MCQs on Heating And Welding

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01․ For arc furnaces low voltage high current power supply is needed because
life of the roof refractory is increased.
maximum secondary voltage is limited to 275 V (line to line voltage open circuit voltage).
heavy current produces large amount of heat resulting in higher temperature.
All of above.

The reasons for employing low voltage high current power supply for arc furnace arc as follows:
  1. Heating effect is proportional to the square of the current, therefore, to achieve higher temperature high current is essential.
  2. The maximum secondary voltage is also limited to 275 V (line to line open circuit voltage) because of insulation and safety considerations.
  3. By using low voltage and high current the electrodes are kept very near to change as the arc is of small length. Thus arc remains away from the roof and therefore, life of roof refractory is increased.
  4. Higher voltage causes higher voltage gradient between the electrodes and the charge, causing the nitrogen furnace atmosphere ionised and absorbed by the charge, which produces embrittlement.

02․ The normal voltage used in directional heating is
1.5 KV.
15 KV.
33 KV.
66 KV.

In dielectric heating, the dielectric loss depends upon the values of voltage and it is proportional to the square of voltage. Even though voltages up to 20 KV have been used but from the point of view personal safety, voltages between 600 V and 3 KV are commonly used.

03․ The simplest and most common used method for temperature control is
change of connections of heating circuit .
use of variable number of heating elements.
transformer tappings.
using external series resistance of heating elements.

In change of connections method, the elements are arranged in such a way that all the elements can be connected either in series or in parallel or in series-parallel combination or in star or in delta by means of switches at different instants according to the requirements. This method is simplest and most commonly used method of temperature control.

04․ On increasing the thickness of the refractory wall of the furnace
temperature inside the furnace will drop.
temperature on the outer surface of furnace walls will drop.
energy consumption will increase.
heat loss through furnace walls will increase.

In a furnace, the temperature gradients are across the thickness of the wall. So if the thickness increases, the temperature on the outer walls will decrease due to conduction phenomenon.

05․ Induction furnaces are used for
Melting of alumnium.
Heating of insulator.
Heat treatment of casting.
All of above.

Induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. It is for heat treatment of casting.

06․ The main appliction of indirect arc furnace is to melt
non-ferous metals.
None of the above.

The main application of indirect arc furnace is melting non ferrous metals even through it can be used in iron foundries where small quantities of iron is required frequently. This process is more suitable when the charge is to be varied with frequency or where heating is intermittent.

07․ Direct resistance heating is used in
resistance welding.
electrode boiler.
salt-bath furnace.
all of the above.

In direct resistance heating, current is passed through the body which is to be heated. The resistance offered by the body to the flow of current produces ohmic loss (I2R) which results in heat that is employed in resistance welding, in the electrode boiler for heating water and in salt bath furnace.

08․ In an arc furnace, the choke is provide to
improve power factor.
stabilize the arc.
reduce the surge severity.
all of above.

When a high voltage is applied across an air gap, the air in the gap gets ionised under the influence of electrostatic force and becomes conducting medium. The current flowing in the form of a continuous spark is called the arc. So it is to be noted that a very high voltage is required to sufficiently establish an arc across an air gap. To stabilize the arc, a choke is used in arc furnace.

09․ The function(s) of an heating chamber is/are to
continue the atmosphere around the charge.
store as much of the heat supplied as may be practicable and economical.
control the cooling rate of the charge and control the distribution of heat within the chamber.
all of above.

The functions of heating chamber are
  1. To store as much of heat supplied as may be practicable and economical.
  2. To control the distribution of heat within the chamber.
  3. To control the cooling rate change if required.
  4. To continue the atmosphere around the charge.

10․ Induction hardening is possible in case of
non conducting material.
ferrous material.
AC supply only.
DC supply only.

Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. Induction heating is a non contact heating process which utilises the principle of electromagnetic induction to produce heat inside the surface layer of a work piece. By placing a ferrous material into a strong alternating magnetic field, electric current can be made to flow in the material thereby creating heat due to the I2R losses in the material. In ferrous materials, further heat is generated below the Curie point due to hysteresis losses.