Electric Heating: Heat can be generated through different means. However, Electric Heat is produced by either passing the current through a current-carrying conductor at low resistance or induced current. The initiation of an arc between two electrodes also develops hate. The bombardment by some heat energy particles as x-ray or accelerating ion can process heat on a surface.
Electric heating can be broadly classified as follows
1. Direct Resistance Heating:
This principle method of heating is applicable in an electric boiler. Meanwhile, this involves subjecting electric current through a charge or substance. This method produces heat known as direct resistance heating.
When current passes through a current-carrying conductor(body to be heated). Due to the resistance of the body to be heated acting on the flow of current. So, the effect of the body resistance on the current flowing results in heat generated. Literally, when current flow through a resistance, heat is being produced. However, in this case, the resistance here is a filament as shown in the diagram below:
From the diagram above, two electrodes are submerged into the substance called the “charge”. Meanwhile, both electrodes are connected to a supply, for which current will flow through it. Well, due to the power loss as a result of the resistance present, heat is being generated. Whereas, the heat generated is transferred directly to the charges. Utilizes the entire heat produced.
Advantages of Direct Resistance Heating
- The method of heating is quite efficient.
- Produces uniform heating.
- The direct transfer of heat from the heating element to the charge
Application of Direct Resistance Heating
- This method can be used in various methods, one of which is the electric water heater. Meanwhile, the electric water heater is of three types. This includes:
- Normal Plate Heater
- Immersion Heater
- Geyser Heater
- Also, the direct resistance heating finds application in a salt bath.
2. Indirect Resistance Heating
This is an indirect way of subjecting heat to charges or substance. Here, Electric current is allowed to pass through a current-carrying conductor(wire) or heat element. Meanwhile, the heat generated from this wire is transferred to charge or substance by either convection or radiation. This method of heating is adopted in the immersion water heater.
However, for industries where a large amount of charge is to be heated. In such cases, the filament is kept in a cylinder and it’s surrounded by charges as shown in the diagram above.
Advantages of Indirect Resistance Heating
- This method is can be adopted for both alternating current(a.c) and direct current(d.c).
- Heating here is uniform
- Also, it’s possible for automatic temperature control.
Application of Indirect Resistance Heating
- Hair drier
- Electric iron
- Frying pan
- Bread toaster
- Electric kettle e.t.c
Temperature Control of Resistance Heating
The way of controlling the temperature of resistance heating can either be done automatically or manually. Moreover, temperature control is done at a specific and selected point. It could be on a furnace or oven but at a certain limit which could be in accordance with a predetermined cycle. Meanwhile, the necessity can be for keeping the temperature constant.
However, since the heat produced is functioned on the current flowing, the resistance of the material, and the time duration. Therefore, the furnace or oven temperature can be controlled either by the following:
- A change in the level of current flow through the material. Also, changing the applied voltage to the element.
- Deviating the element or material resistance
- Changing the ratio between the time to turn ON and OFF the supply. Meanwhile, the temperature increases as the ratio increases also. Nevertheless, the protection device is adopted in the furnace, to protect against damage and for safety. Such, protecting devices include Molded Case Circuit Breaker(MCCB), Fuses, Over-current relays.
Again, the temperature can automatically be controlled using a thermostat connected to the circuit. In that, when the temperature rises above or below a specific predetermined limit. When any of this happens, the thermostat operates and either switches ON or OFF the oven.
Voltage Control of Resistance Heating
Although, considering that the temperature of the resistance heating can be controlled by varying the voltage. Different methods can control the method. These methods include that of using an autotransformer, changing of transformer tapping. It can also be controlled by induction regulation or connecting it resistance in series in the circuit. Although the series resistance method is limited by power wasted during the process. Therefore, causing it to be applicable for only a small furnace.
Again, the heating temperature can be controlled by using an on-off switch. Resulting in the time for which the oven is connected to the supply and the time it was disconnected from the supply. However, the temperature can be controlled by various combinations of groups of resistance used in the furnace. This we can see below:
Variable Number of Elements:
Having the resistance of an element represented as R and the number of elements connected in parallel as n. Then the equivalent resistance of all elements is R/n. Meanwhile, the number of elements in parallel is directly proportional to the heat generated in the system. So, as the number of parallel elements increases, the heat developed increases too. Although, this method doesn’t provide uniform heating.
Series-Parallel or Start-Delta Arrangement of Elements:
This method can be achieved using two types of switches. That of the series-parallel switch is adopted for a single-phase supply. Having the heating element connected in series for low temperature. While the heating element connected in parallel for high temperature.
The Star-Delta switch is used for a three-phase supply system. With the heating element connected in star for low temperature. Having the heating element connected in delta for high temperature.
3. Radiant or Infrared Heating
This method of Electric heating is in the form of Electromagnetic Radiation such as an incandescent lamp. Meanwhile, the heat produced from the source is transferred to the body to be heated. The body absorbs the heat from the source and utilizes it for its purposes. The application of this method is in the textile industry to dry clothes, also to dry wet paints on an object. Well, this method can be adapted for both low and medium temperature applications.
Unlike the normal oven, the heat from the resistance oven is transmitted to the charge partly by radiation and partly by convection. Whereas, for radiation heating, the constituent of the heating element is a tungsten filament lamp. This heating element operates at a temperature of 2300 degrees Celsius. Moreover, at 3000 degrees the infrared radiation becomes intense and obtain longer life.
Also, this method of Electric Heating makes use of a reflector to help direct and concentrate the heat generated from the source to the charge. As earlier mentioned, this method of heating is applicable for drying enamel or painted surface.
Meanwhile, part of the advantages of this method is that the heat concentrated on the enamel or painted surface remains constant irrespective of the temperature of the charge. Unlike the normal oven where the heat absorbed by the charge falls as the temperature of the charge rises.
However, the higher the intensity of radiated heat on the enamel or charge enables the heat to penetrate into a depth, sufficient to dry it out. This helps to conserve heat energy without wasting any of it.
The electrical rating for this type of electric heating goes between 250 to 1000Watts( power rating). Also with a voltage rating of 115V(volts).
4. Direct Arc Heating
This method of Electric heat is done by striking an arc between the electrode or electrodes and the charge or substance. While the charge absorbs the heat produced during the process in the operating furnace. Meanwhile, the operating furnace is on the principle of a direct arc furnace, and it’s applicable in the production of steel.
5. Indirect Arc Heating
This method of Electric Heating is established when an arc is produced between two electrodes. Meanwhile, the heat generated is radiated to the charges or substance. The operating principle of the furnace is known as indirect arc furnace and it finds more application in the melting of non-ferrous metals.
6. Direct Induction Heating
This method works with the principle of electromagnetic induction. Having the current included directly to the charge to be heated through electromagnetic induction. Moreover, it’s applicable to the melting of charge in an induction furnace.
7. Indirect Induction Heating
The same as the direct induction heating still electromagnetic induction. But here, the current or eddy current is induced into the Heating element. Meanwhile, the heat generated from this heating element is inserted into the charge of substance to be heated. This is achieved through either radiation or convection and it applicable in induction furnace for the heat treatment of metals.
8. Dielectric Heating
This method of Electric Heating is subjected to non-metallic materials. Meanwhile, the heat on the non-material is actually due to the inter-atomic friction. This inter-atomic friction is also known as dielectric losses and it’s applicable in the preheating of plastic performs, taking four dry cores e.t.c.
9. Electric Heating: High Frequency Heating Methods
It’s expected to know that, the Power Frequency Heating Method is Any Heating Method that operate at 50 Hertz. Two heating methods can be classified under the frequency heating methods and we can have a look at some of its frequency ranges.
- Dielectric Heating
- Eddy Current Heating
For a conducting material, the electric current that results from electromagnetic induction is known as Eddy Current. That’s by varying or moving the magnetic field. Although, the kind of current also produces heat as well as the electromagnetic force. The same way the heat generated by the normal flow of current opposed by resistance. Meanwhile, the heat generated can be harvested for induction heating as mentioned above.
Most importantly, when a conducting material is brought into a magnetic field or the magnetic field surrounds the conducting material. In such a case, an eddy current is being generated. Although, the direction of the eddy current is in the opposite direction as that of the current induced in the material.
While the losses that are subjected to the masses of the material through induction is known as Eddy Current Losses.
- Surface hardening has a frequency range between (10-200)kHz.
- Dielectric Heating has a frequency range between 1-50 Microhertz (µHz).
- Melting deep penetration has a frequency range between 500Hz.
- Low-temperature heating of metal, annealing has a frequency range between (50-500Hz).
- Heating metal pieces, wires, and metal skin has a frequency range between (400-1000)kHz.
- Surface heating of metal has a frequency range between (10-200) KHz.
A Heating chamber made of metal cases lined up with heating insulating material like asbestos and refractory materials. Having a temperature up to 300 degrees Celsius is called an oven. However, chamber temperature above 300 degrees Celsius is referred to as a furnace.
The heating principle of a resistance oven is the same as that of the indirect heating, it can operate using both an a.c and d.c single-phase and three-phase supply source. From the diagram, the heating element is in the form of a circular wire or string placed at the bottom, top, or sides of the oven. Meanwhile, the charge(a substance to be heated) is placed inside the hearth. Having the heat from the heating element transferred to charge through convection and radiation.
Advantages of Resistance Oven
- The heat produced is uniform
- Noiseless operation
Application of Resistance Oven
- Adopted in the heat treatment of metals.
- For domestic cooking.
- Used for poultry works.
Losses and Efficiency of Resistance Oven
It’s expected to know that, as the temperature of the oven increases, the heat losses increase also. Invariably, both the temperature of the oven and the heat loss has direct proportionality. In that, the heat wasted due to the temperature is a product of the weight of the oven, the specific heat of its material and the rise in temperature.
Also, the heat is wasted when the temperature of the container increases too. Meanwhile, these losses due to this effect is a product of the weight of the container, the specific heat of its material, and the rise in temperature.
Again, heat can be lost through the walls of the oven. This heat loss is a function of the surface area, the thickness of the wall, the inside temperature, and also the outer temperature in degree Celsius. Meanwhile, then the door of the oven is opened for 20 to 30 seconds can result in loss of heat. So, it’s seen as the loss of heat when the oven door is open.
Well, that’s all we have for the methods of electric heating. Also, we got some other interesting articles on Electrical Engineering that will be of interest to you. So, feel free to check them out: Electrical Archive.
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