TECHNICAL ARTICLE:

Increasing quality demands on copper tubes require the development of plants with a view to closer quality tolerances and higher process reliability. The ever increasing automation and process control help to achieve high quality standards while keeping energy consumption and operating costs low.

Within the process chain of copper tube manufacturing, OTTO JUNKER design and manufacture induction melting and pouring furnaces, billet heater plants (both induction heated and gas-fired), roller hearth furnaces,
i.e. important, tailor-made furnace and heater plants which have been continuously developed further together with our customers and which contribute significantly to the quality of the final product .

The melting and pouring process at the beginning of the manufacturing cycle irreversibly determines the material quality with regard to analysis accuracy and purity.

The technical and economical advantages offered by the induction melting and pouring furnaces have made them ever more popular. Induction heating allows for an accurate temperature regime and process control, low firing losses and precisely controllable bath movement.

The transition towards the digitally controlled medium-frequency technology has vastly improved process technology (e.g. by using the OTTO JUNKER multi-frequency technology) and considerably increased the power density.

The two basic designs - coreless furnace and channel furnace - allow for picking the furnace type best suited for the requirements. Special furnace designs have been developed for producing alloys with particularly low oxygen and gas contents.

For the heating of extrusion billets, OTTO JUNKER is the only manufacturer worldwide supplying heater plants with either induction heating or gas-firing so that the best-suited type of equipment for a particular requirement, or a combination of the two, can be selected.

The main advantages of induction heating are the exact temperature regime, better flexibility, higher temperatures and smaller space requirements. Gas-fired heaters mainly distinguish themselves by their low energy costs.
On induction billet heaters a saving of 20 % in electric energy could be achieved by using the OTTO JUNKER multi-layer coil. Recent developments for the preheating of billets in gas-fired heater plants have considerably increased the overall efficiency to up to 70 %.

The annealing at the end of the treatment process crucially influences the mechanical properties as well as the surface quality. The obvious advantages of the roller hearth furnaces as compared to bell-type and chamber-type furnaces such as low energy consumption, high throughput, low space requirement and good automation possibilities have made them increasingly popular. Out of the huge number of OTTO JUNKER developments we wish to refer to the OTTO JUNKER Copper Tube Purging system (CTP system). This system not only automatically reduces the oil residues but also allows for a precise process control.

Induction Melting and Pouring Furnaces

In the past couple of decades induction furnaces have become increasingly popular. As compared to fuel-fired furnaces, their main advantages are the direct heating of the load (little overheating), the exact temperature regime and the precisely controllable bath movement resulting in low fire losses and being friendly on the environment and working conditions regarding heat, dust and noise. Last but not least, the induction furnace stands out because of its extremely neutral metallurgical behaviour. Induction heating allows for an efficient and high-quality melting control and process automation. These advantages have been extended further with the transition from the traditional mains-frequency technology to the digitally controlled medium-frequency technology.

1. Designs and their Applications

The two basic types of induction melting and holding furnaces - the coreless furnace and the channel furnace - are considerably different in design and allow for picking the type of furnace suitable for a particular requirement.

Due to the better electric efficiency, the power consumption of the channel furnace related to copper is lower by almost 100 kWh/t. The possible power density of a channel furnace is limited by the max. power per inductor and, of course, by the number of inductors. Currently, the max. inductor rating for the melting of copper is approx. 2,500 kW. For design reasons, no more than two high-power inductors per furnace vessel should be used.

The power density of the coreless furnace is limited by the flux level or bath movement which in turn goes up with increasing power and goes down with increasing frequency. The upper limit for mains-frequency plants is 200-250 kWh/t, but with medium-frequency plants with 250 Hz power densities of 500 kWh/t are fairly common without the risk of metal throw-outs. The coreless furnace also offers the possibility of setting or changing the metallurgically required bath movement by selecting or changing the frequency (multi-frequency technology).
Due to the intensive bath movement, the coreless furnace is undoubtedly the ideal melting tool for small-size material and swarf

The transfer of molten copper to the continuous casting machine can be direct from the melting furnace via launders or by putting in a pouring or holding furnace. Separate pouring and melting furnaces might make sense for metallurgical and production technology reasons. This furnace can both be used as an alloying furnace, e.g. for desoxidation, or as a buffer vessel and for homogenizing. If there are special requirements to the copper quality, a pouring furnace with fore hearth and nozzle/stopper discharge offers advantages.

2. Examples of Application We had a special requirement for our medium-frequency coreless furnace concept where a free falling pouring stream during transfer of the copper to the mould support table was supposed to be avoided in order to keep gas pick-up to a minimum. Pouring is effected through the pivot joint of the furnace directly into the tundish (see fig. 1) This technology considerably improves the quality of the oxidation-prone alloy. Such a medium-frequency furnace plant consisting of two furnaces (16 t and 18 t) with a 3,000 kW switchgear, a frequency of 250 Hz and a JOKS furnace control system for fully automatic melting control has been supplied to Messrs. Hitachi Cable.

Figure 1: Example of a melting furnace with puring through the pivot joint

A similar plant, consisting of a 20 t furnace with a power rating of 3,500 kW has been supplied to Messrs. Kobe Steel recently. An energy consumption of 380 kWh/t for the melting of pure copper to 1,200°C was achieved.

Now let's look at an application for channel induction furnaces for the melting and pouring of oxygen-free copper, i.e. a combined melting and pouring plant with particularly stringent requirements with regard to the protection against gas pick-up and avoiding metal transfer locations. Fig. 2 shows the recently commissioned plant at Norddeutsche Affinerie in Hamburg. The plant consists of a drum-type melting furnace with a capacity of 55t (useful capacity 35t) with two channel inductors of 2,050 kW each and a melting rate of 15 t/h. Discharging is via a pouring siphon through the pivot joint via a ceramic tube into the pivot joint of the pouring furnace. The pouring and holding furnace has got a capacity of more than 45 t at a useful tapping weight of 35 t.

Figure 2: Melting and pouring furnace at Norddeutsche Affinerie in Hamburg

This furnace is heated by a 2,050 kW inductor identical in design. The melt is supplied to the continuous casting machine via the nozzle/stopper system in the fore hearth. For modification and maintenance work the pouring furnace can be moved by 6 m. The contractually agreed energy consumption for melting to 1,200°C of 285 kWh/t was fallen below during all the tests.

Induction and gas-fired rapid billet heaters

The most common types of heaters for the heating of copper billets for extrusion presses are gas-fired rapid heaters and induction billet heaters. Both offer a number of significant advantages i.e. short heating-up times, fully automated operation, homogeneous temperature distribution, good adaptation to various operating conditions and easy use, little maintenance and low energy consumption.

1. Induction billet heaters

An induction billet heater induces electromagnetic energy directly into the billets and not only to its surface. The heater usually has several individually controllable heating zones, each with a coil, a thyristor switch and a pyrometer. To reduce energy consumption by 20% compared to conventional induction heaters the OTTO JUNKER coils are usually designed as multi layer coils. Heating copper to 900°C requires approx. 180 kWh/t.
In induction billet heating two different types of heater are used, each with a well defined field of application.

A single billet heater has up to 6 independent sections which allow for the production of the desired axial temperature distribution in the billet. This is often required to optimize the poor temperature distribution of billets coming from older gas-fired heaters and increase press throughput. The main advantage, apart from the good temperature quality, is the flexibility of the heater. It is perfect for short heating-up times and huge variations in temperature and alloy. As compared to multi-billet heaters, the productivity is reduced because of loading and unloading times. To increase production several single billet heaters can be arranged in parallel.

The most common and fastest type of induction heater in the brass and copper industry is the multi-billet heater (see fig. 3). This heater is designed as continuous heater plant. The row of billets in the heater shows an almost linearly rising temperature along the length of the heater. The billets are pushed forward in small steps through the heater until they reach the resistance-heated soaking chamber at the hot end. This chamber is a buffer for at least 3 billets and compensates for a varying demand of billets, press problems or other breaks. In this case the production is automatically slowed down by the heater control always ensuring the correct temperature gradient. Therefore a restart after longer breaks is possible even without being forced to sort out billets. The latest installation of such a heater for 14 t/h has been commissioned at IUSA, Mexico.

Figure 3: Multi billet heater for brass billets at Nordic Brass, Sweden

2. Gas-fired rapid heater

The gas-fired rapid heater is the counterpart to the multi billet induction heater in the fuel heating field. As far as their design is concerned, both types of heater are similar to each other and regarding their modes of procedure they are even virtually identical. However, gas-fired heaters are considerably longer than induction heaters as their power density is approx. 3 to 4 times lower.

Figure 4: Gas fired rapid heater with preheating zone

Heating is effected by means of a large number of swirl-type burners with a gas/air premix. Alternatively, heating can also be effected by a considerably lower number of flat flame burners operating with preheated combustion air (see fig. 4), where the mixing of gas and air takes place in the burner proper, thus achieving a better efficiency in the heating section.

The hot exhaust gases from the heating section are used to preheat the row of billets in order to improve the overall efficiency. OTTO JUNKER offers two preheating zone alternatives, i.e. the high convection preheating zone and the counterflow preheating zone.

For conveying through the heater plant, OTTO JUNKER offers two types of conveyor system, i.e. the walking beam for billet temperatures up to 1,050°C and the roller transport system for lower temperatures up to 800°C.

10 gas fired heaters with throughput rates of up to 30 t/h have been commissioned within one year for Wolverine Tube, Mueller Copper Tube, Chase Brass and Cupro San Luis.

3. Heater combinations

Looking at the dimensions of induction heaters and gas-fired rapid heaters, it is pretty obvious that the induction heater is considerably better as far as its flexibility is concerned. Apart from that, the temperature measurement and control are easier on the induction heater as it is not affected by the flame influence. Another advantage offered by the induction heater is the possibility of running higher heating temperatures.

Considering higher energy cost for the induction heater it sometimes makes sense to use a combination of both types of heater in order to meet the requirements. In that case, of course, each type of heater will be used in areas where its advantages dominate, i.e. basic heating to as high a temperature as possible in the gas-fired rapid heater and final heating to set point temperature in the flexible induction heater. Such an installation has been supplied to Halcor, Greece.

High-convection roller hearth furnace

Copper tube annealing has in recent years been carried out almost exclusively in roller hearth furnaces, that is in continuous furnace lines. Over the last couple of decades OTTO JUNKER has supplied a large number of high-convection roller hearth furnaces (see fig 5) for the bright annealing of copper tubes with throughput capacities ranging from 1.5 to 6.5 t/h.

Figure 5: Roller hearth furnace with automatic materials handling system

1. Layout and working principle

Product arrangement is similar to that in a bell furnace, that is multi-tray stacks. This applies primarily for level-wound coils but the same principle can also be used for pancakes and straight length tubes.

Heat transfer is almost exclusively by high convection. For this purpose the furnace has powerful atmosphere circulating fans which create an atmospheric flow through the load from the bottom to the top. This offers the following benefits:

Due to the high air velocity a low temperature head of normally 5 K above the required product temperature is sufficient to achieve full throughput. Temperature distribution in the furnace is extremely uniform due to the defined atmosphere flow circuit. This provides for uniform heating of the product.

The equipment comprises of the following items (see fig. 5 and 6):

vacuum lock chamber on both ends of the furnace high-convection furnace and cooling zone tray return track with cross conveyor, stacking and destacking devices CTP system atmosphere cleaning system water recooler system electrical and temperature control equipment by PLC, visualisation and data logging

Figure 6: 3 t/h stacker type furnace for the annealing of level wound copper tube coils

Fig. 5 shows a furnace installation for the bright annealing of 5 t/h of copper tubes. It was installed and commissioned in 2001 at one of the largest copper tube producers in China. The equipment is controlled by two operators. It is arranged on a floor area of 60 m x 12 m and has a total energy consumption of approx. 220 kWh/t.

2. The copper tube bore purging system

The increasing quality demands on copper tube manufacturers especially by the air conditioning and refrigeration industries lead to the development of the OTTO JUNKER copper tube purging system.
It reduces the carbon deposits and residual oil on the inside surface of the heat-treated tube by up to 90% as compared to furnaces without this purging system.

Expulsion of the drawing oil residues is achieved by purging the coiled tube with inert gas. During loading the tubes are connected to gas distribution manifolds in the tray at one or both ends via flexible gas hoses. Inside the furnace the tray is automatically connected to a purging system in selected positions during the heating-up phase.

The principle of purging at one end with blowing out the oily purging gas into the furnace atmosphere has been a proven mode of procedure for many years. A further development is a tube bore purging system with both coil ends connected to an inert gas supply and discharge system. In this case the inert gas blown through the coils is discharged outside the furnace. The advantage of this system is that exact monitoring of the purging operation is possible.

Credits

Paper by: Steffan Dappen, Willi Johnen, Dietmar Trauzeddel and Gunther Voswinckel of The Otto Junker Group, Germany