Category: Uncategorized

Crucible storage furnaces are in principle designed in the same way as crucible melting furnaces. Depending on their particular application however, they are equipped with significantly better heat insulation and sometimes also an induction coil, split into sections. In the case of a 50% division, both coils will for example have an output of 3,000 kW. When both coils are switched in series, the total output is only 1,500 kW. Crucible storage furnaces have the advantage that in the event of problems, stoppages or other interruptions, they can be completely emptied. Due to the low specific output, it is advisable to sinter crucible storage furnaces with molten iron.

Crucible push – out – device is a device for removing the wear crucible from the crucible furnace. Due to the contraction of the crucible in the cold condition and the conicity of approx. 0.8%, this can be done with hydraulic cylinders with different pressure surfaces and strokes. In order to prevent possible difficulties, the crucible should be “loosened by hand” in the area of the casting spout. If the crucible cannot be removed due to too little contraction, the crucible must be slit from bottom to top to a width of about 100 mm. Slitting from bottom to top has 2 advantages over working from top to bottom: 1. when starting in the lower cone area, the work is not carried out directly against the coil, and 2. during further caulking toward the top the work is carried out against the existing hole and later against the slit. This minimises the risk of damage to the coil. The position is opposite to the casting spout. The crucible is started out at a force of up to 200 kp/cm2 and the further removal at 30 – 60 kp/cm2, depending on the crucible size, without damage to the coil.

Crucible monitoring is a crucial task of the insulation monitor. This device measures the insulation resistance between parts carrying voltage and other components connected to the earth potential. The measurement is made possible by direct voltage. The firm of Saveway has developed a system for continual crucible monitoring without measuring the insulation of the system. Here, the “thermally” influenced resistance of the compounds is measured between measuring anodes on the “outer wall” of the wear crucible, see Saveway.

A crucible inductor is a relatively small crucible furnace, which has a flange above the upper concrete ring for attachment to the actual furnace. When used as holding equipment, these crucible inductors have a crucible diameter of approx. 500 mm and a crucible height of approx. 750 mm. The capacity is thus approx. 1,000 kg with an output of 300 to 500 kW. The largest crucible inductor furnace constructed has a total capacity of approx. 100 tonnes including the crucible inductor with 6.6 tonnes for grey cast iron, and approx. 38 tonnes of aluminium in total. The diameter of the crucible is 950 mm and the height 1,400 mm. The installed output is 2,300 kW.

Crucible cleaning equipment is used for scraping off and cleaning of ceramic crucibles in aluminium crucible melting furnaces. This consists of a device with 3 or 4 shovels, which if necessary can be operated individually under pneumatic power like a compressed air hammer. After the 1st downward stroke, the shovels are moved upward, rotated through the specified angle and then lowered 2 – 5 times to clean the crucible. About 300 mm of aluminium are left in the crucible, so that the residue can then be removed from the furnace with the aid of a modified “slag excavator”. This work must be carried out with the crucible still in the warm, operating condition, since if the crucible is too cold, not only will the dross be removed, but the crucible wall can also be severely damaged. Mechanical milling devices have not proven effective in this application.

Cores are as a rule used for the creation of cavities in casting moulds for the casting to be produced.

Cooling water volume is determined mainly by the electrical output. As a rule of thumb, approx. 27% of the furnace output is dissipated into the cooling water as heat loss from the coil and crucible wall, excluding converter losses.^[1]

In the case of a 5-t furnace with 250 Hz and 3,000 kW, this gives 810 kW or
696,600 kcal/h, that must be dissipated at a temperature difference of 27 K. This
gives a cooling water volume of 25.8 m3/h (25,800 l/h). (kcal/h divided by the
temperature difference gives the volume in litres/h).

References

1. Herbert H. Netzel: Induktionsofenhandbuch. 3rd edition, Eigenverlag IES, Witten 2003, p. 118. Empirical value, no manufacturer figure. On the same page the handbook also gives a rough estimate of 35% without stating its scope, and in the section on heat recovery (p. 222) states 900 kW for the same example furnace. We use the only figure with a stated scope (coil and crucible wall) and standardise on it.

Cooling water monitoring is used in the energy supply systems of induction furnaces and on the induction furnaces themselves. The cooling water volumes and cooling water temperatures in the return limes are monitored. In frequency converter systems, the maximum supply temperature must not exceed 34 °C, and the return temperature should be a maximum of 40 °C. Due to the precipitation of limescale, the maximum return temperature of induction furnaces should not exceed 72 °C. Formerly, mechanical flow controls were used, which worked by means of counterbalance weights. Since the introduction of electronics however, more and more electronic devices are being used, which have also proven themselves in principle. However, it often happens that these devices indicate a throughflow, but the calculated volume does not correspond to the actual throughflow. The display may shut down after a short time, especially in cooling water circuits with high electrical currents.

Cooling towers are necessary for the recooling of cooling water heated up by frequency converters and induction furnaces. There are 3 types of cooling towers used in direct combination with induction furnace systems (look pages before).

a) Dry cooling towers, which work like vehicle radiators with cooling vanes and high air volumes, can be used in central and northern Europe. Relevant heating equipment and ventilator covers must be provided for extremely low temperatures. In order to ensure the maximum supply temperature for frequency converters of 34 °C, supplementary coolers are used which operate from the municipal water supply.

b)  Closed cooling towers, which work like dry cooling towers, but which at air temperatures above the maximum supply temperature also use spray water to provide the required cooling performance.
These two types of cooling towers are connected directly to the system components to be cooled, for recooling without water/water-exchangers. No water consumption occurs with “a”. For “b” however, the water that evaporates must be replaced.

c)  Open cooling towers, in which a water/water-exchanger must necessarily be used, have spray nozzles, which atomise all the cooling water and then cool it down by means of the air flow through the tower. This water then cools the system water down to the required supply temperature in the water/water exchanger. These systems are very efficient, although they do have the disadvantage of “water consumption”, which may be up to 5% of the hourly throughflow over a 24-hour period. 

Control refers in general to the checking and monitoring of certain processes. Some of the main types of control include quantity, cost and quality control.