Metallurgical apparatus – Process
Patent
1996-02-16
1998-03-24
Kastler, Scott
Metallurgical apparatus
Process
266252, 266256, C21D 967
Patent
active
057309302
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for exchanging the atmosphere in a hood-type annealing furnace, the fresh gas being blown into the furnace from below and the gas mixture forming being evacuated from the furnace from below. Furthermore, the present invention relates to a hood-type annealing furnace to perform this method, with a pedestal, a plate to carry the annealing charge arranged on the pedestal, a blower arranged below a central aperture of the annealing charge plate, a hood surrounding the annealing charge and the annealing charge plate at a distance and at least one gas inlet and one gas outlet arranged in the pedestal.
Hood-type annealing furnaces are used in particular for heat-treatment of sheets which are wound in coils. The latter are stacked on top of each other on the annealing charge plate, their central "eye" forming a channel in the continuation of the central aperture of the annealing charge plate. Once the hood has been placed over the fresh charge, the air present in the furnace is exchanged for protective gas. Then the annealing treatment begins, the blower sucking in the gas centrally downwards from above and carrying the gas sideways into the ring-shaped space between the annealing charge and the hood.
At least one exchange of the atmosphere of the hood-type furnace is necessary per charge. If hydrogen is used, this procedure must be performed many times. The air must be evacuated from the furnace before heating commences. This is performed by blowing in nitrogen. Then hydrogen is fed in which dispels the nitrogen. After the annealing treatment has been completed, the hydrogen has to be dispelled by blowing in nitrogen before the hood can be removed.
When the atmosphere is exchanged, the fresh gas is blown into the furnace below the blower and carried by the blower into the ring-shaped space between the annealing charge and the hood. At the same time, the gas mixture forming is evacuated through the gas outlet.
The amount of gas entering the furnace when the atmosphere is exchanged is quite considerable. The volume flow rate is 100 to 180 m.sup.3 /h for a period of 20 to 40 minutes under normal conditions. However, this presupposes that the blower is fully functionable. If the blower fails, the rinsing time is approximately 24 hours. This loss of operating time is a very important factor. In addition, the gas consumption is quite nconsiderable.
The object of the present invention was to find a remedy and permit more cost-effective and, above all, quicker exchange of the atmosphere.
This object is solved by the method according to the present invention, characterised in that the fresh gas is blown in as an upwardly directed high-speed jet in a flow pathway leading upwards and that the following equation applies to the velocity of the jet. ##EQU1## where V.sub.f =free furnace chamber volume in litres
It may, under certain circumstances, be advantageous to generate a pulsating high-speed jet.
The inventive device is characterised in that the gas inlet is designed as an upwardly directly nozzle which terminates in a flow pathway leading upwards.
The present invention is based on the knowledge that the velocity of the jet entering the furnace decisively influences the exchange of the atmosphere and that, when optimising the velocity, the density of the gas to be rinsed out and the density of the fresh gas are to be taken into consideration in accordance with the above equation.
Surprisingly, it is possible with the inventive measures to accelerate exchange of the atmosphere to an extent which could not have been predicted. After only approximately 3 to 10 minutes, the desired residual content of gas to be replaced of approximately 4% has been achieved. A particularly surprising effect is that this result is achieved more or less regardless of whether the blower is running or not. This means that the rinsing time is shortened from 24 hours to approx. 5 minutes in cases where the blower is not working.
Apart from this really enormous time-saving, there is also a corresponding savi
REFERENCES:
patent: 4596526 (1986-06-01), Soliman
patent: 4846675 (1989-07-01), Soliman
patent: 5112030 (1992-05-01), Tahara et al.
patent: 5380378 (1995-01-01), Hemsath
Kuhn Friedhelm
Scheuermann Walter
Velten Georg
Kastler Scott
LOI Thermprocess GmbH
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