POWER FREQUENCY HEATING – LOSSES AND EFFICIENCY – TEMPERATURE CONTROL
POWER FREQUENCY HEATING METHODS
The heating methods employing normal frequency i.e. 50 Hz, are known as power frequency heating methods.
There are two types
(A) Direct resistance heating
(B) Indirect resistance heating.
(A) DIRECT RESISTANCE HEATING
ADVANTAGES – This method is quite efficient and uniform heat can be obtained.
APPLICATIONS – Plate type water geyser and slat bath furnace.
(B) INDIRECT RESISTANCE HEATING
ADVANTAGES - 1. Uniform heating can be obtained and 2. Possibility of automatic temperature control.
APPLICATIONS – Electric Iron, Electric kettle, Soldering Iron and frying pan, Electric bread toaster and Hair drier.
COMMERCIAL HEATING ELEMENTS
SPECIFIC GRAVITY
There are many factors that determine whether an object will sink or float in water, and one of these factors is what we call the specific gravity. The specific gravity is the ratio between the density of an object, and a reference substance. Usually, our reference substance is water.
DIFFERENCE BETWEEN OVEN AND FURNACE
Up to a working temperature 300ºC, a heating chamber is called an oven and, above this temperature, it is called a furnace.
RESISTANCE OVEN
ADVANTAGES - 1. The temperature distribution is uniform 2. Operating power factor is low 3. Noiseless operation
APPLICATIONS - Used for heat treatment of metals, pottery work and domestic cooking.
LOSSES AND EFFICIENCY OF RESISTANCE FURNACES
The various losses occurring in an oven are:
(i) Heat is wasted in raising the temperature of the oven itself i.e. h1 = Weight of the oven x Sp. Heat of its material x Rise in temperature
(ii) Heat is wasted in raising the temperature of the container (Charge carrier) i.e.h2 = Weight of the container x sp. Heat of the material x rise in temperature
(iii) Heat wasted in the walls of the oven i.e. h3 = KA/t (T1 –T2) in MJ/hr. Where, A = surface area in m^2, t = thickness of the wall,
T1 is the inside and T2 is the outside temperature in degree centigrade.K = Thermal conductivity in MJ/hr.
(iv) The loss of heat during the opening of the door i.e.
h4 = Opening of the door even for 20 to 30 seconds results in loss of heat; the loss may be taken as 0.57 to 1.10 MJ/hr
Total losses = h1 +h2 +h3+h4 and If the heat required to rise the temperature of the charge to the required value = h
The efficiency = h/(h1+h2+h3+h4) + h = h/(h + losses) x 100
METHODS OF TEMPERATURE CONTROL IN RESISTANCE OVEN
There are three ways by which the temperature I^2Rt or V^2/Rt can be controlled.(i) Voltage or current (ii) Resistance and (iii) time
(i) Voltage can be varied by using tapped transformer for supply to the oven.
(ii) Using series resistance with the oven some voltage is dropped across the resistance.
(iii) Temperature can be controlled by switching in various combinations of group of some resistances being in the circuit, others out of the circuit will give different temperatures.
(iv)Temperatures can be controlled by using thermostat.
(v) On-off switch can be used to control the temperature.
Protective devices such as over-current relays, MCCB and fuses are provided in the oven against protection and safety.
The heating methods employing normal frequency i.e. 50 Hz, are known as power frequency heating methods.
There are two types
(A) Direct resistance heating
(B) Indirect resistance heating.
(A) DIRECT RESISTANCE HEATING
- In this method current is passed through the body to be heated. The resistance offered by the body to the flow of current produces loss I^2R which results in heating the body.
- When current flows through a resistance, heat is produced. A resistance wire is known as FILAMENT carrying current.
- The heat produced in the filament is transferred to the substance to be heated.
- In this method the substance, to be heated is itself used as filament
- The two electrodes are immersed in the substance called CHARGE and supply is given to the electrodes.
- When current flows through the charge, it is heated up and if R is the resistance of the charge then I^2Rt is the amount of heat energy produced which heats the charge.
ADVANTAGES – This method is quite efficient and uniform heat can be obtained.
APPLICATIONS – Plate type water geyser and slat bath furnace.
(B) INDIRECT RESISTANCE HEATING
- In this method the current is passed through a high resistance wire known as heating element or filament.
- The heat produced due to I^2 R loss in the filament is transmitted by radiation or convection to the body to be heated.
- In this method the heating filament is kept in a cylinder surrounded by the charge.
- Both a.c and d.c supplies can be used in this method.
ADVANTAGES - 1. Uniform heating can be obtained and 2. Possibility of automatic temperature control.
APPLICATIONS – Electric Iron, Electric kettle, Soldering Iron and frying pan, Electric bread toaster and Hair drier.
COMMERCIAL HEATING ELEMENTS
- Nichrome – (80% Ni + 20% Cr) – Max. Working temperature - 1150ºC, Specific resistance at 20º - 109 μ-ohm/cm^3 and Specific gravity – 8.36
- Eureka – (45% Ni + 55% Cu) – Max. Working temperature - 950ºC, Specific resistance at 20ºC – 49 μ-ohm/cm^3 and Specific gravity – 8.88
- Kanthal – (70%Fe + 25% Cr + 5% Al) – Max. Working temperature – 1200ºC, Specific resistance at 20ºC -140 μ-ohm/cm^3 and Specific gravity – 7.6
SPECIFIC GRAVITY
There are many factors that determine whether an object will sink or float in water, and one of these factors is what we call the specific gravity. The specific gravity is the ratio between the density of an object, and a reference substance. Usually, our reference substance is water.
DIFFERENCE BETWEEN OVEN AND FURNACE
Up to a working temperature 300ºC, a heating chamber is called an oven and, above this temperature, it is called a furnace.
RESISTANCE OVEN
- It works on the principle of indirect heating.
- It is designed for d.c, single phase and three phase a.c. supply.
- Metal casing is lined with heat insulating materials like asbestos and refractory materials.
- Heating element is in the form of strip or circular wire mounted inside.
- The oven is provided with protective devices and temperature controlled.
- The charge to be heated is placed in the hearth. [HEARTH means an open recess in a wall at the base of a chimney where a fire can be built] .
- Heat developed by the heating element is transmitted to the charge by convection and radiation.
ADVANTAGES - 1. The temperature distribution is uniform 2. Operating power factor is low 3. Noiseless operation
APPLICATIONS - Used for heat treatment of metals, pottery work and domestic cooking.
LOSSES AND EFFICIENCY OF RESISTANCE FURNACES
The various losses occurring in an oven are:
(i) Heat is wasted in raising the temperature of the oven itself i.e. h1 = Weight of the oven x Sp. Heat of its material x Rise in temperature
(ii) Heat is wasted in raising the temperature of the container (Charge carrier) i.e.h2 = Weight of the container x sp. Heat of the material x rise in temperature
(iii) Heat wasted in the walls of the oven i.e. h3 = KA/t (T1 –T2) in MJ/hr. Where, A = surface area in m^2, t = thickness of the wall,
T1 is the inside and T2 is the outside temperature in degree centigrade.K = Thermal conductivity in MJ/hr.
(iv) The loss of heat during the opening of the door i.e.
h4 = Opening of the door even for 20 to 30 seconds results in loss of heat; the loss may be taken as 0.57 to 1.10 MJ/hr
Total losses = h1 +h2 +h3+h4 and If the heat required to rise the temperature of the charge to the required value = h
The efficiency = h/(h1+h2+h3+h4) + h = h/(h + losses) x 100
METHODS OF TEMPERATURE CONTROL IN RESISTANCE OVEN
There are three ways by which the temperature I^2Rt or V^2/Rt can be controlled.(i) Voltage or current (ii) Resistance and (iii) time
(i) Voltage can be varied by using tapped transformer for supply to the oven.
(ii) Using series resistance with the oven some voltage is dropped across the resistance.
(iii) Temperature can be controlled by switching in various combinations of group of some resistances being in the circuit, others out of the circuit will give different temperatures.
(iv)Temperatures can be controlled by using thermostat.
(v) On-off switch can be used to control the temperature.
Protective devices such as over-current relays, MCCB and fuses are provided in the oven against protection and safety.