This report intends to analyze the energy losses in an Eletric Arc Furnace (EAF) due to its panels. The energy saving panels are made by two overlapped panels with the inner layer made with spaced tubes and the outer layer made of standard panel. Between the two layers, slag accumulation provides thermal insulation, minimizing energy loss. As result, these painels increase the arc power e reduct the energy loss, increasing the productivity.
In 2005, MacRosty wrote an article about EAF process and described the water-cooled panels process. According to him, the heat extracted by the cooling water from the roof and walls panels depends on the mass flowrate of water, its heats capacity and the inlet and outlet cooling water temperature diference. In this same article is explained that during the heat, the walls and roof do not heat up at the same rate as the steel due to cooling water pumped through the panels. In his experiment, for the first 15 min, the steel is heated via the burners and increasingly radiates heat to the walls and roof. After the power is turned on, the net radiative transfer is dominated by the radiation from the arc to the steel and other furnace elements. When the arc is boring into the scrap, the arc is shielded by the scrap, and therefore the majority of the energy from the arc is radiated to the scrap. From time 53min, the ability of the scrap to shield the walls decreases quite rapidly, and the incident radiation to the walls increases accordingly. The increase is a result of the walls being exposed as the steel melts, and the dead-time in the wall exposure is a result of the cone shaped void which results when the electrodes bore into the steel; thus the walls are largely protected until most of the scrap is melted. The presence of a foaming slag, which shields the walls from the arc, prevents the radiation to the walls from increasing even further. Finally, toward the end of the heat when the power is turned off, there is a net loss of radiative energy from the bath to the roof and walls.
Based on MacRosty’s considerations, this section will analyze the water-cooled panels behavior for the Gerdau’s EAF. The dataset used contains data from 3 different EAFs. The graph below shows the behavior of the water-cooled panels for each EAF.
As we can see in the plot, for each EAF the panel losses appears with a singular feature. As said by MacRosty, once there’s slag in the bath, the water cooled panels are covered by it and the energy that the system loses to the panels is reduced. A similar behavior appears to exist in the EAF 2. From time 41 e 48 min, there’s a fall in the energy transfered to the panels. For the EAF 3, this fall happens at the time 19 min. However it’s not enough to claim that at this point there’s a certain amount of slag.
Gerdau’s current model estimate the slag generation as the plot below shows.
##
## Pearson's product-moment correlation
##
## data: WCP11 and Slag11
## t = -6.1664, df = 298, p-value = 2.269e-09
## alternative hypothesis: true correlation is not equal to 0
## 95 percent confidence interval:
## -0.4331354 -0.2319908
## sample estimates:
## cor
## -0.3363942##
## Pearson's product-moment correlation
##
## data: WCP21 and Slag21
## t = -22.068, df = 298, p-value < 2.2e-16
## alternative hypothesis: true correlation is not equal to 0
## 95 percent confidence interval:
## -0.8271102 -0.7404437
## sample estimates:
## cor
## -0.787642##
## Pearson's product-moment correlation
##
## data: WCP31 and Slag31
## t = -16.918, df = 298, p-value < 2.2e-16
## alternative hypothesis: true correlation is not equal to 0
## 95 percent confidence interval:
## -0.7534692 -0.6372175
## sample estimates:
## cor
## -0.6999506As we can see, the Pearson correlation coefficient and its test prove that there’s a correlation between the slag generation and the water-cooled panels energy losses, although it varies as the EAF changes. However, the correlation seems to be always negative, indicating that when the panels aren’t covered the slag generation is smaller, which makes sense according to MacRosty’s explanation.
The graphs belows show some other variables, such as coke injection, total liquid phase, O2 injection decarburization, offgas and the arc stability.
To facilitate the comparison among the variables for each EAF the next plots can be used.
It is also interesting to take a look at the correlation between the slag generation and the other variables, as it follow.
## WaterCooledParts Slag Coke O2Decarburization
## -0.3363942 1.0000000 0.5200558 0.4315696
## OffGas ArcStability
## 0.1912613 -0.4593716## WaterCooledParts Slag Coke O2Decarburization
## -0.7876420 1.0000000 0.2603367 0.5317438
## OffGas ArcStability
## 0.3444622 -0.6919149## WaterCooledParts Slag Coke O2Decarburization
## -0.69995059 1.00000000 0.38213466 0.41096843
## OffGas ArcStability
## 0.02279609 -0.72651201Basically, the slag generation is correlated with all variables. However, it seems to be more significant with the water cooled parts and the arc stability. The liquid mass were ignored beacuse it’s a estimated variable. It is possible to conclude that the water cooled panels and the arc stability can be important variables to estimate the EAF temperature and the slag generation.