June 17, 2014

Case Study: A New Design for a Water-Cooled Furnace Brings to Life the Concept of “Conservative Innovation”

When a CECON consultant specializing in chemical and process engineering presented his idea for a new design for a water-cooled furnace to the expert designers he worked with they said one word: IMPOSSIBLE.  But employing his concept of “conservative innovation design,” he persisted and built a furnace that not only operated successfully, it became far more practical to maintain and repair than the traditional design.

Over the course of five years, I have built and operated a total of seven rotary induction water cooled furnaces—giant furnaces capable of processing hundreds of tons of ore per hour—in two different factories.  

When No. 6 was running, my electrician, who I had been working with since No. 1, came into my office and declared: “We must get rid of this water cooling nuisance. You must make an air-cooled coil!” He had a point. Water leakages and sometimes blocked water passages, created safety risks, demanded problematic repairs and consequently caused lengthy down time of the furnaces.

I immediately responded: “Our furnaces are not the first in the world. If this were possible, the Chinese experts who supplied the equipment would have done it long ago. Forget about it! We have enough on our hands.”

He was disappointed, but he understood my point.

But that evening I found myself considering the possibilities. I started writing notes and making sketches. I assumed that if this was really impossible, my engineering calculations would run into a dead end, or some expert will convince me to let go of the idea.

My criteria were that I need the new design to be suitable for conversion of the existing furnaces without major modifications including the need to modify the power supply and control mode. Getting rid of the cooling water nuisance was tempting but not enough to justify a major design revolution, especially considering that we already 6 furnaces running.

It took some calculations and consulting with experts of heat exchange and electrical bus bar designers to come up with an alternative design of an air-cooled coil that would have the same Ohmic resistance as the original despite of the higher working temperature. The copper weight was 2.5 times the original. More expensive but still not prohibitive. I now knew that I had a suitable design.

We knew the maximum current in the coils so we could calculate the power needed to overcome the Ohmic resistance of the coils. We then measured the cooling water flow rate and temperature difference and thus verified that we know the heat load to be removed by the surrounding air. With this in hand, and using some heat exchange coefficients found in literature, we could calculate the surface temperature of any suggested copper profile to be used for the construction of the new AIR-COOLED INDUCTION COIL. 

Knowing the service temperature of the Copper we could now find the new specific resistance of the Copper and calculate the required cross section in order that the total coil resistance will be the same as the original. The copper weight was 2.5 times the original. More expensive but still not prohibitive. I now compared my results to engineering tables which I received from an electrical engineer showing service conditions and loads for bare copper cables and bus bars used in electrical panels. The comparison was encouraging. I now knew that I had a suitable design.

What is Conservative Innovative Design?
At this stage I would like to spell out the main characteristics of what I call Conservative Innovative design. The innovative part of this is easy to understand. Once you face a problem, which the existing technology cannot solve satisfactorily, keep your mind open to innovative ideas even if, at first glance they look unsuitable. Do not be afraid of innovation. Have the guts to try new things.

How do you apply innovation in a conservative way? We must remember that we are within an operating facility where a lot of money was invested and time is highly valued. We must respect the “old” technology even if we are critical of it, as this is actually what we have and what is paying the bills and providing pay checks.

We must not be too adventurous or too arrogant. We must consult with many experts, listen to many opinions, especially those which reject our ideas and predict failure. We must be critical to prove to ourselves that the rejecting opinions are wrong–or maybe they are right or partially right and maybe we can learn something and improve. This requires patience and hesitation. A thinking process that does not go through the hesitation stage had simply not been examined deeply enough. Never assume that things will go right. Prove it to yourself!

 Conservative Innovative Design in Execution

Sketching the new design, I uncovered additional bonuses. I started with a coil made of a flat copper bar, on which I could do bolted connections, avoiding copper welding—the culprit for many leaks. It took a few iterations to come up with a design where the coil, its support and the “decorative safety cover were all one split-able unit. Dismantling such a coil from the furnace should not even need to stop the furnace. Just bridge the electrical connection over the coil, unbolt the two halves, and split it open. All other elements stay in place. The lower tire stays in place. No cooling water issues. The assembly of a replacement coil would also be very simple. The safety panel which now became part of the coil support did not bother any more. It did not impose inconvenience anymore.

Stopping and cooling the furnace would now only be required when a substantial repair to the thermal insulation was needed. However, such a shutdown would require only 1.5 days versus the standard 6+ days for the water cooled furnace. In this respect, this design became A REVOLUTION.

The problem of rust chips falling onto the copper was also reduced as the short circuits would now not result in water leaks. A short study indicated that we could apply a certain metallic coating onto the outside of the furnace tube that will prevent severe rusting even when the red hot surface is exposed to atmospheric air as a result of a faulty cladding of the thermal insulation.

Pitching to the experts
I was now convinced that the new design was superior to the original. I knew that electrically, the new coils had exactly the same features as the former. The dimensions were similar so the magnetic fields were expected to be the same, the resistance was identical so the power supply regulation would not have to be changed and the operators should not feel any difference, the need for emergency cooling water supply was removed so quite a few control alarms were cancelled. This was the conservative part of the innovation.

A meeting was set up with the Chinese experts whom I knew well and have been working with them for 5 years on 6 furnaces. I presented the subject, first explaining why I wanted the cooling water system replaced by air-cooling. There was a unanimous response: “IMPOSSIBLE!”

The discussion went on for many hours. Chinese people are very polite to a customer. But in the evening they still claimed I should forget about it. After the meeting, my interpreter received a phone call. An electrical engineer, a junior member of the Chinese expert team, asked for a meeting. At the meeting, she asked not to be quoted, and said: “Unofficially, I could find no mistake in your presentation. It will cost you some money and you face the risk of the innovator but it should work.”

Putting the design to work
Now I had to convince management. They approved my suggestion to install 5 such elements on the lower zone of furnace No. 6 and hold the production of the coils for No. 7. The coil producer was set to produce these 5 coils working 3 shifts a day.

Installation went on immediately and the furnace was re-started with 2 water cooled zones and the lower zone, which is always at highest power load, AIR COOLED. The operators did not notice any difference. We studied all parameters carefully and everything was fine.

My electrician reminded me whose idea it was in the very beginning.

Furnace No. 7 cost $100,000 USD more than No. 6. It was air-cooled, without any cooling water piping and no emergency cooling water supply and alarms. It operated as usual. No special excitement. Nothing to write home about. EXACTLY the result we wanted.

Technical background
Giant rotary furnaces are known all over the world as the main equipment in Portland cement factories. A special and different family of the rotary furnaces are the relatively small and short units used for roasting of small quantities of minerals.

This article refers to roasters used to convert Molybdene Sulphide (also called Moly Concentrate – MoS2 ) into Molybdene Trioxyde (MoO3). This process results in emission of Sulphur Dioxide gas. We intend to make Sulphuric Acid out of it, the concentration of the SO2 in the flue gas must be kept high. This can only be achieved by avoiding heating by fire and also by using enriched air ( ~ 90% Oxygen ) instead of atmospheric air. How do we heat such a furnace without a fire?  We need indirect heating so here comes in the design of Electrical Induction Heated Rotary Furnace. It does not use direct heating by fire nor external heating by radiative elements.

Description of the Electrical Induction Rotary Roaster
Electrical inductors are very common in the metallurgic industry. They usually consist of a Copper coil surrounding a tube, where AC electrical current in the coil induces a varying magnetic field that in turn induces stray electrical currents in the furnace body thus heating it. This design avoids the traditional heating elements with the associated ceramic insulation and radiative high temperatures, by using the furnace body itself as the heating element.

A heavy duty thick wall special Carbon Steel rotary tube serves as the furnace body. The tube is thermally insulated by high performance ceramic insulating material externally protected by glass fabric. The thickness of the insulation is ~ 25 mm. It is intended to enclose the hot furnace tube, prevent heat loss, protect the environment and prevent atmospheric Oxygen access to the hot iron tube. It must however be totally non-magnetic and therefore, its cladding cannot be made of any metallic plate.

The induction coils are located outside of the thermal insulation. A square Copper tube is formed into coils. Usually the Copper tube will be 20X20X2 mm. The coils are built as elements, each consisting of 7-12 rounds, each round spaced ~ 50 mm from the former round. The diameter of the coil is ~ 125 mm larger than the O.D. of the furnace steel tube. Each element is between 400-600 mm long. A furnace may have ~ 16 elements, usually grouped into 3 zones – Upper, Middle and Lower. Within each zone the elements are electrically connected in series.   Each element is cooled by cooling water flowing inside the square copper tube.

The electrical system is designed to heat each zone separately. It is a sophisticated electrical system, converting the 3-phase power into single phase by the use of chokes and capacitors. For temperatures of the Carbon steel body up to 600-750 degC a frequency of 50-60 Hz is sufficient. Higher temperatures require higher frequencies because of the specific magnetic properties of the steel changing at high temperatures. The 50-60 HZ system is very simple which gives it a big advantage.

The process:
Roasting of Molybdene Concentrate is performed around 600 degC. The reaction is exothermic. Therefore, at start-up, full power is applied all along the furnace to heat it to the process temperature. The heavy furnace requires some 24 hours to heat up. Once hot, raw material is fed into the furnace. At the upper side the material is gradually heated while losing its volatiles. At approximately 30% of its way down the material reaches 400 degC and starts burning. Sulphur Dioxide (SO2 ) is emitted. While burning takes place the material produces heat and does not need additional heat supply.  The main reaction is completed in the middle section of the furnace. At the lower section additional heat must be continuously supplied as the very small burning rate of the remaining Sulphur cannot provide the needed heat. For these reasons, the furnace heating is divided into zones which are separately regulated. The ability to do this enables considerable power saving by utilizing the exothermic reaction heat to serve as the main heat source for the process. Variations in the chemical composition of the feed material and repeated roasting of off-spec product do however require that the furnace will be properly equipped for full scale heating.
During normal operation the temperature profile is as follows :
-        Inside the furnace : ~600 degC.
-        The furnace steel body under the insulation : ~ 580-650 degC.
-        Outside the thermal insulation : 40-50 degC.
-        The Copper coil : Actually same as the cooling water i.e. 20-25 degC.

Problems and corrosion.
Internal corrosion is unavoidable with this type of furnace. However, sticking of material to the furnace surface greatly reduces this effect. Typically, a furnace tube may need replacement after continuous running of 1.5-2 years. However, service times as short as 6 months have also been recorded.

External corrosion should be prevented by the thermal insulation which practically prevents access of Oxygen to the hot iron surface. Faulty thermal insulation may be a contributing factor to considerable external rusting resulting in short furnace tube lifetime.

The cooling of the furnace takes a few hours. Burning inside the furnace continues even when the feeding is stopped.  In case of power failure, emergency power is provided for furnace rotation and running of the I.D. fan. Operation staff do not like to interfere with the operation of a stable running furnace so they try not to stop feeding. It is therefore required to provide cooling water to the furnace even at times of power failures. Such a design requirement further complicates the system.
-        Any blockage of the water flow through an induction coil is problematic.
-        Any water leakage from an induction coil is a safety hazard.
-        If the thermal insulation is damaged heavy rust pieces fall off the furnace shell onto the internal side of the induction coils. These cause short circuits which, in turn, might perforate the Copper tube causing water leakage.
-        The induction coils are hidden behind safety panels also called decorative panels. Any maintenance work on the furnace requires temporary removal of these panels. Once removed, and especially if maintenance becomes frequent, they tend not to come back – causing further safety issues.

Maintenance and management
The furnace is a heavy piece of equipment. If an induction coil has to be repaired or replaced or if the thermal insulation requires attention, the furnace must be cooled down and stopped. Now some coils must be removed. This requires temporary support of the lower side of the tube, removing the low end tire and pair of rollers, dismantling of the discharge hood and associated equipment, disconnecting the coils from electricity and cooling water, dragging the coils out until the problematic area is reached, maintaining whatever needs to be done and then re-installing everything and re-heating of the furnace. This means a minimum shutdown period of 6 days before feeding could resume.

When such preventive maintenance is required, it is typically happening when production is urgently required and customers are yearning for product. The management will not approve a 6 day shutdown. Therefore, the problematic coil is bridged electrically, cooling water flow to the damaged coil is stopped and the furnace continues to run with less one coil. The problem which was thus ignored does not go away. It causes more damage until there is no more an option to delay.

The air-cooled split-able induction heating coil design changed our plant’s life.

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