Killn construction process improvemets blow off system

Kilnns are used for giving stainless steel its heat treatment. Also the shiny surface of the steel arises due to the heating up in the killn. For this a wide steel plate is pulled through a killn. At first the strip of metal is heated up to 1000 degree Celsius. After that the strip is cooled with air. When the strip reaches a temperature of 600 degree Celsius  the strip is coolded with water by spay nozzles. This water should be blown off of the strip before the strip reaches the scale breakers at the end of the killn. Scale breakers are needed to break of the polluted top surface of the steel plate so the shiny surface becomes visable. The scalebreakers pull hard on the plate to remove this top layer. If the plate is moist the scale breaker will slip not making it possible to get off the polluted top layer.

The toplayer is passivated by dipping the steel strip into an annealing bath. We do not want to change the concetration of chemicals in this annealing bath. That is also a reason we want a dry strip when the steel comes out of the killn.

Due to a wrong design of the dryer section all customers could not produce at top speed due to the fact the water would be a problem. The biggest problem in the design of the old blow off is that the compressed air delivered by a fan could not find the excit way. There was no rounding added in the design. This can be seen in the pdf. The second problem, also related to friction, is the quantity of bends in the old blow off. The air has to change from direction many times. This raises power consumption. The company did not even find it strange that the middle of the strip often stays wet. If you look at the first page of above pdf it is logical the steel plate stays wet in the middle.

With the above shown redesign (pages 2 and 3) customers could now produce at maximum speed. Due to the redesign the blow off functions at least 5 times as good as the old one. Also the production of the new blow off was 2,5 times cheaper as the old one. This saved about 80.000 euro per killn. Before the redesign customers could produce at maximum 80% of maximum speed due to dryer problems. Now with this new tool placed in a killn the customer has no questions about the dryer section. No adjustments had to be done this time. The blow off angle can now be adjusted. This gives the customer to adjust the blow off to his process reducing energy costs. Due to a better control of the strip temperature at the excit of the killn, we now do not have to heat up the annealing bath. This will again bring energy savings for the customer. We can now control the temperature of the steel plate leaving the killn much better.

Also a new carrousel was invented. The caroussel is needed to exchange the rolls that carry the strip in the killn section. If the strip gets damaged by a damaged carrying roll the price for selling the steel reduces. That is why the carrousel was invented. If one roll in the killn section was damaging the steel strip the carrousel could rotate 90 degree so a new clean roll could carry the steel strip without letting the killn cool down. So production can go on even with damaged rolls in the killn were temperatures reach 1000 degree Celsius.

In the development of a new carrousel all kind of strength problems occured. Too high stresses were present at the transition from curved central area to the flat side plates. This can be seen here. These side plates are positioned in the walls of the killn and are water cooled. Also the central section of the carrousel is water cooled. When high forces / stresses are available in a steel construction most people will raise the thickness of the steel plates and add stronger welds. I was thinking in another direction. I thought that we could lower the thickness of the flat steel plate. By doing this a stress redistribution would take place lowering peak stresses.

The engineering company that was producing drawings of the carrousel and making strength calculations told us that adjusting the plate thickness one time would mean half a day extra drawing time. They draw in Solid Works and this programm can only do a strength calculation if all parts of the construction are connected with each other. For finding the optimum plate thickness the plate thickness has to be adjusted several times meaning 3 days of extra drawing time. I asked the engineers if i was possible to change the properties of the flat steel plates. They said this was possible ( I already new from my aerodynamics study that this would be possible). The flat plate was at first 15 mm thick and made out of steel. Lowering the  elasticity modulus of the plate can also be seen as lowering the thickness of this plate. On the second picture of the pdf we can see the stresses in the steel plate with elasticity modulus of 280.000 at thickness of 15 mm. On the third page of this pdf the elasticity is lowered to 100.000 N/mm*mm. The last page shows the plate 15 mm thick with a elasticity of 30.000. At that elasticity modulus a very good redistribution of stresses takes place. Peak tensions are now lowered with 40%. Now we can find the according plate thickness very fast. We do want to use steel with a elasticity of 280.000. This can be done by using a mechnical formula for calculating plate stiffness. This formula says that the ratio between optimum elasticity and starting elasticity (30.000/280.000) is equal to third root of the ratio of optimal thickness devided by starting thickness.  As the starting thickness is 15 mm the only unknown factor in this formula is the optimum steel thickness. The optimum steel thickness is about 8 mm. The calculating people were surprised of my clever way to save time and money.

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