TPT March 2007

The system has conclusively demonstrated its reliability, accuracy and economic efficiency and, needless to say, it is likewise suit- able for temperature monitoring in channel-type induction furnaces, eg in monitoring the inductors.

3.1 The Otto Junker high-convection heating and cooling system Product arrangement is similar to that in a bell furnace, which has multi-tier stacks. This applies primarily to layer-wound coils but the same principle can also be used for pancakes and straight tubes. Heat transfer is almost exclusively by convection. For this purpose the furnace has powerful circulating fans. Atmosphere flow through the load is from bottom to top. This offers the following benefits: • Due to the high air velocity a high temperature head is not needed to reach full throughput (5 Kelvin above product temperature is normally sufficient) • Temperature distribution in the furnace is extremely uniform due to the short air flow circuit. This provides uniform heating of the product • No overheating of the product • Short overall furnace length • The high-convection

€ Figure 8 :

Visualisation of temperature profile

3. Final annealing Finished pancakes and level-wound coils are normally delivered in soft state, which is why they must undergo an annealing process. Copper tube annealing has in recent years been carried out almost exclusively in roller hearth furnaces that are in continuous furnace lines. Over the last couple of decades, Otto Junker has supplied a large number of high-convection roller hearth furnaces for the bright annealing of copper tubes with throughput capacities ranging from 1.5 to 7.2t/h. The main advantages over batch furnaces (chamber and bell type) are: a) Reduced heating energy consumption as the furnace is maintained at a constant temperature. The continuously operating roller hearth furnace dispenses with the need for cyclical heating and cooling of the inner bell and furnace base and the repeated heat-up of the furnace bell to setpoint temperature. b) Significantly reduced process cycle times by elimination of heating and cooling of the furnace inner bell and base station. This leads to a handling time otherwise required for positioning of heating and cooling bells. c) Reduced capital cost for throughput capacities in excess of about 1t/h d) Reduced floor space requirements Figure 10 shows a furnace installation for the bright annealing of 5 t/h of copper tubes. It was installed 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.

system takes the heat from the electric or gas-fired radiant tubes uniformly and thus keeps their outside temperature low. This adds to the long radiant tube lifetime The cooling zone is of same design as the hot zone of the furnace line. The radiant tubes are replaced by water- carrying cooler banks.

› Figure 11 : Cross sectional view of the furnace

Comparison of heat transfer by radiation versus high convection Many conventional roller hearth furnaces for copper tube annealing transfer the heat in the hot zone by radiation. However, for heat transfer by radiation it is necessary to have a furnace temperature well above the product temperature in order to reach a reasonable throughput rate. This system works to some extent with straight tubes and single-layer pancake loads.

fi Figure 12 : Conventional technology (radiation) + Junker technology (high convection)

€ Figure 10 :

5t/h stacker type furnace for the annealing of level wound copper tube coils

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M arch /A pril 2007