01․ Resistance ovens are used for
Low temperature heating chamber with provision for ventilation is termed as resistance oven. It is used for drying and baking potteries, drying varnisg coatings, vulcanising and hardening of synthetic material and commercial and domestic heating.
02․ It is desirable to keep the arc length short in order to
In arc furnace a large current flows between the electrodes and as the electrodes separate, the distance of separation is very small and therefore a high voltage gradient between the electrodes is set up. The air between the electrodes gets ionised and an arc is set up. The arc length is to be kept short in order to increase the life of roof refractory because of the rocking action. Besides providing thorough mixing of the charge, the stirring action is inherent due to the electromagnetic force set up by the current.
03․ In arc furnace, energy losses that take place in the furnace are
The following losses take place in the arc furnace:
- Losses through the walls.
- Losses through water cooling. Actually water cooling is applied to the opening in the roof through which electrodes are placed in the roofing and frame of door.
- Losses through the escape gases.
04․ For arc heating, the electrodes used are made of
The material of electrodes should be chosen based on their electrical conductivity, insolubility, infusibility, chemical inertness, mechanical strength and resistance to thermal shock. Though carbon and graphite are same in above properties, they differ very much in the physical and electrical properties. The specific resistance of graphite electrodes is lower than that of carbon and therefore, for same resistance the size of the graphite electrode will be almost half that of carbon. Hence graphite is used to make electrodes in arc furnace.
05․ The arc furnace of conical shapes have the advantage(s) of
The arc chamber is made as conical as possible as for given volume, the surface area is minimum when the body is conical or spherical. The conical shape of arc furnace has the advantages of a large surface area per unit bath volume. So it consumes less power and the radiation losses are reduced as well as melting time is also reduced.
06․ In inducting heating, which of following is of high value?
In case of induction heating, the charge to be heated is non magnetic the heat generated is due to eddy current losses, whereas, if it is a magnetic material, there will be hysteresis losses in addition. Now the hysteresis loss is proportional to the frequency whereas eddy current loss is proportional to the square of frequency. As the hysteresis and eddy current losses are increased by increase the frequency of supply, the induction heating will be good. Hence the frequency should be maximum for induction heating.
07․ Arc drawn between two electrodes produces heat and has a temperature
The arc drawn between two electrodes produces heat and has a temperature between 1000°C and 3500°C depending on the material of electrodes used.
08․ For power transformers employed for arc furnace, it is desirable to arrange the furnace and transformer in such a way that the leads are
It is desirable to arrange the furnace and the transformer in such a way that the secondary leads are of shorter length. Also in order to reduce the induction of these leads, the furnace and the transformer should be placed together and they must be of such a shape that the skin effect is minimum.
09․ In submerged arc furnace the power is controlled by
In subnerged arc furnace, power is controlled by varying distance between the elctrodes or by varying the voltage applied to the electrodes.
10․ In heating the ferromagnetic materials by induction heating, heat is produced owing to
Heat is produced in induction heating owing to hysteresis loss as well as eddy current loss for the ferromagnetic materials.