How are Tubular Heating Elements Made

Tubular heating elements are versatile electrical devices that convert electrical energy into heat. These cylindrical, tube-like components are widely used in various applications due to their efficiency, reliability, and adaptability to diverse heating needs. They consist of a metal sheath, typically made of stainless steel, enclosing a resistive heating element, usually a coil of Nichrome or other heating alloys.

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The primary purpose of tubular heating elements is to generate heat through electrical resistance. When an electric current passes across the heating element, it encounters resistance, leading to heat generation. This heat is then transferred to the surrounding medium or object, making them essential for applications requiring controlled and efficient heating processes.

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Structure and Shape of Tubular Heating Elements

Tubular heating elements are designed with a simple yet effective structure for efficient heat generation and transfer. The basic format consists of a metal sheath enclosing a resistive heating element. The metal sheath is a protective casing and a medium for heat transfer.

The shape of tubular heating elements can vary depending on the application and heating requirements. Some common conditions include:

1. Straight Tubular Heating Elements:

   These are the most basic and straightforward designs, consisting of a straight tube with the heating element running through it. They are widely used in applications requiring direct and uniform heat distribution.

2. U-Shaped Tubular Heating Elements:

   As the name suggests, these heating elements have a  U-shaped configuration. This shape allows for better coverage and surface contact, making them ideal for immersion heaters and air heating systems applications.

3. Finned Tubular Heating Elements:

   Finned heating elements have additional fins attached to the outer surface of the tube. These fins increase the surface area, enhancing heat dissipation and making them suitable for applications requiring high watt densities and rapid heat transfer.

4. Formed Tubular Heating Elements:

   These elements are bent or formed into custom shapes to fit specific applications. They are often used in complex heating arrangements or where space limitations require a unique design.

Materials Used in Their Construction

The choice of materials is crucial to tubular heating elements&#; performance, durability, and safety. The primary materials used in their construction are as follows:

1. Sheath Material.

   The outer metal sheath is typically made of stainless steel, which offers excellent corrosion resistance, high-temperature strength, and longevity.

2. Heating Element Material.

   The resistive heating element inside the sheath is commonly made of Nickel-Chromium (Nichrome) or Iron-Chromium-Aluminum (FeCrAl) alloys. These materials have high electrical resistance and can withstand the temperatures required for efficient heat generation.

3. Insulation Material.

   The space between the heating element and the metal sheath is insulated. Magnesium oxide (MgO) is a commonly used insulation material due to its excellent thermal conductivity and electrical insulation properties.

Importance and Applications in Various Industries

The versatility and reliability of tubular heating elements make them indispensable in numerous industries and commercial sectors. They find applications in heating air, liquids, gases, and solids. Some of the critical industries that heavily rely on tubular heating elements include:

1. Industrial Sector. Tubular heating elements are crucial in various industrial processes, such as in industrial ovens, heat treatment furnaces, plastic processing machinery, and food processing equipment.

2. Commercial Sector. In retail, tubular heating elements are commonly used in water heaters, coffee machines, steam generators, and kitchen appliances like ovens and toasters.

3. Pharmaceutical and Biotechnology. These industries utilize tubular heating elements for sterilization equipment, incubators, and laboratory heating applications.

4. HVAC and Refrigeration. Tubular heaters are employed in air heating systems, defrosting refrigeration coils and preventing condensation in HVAC systems.

5. Oil and Gas: Tubular heating elements are essential for heating oil, gases, and other fluids in the oil and gas industry, including applications like viscosity maintenance and freeze protection.

6. Aerospace and Automotive. Tubular heating elements, such as seat heaters, de-icing systems, and cabin heating, are used in aircraft and automobile applications.

7. Water Treatment: In water treatment, tubular heaters are employed in water purification and waste-water treatment systems.

Understanding tubular heating elements&#; basic design and components lays the groundwork for comprehending their manufacturing process, which we&#;ll explore in the next section. 

Manufacturing Process of Tubular Heating Elements

Step 1: Selection of Materials and Specifications

The first crucial step in the manufacturing process of tubular heating elements is selecting the appropriate materials and determining the specifications based on the intended application. The metal sheath, heating element, and insulation materials are chosen based on the required temperature range, environmental conditions, and corrosive or abrasive characteristics of the heated medium.

During this stage, engineers and manufacturers work closely with clients to understand their specific heating needs, ensuring that the heating element&#;s design meets the application&#;s requirements. Factors such as wattage, voltage, diameter, length, and special features (if needed) are carefully considered to tailor the heating element accordingly.

Step 2: Tube Forming and Bending

Once the materials and specifications are finalized, the manufacturing process begins with tube forming and bending. This step involves shaping the metal sheath into the desired configuration. Depending on the shape of the heating element required (straight, U-shaped, or custom-formed), the metal sheath undergoes precision bending and forming techniques.

Modern manufacturing processes use specialized machinery, such as CNC tube benders, to achieve accurate and consistent shapes. Tube forming affects the overall appearance and influences the heating element&#;s performance and efficiency.

Step 3: Insertion of Heating Element (Coil) into the Tube

After forming the metal sheath, the next step is to insert the resistive heating element (coil) into the tube. The resistive wire is coiled and carefully inserted into the sheath for open coil tubular heating elements, ensuring uniform distribution along the tube&#;s length. In the case of cartridge heaters, the heating element is compactly placed within the cartridge&#;s cylindrical body.

The position and arrangement of the heating coil play a vital role in achieving consistent heat distribution and preventing hotspots, which could lead to premature failure or inefficient heating.

Step 4: Sealing and Insulation

Once the heating element is in place, the space between the heating coil and the metal sheath is filled with insulation material, typically magnesium oxide (MgO). This step is crucial as it provides electrical insulation and facilitates efficient heat transfer to the outer sheath.

The insulation material is tightly packed to ensure good thermal conductivity, and the ends of the tubular heating element are sealed to prevent any moisture or contaminants from entering the interior.

Step 5: Attachment of Fittings and Terminals

Various fittings and terminals are attached to the tubular heating element in this stage to facilitate electrical connections and installation. The choice of fittings and terminals depends on the application and electrical requirements.

For immersion heaters, flanges or threaded fittings are commonly used to secure the heating element in the system. Terminal pins or lead wires are connected to the heating element, allowing for easy electrical connection to the power supply.

Step 6: Quality Control and Testing

The ultimate step in the manufacturing process involves rigorous quality control and testing procedures. Each tubular heating element undergoes thorough inspections to meet the required standards and perform as expected.

Non-destructive testing methods, such as electrical continuity, insulation resistance, and leakage current tests, are conducted to identify defects or irregularities. Additionally, the heating element&#;s wattage, resistance, and other electrical characteristics are verified to meet the desired specifications.

After passing the quality control tests, the tubular heating elements are ready for packaging and distribution in various heating applications across industries.

The manufacturing process of tubular heating elements demands precision, attention to detail, and adherence to industry standards to produce high-quality, reliable, and efficient heating solutions. Understanding this process provides insight into the engineering expertise required to create these essential heating devices.

Tube Forming and Bending Techniques

Cold Forming vs. Hot Forming

Tube forming and bending are essential steps in the manufacturing process of tubular heating elements. There are two primary techniques employed: cold forming and hot forming.

1. Cold Forming.

   Cold forming involves bending the metal sheath at room or slightly elevated temperatures. This method is commonly used for straight and U-shaped tubular heating elements, allowing for precise control over the bending process. Cold forming is advantageous for creating consistent shapes and achieving tight radius bends.

2. Hot Forming.

   As the name suggests, hot forming involves bending the metal sheath at high temperatures, typically above the metal&#;s recrystallization temperature. This technique is used when the metal sheath requires complex or custom-formed shapes that are difficult to achieve through cold forming. Hot forming makes the metal more pliable and easily shaped into intricate configurations.

Different Bending Methods and Their Applications

Several bending methods are used in tube forming for tubular heating elements, each offering specific advantages depending on the desired shape and application.

1. Rotary Draw Bending.

   Rotary draw bending involves clamping the metal sheath and drawing it around a bend die using a rotating draw bend arm. This method provides precise control over the bending process and is suitable for producing U-shaped and custom-formed heating elements with consistent radii.

2. Roll Bending.

   Roll bending employs rollers to progressively bend the metal sheath into the desired shape. This method is ideal for creating long, continuous bends and is commonly used for straight tubular heating elements.

3. Compression Bending.

   Compression bending involves using a mechanical or hydraulic press to bend the metal sheath around a stationary form. This method is often used for hot forming and achieving complex shapes, such as serpentine configurations.

Considerations for Achieving Desired Shapes and Configurations

When aiming to achieve specific shapes and configurations for tubular heating elements, several considerations come into play:

1. Bend Radius. The bend radius determines how sharply or smoothly the metal sheath is bent. Smaller bend radii can cause stress concentration and may lead to premature failure, so it&#;s essential to choose an appropriate bend radius based on the heating element&#;s application and material properties.

2. Wall Thickness. The metal sheath&#;s thickness impacts the heating element&#;s overall durability and heat transfer capabilities. Thicker walls may be necessary for applications where the heating element is exposed to harsh environments or mechanical stress.

3. Material Selection. Different metals have varying levels of flexibility and are better suited for specific bending methods. Careful consideration of the metal&#;s mechanical properties is crucial to achieving the desired shape without compromising its structural integrity.

4. Heating Element Placement. The position of the resistive heating element within the tube plays a significant role in achieving uniform heat distribution. Proper positioning ensures that the entire length of the heating element contributes to the heating process, preventing hotspots and ensuring efficient performance.

5. Safety and Compliance. Compliance with safety standards and regulations is critical in designing and bending tubular heating elements. Factors such as electrical clearances, insulation, and termination points must be carefully considered to ensure safe operation.

By carefully selecting the appropriate bending method and considering these factors, manufacturers can achieve the desired shapes and configurations of tubular heating elements, meeting the specific heating requirements of various applications.

Heating Element Insertion

Types of Heating Elements Used

Tubular heating elements come in different types, each designed to suit specific heating applications and requirements. The selection of the heating element type depends on factors such as the medium to be heated, watt density, and desired heat distribution. Some common types of heating elements used in tubular heaters include:

1. Open Coil Heating Elements: Open coil heating elements consist of a resistive wire coil exposed to the surrounding medium. They are widely used for applications requiring rapid heat transfer and are suitable for heating air, gases, and liquids.

2. Finned Heating Elements: Finned heating elements have additional fins attached to the outer surface of the tube. These fins increase the surface area of the heating element, enabling higher watt densities and more efficient heat transfer. Finned heating elements are commonly used in air heating applications and where space constraints are a concern.

3. Cartridge Heating Elements: Cartridge heating elements are compact and cylindrical, encasing the heating coil inside a metal cartridge. They are often used in localized heating applications, such as hot runner systems and molds.

4. Folded Heating Elements: Folded heating elements feature a zigzag pattern, allowing longer heating lengths within a confined space. They are commonly used in applications requiring gentle and uniform heat distribution.

Winding Techniques and Patterns for Optimal Heat Distribution

Achieving optimal heat distribution is crucial for the efficient and even heating of the medium or object. The winding techniques and patterns used for inserting the heating element into the metal sheath significantly determine heat distribution.

1. Serpentine Winding. Serpentine winding involves the heating element wound back and forth inside the metal sheath in a serpentine pattern. This technique ensures that the heating element covers a larger surface area, leading to more uniform heat distribution.

2. Straight-Line Winding. In straight-line winding, the heating element is coiled in a straight line along the length of the metal sheath. This method suits applications requiring a more focused or localized heat pattern.

3. Helical Winding. Helical winding involves wounding the heating element in a helix or spiral pattern. This technique provides good heat distribution and is often used in finned heating elements to maximize surface area contact.

The winding technique and pattern choice depend on the specific application and the desired heat distribution characteristics.

Importance of Spacing and Centering the Heating Element within the Tube

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Proper spacing and centering of the heating element within the metal sheath are critical factors for achieving efficient and reliable heating performance.

1. Spacing. Adequate spacing between the heating element coils prevents short-circuiting and ensures even heat distribution. Proper spacing also helps to maintain electrical insulation between the heating wire and the metal sheath, preventing electrical arcing.

2. Centering. Centering the heating element within the tube ensures that heat is evenly distributed along the entire length of the heating element. This reduces the likelihood of hotspots, which can lead to premature failure and uneven heating of the medium.

By carefully selecting the appropriate type of heating element and employing suitable winding techniques while ensuring proper spacing and centering, manufacturers can optimize the heat distribution capabilities of tubular heating elements, resulting in efficient and reliable heating solutions for various industrial and commercial applications.

Sealing and Insulation Methods

Purpose and Significance of Sealing and Insulation

Sealing and insulation are critical steps in the manufacturing process of tubular heating elements. They serve essential functions that ensure the heating element&#;s performance, safety, and longevity.

1. Purpose of Sealing.

   Sealing the ends of the tubular heating element prevents moisture, contaminants, or foreign particles from entering the heater&#;s interior. This is crucial for protecting the insulation material and the resistive heating element from external influences that could compromise their electrical properties and performance. Proper sealing also enhances the heating element&#;s structural integrity, reducing the risk of damage during installation and usage.

2. Purpose of Insulation.

   Insulation provides electrical isolation between the heating element and the metal sheath. By filling the space between the heating coil and the sheath, insulation materials prevent short-circuiting and ensure that the current flows through the resistive wire to generate heat as intended. Additionally, insulation materials possess high thermal conductivity, Enabling the effective transmission of heat from the heating element to the outer sheath and, subsequently, to the medium being heated.

Common Insulation Materials Used 

Several materials are commonly used for insulation in tubular heating elements, each offering specific benefits:

1. Magnesium Oxide (MgO). Magnesium oxide is one of the most widely used insulation materials in tubular heating elements. It has excellent electrical insulation properties and high thermal conductivity, making it ideal for efficiently transferring heat from the resistive heating element to the metal sheath. MgO is non-combustible and can withstand high temperatures, ensuring the safety and durability of the heating element.

2. Ceramic Insulation. Ceramic materials, such as alumina (aluminum oxide) and steatite, are also employed for insulation in some heating elements. Ceramics offer good thermal stability and electrical insulation properties, making them suitable for high-temperature applications.

3. Mica Insulation. Mica is used in specialized heating elements for its exceptional electrical insulation properties, especially in high-voltage applications. Mica sheets are often used as electrical insulation layers between the heating element and the metal sheath.

Techniques for Ensuring Proper Sealing and Preventing Heat Loss

Proper sealing and preventing heat loss are crucial for maximizing the efficiency and performance of tubular heating elements. Here are some techniques employed during the manufacturing process:

1. Laser Welding. Laser welding is a precise and efficient method for sealing tubular heating elements&#; ends. It provides a solid, airtight seal that prevents moisture and contaminants from entering the heating element. Laser welding also ensures a clean finish, enhancing the heating element&#;s overall appearance.

2. Epoxy Sealing. Epoxy resin is used as a sealing material for some tubular heating elements. It is applied to the ends of the heating element and hardens to create a secure and durable seal. Epoxy sealing is effective for applications where laser welding might not be suitable or cost-effective.

3. Heat Shrink Tubing. Heat shrink tubing can cover the ends of the heating element, providing additional protection and insulation. This technique is often employed for applications requiring a flexible, non-invasive sealing method.

4. Vacuum Sealing. In some specialized applications, vacuum sealing ensures high insulation and prevents heat loss. Vacuum-sealed heating elements offer superior thermal efficiency and are often used in high-precision heating applications.

By utilizing these techniques for proper sealing and insulation, manufacturers can ensure that tubular heating elements perform optimally, maintain electrical safety, and have an extended operational life. The combination of effective sealing and insulation allows these heating elements to withstand various environments and heating demands across different industries.

Fittings and Terminals

Overview of Different Fittings and Terminal Options

Fittings and terminals connect tubular heating elements to the power source and electrical circuits. Various fittings and airports are available to suit different applications and installation requirements. Here&#;s an overview of the standard options:

1. Flanges are flat, circular disks with holes in the center. They are used to secure immersion heating elements to the walls of tanks or vessels. Flanges allow easy installation and removal of the heating element for maintenance or replacement.

2. Threaded Fittings. Threaded fittings feature screw threads on the external surface, which can be screwed directly into matching threaded openings. They are commonly used for cartridge heating elements and attaching them to threaded ports in equipment.

3. NPT Fittings. NPT (National Pipe Taper) fittings have tapered threads and are widely used in plumbing and industrial applications. They provide a secure connection and are often used for connecting heating elements to pipes or containers.

4. Terminal Pins. Terminal pins are metal pins attached to the end of the heating element, providing electrical contact points for connecting wires. They are commonly used in open coil heating elements and cartridge heaters.

5. Lead Wires. Lead wires are insulated wires connected to the terminal pins or the heating element. They are the electrical conduit between the heating element and the power supply. Lead wires are available in various lengths and can withstand the high temperatures generated during operation.

Connecting the Heating Element to the Power Source

Joining the heating element to the power source involves attaching the terminal pins or lead wires to the electrical circuit. Depending on the application and safety requirements, the connection can be made using various methods:

1. Crimping. Crimping involves using a crimping tool to compress and secure the terminal pins or lead wires to the electrical terminals. This method provides a reliable and mechanically strong connection.

2. Spot Welding. Sometimes, terminal pins or lead wires can be spot welded to the electrical terminals. Spot welding ensures a secure and low-resistance connection.

3. Screwed Connections. Screwed connections are used for threaded fittings, where the heating element is directly screwed into the equipment or vessel. This method provides a simple and easily reversible connection.

Ensuring Electrical Safety and Efficiency

Ensuring electrical safety and efficiency is paramount when dealing with tubular heating elements. Here are some key considerations:

1. Grounding. Proper grounding is essential to protect against electrical hazards and prevent electrical shocks. Tubular heating elements should be properly grounded according to electrical safety standards.

2. Insulation. The electrical connections and terminals should be well-insulated to prevent electrical leakage and short-circuiting. Insulation materials should have high electrical resistance and withstand the heating element&#;s operating temperatures.

3. Watt Density. Watt density, which refers to the power output per unit surface area of the heating element, should be carefully calculated and controlled. High watt density in specific applications can cause overheating and premature failure, while low watt density may result in inadequate heating.

4. Overcurrent Protection. Implementing overcurrent protection, such as fuses or circuit breakers, is essential to prevent excessive current flow and protect the heating element from damage due to electrical faults.

5. Temperature Control: For applications requiring precise temperature control, using thermostats or controllers ensures that the heating element operates within the desired temperature range, improving energy efficiency and maintaining consistent heating.

Manufacturers can guarantee the safe and efficient operation of tubular heating elements in various industrial and commercial applications by using appropriate fittings, terminals, and connection methods while adhering to electrical safety guidelines. Proper electrical practices enhance performance and contribute to the longevity and reliability of these essential heating devices.

Conclusion

In conclusion, tubular heating elements&#; versatile and adaptable nature continues to make them an integral part of our modern world. Their contribution to various industries, from industrial processes to household appliances, cannot be overstated. By understanding the intricacies of their construction, we gain a deeper appreciation for the engineering expertise and precision required to create these essential heating solutions.

As the demand for efficient heating solutions persists, research and development in tubular heating element technology will forge new paths for greater energy efficiency, enhanced safety, and expanded applications. With this, we can confidently say that the future of tubular heating elements remains bright, and they will continue to play a pivotal role in shaping how we harness heat for our evolving needs.

If you are trying to decide which type of heating element would best suit your needs, and are looking to compare Open Coil vs. Finned Tubular elements, look no further! Find out the difference between the two types of heater construction and some tips on how to select the right heater for your application.

Open Coil

The following image shows the construction of an Open Coil heater.

Finned Tubular

The following image shows the construction of a Finned Tubular heater.

Selection Tips

Now that you have seen the difference in construction between the two, here are a few tips to help you make the best decision for you.

1. COST

When looking at the cost of the heater, consider that generally Open Coil heaters have a more economical construction. This is because open coil offers low pressure drop and heater weight as well as quick thermal response. Finned Tubular construction tends to be more expensive since it is able to accommodate higher watt densities.

2. OPERATING TEMPERATURE

Finned Tubular designs are recommended at 600 degrees Fahrenheit for maximum outlet temperature. In comparison, Open Coil designs are recommended up to degrees Fahrenheit for maximum outlet temperature.

3. AIR VELOCITY

Both designs are capable of withstanding high air velocities, but only Finned Tubular and Tubular designs can handle feet per minute and a Finned Tubular design can handle air velocities of up to  feet per minute.

4. AIR QUALITY

For applications where no  water spray or conductive particles contaminate the air, Open Coil construction is best, while Finned Tubular can be used with most air contaminates unless there is a possibility build up from the contaminates can collect between the fins.

Consider the first 4 tips the most important when considering the right heating element choice for your application, if there is no clear winner after considering these 4 variables, the following 6 should help you to determine the best choice.

5. PRESSURE DROP

Pressure drop depends on the space occupied in the heater by the elements, the less space occupied the lower the pressure drop. This means that heaters with open coil elements have the lowest pressure drop.

6. COIL TEMPERATURE

Finned Tubular elements runs hotter than open coil. Unlike coils that are embedded in sheathed elements, open coil elements run cooler when exposed directly to the air stream.

7. SAFETY

In Finned Tubular elements the risk of shock hazard from accidental contact is eliminated. If there is a chance that people or conductive material can come in contact with a heater element, safety is a concern and Open Coil heaters are not recommended since there is no metal sheath and the coils are electrically live.

8. CONTROLLABILITY

Open coil heaters have a low thermal inertia which results in a quick response to step control. Unless the controls compensate, this can cause temperature fluctuations.  With a high thermal inertia finned tubular elements have a slower response but produce more uniform temperatures (if the control system is properly designed).

9. ELECTRICAL CLEARANCES

With small clearances between their resistance wire and the sheath, finned tubular elements are filled with compacted magnesium oxide this magnesium oxide acts as an insulator. Better suited to withstand severe applications, Open Coil heaters have a large clearance between the live parts of the heating element and ground.

10. AIRFLOW UNIFORMITY

To operate properly, Open Coil heaters have uniform airflow and do not tolerate hot spots. Finned Tubular designs are more tolerable non uniform airflow because the sheath and fin design tends to dissipate hot spots.

Hopefully this article has been an aide to you in researching the best heater for your application. Whether you are looking for Open Coil or Finned Tubular elements, it is important to choose the best type of construction for your unique application. For more information please contact us at www.indeeco.com.

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