Continuous homogenizing

The diagram below shows the temperature cycle in a continuous homogenizing plant. Logs travel through the furnace and the cooling station on one level. First up, the furnace comprises a heating and a holding compartment. The heating compartment ensures the logs reach a uniform specified temperature. Installed at the end of the heating compartment, sting thermocouples verify the temperature of each log. Next, the logs travel to the holding compartment. Here they are kept to precisely the set temperature throughout the holding time. This period can be from 2 to 9 hours. Finally, the cooling station cools the logs according to the selected regime. Simply adjusting the fan speed controls the cooling rate in the air cooler. All this ensures the heating, holding, and cooling parameters are exactly the same for every log.

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Heating regimes

Usually, logs are heated swiftly to homogenizing temperature within around 1.5 to 4 hours – depending on the log diameter. Holding times vary according to the alloy and recipe. Also available are plants engineered to meet special heating requirements. There is a plant in Norway for instance that processes hard ZnMg alloys. The furnace is capable of slow heating in the critical temperature range to avoid cracks caused by inner tensions (green curve (b) instead of standard red curve (a)).

Perfect holding means precise temperature control over the entire length of every log. Temperature deviations are within 2°K, and guaranteed within +3°K. Current log temperatures are measured by several sting thermocouples installed in strategic places, i.e. at the entry to the holding zone and before transfer to the cooling station. A dedicated algorithm controls the temperature.

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Cooling regimes

Heating and holding are not the only factors required for top extrudability and mechanical properties. Equally important is billet cooling after homogenizing. There are many different cooling regimes in use – depending on metallurgical requirements and your recipes. So a wealth of different coolers have been built to date.

Typically, log temperature uniformity is superior in a continuous cooler than in a batch-type cooler. Due to this variety of cooling regimes, cooling stations vary widely in design and construction. We distinguish between open and closed types, as well as coolers with built-in heating devices. Specifically for countries with cold climates, HE builds cooling stations with integrated waste-heat recovery. Here, cooling air is recirculated until it is suitable for heating buildings.

Key benefits

  • Perfectly uniform metallurgical property of the billet due to precise and reproducible temperature regime during heating, holding, and cooling.
  • Personnel cost savings
    Batch furnaces require one or two operators per shift (to build the log stacks, transport and remove the batches). In contrast, continuous homogenizing plants operate with practically no personnel. Logs stripped from the casting pit enter the log laydown station. Then, after homogenizing, heat-treated logs automatically arrive at the billet saw. The plant operates fully automatically, including heat treatment and cooling as well as all integrated log handling operations. No operators are required.
  • Best log straightness
    Perfect straightness is an important issue whenever a hot shear is used at the extrusion press. A conti- furnace straightens bent logs, effectively reducing the reject rate.
  • Low downtime maintenance
    Plant availability is 99%! Batch-type furnaces are typically subjected to periodic temperature change cycles. Crucially, continuous furnaces operate at a practically constant temperature level, which means much lower thermal strain and wear on the equipment.
  • Control software
    Invaluable to every Hertwich plant is the extensive automation. It ensures fully automatic operation with no operator intervention. That includes re-starting the plant. This completely eliminates human error. Malfunctions are reported on screen, with all the necessary information for easy troubleshooting.
  • Low energy consumption
    An electric conti- furnace consumes around 210 kWh of electric energy per ton of aluminum. In contrast, a gas-heated furnace of the same size with recuperative burners requires 230kWh of natural gas and 35 KWh of electric energy per ton of aluminum. Specific electric power consumption is considerably lower than for a batch-type furnace.