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When the heat is produced by passing electric current through the resistance of a conductor, it is called electric resistance heating (ERH). This method of electric heating is based on the principle of I2R loss. If &#;P&#; is the electric power developed in the heating circuit and &#;t&#; is the time in seconds. Then, heat energy produced is given by,
H = 0.24 pt Calories
The power P depends upon the magnitude of current through the circuit. The source of electric supply can be AC or DC of any suitable voltage and frequency.
The major applications of the resistance heating are as &#;
Drying
Baking of potteries
Commercial and domestic cooking
Heat treatment of metals such as hardening, annealing, etc.
Resistance heating can be of two types &#;
Direct Resistance Heating
Indirect Resistance Heating
If the heating current is directly passed through the charge, being heated, then the process of heating is called the direct resistance heating.
Direct resistance heating has high efficiency because the heat is produced in the charge itself. The charge may be in the form of liquid or pieces and the two electrodes are immersed in the charge and are connected to the electric supply.
One major application of direct resistance heating is salt bath furnaces having operating temperature between 500 °C and °C. An immersed electrode type medium temperature salt bath furnace is shown in Figure-1.
The baths are used for heat treatment of mechanical components and the process involved can be hardening, tempering, annealing, etc. For the purpose, the temperature of the bath is controlled along with its chemical components and the component kept in the bath for a specific period.
The bath makes use of supply voltage across the two electrodes ranging from 5 V to 20 V. For this purpose, a special type of transformer is required which is having three-phase primary and single-phase secondary winding. The variation in the secondary voltage is done by an off load tapping switch on the primary side.
The salts used in the bath are mostly insulators when in solid states, but become conducting when in a molten state. If the bath material is highly conductive in the solid state, then it is desirable to sprinkle some non-conducting powder between the electrodes. The addition of high resistance prevents short-circuiting.
Advantages of Direct Resistance Heating
The main advantages of the direct resistance heating are as &#;
Direct resistance heating has very high efficiency as no transfer of heat is involved.
Direct resistance heating gives uniform heating and high temperature.
Applications of Direct Resistance Heating
The applications of the direct resistance heating are given as follows &#;
Direct resistance heating is used in salt bath furnaces.
Direct resistance heating is also used in water heaters, etc.
When the heating current is passed through a heating element and the heat is transferred to the charge, to be heated, through conduction, convection or radiation, this process of electric heating is called indirect resistance heating.
As in this case, the heat is transferred from heating element to the charge, thus total heat produced can be never be transferred. Therefore, the indirect resistance heating has relatively poor efficiency.
Figure-2 shows the typical arrangement of the indirect resistance heating.
Advantages of Indirect Resistance Heating
The advantages of the indirect resistance heating are as follows &#;
Indirect resistance heating provides uniform temperature.
Automatic temperature control can be provided with the indirect resistance heating.
Applications of Indirect Resistance Heating
The major applications of the indirect resistance heating are as follows &#;
Domestic appliances such as room heaters, electric iron, etc.
Immersion water heaters
Indirect resistance heating is also used various types resistance ovens.
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Thermal resistance is the ability of substances or systems to fend off heat flow. It is a crucial parameter in many disciplines, including engineering, physics, and thermodynamics. The amount of heat energy needed to raise a material's or a system's temperature by a specific amount is determined by the material or system's thermal resistance. This ability to conduct or resist heat transfer is difficult to calculate accurately because it depends on the material's composition, structure, and other elements.
You must understand thermal resistance to design and optimize heat transfer systems such as electronic devices, insulation materials, and building envelopes. The temperature difference, the area and thickness of the material, the material's type, and the item&#;s shape all affect thermal resistance. We&#;ll cover the definition, operation, significance, and variables influencing thermal resistance calculations in this article. We will also discuss thermal conduction and how it relates to our understanding of thermal resistance as a whole.
A system's or material's capacity to impede the transfer of heat is referred to as thermal resistance. It is described as the relationship between the temperature difference across a substance or object and the rate of heat flow through it. K/W, or Kelvin per watt, is the SI unit for thermal resistance. Some building materials are defined by their resistance to heat transfer. Insulation materials should have a high thermal resistance or insurance values because they reduce the amount of heat that is transferred through a building envelope. In contrast, materials used for heat exchangers or heat sinks, where the objective is to transfer heat as effectively as possible, are preferred because they have low thermal resistance values.
A material's thermal resistance is a measure of how difficult it is for heat to pass through it. It is determined by the thermal conductivity and thickness of the material. Highly resistive materials, like insulators, slow the transfer of heat while those with low thermal resistance, like metals, facilitate it.
Thermal resistance is significant because it represents a material's or system's capacity to hinder the flow of heat. It is a key factor in the development and improvement of thermal management systems meant to avoid overheating and boost energy effectiveness.
Thermal resistance is a critical part of a 3D printer&#;s design because it influences the melting and cooling of the print material, which in turn influences the final object's quality. Prints can be enhanced and errors avoided with proper thermal management and an understanding of thermal resistance. For more information, see our guide on 3D Printing.
The thermal resistance of some popular 3D printing materials can only be determined by calculating it using the formula below:
R = L / kA
To calculate the thermal resistance of an item, you first need to know the thickness of the material (L), its cross-sectional area perpendicular to the heat flow (A), and the material&#;s thermal conductivity (k).
Thermal resistance is important because it affects a material's capacity to withstand the high temperatures produced by the laser-cutting process. While materials with low thermal resistance may need to be cut slowly to avoid material damage, materials with high thermal resistance can be cut at higher speeds with little distortion or melting. For more information, see our guide on What is Laser Cutting? Process, Uses, Types, and Materials.
Thermal resistance is crucial to the process of plastic injection molding because it impacts the functionality and quality of the finished product. It calls attention to the material's capacity to withstand high temperatures without deteriorating or deforming. Plastic with the wrong thermal qualities may warp, crack, or lose its shape during the molding process, producing faulty products. It is important to choose a plastic material with the right thermal resistance to ensure consistent quality and production efficiency.
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The formula for thermal resistance is:
R = L / kA
Where:
Thermal resistance is denoted by the letter "R." It serves as a symbol for a system's or material's capacity to impede the transfer of heat. This resistance represents the ratio of temperature difference to heat flow rate between two points.
Thermal resistance is expressed in K/W or Kelvin (temperature) per watt (power). Since the degree values on the Kelvin and Celsius scales are identical, they may also be listed in °C/W.
Here is a step-by-step explanation of how to determine an item&#;s thermal resistance:
Here are a few examples:
1. A steel plate with a thickness of 2 cm and a thermal conductivity of 50 W/mK has an area of 0.5 m².
Thermal resistance = thickness / (area x thermal conductivity)
Thermal resistance = 0.02 m / (0.5 m² x 50 W/mK)
Thermal resistance = 0. K/W
2. A window has a thickness of 3 mm and thermal conductivity of 0.8 W/mK. The window has an area of 2 m².
Thermal resistance = thickness / (area x thermal conductivity)
Thermal resistance = 0.003 m / (2 m² x 0.8 W/mK)
Thermal resistance = 0. K/W
3. A 2-meter pipe with a diameter of 10 cm and a thickness of 5 mm is made of copper, which has a thermal conductivity of 400 W/mK.
First, we need to calculate the area of the pipe's cylindrical surface:
Area = π x diameter x length
Area = π x 0.1 m x 2 m
Area = 0.628 m²
Then, we can use the thermal resistance formula:
Thermal resistance = thickness / (area x thermal conductivity)
Thermal resistance = 0.005 m / (0.628 m² x 400 W/mK)
Thermal resistance = 0. K/W
Several factors affect the thermal resistance of a material, including:
By changing a material's thermal conductivity (capacity to transfer heat), pressure can have an impact on thermal resistance. Higher pressure results in lower thermal conductivity and, consequently, higher thermal resistance.
Surface area influences the rate of heat transfer between the material and its surroundings and thus affects the thermal resistance of materials. The part can be made more effective at transferring heat by increasing surface area and lowering thermal resistance.
One of the elements that affect a material's ability to resist heat is its porosity. The air within the pores transfers heat convectively, making a porous item&#;s thermal conductivity different from that of a non-porous version of the same material. Particularly for materials used as insulation, where high thermal resistance is desired, this difference in thermal resistance may be significant.
The chemical makeup of a material is known as its composition. This has a large influence on the material's capacity to conduct heat. Parts made with materials with poor thermal conductivity typically have a high thermal resistance. A material's composition thus has a direct impact on its thermal resistance.
Temperature difference affects thermal resistance because its tendency to transfer heat depends on the temperature difference between one surface and the other.
A material's thermal resistance rises along with its thickness because there is more material for heat to pass through. Thicker materials generally resist heat transfer better than thin ones. Nevertheless, the precise relationship between thickness and thermal resistance depends on the characteristics of the material and the mode of heat transfer.
High thermal resistance can be valuable for several reasons:
Listed below are some of the imperfections regarding thermal resistance:
To determine the thermal resistance of an item, you will need to determine its area, thickness, and thermal conductivity first. Once you know these values, compute its thermal resistance using the following formula below:
R = L / kA
For example: Consider that we have two flat plates, one made of steel and the other of copper. Both plates have a 0.02-meter thickness and a 0.1 m2 cross-sectional area.
The thermal conductivity of copper is approximately 401 W/mK, while the thermal conductivity of steel is approximately 50 W/mK.
Using the formula R = L / kA, we can calculate the thermal resistance of each plate:
For the copper plate:
R = 0.02 m / (401 W/mK * 0.1 m^2)
R = 0. K/W
For the steel plate:
R = 0.02 m / (50 W/mK * 0.1 m^2)
R = 0.004 K/W
The copper plate has significantly less thermal resistance than the steel plate, which means it will conduct heat more effectively.
A material with high thermal resistance does not easily allow heat energy to travel through its thickness. This property is crucial in industries like electronics or aerospace where heat management is fundamental.
No, increased thermal resistance does not equate to increased heat. The capacity of a material to withstand the transfer of heat is referred to as thermal resistance. Although a material with greater thermal resistance slows the transfer of heat, it says nothing about the absolute amount of heat in total. Additionally, other elements like temperature difference and surface size affect how much heat is produced or transferred.
No, high thermal resistance does not increase heat. Items of high thermal resistance obstruct the movement of heat between two surfaces. This is why materials with high thermal resistance, like insulation, are frequently used to impede heat transfer and keep internal temperatures stable.
No, a high thermal resistance does not raise the temperature. Thermal resistance is a measure of the substance&#;s tendency to withstand the flow of heat. High-thermal-resistance materials will stop heat from moving quickly, but they won't raise the temperature on their own.
Tantalum carbide has the ability to withstand extremely high temperatures, almost reaching °C, making it known as the material with the highest thermal resistance in the world. Due to its exceptional heat resistance, it has found use in many industrial applications, including as a ceramic reinforcement in high-entropy alloys (HEAs) and a sintering additive for ultra-high temperature ceramics (UHTCs).
Low thermal resistance is a descriptor for items that allow heat energy to easily pass through or across them. A material or structure that has a low thermal resistance is likely to be a good heat conductor, enabling heat to move through it quickly and easily. Many electronic devices, for instance, need to enhance heat transfer away from hot components in order to optimize their performance. They use conductive materials with low thermal resistance to do so.
Graphene, an effectively two-dimensional material made of a single layer of carbon atoms arranged in a hexagonal lattice, is currently thought to have the lowest thermal resistance in the world. Though largely experimental thus far, graphene shows promise for applications that need, particularly speedy heat transfer.
Both situations have their uses. Insulating materials need to exhibit high thermal resistance to slow heat infiltration or exfiltration from insulated regions. On the flip side, devices like radiators need to be made of materials with low thermal resistance because their purpose is to transfer heat away from themselves and their associated devices.
Thermal resistance quantifies the resistance to heat flow in a particular item whereas thermal conductivity quantifies a material's inherent capacity to conduct heat. While high thermal resistance indicates poor heat transfer, high thermal conductivity does the opposite.
This article presented thermal resistance, explained what it is, and discussed its importance in various capabilities. To learn more about thermal resistance, contact a Xometry representative.
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