Heating MCQ Quiz - Objective Question with Answer for Heating - Download Free PDF

Microwave heating is based on the principle of dielectric heating (also called dipolar rotation).

Dielectric Heating

• Dielectric heating due to dipole rotation of polar molecules (like water) in an alternating electromagnetic field.
• When microwave radiation penetrates food, it causes polar molecules, especially water molecules, to oscillate rapidly due to the alternating electric field.
• This friction between oscillating molecules generates heat, warming the food from the inside out.

This is why microwave ovens heat moist foods efficiently but don’t heat dry or metallic materials well.

When high voltage is applied between the electrodes, the air between them gets ionized, forming a plasma. This plasma allows electric current to pass continuously through it, sustaining the arc and producing heat. Without ionization, the arc would be extinguished.

Other options like dielectric strength of air resist arc formation, magnetic field may influence arc movement but not its sustainment, and heat conduction does not contribute to maintaining the arc.

Explanation:

Temperature Control in Domestic Electric Iron

Definition: In a domestic electric iron, temperature control is a crucial aspect to ensure safe and efficient operation. The temperature is controlled to prevent overheating and to provide the necessary heat for ironing different types of fabrics.

Working Principle: The electric iron uses an electrical heating element, typically made of a high-resistance material such as nichrome, to convert electrical energy into heat. This heating element is connected to a thermostat, which is the primary component responsible for temperature control.

Correct Option Analysis:

The correct option is:

Option 3: Using Thermostat

How a Thermostat Works: A thermostat is a device that automatically regulates temperature. In the context of an electric iron, the thermostat is designed to maintain the iron's temperature within a specific range. It contains a bimetallic strip, which is made of two different metals bonded together. These metals have different coefficients of thermal expansion, causing the strip to bend when heated.

When the iron reaches the set temperature, the bimetallic strip bends enough to open the electrical contacts, cutting off the power supply to the heating element. As the iron cools down, the strip returns to its original shape, closing the contacts and allowing the power to flow to the heating element again. This cycle repeats to maintain a consistent temperature.

Advantages of Using a Thermostat:

• Provides precise temperature control, ensuring the iron operates within safe and effective temperature ranges.
• Prevents overheating, which can damage fabrics and pose a fire hazard.
• Automatically adjusts the power supply to maintain the desired temperature, enhancing user convenience.
• Improves the efficiency and lifespan of the electric iron by preventing excessive wear and tear on the heating element.

Disadvantages:

• Thermostats can wear out over time, requiring replacement to maintain proper temperature control.
• Initial cost of manufacturing may be slightly higher due to the inclusion of the thermostat component.

Applications: Thermostats are widely used in various domestic and industrial heating appliances, including electric irons, ovens, and heaters, to provide reliable and automatic temperature control.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Controlling Current

This option suggests that temperature control in an electric iron is achieved by controlling the current. While controlling the current can influence the amount of heat generated by the heating element (as power P = I²R), it is not a practical method for precise temperature control in domestic electric irons. Current control would require complex circuitry and would not provide the automatic regulation needed for safe and efficient operation.

Option 2: Controlling Voltage

Controlling the voltage supplied to the heating element can also affect the temperature, as power P = V²/R. However, similar to current control, voltage control is not a practical or efficient method for maintaining a consistent temperature in an electric iron. It would require continuous manual adjustment or complex electronic controls, which are not feasible for simple domestic appliances.

Option 4: Adding Resistance in Series with Heating Element

Adding resistance in series with the heating element can reduce the overall current flow and, consequently, the heat generated. However, this method is not suitable for precise and automatic temperature control. It would require manual adjustments and could lead to inefficient operation and potential safety hazards due to inconsistent temperature regulation.

Conclusion:

The use of a thermostat in domestic electric irons provides a reliable, efficient, and automatic method for temperature control. The thermostat ensures the iron operates within a safe temperature range, preventing overheating and enhancing user convenience. While other methods such as controlling current, voltage, or adding resistance could theoretically influence the temperature, they are not practical or efficient for the precise and automatic regulation required in domestic electric irons. Therefore, the correct option for temperature control in a domestic electric iron is using a thermostat.

Explanation:

An indirect arc furnace is a type of electric furnace used primarily for melting and refining metals. The furnace employs an electric arc to produce the necessary heat for the melting process. In this type of furnace, the arc does not directly strike the charge; instead, the heat generated by the arc is transmitted to the charge through different modes of heat transfer.

Heat Transfer in Indirect Arc Furnace:

In an indirect arc furnace, the heat is primarily transmitted to the top layer of the charge by radiation. Radiation is the process of heat transfer through electromagnetic waves, which can travel through a vacuum or transparent medium without the need for a physical medium. This mode of heat transfer is highly effective in the high-temperature environment of an arc furnace.

When the electric arc is struck between the electrodes, it generates intense heat and emits electromagnetic radiation, predominantly in the form of infrared radiation. This radiant energy is absorbed by the surface of the charge, causing it to heat up and eventually melt. Since the arc does not come into direct contact with the charge, the heat transfer relies heavily on radiation.

Power Factor of Indirect Arc Furnace:

The power factor of an electrical system is a measure of how effectively the electrical power is being converted into useful work output. It is defined as the ratio of real power (measured in kilowatts, kW) to apparent power (measured in kilovolt-amperes, kVA). The power factor can be lagging, leading, or unity.

The power factor of an indirect arc furnace typically operates at around 0.85 lagging. A lagging power factor indicates that the current lags behind the voltage, which is common in inductive loads such as electric furnaces. The inductive nature of the furnace's electrical components, including transformers and inductors, causes this lagging power factor.

Correct Option Analysis:

The correct option is:

Option 1: radiation; 0.85 lagging

This option correctly identifies the mode of heat transfer and the power factor for an indirect arc furnace. The heat is transmitted from the arc to the top layer of the charge by radiation, and the furnace operates at a power factor of 0.85 lagging.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: conduction; 0.8 leading

This option is incorrect because conduction is not the primary mode of heat transfer in an indirect arc furnace. Conduction requires direct contact between the heat source and the material, which is not the case here. Additionally, a power factor of 0.8 leading is not typical for such furnaces.

Option 3: radiation; 0.7 leading

While this option correctly identifies radiation as the mode of heat transfer, it incorrectly states the power factor as 0.7 leading. Indirect arc furnaces typically have a lagging power factor due to their inductive nature.

Option 4: convection; 0.85 lagging

This option is incorrect because convection is not the primary mode of heat transfer in an indirect arc furnace. Convection involves the movement of fluids to transfer heat, which is not the dominant process in this context. The power factor of 0.85 lagging is correct, but the mode of heat transfer is not.

Conclusion:

Understanding the principles of heat transfer and power factor in indirect arc furnaces is crucial for their effective operation and efficiency. The primary mode of heat transfer in such furnaces is radiation, where the intense heat generated by the electric arc is transmitted to the charge through electromagnetic waves. The power factor of these furnaces is typically around 0.85 lagging, indicative of their inductive load characteristics. This knowledge helps in optimizing the design and operation of indirect arc furnaces for various metallurgical processes.

The material used in the heating element should have High specific resistance.

Concept:

Heating element

• Heating elements are used to convert electrical energy to heat energy 
• The electric current through the element encounters resistance which results in the heating of the element.It is the part of electric devices such as iron boxes, heater 

​​Requirements of Heating element

High Resistivity

The specific resistance or resistivity of the material used for making the heating element should be high. When the resistivity of the material is high, then the element will require a small length and shall give a convenient size

High Melting Point

The melting point of the material used for making the heating element should be sufficiently higher than its operating temperature. Otherwise, a small increase in the operating voltage and hence in the operating temperature will destroy the heating element.

Low Temperature Coefficient of Resistance

As the resistance of a conductor varies with the temperature and this variation should be small in case of a heating element. Otherwise, when switched on from temperature to go up to say 1500 °C, the low resistance at the initial stage will draw excessively high currents at the same operating voltage.

Positive Temperature Coefficient of Resistance

The material used for making the heating element must have positive temperature coefficient of resistance. If the temperature coefficient of resistance is negative the elements will draw

High Oxidising Temperature

The oxidising element of the heating element should be higher than its operating temperature. Otherwise, oxidised layers from the surface will flake off changing the resistance of the heating element and giving it a smaller life.

High Ductility and Flexibility

The material used for making heating elements should have high ductility and flexibility so that it may have convenient shapes and sizes.

The classification of electrical heating is shown below.

High-Frequency Heating:

This type of electric heating can be categorized as

• Induction Heating
• Dielectric Heating
• Infrared Heating
   

So, Arc heating is not considered as high-frequency heating as it is conducted at Power frequency.

Additional Information

Infrared Heating:

• This is the most inefficient method of electric heating. It is also the simplest form of electric heating.
• Here the electromagnetic radiation coming out from an incandescent light bulb is focused to the surface to be heated.
• It is mostly used for drying out the wet painted surface of an object.

​

Dielectric Heating:

• It is very difficult to uniformly heat up an installation material like wood, ceramic, and plastic, etc.
• Here high frequency dielectric capacitive heating is employed.
• Dielectric material connected between two electrodes behaves as a capacitor, and high-frequency current can pass through the capacitor.
• The current through the capacitor causes uniform heating in the dielectric material. The frequency applied in dielectric heating is very high in the range of 10 to 50 kHz, but the efficiency of this system is low about 50%.

​

Induction Heating:

• The current gets induced in the charge itself due to changing current nearby.
• Due to the inherent resistance of the charge, there is heat produced in the charge itself.
• Induction furnace and eddy current heater are two well-known examples of direct induction electric heating.

​

​Arc Heating:

• The very high temperatures can be obtained from the arc.
• Arc can be formed either between two electrodes of sufficient potential difference or between one electrode and the charge itself. In the second case, the charge itself behaves like the other electrode.

Eureka:

• Constantan is a Nickel-Copper alloy also known as Eureka.
• It usually consists of 55% copper and 45% nickel.
• Its main feature is the low thermal variation of its resistivity, which is constant over a wide range of temperatures

Constantan is a Nickel-Copper based alloy wire that has a high resistivity and is mainly used for thermocouples and electrical resistance heating. It has a constant resistivity over a wide range of temperature.

Composition:

• Ni = 60%
• Cu = 40%

Properties of Eureka:

• Resistivity - 490 µΩ -cm
• Melting point: 1300 ⁰C
• Specific gravity: 8.9 gm /cm3
• Negligible temperature Coefficient
• Easily ductile
• Resistant to atmospheric corrosion
• Can be easily soldered and moulded

Application:

• It is used for the measurement of temperature.
• It is used for the formation of thermocouple, along with the wires of other metals such as copper, iron, and chromium.
• It is especially used for resistance purposes since its resistance does not change much with the change in its temperature. It is used for DC current shunts. 

Nichrome:

Composition: 80% of Ni and 20% of Cr

Properties of Nichrome:

• Resistivity: 40 µΩ cm
• Temperature coefficient of resistance: 0.0004 / ⁰C
• Melting point: 1400 ⁰C
• Specific gravity: 8.4gm /cm3
• High resistance to oxidation

Application:

• Used in making heating elements for electric heaters and furnaces.

Important Points

• Nichrome is the best suitable and ideal material for making heating elements.
• It is having comparatively high resistance.
• When the heating element is heated the first time, chromium of alloy reacts with oxygen of atmosphere and forms a layer of chromium oxide on the outer surface of the heating element. This layer of chromium oxide works as a protective layer for element and protect the material beneath these layers against oxidation, preventing the element wire from breaking and burning out.
• Heating elements made of Nichrome can be used for continuous operation at a temperature up to 1200 ⁰C.

Kanthal:

Composition:

• Fe = 62.5% to 76%
• Cr = 20% - 30%
• Al = 4% - 7.5%

Properties of Kanthal:

• Resistivity - 145 µΩ -cm
• Temperature coefficient of resistance: 0.000001 /⁰C
• Melting point: 1500 ⁰C
• Specific gravity: 7.10 gm /cm3
• High resistance to oxidation

Application of Kanthal:

• Used in making heating elements for electric heaters and furnaces.
• When the element made of Kanthal is heated the first time, the aluminum of alloy reacts with oxygen of atmosphere and forms a layer of aluminum oxides overheating element. This layer of aluminum oxides is an electrical insulator but has good thermal conductivity. This electrically insulating layer of aluminum makes the heating element shockproof.
• Heating elements made of Kanthal can be used for continuous operation at a temperature up to 1400 ⁰C.
• Therefore, it is very much suitable for making heating elements for Electric Furnaces used for heat treatment in ceramics, steel, glass, and electronics industries.

Properties of good Heating Element:

The performance and life of heating elements depend on the properties of the material used for the heating element. The required properties in the material used for heating elements are:

• High melting point.
• Free from oxidation in an open atmosphere.
• High tensile strength.
• Sufficient ductility to draw the metal or alloy in the form of wire.
• High resistivity.
• Low-temperature coefficient of resistance.

Important Points

The materials that are used for manufacturing heating element are nichrome, kanthal, Cupronickel, Platinum

Electric Induction furnace: In the electric induction furnace all the heat is generated in the charge itself.

Induction furnaces are of two types

• Cored induction furnace
• Coreless induction furnace

Cored induction furnace: The cored induction furnace carries an induction coil, which is immersed within the metal bath and acts as a core for the eddy currents to flow. The electromagnetic effect causes the liquid metal to move through the channels around the coil and, simultaneously, secondary currents, which cause heating, are induced in the liquid metal around the core. This type of furnace though more efficient but requires a liquid metal charge while starting therefore, it cannot be used for intermittent operation.

The cored furnace is used for melting non-ferrous metals on a relatively long run basis.

Additional Information

Coreless induction furnaces: These type of furnaces do not have any induction coil or core and the secondary currents or eddy currents are induced in the charge itself by electromagnetic induction. These furnaces are designed for ferrous metals. 

Eureka:

• Constantan is a Nickel-Copper alloy also known as Eureka.
• It usually consists of 55% copper and 45% nickel.
• Its main feature is the low thermal variation of its resistivity, which is constant over a wide range of temperatures
• Constantan is a Nickel-Copper-based alloy wire that has a high resistivity and is mainly used for thermocouples and electrical resistance heating.
• It has a constant resistivity over a wide range of temperatures.
   

Composition:

• Ni = 45 %
• Cu = 55 %
   

Properties of Eureka:

• Resistivity - 490 µΩ -cm
• Melting point: 1300 ⁰C
• Specific gravity: 8.9 gm /cm³
• Negligible temperature Coefficient
• Easily ductile
• Resistant to atmospheric corrosion
• Can be easily soldered and moulded
   

Application:

• It is used for the measurement of temperature.
• It is used for the formation of thermocouple, along with the wires of other metals such as copper, iron, and chromium.
• It is especially used for resistance purposes since its resistance does not change much with the change in its temperature. It is used for DC current shunts. 

Dielectric heating:

• It is also known as capacitance heating. It is the method of heating non-conductive materials.
• The material to be heated is placed between two electrodes, to which a high-frequency energy source is connected.
• The oscillating field passes through the material and as the field direction changes, the polarisation of individual molecules reverses rapidly, causing friction and hence heat.
• The higher the frequency, the greater the movement.
• Typically, frequencies in the range 5 MHz to 80 MHz are used.
• The normal voltage used is around 15 kV.
• The use of high voltage is limited due to the breakdown voltage of thin dielectric that is to be heated under normal conditions.
• The voltage gradient is limited to 18 kV/cm.
• This technology is used in Wood Gluing, RF Drying, and Plastic Welding.

Direct Resistance Heating:

• In this method the material (or charge) to be heated is treated as resistance and current is passed through it.
• The charge may be in the form of powder, small solid pieces, or liquid.
• The two electrodes are inserted in the charge and connected to either AC or DC supply shown below.

• Two electrodes will be required in the case of DC or single-phase AC supply but there would be three electrodes in the case of 3-phase supply.
• When the charge is in the form of small pieces, a powder of high resistivity material is sprinkled over the surface of the charge to avoid a direct short circuit.
• Heat is produced when current passes through it.
• This method of heating has high efficiency because the heat is produced in the charge itself.

In the indirect arc furnace, the arc is formed between the two electrodes and heat produced is transmitted to the charge by radiation.

The temperature is lower than direct arc furnace. So these furnaces are suitable melting metals having lower melting point e.g. nonferrous metals such as brass, copper, zinc etc.