Hood-type annealing furnace and method for operating the same

A hood-shaped annealing furnace for the thermal treatment of cold-rolled steel strip, the hood-shaped annealing furnace comprising:

• a base;
• a protective hood configured to be placed on said base and to receive the steel strip under said protective hood;
• a heating hood surrounding said protective hood and defining a combustion space between said protective hood and said heating hood;
• said combustion space configured to contain a flame and hot exhaust gas of said gas burner acting with thermal energy upon an outside of said protective hood;
• a gas burner having a heating capacity and being configured to heat said protective gas atmosphere;
• a blower configured to circulate the protective gas atmosphere heated by said gas burner under said protective hood and to heat the steel strip under said protective hood to a temperature of at least 500° C. in the protective gas atmosphere;
• and
• an electric heater fed by a public power network and having a heating capacity corresponding substantially to said heating capacity of said gas burner, said electric heater configured to independently heat the protective gas atmosphere as an alternative to said gas burner to a temperature above a recrystallization annealing temperature with said gas burner switched off;
• said electric heater being accommodated at least one of outside a stream of the exhaust gas below said gas burner or in a portion of said heating hood set back radially with respect to said combustion space;
• said electric heater including a multiplicity of NIR emitters configured to act directly with infrared radiation upon an outside of said protective hood.

A hood-shaped annealing furnace for the thermal treatment of cold-rolled steel strip, the hood-shaped annealing furnace comprising:

• a base;
• a protective hood configured to be placed on said base and to receive the steel strip under said protective hood;
• a gas burner having a heating capacity and being configured to heat a protective gas atmosphere;
• a blower configured to circulate the protective gas atmosphere heated by said gas burner under said protective hood and to heat the steel strip under said protective hood to a temperature of at least 500° C. in the protective gas atmosphere;
• an electric heater fed by a public power network and having a heating capacity corresponding substantially to said heating capacity of said gas burner, said electric heater configured to independently heat the protective gas atmosphere as an alternative to said gas burner to a temperature above a recrystallization annealing temperature with said gas burner switched off;
• and
• a closed circuit including a heat exchanger disposed outside said protective hood and a further blower configured to circulate the protective gas atmosphere through said heat exchanger;
• said electric heater configured to act with thermal energy upon said heat exchanger from outside to heat the protective gas atmosphere circulated through said heat exchanger.

A method for operating a hood-shaped annealing furnace for the thermal treatment of cold-rolled steel strip, the method comprising the following steps:

• providing a hood-shaped annealing furnace including:
• a base,
• a protective hood configured to be placed on the base and to receive the steel strip under the protective hood,
• a gas burner having a heating capacity and being configured to heat a protective gas atmosphere,
• a blower configured to circulate the protective gas atmosphere heated by the gas burner under the protective hood and to heat the steel strip under the protective hood to a temperature of at least 500° C. in the protective gas atmosphere, and
• an electric heater fed by a public power network and having a heating capacity corresponding substantially to the heating capacity of the gas burner, the electric heater configured to independently heat the protective gas atmosphere as an alternative to the gas burner to a temperature above a recrystallization annealing temperature with the gas burner switched off;
• upon a surplus of electrical energy occurring in the public power network, connecting the electric heater to the public power network using a controller until the coils have assumed the recrystallization annealing temperature or a stipulated maximum temperature lying above the recrystallization annealing temperature;
• and
• subsequently breaking the electrical connection between the electric heater and the public power network using the control device and making at least one further electrical connection to an electric heater of at least one substantially identically constructed further hood-shaped annealing furnace, until reaching a reduction in the surplus of electrical energy in the public power network.