Transfer type plasma heating anode

Details Transfer type plasma heating anode Transfer type plasma heating anode

2001-08-10

2003-11-18

Paschall, Mark (Department: 3742)

Electric heating

Metal heating

By arc

C219S121520, C219S075000, C219S119000

Reexamination Certificate

active

06649860

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an improvement in a transferred plasma heating anode and, particularly, to a transferred plasma heating anode suitable for heating a molten steel in a tundish.
BACKGROUND ART
FIG. 1
shows a direct current twin-torch plasma heating device used for heating a molten steel in a tundish. Two plasma torches, an anode
3
and a cathode
4
, are inserted through a tundish cover
2
, and a plasma arc
6
is generated between the torches
3
,
4
and a molten steel
5
to heat the molten steel. An electric current
7
flows from the cathode
4
to the anode
3
through the molten steel
5
.
One example of an anode plasma torch is shown in FIG.
2
.
FIG. 2
shows a cross section of the tip end portion of the anode torch. For example, oxygen-free copper is used as a material for the anode
3
. The anode torch comprises an outer cylinder nozzle
8
that is made of a stainless steel or copper and that covers the outside and the anode
3
that is made of copper and that is situated inside the torch. The tip end portion of the anode
3
is in a flat disc-like shape. Both the anode
3
and the outer cylinder nozzle
8
each have a cooling structure. The inlet side and outlet side water paths of cooling water of the anode
3
are partitioned with a partition
9
; the inlet side and outlet side water paths of cooling water of the outer cylinder nozzle
8
are partitioned with a partition
11
(reference numerals
10
,
12
in
FIG. 2
indicating the flows of cooling water). There is a gap
13
between the outer cylinder nozzle
8
and the anode
3
, and a plasma gas is blown from the gap
13
.
One of the problems associated with the direct current anode plasma torch is that its life is short because the anode tip end is damaged. Because the anode becomes a receiver of electrons during plasma heating operation, electrons strike the external surface of the anode tip end, and the thermal load applied to the tip end external surface becomes significant.
Moreover, the thermal load applied to the anode tip end is as large as several tens of megawatts/m
2
, and the form of heat transfer on the cooling side at the anode tip end is thought to be a heat transfer through forced-convection nucleate boiling. When the heat transfer is through forced-convection nucleate boiling, the heat transfer rate is a magnitude of 10
5
[W/m
2
K], and is about 10 times as large as that of a forced-convection heat transfer. When the thermal load applied to the external surface of the anode tip end becomes excessive, the temperature of the heat transfer surface on the cooling side rises, and a burnout phenomenon in which the heat transfer form changes from nucleate boiling to film boiling takes place. When the change takes place, the heat transfer rate rapidly lowers on the heat transfer surface, and the heat transfer surface temperature rises. Finally, the temperature of the anode tip end exceeds the melting point, and there is a possibility that the anode tip end is melted and lost.
For the conventional anode cooling water path structure shown in
FIG. 2
, a thermal load that causes burnout, namely, a burnout critical heat flux is shown in FIG.
31
. In the graph shown in
FIG. 31
, a radius on the tip end cooling side of the anode
3
in which the maximum radius Rcool on the tip end cooling side thereof is 22 mm is taken as abscissa, and a burnout critical heat flux is taken as ordinate. Zenkevich's formula (Zenkevich et al, J. Nuclear Energy, Part B, 1-2, 137, 1959) is used for estimating the burnout critical heat flux, and the burnout critical heat flux W
B0
[W/m
2
] is expressed by the formula (1):
W
BO
=L
{square root over ((&sgr;
G
/&sgr;))}(2.5+184(
i−i
cool
)/
L
)×10
−5
  (1)
wherein L, &sgr;, G, &ngr;, i and i
cool
in the formula (1) are physical quantities, L is a heat of vaporization [J/kg], &sgr; is a surface tension [N/m], G is a weight speed [kg/m
2
s], &ngr; is a kinematic viscosity [m
2
/s], i is an enthalpy [J/kg] and i
cool
is an enthalpy [J/kg] of a main stream. It is seen from the graph in
FIG. 31
that the burnout critical heat flux near the center is low. The heat flux is low because the influence of the flow rate of the cooling water flowing in the anode
3
is significant. The cooling water flowing from the upper side of the anode in the central portion strikes the anode tip end to lower the flow speed. As a result, the burnout critical heat flux is also lowered. When the thermal load applied to the external surface of the anode tip end exceeds the burnout critical heat flux, it is estimated that burnout takes place on the cooling side of the anode tip end to raise the heat transfer surface temperature and to melt the anode tip end. The central portion of the anode tip end where the burnout critical heat flux is low therefore tends to be melted and lost.
Moreover, when transferred plasma heating is conducted, heat tends to concentrate on the central portion of the external surface of the anode tip end. Furthermore, when a current concentration site (anode spot) is once formed on the anode surface, current further tends to concentrate on the anode spot. That is, when damage begins to be formed on the external surface of the anode tip end due to melting, formation of the damage is further promoted, and the damage finally reaches the cooling water side to end the life of the anode.
FIG. 3
illustrates the pinch effect associated with plasma. A flow
14
of a gas having temperature sufficiently lower than that of plasma
15
blown from a gap
13
between an outer cylinder nozzle
8
and an anode
3
concentrates the plasma
15
in the central direction (thermal pinch effect). In general, the current density in plasma is described as an increasing function of temperature, and the current density in a plasma central portion
16
is large in comparison with the average. As a result, the current density incident on a central portion
17
of the external surface of the anode tip increases. Accordingly, the degree of damage is large in the central portion
17
on the external surface of the anode tip end in comparison with a peripheral portion
18
of the external surface at the tip end. Moreover, electrons
21
moving toward the anode in the plasma receive a force
22
directing toward the central portion by interaction with a rotating magnetic field
20
produced by a current
19
flowing in the plasma (magnetic pinch effect).
Furthermore, as shown in
FIG. 4
, the anode tip end is outwardly deformed in a protruded shape by the pressure of the cooling water flowing inside, thermal stress and creep. The protruded deformation forms a projection
23
in the central portion
17
of the external surface of the anode tip end. As a result, an electric field
32
is concentrated on the projection
23
. Since electrons
21
moving in the plasma are accelerated in the direction of the electric field
32
, the current
19
is concentrated on the projection
23
. Accordingly, the electric current is further concentrated on the central portion
17
of the external surface at the anode tip end. That is, the central portion
17
of the external surface at the anode tip end is further likely to be damaged. When the damage is increased in the central portion
17
of the external surface at the anode tip end, a cooling water path
25
of the anode is finally broken, and operation becomes impossible. As explained above, as a result of concentrating an electric current on the central portion
17
of the external surface at the anode tip end, the life of the anode is significantly shortened.
FIGS.
5
(
a
) to
5
(
d
) illustrate the concentration of an electric current on an anode spot. In an initial state (FIG.
5
(
a
)) in which the cleanness of an external surface
26
of the anode tip end is excellent, electrons
21
are approximately vertically incident on the external surface
26
. However, as explained above (see FIG.
4
), an electric curren

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