60° |
| Height of snout (mm) | 4738 |
During the galvanizing process, zinc vapor first evaporates at the bath surface and then flows upward. The steel strip moves downward at a speed of 2 m/s. For convenience of description, the conditions are depicted as without-curtain. In the present calculations, the fluid is zinc vapor and nitrogen. Velocity at the bath surface and the wind curtain’s inlet is specified.
The pressure at the outlet and the wind curtain’s outlet is imposed. The steel strip is set as a moving wall, where no slip and adiabatic conditions are imposed on other walls. The detailed setting of boundary conditions is listed in Table 2, and the symbol (–) in Table 2 represents the variable is extrapolated from the interior of the computational domain. For spatial discretization, a second-order upwind scheme is applied to the convection terms, while the central difference is used for the diffusion terms.
| Zones | Velocity V (m/s) | Pressure P (pa) | Temperature T (K) | Concentration C(/) |
|---|---|---|---|---|
| Bath surface | α (Psat-P) | – | 873.15 | CZn=1 |
| Curtain’s inlet | 3/7/10 | – | 298.15/573.15/873.15 | CN2=1 |
| Curtain’s outlet | – | 0 | – | – |
| Outlet | – | 0 | – | – |
| Snout walls | 0 | ∂p/∂n=0 | 673.15 | ∂C/∂n=0 |
| Steel strip | 2 | ∂p/∂n=0 | 733.15 | ∂C/∂n=0 |
| Others | 0 | ∂p/∂n=0 | adiabatic | ∂C/∂n=0 |
Figure 2 shows the mesh distribution of the snout. Unstructured polyhedral meshes are used for the calculation, with mesh refined on the bath surface, wind curtain, and outlet. In order to obtain the mesh-independent solution, three meshes are tested to analyze the variation of velocity components at a specific location in the snout, as plotted in Fig. 3. It can be seen from Fig. 3 that the mesh, which has 2304224 elements, is fine enough to produce mesh-independent results, and the variation of velocity components is weak when the mesh is further refined to 3112144 elements. Therefore, the mesh, which has 2304224 elements, is employed in the following calculations.
The numerical model is validated by comparing the computed velocity vectors above the bath surface with experimentally measured velocity,6) as indicated in Fig. 4. It can be observed from Fig. 4 that the airflow close to the steel strip flows downward, as shown in the C region of Fig. 4. In the A region, the airflow turns the direction from oblique to horizontal flow and then moves upward along the snout walls, as presented in the B region of Fig. 4. Furthermore, a clockwise vortex is identifiable between the steel strip, bath surface, and snout walls. The flow phenomenon in the present calculations are consistent with the experiment.
The velocity, position, and temperature of the wind curtain are the critical factors affecting the concentration of zinc vapor in the snout, and these factors are divided into three grades in magnitude (1, 2, and 3) in the present study. For example, the velocity of the wind curtain is set to 3, 7, and 10 m/s, respectively, as listed in Table 3.
| Grades | Velocity V (m/s) | Position a (mm) | Temperature T (K) |
|---|---|---|---|
| 1 | 3 | 250 | 298.15 |
| 2 | 7 | 500 | 573.15 |
| 3 | 10 | 750 | 873.15 |
It is necessary to find out the optimal case of the minimum zinc vapor concentration in the snout within the given velocity, position, and temperature in the numerical simulation. If control variates are adopted for the calculation, 27 cases need to be calculated. The orthogonal experiment is carried out to understand the law of general variation and reduce the cases of numerical simulation. The interactions between velocity, position, and temperature are neglected, and the standard orthogonal table with 4 factors and 3 grades (L9 (34)) is chosen in the present study. The number of cases equals factors multiplied by (grades minus 1) plus 1, so only 9 cases are calculated in the numerical simulation. The arrangements of orthogonal test are listed in Table 4. The frequency of each grade is the same in any column. For example, grade 1 appears three times in any column. In addition, the numbers paired in the same row are the same for any two columns in the table. For example, the combination of grades 2 and 3 appears two times in any two columns. The purpose of the arrangements is to ensure the uniform distribution of numerical points within the entire range. The results are based on the maximum zinc vapor concentration (Cmax) at Y=2 m, as shown in Fig. 8. It can be seen from Table 4 that the concentration of zinc vapor in different cases varies a lot, so the velocity, position, and temperature of the wind curtain play an important role in the concentration of zinc vapor.
| NO. | Velocity V (m/s) | Position a (mm) | Temperature T (K) | Blank | Maximum zinc vapor concentration Cmax |
|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 1.470e-2 |
| 2 | 1 | 2 | 3 | 2 | 1.454e-2 |
| 3 | 1 | 3 | 2 | 3 | 8.843e-3 |
| 4 | 2 | 1 | 3 | 3 | 1.554e-2 |
| 5 | 2 | 2 | 2 | 1 | 5.880e-3 |
| 6 | 2 | 3 | 1 | 2 | 5.536e-3 |
| 7 | 3 | 1 | 2 | 2 | 1.437e-2 |
| 8 | 3 | 2 | 1 | 3 | 4.667e-3 |
| 9 | 3 | 3 | 3 | 1 | 6.874e-3 |
Intuitive analysis is used to calculate the averaged maximum zinc vapor concentration (
| Factors | F | |||
|---|---|---|---|---|
| Velocity | 1.269e-2 | 8.986e-3 | 8.637e-3 | 3.392 |
| Position | 1.487e-2 | 8.362e-3 | 7.085e-3 | 11.694 |
| Temperature | 8.302e-3 | 9.698e-3 | 1.231e-2 | 2.789 |
The F value in Table 5 represents the impact of the three factors on the concentration of zinc vapor. It can be indicated from the F-value that the position of the wind curtain has the most significant impact on the concentration of zinc vapor, followed by velocity, and the least influential factor is temperature. Therefore, the position of the wind curtain is the critical factor in decreasing the concentration of zinc vapor used by the wind curtain.
The curves of
In summary, the conditions, with velocity of the wind curtain at 10 m/s, position at 750 mm, and temperature at 298.15 K, are optimal. For convenience of the description, these conditions are denoted as the curtain.
A line denoted by A-B is extracted to analyze the variation of zinc vapor concentration from the bath surface to the top of the snout, as shown in Fig. 6. The transformed coordinate, NY, is calculated as follows:
| (11) |
where X, Y is the Cartesian coordinate, θ is the angle of snout.
Shown in Fig. 7 is the concentration of zinc vapor along A-B line. The red solid line and green dashed line are the concentration of zinc vapor in the without-curtain condition and the curtain condition, respectively. A significant decrease of zinc vapor concentration is observed at nearby NY = −2.3 m, which is related to the confinement of the wind curtain. The decrease of zinc vapor concentration indicates that the wind curtain can successfully prevent the upward flow of zinc vapor. Although the evaporation rates of zinc vapor in the curtain condition are increased, the concentration of zinc vapor at the top of the snout is decreased by 1 order, according to Table 6 and Fig. 7. It can also be found from Fig. 7 that the concentration of zinc vapor in two cases shows a similar trend. The maximum concentration of zinc vapor is observed at the bath surface, and then decreases with the increase of NY. The concentration of zinc vapor at the top of the snout is decreased from 1.4e-2 to 4.4e-3 under the effect of the curtain.
| Cases | Evaporation rates of zinc vapor (kg/s) |
|---|---|
| Without curtain | 6.363e-7 |
| Curtain | 2.358e-4 |
The distribution of zinc vapor concentration in the snout is plotted in Fig. 8, where the zinc vapor with high concentration is depicted by the red contour while the low concentration zinc vapor is colored by the cyan contour. The figures on each slice in Fig. 8 are the maximum zinc vapor concentration. For example, the maximum zinc vapor concentration on the slice of Y=2 m is 0.0161 in the without-curtain condition. The concentration of zinc vapor is decreased with the snout’s height, with the lowest concentration of zinc vapor on the slice of Y= 2 m. For the without-curtain condition, the zinc vapor with high concentration is filled with the entire snout. After the wind curtain is established, the high concentration of zinc vapor is effectively blocked, resulting in a dramatic decrease of zinc vapor concentration at the top of the snout, and the maximum zinc vapor concentration at each slice is lower than that in the without-curtain condition.
Figure 9 shows the distribution of temperature in the snout, where high temperature gas is depicted by the red color while the low temperature gas is shown by the green color. It can be noticed that the temperature in two cases is different. During the galvanizing process, the temperature of steel strip may reach up to 883.15 K, and the temperature of snout walls is 853.15 K. Therefore, the temperature for the without-curtain condition is high. For the curtain condition, the nitrogen is injected with the temperature of 298.15 K, resulting in the decrease of temperature from the bath surface to the top of the snout. The injected gas with low temperature significantly decreases the temperature on the right side of the snout, as shown in the green region in Fig. 9(b).
The velocity component in the Y direction is illustrated in Fig. 10, and the positive and negative values of the velocity component represent the upwards and downwards flow of the fluid, respectively. The airflow in the snout is driven by the movement of the steel strip, the airflow injected by the wind curtain, and the convection caused by temperature gradients. The airflow close to the steel strip flows downward, and the flow direction is parallel to the steel strip. The airflow away from the steel strip is driven by convection, and the airflow injected by the wind curtain. For the without-curtain condition, the airflow away from the steel strip flows upward, with a magnitude of around 0.4 m/s. While the velocity of the airflow away from the steel strip is increased under the impact of the wind curtain, as shown in the pink region in Fig. 10(b).
Shown in Fig. 11 is the flow state of zinc vapor in the snout, where the streamline is colored by the concentration of zinc vapor. The red streamlines in Fig. 11 represent high-concentration zinc vapor, while the zinc vapor with low concentration is depicted by the cyan. The high concentration of zinc vapor is filled with the entire snout in the without-curtain condition, while the zinc vapor with high concentration is prevented by the curtain after the wind curtain is established. The airflow enters from the wind curtain’s inlet and then impinges on the snout walls. A clockwise vortex is identifiable near the exit of the wind curtain. Therefore, high-concentration zinc vapor is drawn from the right side of the snout to the left side. At this moment, the concentration of zinc vapor above the curtain is significantly decreased, which indicates that the wind curtain can effectively prevent the upward flow of zinc vapor.
In this paper, numerical simulations are performed to analyze the high-temperature zinc vapor flow in the snout during the galvanizing process. The method of using wind curtain is proposed to prevent the upward flow of zinc vapor. Attentions are paid to the impacts of the inflow velocity, position, and temperature of the curtain on the concentration of zinc vapor in the snout. The main conclusions are summarized as follows:
(1) The upward flows of zinc vapor are effectively prevented by the wind curtain. The concentration of zinc vapor in above of the curtain in the snout is lower than that in the without-curtain condition.
(2) The concentration of zinc vapor above the curtain decreases with the increases of the inflow velocity and the height of the wind curtain, but it increases when the temperature of the wind curtain’s inflow is raised.
(3) The position of the wind curtain has the most significant impact on the concentration of zinc vapor, followed by the velocity. The least influential factor is the temperature. Therefore, the position of the wind curtain is the critical factor of confining the zinc vapor by using the wind curtain.
The authors declare no conflicts of interest.
