Air mist cooling in continuous casting
Air mist cooling in continuous casting
Continuous casting machines are now required to cast a wide range of steel grades which are ranging from ultra low carbon and low carbon grades to high carbon and high quality pipeline grades. Consistent production of prime quality of these products require increased operational and maintenance flexibility of a casting machine so that the optimum casting parameters can be maintained for each steel grade. This flexibility extends not only to the machine elements and control systems, but also to the secondary cooling system and demands more efficient and reliable spray cooling.
Cooling by water plays an important role in extracting heat from both the mold and solidifying liquid steel during the continuous casting of steel. It is characterized by complex boiling phenomena. Heat extraction rates during water cooling, which have strong dependence on the metal surface temperature and it can rapidly change with time as the strand cools down. Hence uncontrolled cooling may cause fluctuations in the temperature gradients inside the solidifying shell of steel and generate tensile thermal stresses at the solidification front that can ultimately lead to the appearance of hot tears/cracks in the final product.
The necessity of having a sound quality of cast steel product and increased productivity of continuous casting machine has focused attention on the need for more efficient systems of secondary cooling during continuous casting of liquid steel. Air mist cooling (AMC) in secondary cooling zone of continuous casting machine is a step in this direction. Air mist nozzles utilize compressed air in combination with water pressure to atomize secondary cooling water. This provides a much wider turn down / control ratio which is necessary in case a product mix covers a wide range of steel grades. Air mist nozzles also offer much larger internal free passage compared to single fluid nozzles of the same flow rate size.
Air mist cooling works by forcing water through specially designed mist nozzles. This creates a mist (fog) of ultra fine water droplets with an average size of 25 microns (0.025 mm) or less. With high pressure mist cooling, one can get an even smaller droplet size which is as small as 5 microns (0.005 mm). This creates a surface area larger than a big field from just one litre of water. Higher surface area helps water to evaporate very quickly. These tiny water droplets (fog) quickly absorb the energy (heat) present in the environment and evaporate, becoming water vapor (gas). The energy (heat) used to change the water to water vapour is eliminated from the environment hence cooling the environment.
Relative humidity of the air in the environment plays an important role in air mist cooling. It is the amount of moisture (water) in the air compared to the amount of moisture the air could absorb at the same temperature. This is a crucial factor in determining the maximum air mist cooling potential. The lower the relative humidity, the more water can be vaporized allowing more heat to be removed.
In case of water spray cooling when water is sprayed onto the steel surface above a particular temperature it produces a thin layer of steam between the steel surface and the water. This condition is often referred to as “film boiling”. Studies that have used hydraulic spray nozzles suggest that the heat transfer coefficient is largely dependent on the mass water flux generated by the spray nozzle. However the addition of air to the water spray creates a complex situation. The air causes the atomization of the water which aids in the cooling of a steel surface.
Air mist nozzle
The essential features of modern air mist nozzles are the mixing chamber, extension pipe, water and air inlet adapters and their internal geometries and geometry of nozzle tip. These components are to be precision designed to ensure a very high heat transfer coefficient, stable spray angles and uniform water distribution. The nozzles have non clogging characteristics and there are no wear parts in the mixing chamber of air and water. The spray width of these nozzles is stable within a wide range of water pressure. Thus these nozzles have constant and uniform spray characteristics.
A higher specific water density is not the only factor decisive for the heat transfer coefficient. The air/water ratio is also to be considered with compressed air providing the kinetic energy necessary for penetration through the steam layer above the strand surface. This is important beyond 650 deg C because of Leidenfrost phenomenon.
The Leidenfrost phenomenon is a phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid’s boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly. The Leidenfrost point signifies the onset of stable film boiling. It represents the point on the boiling curve where the heat flux is at the minimum and the surface is completely covered by a vapor blanket. Heat transfer from the surface to the liquid occurs by conduction and radiation through the vapor.
Air mist nozzle should meet the following requirements.
- Atomization of cooling water into a fine mist for uniform cooling of the steel
- Wide angle discharge of the mist stream in order to reduce the installation of number of nozzles
- Increase in the size of the nozzle outlet to have reduction in the nozzle clogging and increase in the discharged water volume range.
- The nozzle size should facilitate its installation between the rolls
The important factors in the air mist cooling which contributes to the effective heat transfer conditions are i) flux density of air mist spray and ii) velocity of the spray
Fig 1 gives a full cone type air mist nozzle for billet casting machine while Fig 2 gives location of two air mist nozzles in a continuous casting machine for casting beam blanks.
Table 1 gives comparison of the performance of air mist cooling and spray cooling in some of the Japanese plants
Fig 1 Full cone type air mist nozzle for a billet casting machine
Fig 2 Location of two air mist nozzles in a beam blank casting machine
|Tab 1 Comparison of air mist cooling and spray cooling in some Japanese plants|
|Air mist cooling||Spray cooling||Air mist cooling||Spray cooling|
|Plant A||0.89 % in 15 days*||Ranging from 1.5 % to 19.8% in 15 days||Cleaning of clogged nozzles in 15 days*||Cleaning of clogged nozzles in 15 days|
|Plant B||Small||Around 20 % in 5 months||No clogging||Changing of clogged nozzle in 3 to 12 months|
|Plant C||Small||Use of walking bar||Change of 15 nozzles in 2 months||Use of walking bars|
|Plant D||Small||Around 20 % in 4 months||Check in 15 days||Check in each cast|
|* Air injection nozzle|
The benefits of air mist cooling in a continuous casting machine are given below.
- Reduced insidence of surface and corner cracking and core segrigation due to imrovement in the liuid distribution and reduction in the cooling water flow.
- Increase in casting speeds and production capacity
- Enhancement of the casting machine operating condition for an enlarged product mix due to wider turn down ratio and optimization of air/water ratio.
- Significantly reduced maintenance and pipe costs due to simple and rigid nozzle mounting and spray piping
- Improvement in operational safety due to perfect alignment of the nozzles and spray piping and to reduction in nozzle clogging
Nozzle characteristics must be investigated and test procedures developed to measure cooling patterns and heat transfer. Improved nozzle design and air/water systems gives in better water distribution and this reduces surface defects, corner cracking and core segregation.