What is Non-Oxidizing Furnace?
A furnace is a device used for high-temperature heating. The device derives its name from Greek term ‘fornax’, which means oven.
In American and Canadian English, the term furnace refers to the household heating system based on a central furnace which also has synonyms like ‘kiln’- a device used for the production of Ceramics. It is either known as a boiler or a heater in British English.
An Industrial Furnace
It is usually used for many purposes like extraction of metal from ores (called Smelting) or in oil refineries and other chemical plants as the heat source for fractional distillation columns.
The term furnace may also refer to a direct fired heater. It is used in boiler application in chemical industries for providing heat to chemical reaction for the process of cracking.
Hot dip galvanizing is a commonly used process today. It is applied to take exact care of all the problems of corrosion. Although hot dip galvanizing is highly detrimental to nature, it is applied for zinc coating to give it more stability.
Continuous strip galvanizing by hot dip galvanizing method is executed in a series of processes. These are applied for the preparation of strip, annealing of the strip, cooling, controlled zinc coating, skin passing, bringing flatness and passivation.
In the case of CR strip galvanization, it needs to be cleaned properly. The cleaning of scale, iron fines, oil, and other unnecessary things on the surface are included here. To do so, it is passed through the alkaline solution. After that, another cleaning action is conducted by rotating brushes. Finally, the strip then is put into the non-oxidation furnace where it is exposed to a set of burners for heating extremely.
The burners are set on a fixed air in order to enable an oxidizing atmosphere in the first zone and slightly reducing atmosphere in the next zones. The strip gets decreased when it passes through the first zone of the non-ox furnace. These oxides are reduced in the last part of the non ox furnace, especially where the ambient temperature is kept between 1200 and 1300 degree centigrade, depending on line speed and thickness of the strip.
After that, the strip is passed through a radiant tube furnace. Here the strip is put into an annealing temperature, creating an atmosphere of nitrogen and hydrogen. The strip temperature is kept between 550 and 750-degree centigrade, depending on the end use of the strip.
Then the strip is passed through a precisely controlled gas. The gas is fired at soaking zone to get the desired properties which already remains in the material. After that, the strip passed through a controlled jet cooling section. This step brings down the temperature of the strip very near to the zinc bath temperature prior to entering the zinc bath.hen, the strip is wrapped up in the molten zinc bath. It creates a thin layer on the strip. This zinc-iron alloy layer formation takes place over the strip. It comes in the form of zinc coating. The strip is removed and passed through an air jet air knife. This knife removes the excess zinc from the surface in order to make it smooth and obtain the desired coating thickness.
The Non-oxidising furnace (NOF) of a galvanizing line is layered with insulation bricks or ceramic fibres. Usually, the non-oxidising furnace operates at extremely high temperatures between 12000C and 13500C .There is a number of criteria in selecting an appropriate layer for the heating furnace. These are process conditions, health and safety regulations, required availability, cost, etc.
Since 2006, bio-soluble fibres for temperature above 12600C were not available in the market. Therefore, the lining of the non-oxidising furnace included bricks.
The lining has a few disadvantages. Say, for an example, it has a very short life. It has a poor effect on the availability. However, there is a doubt that it provides soiling to the steel strip. To fight out the disadvantages a project was layed out to find a customary refractory lining for the non-oxidising furnace of the galvanizing line. It will, therefore, have an increased lifetime with no negative effects on the steel strip.
If the lining gets no exposure to erosion and mechanical damage it is sure to get thermal related stresses. These are the major causes of failure in the lining. Commonly the elements from the Hasselman’s equations are used in such cases in order to select the optimal materials for lining with high thermal stresses.
Hence, materials with high strength, high flexibility, high thermal conductivity and low expansion are preferred. But there comes a problem with this approach. It only compares the thermal shock resistance of different materials at a particular temperature. It is still vague that at what temperature the highest stresses really occur.
Needs of non-oxidising furnace:
Apart from high temperatures other main needs for the non-oxidising furnace lining are
- Long life – which means no repairing problems for long-time.
- No negative effect on the quality of the steel strip.
- Comparable heating and cooling curves – fpr the original lining with insulation brick.
In the case of industrial process zinc bath composition analysis is done to produce smooth, uniform and better quality zinc coating.
The inline coating gauge makes sure monitoring of the uniform controlled coating. With it, thickness is measured to be sure whether it has met the requirement. Now the strip is passed through a process called the skin pass mill & tension leveler. This is done to get the desired elongation and flatness.
In the final step, the strip is passed through chromating section & chemical coater. This is done to ensure a uniform chrome coating over the entire surface in order to prevent formation of the white rust. Now all is done. Sometimes, a thin organic anti-fingerprint solution is applied to keep its appearance neat and clean.