Determinación de falencias en competencias científicas de los estudiantes de la

As the induction furnace is operated with the large amount of temperature, heat balance of the furnace must be understood fully to make the proper decision about cooling effects inside the induction coil to resist the overheating condition and power source side such as frequency conversion equipment and power-factor improving capacitor. Efficiency of induction furnace is expressed as a total, deducting electrical and heat transfer losses. Heat balance diagram of crucible type induction furnace is shown is Figure 2.7.

Input Input 100% 100% Water-cooled Transformer Transformer (1)

cable _{Water-cooled cable} (1.5) (1) (1.5) Bus bar Coil Inverter _{Coil (17)} condenser (16) (4) (2) Bus bar condenser (2) Slag, etc. Heat conduction (1.5) ₍₇₎ Heat conduction Heat radiation (4.5) Total Total (3) efficiency efficiency Heat radiation 67% 69% (2) (b) Distribution of losses in (a) Distribution of losses in

low-frequency furnace. high-frequency of furnace.

Heat loss (%) is given in ( ). Heat loss (%) is given in ( ).

Figure 2.7. Heat Balance Diagram of Crucible Type Induction Furnace Source: Energy Conservation in Iron Casting Industry (1998)

In above figure, 100 percent of input energy is used fully in both of these furnaces; high-frequency and low-frequency crucible type furnace which have electrical and heat losses. Electrical losses consist of transformer, frequency converter, water-cooled condenser, bus bar, wiring, cable and coil. Loss in coil is an essential factor, on which the furnace capacity depends. Heat losses in induction furnace

consist of conduction loss of heat escaping from furnace wall to coil side, radiation loss of heat released from melt surface, absorption loss in ring hood and slag melting loss. Heat efficiency of high-frequency furnace (69%) is slightly larger than that of low-frequency furnace (67%). Low-frequency furnace is larger in heat loss (conduction and radiation) due to long melting time, while high-frequency furnace is larger in electrical loss (transformer, inverter and bus bar) due to short melting time.

To improve heat efficiency of furnace, the proper decision about the kind of material, size and shape of charging materials to be melted, melting amount, connection with pouring line and layout of the melting shop should be made and adjusted carefully by user’s side. Induction furnace equipment should be melted with minimum distance between each of equipment to reduce wiring losses. To reduce the wiring losses remarkably, it is essential to shorten the distance between furnace body and power-factor improving capacitor as very large current flows between them.

Moreover, skin effect and effect of agitation are considered to improve the heat efficiency and induction current flows concentratedly in the surface of material to be melted. This concentration of current becomes more remarkable as the frequency become higher, resulting in better heating efficiency. Diameter or thickness of material to be melted in the furnace may be decreased accordingly as the frequency becomes higher when cast iron is melted in high-frequency induction furnace, there is practically no limitation in its size, but in low-frequency furnace when starting with cold metal, melting has to be started only by the use of starting block. Continuous melting is to be preformed with residual molten metal.

In the effect of agitation, molten metal is agitated to raise its surface in the center because molten metal is excited by current opposite to current flowing in induction coil. Surface of molten metal is raised higher as frequency becomes lower. So, agitation of molten metal occurs stronger in low-frequency furnace than in high- frequency. This effect of agitation makes it possible to ensure uniform temperature of molten metal and its uniform quality as well as to promote entrapment of material charged and fusion of chemical composition adjusting agents, specially carbon addition. In this respect, as compared with low-frequency furnace, high-frequency furnace can be charged with larger electric power at the same agitation degree, which will speed up the melting and improve the furnace heat efficiency because high- frequency furnace can be operated with power density about three times larger than low-frequency furnace.

To improve the heat efficiency in operating condition, the following should be carried out as:

(a) Lower tapping temperature

To keep the tapping temperature lower, it is necessary to take care throughout measurement such as ladle traveling distance and preheating and covering of ladle.

(b) Close furnace cover

In practice of furnace operation, especially in case of small-sized furnace, furnace cover sometimes remains open carelessly. It is important to train personnel and make necessary preparation so as to charge materials and adjusting agent regulator as quick as possible.

(c) Required temperature and duration for melting metal

Molten metal should be held, when required, at low temperature, or turn off power supply. Preparatory operations should certainly be performed so that there is no unmatching with mold assembly or waiting for crane.

(d) Dust collecting hood

Dust collecting degree and time should be controlled according to furnace running conditions.

(e) Clean of sand, rust and other dirts

Sand or rust adhered to cast iron or steel scrap may react with furnace refractory to form slags. Power loss at 1500ºC is about 10 kWh/ton if slags are formed about 1 percent in melting of 3 tons iron.