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Air mist cooling in continuous casting

• satyendra
• April 20, 2013
• 0 Comments
• Air mist cooling, Air mist nozzle, Heat flux, Heat transfer co-efficient, Leidenfrost phenomenon, secondary cooling zone, spray cooling,

Air mist cooling in continuous casting

A continuous casting machine (CCM) is required to cast efficiently a wide range of steel grades in the present day environment. This range varies from ultra low carbon and low carbon grades to high carbon to low alloy and high quality pipeline grades. Consistent production of prime quality of these products needs increased operational and maintenance flexibility of the CCM so that the optimum casting parameters can be maintained in the CCM for each of the steel grade. This flexibility extends not only to the machine elements and control systems, but also to the secondary cooling zone of the machine (Fig 1) and demands more efficient and reliable spray cooling in the zone. Hence, the secondary cooling zone has also become an area of focus in the present environment because of the demand for improved product quality and increased productivity of the CCM. The CCMs now need very efficient system for spray cooling in the secondary cooling zone.

Fig 1 Schematic diagram of secondary cooling zone of CCM

Cooling by water plays an important role in extracting heat from the solidifying liquid steel both in the mould and after the cast steel leaves the mould during the continuous casting of the 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 can cause fluctuations in the temperature gradients inside the solidifying shell of steel and generate tensile thermal stresses at the solidification front which can ultimately lead to the appearance of hot tears/cracks in the final product. The schematic view of the secondary cooling and the relationship between heat flux and steel surface temperature is shown in Fig 2.

Fig 2 Schematic view of secondary cooling and relationship heat flux and steel surface temperature

The spray cooling is the only controllable part of the secondary cooling process and is therefore is the major factor in determining the product quality and the productivity of the continuous casting process. The spray cooling system uses nozzles for spraying of water. The main purpose of nozzles is the cooling of the strand surface. The spray nozzle arrangement and process parameters determine the characteristics of the spray water cooling. The spray nozzle arrangement defines the area of the strand surface where the spray cooling takes place while the process parameters such as operating pressure and flow rates determine the cooling intensity and distribution on the strand surface. Heat extraction from the strand surface which defines the cooling and the solidification process is a result of both the nozzle arrangement and the process parameters.

The major criterion for the selection of the spray nozzle is the heat transfer co-efficient which is determined by the spray pattern, liquid distribution (density), and volume. The optimization potentials in nozzle arrangement include (i) nozzle alignment, (ii) header and segment pipe design, and (iii) liquid distribution optimization. For process parameters optimization, potentials lie in the choice of (i) the atomization type (air mist or water only), (ii) spray kinetics, (iii) spray potential, and (iv) the correlated cooling efficiency which is measured by the heat transfer coefficient.

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 in the secondary cooling zone of a CCM 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 the 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.

Principle of air mist cooling

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 vapour (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 can 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 vapourized 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’ (Fig 2). Studies which 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.

The term Leidenfrost phenomena is given to the body of phenomena observed when a small amount of liquid is placed or spilled on a very hot surface. It is named after the German medical doctor J. G. Leidenfrost. 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 vapour layer which keeps the 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 vapour blanket. Heat transfer from the surface to the liquid occurs by conduction and radiation through the vapour.

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. Also, the nozzle spray angle and spray height play important roles. Both determines the spray foot prints (width and depth of spray) and are therefore factors influencing the water jet density (water flux) and the jet impact. Besides these two variables, the ratio between the compressed air volume and the water flow is to be considered as another factor in the secondary cooling process. Spray cooling on the strand involves boiling and the formation of steam layer on the steel surface. The compressed air provides the kinetic energy necessary for the penetration of droplets through the steam layer.

Air mist nozzle

The initial approach for the secondary cooling system for the CCMs was based upon the utilization of single fluid spray nozzles. The nozzle technology has undergone improvements since then although several machines are still running on only water based cooling systems and achieving high productivity and high product quality. However, in the present day environment, CCMs need high flexibility in terms of steel grades and section size variation and this necessitates a high flexibility in the secondary cooling system and hence the CCMs to be equipped with air mist nozzles.

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 air mist 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.

Air mist nozzle is to meet the requirements of (i) atomization of cooling water into a fine mist for uniform cooling of the steel, (ii) wide angle discharge of the mist stream in order to reduce the installation of number of nozzles, (iii) increase in the size of the nozzle outlet to have reduction in the nozzle clogging and increase in the discharged water volume range, and (iv) the nozzle size is to 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.

It is desirable to have air mist nozzles with a wide turn down ration in order to keep the types of nozzles installed in a CCM at a minimum numbers. This helps both in the maintenance as well as keeping of the nozzle inventory at low levels.

Since air mist nozzles operate with compressed air in addition to water, the required free cross sections to provide the same water flow rate are increased compared to single fluid nozzles. The increased free cross sections are less prone to internal nozzle clogging generally caused by poor spray water quality and as such show increased nozzle life time and reduced maintenance work load. Fig 3 and Tab 1 compares single fluid nozzle with the air mist nozzle.

Fig 3 Comparison of single fluid nozzle and air mist nozzle

In secondary cooling system of CCMs, it is necessary that the nozzles provide uniform water distribution across the strand surface and over the entire turn down ratio. Tolerances of + / – 15 % from the mean value can be achieved with a multi-nozzle arrangement at water pressures ranging between 1 kg/sq cm and 7 kg/ sq cm. The uniform spray distributions provided by the air mist nozzles both at the minimum and maximum turn down ratios are shown in Fig 4.

Fig 4 Spray distribution performance of Air mist nozzle

The principle advantage of air mist nozzle over the single fluid nozzle is an increased water turn down ratio.   The water turn down ratio is calculated from the flow rate at the maximum operating water pressure (typically 7 kg/sq cm) divided by the flow rate at minimum operating pressure (typically 0.5 kg/sq cm for air mist nozzles and 1 kg/sq cm for single fluid nozzles).The nozzles show a stable spray water distribution within these operating pressure ranges. Typical water turn down ratios for air mist nozzles vary in the range of 10:1 to 30:1, while those of the single fluid nozzles vary typically in the range of 2.6:1 to 3.7:1. The increased turn down ratio provides a higher flexibility in terms of heat transfer variation. This is illustrated in Fig 5.

Fig 5 Comparison of turn down ratios and heat transfer co-efficient of singe fluid and air mist nozzles

Air mist nozzles for billet and bloom casting

When air mist cooling becomes for a billet or bloom CCM, the flat jet nozzle is generally not the best choice. This is especially when the ‘halfway cracks’ are experienced. Halfway cracks are formed because of the reheating of the strand surface after it has passed the sharp heat extraction zone beneath a spray jet. During this reheating process, the surface expands and imposes a tensile strain on the hotter and weaker inner material which can then crack. The use of flat jet nozzles intensifies this effect.

Full cone nozzles or oval nozzles provide a softer cooling by extracting heat over an extended surface area. These two spray patterns are the standard for single fluid water secondary cooling systems, however, there has not been an adequate version using air mist. Common full cone air mist nozzles show unstable spray performances, very high air consumptions, and a tendency to clog very easily. Oval cone air mist nozzles with multi-slot orifices. Non uniform spray patterns and the very narrow easy to clog slots, have made these nozzles barely more than a compromise.

With the development of a new generation of full and oval cone air mist nozzles, it is now possible to effectively use air mist cooling in the billet and bloom CCMs. The compact block design allows mounting both on horizontal spray bars and on vertical ‘banana’ nozzle header. A full cone type air mist nozzle is shown in Fig 6.

Fig 6 Typical full cone air mist nozzle

With these nozzles, turn down ratios of 1:14 have been achieved at water pressure range of 1 kg/sq cm and 10 kg/sq cm at 2 kg/sq cm air constant pressure. Nominal spray angles for circular full cone nozzle range from 0 degrees to 90 degrees. Free passages with 2 mm in diameter are around 3 times higher than before for a nozzle size with flows ranging from 0.5 litres per minute (l/min) at 1 kg/sq cm water pressure and 5 l/min at 7 kg/sq cm water pressure at a constant 2 kg/sq cm air pressure.

Tab 2 gives comparison of the performance of air mist cooling and spray cooling in some of the Japanese steel plants

The benefits of air mist cooling in a continuous casting machine are (i) reduced incidence of surface and corner cracking and central segregation due to improvement in the liquid distribution and reduction in the cooling water flow, (ii) increase in casting speeds and production capacity, (iii) enhancement of the operating condition of the CCM for an enlarged product mix due to wider turn down ratio and optimization of air/water ratio, (iv) significantly reduced maintenance and pipe costs due to simple and rigid nozzle mounting and spray piping, and (v) improvement in operational safety due to perfect alignment of the nozzles and spray piping and to reduction in nozzle clogging.