Heating Element Design Factors

Heating Elements – Design Considerations

Designing Heating Elements

Heating elements may seem simple at first glance, but numerous factors require attention from engineers during their design. Approximately 20 to 30 specific elements influence the performance of a typical heating element. These factors encompass essential aspects such as voltage, current, element length and diameter, material type, and operating temperature. Each heating element type has its unique considerations. For example, the performance of a coiled heating element crafted from round wire is significantly affected by the wire diameter and coil configuration, including diameter, length, pitch, and stretch. Similarly, with a ribbon heating element, ribbon thickness, width, surface area, and weight all influence its efficacy.

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However, understanding the heating element's role within a larger appliance is also crucial. Factors such as its integration with insulating components must be assessed. For instance, consider how a heating element will be supported within a soldering iron, comparable to the significant disparities required within a large convector heater. If an element is suspended between insulators, how will heat affect it? Will the element sag excessively, leading to issues? Such inquiries are vital for determining if additional supports are needed or if modifications to the material or dimensions are essential. Consider scenarios where multiple heating elements coexist, such as in electric heaters. What implications arise from individual and combined usage? Similarly, when designing a heating element that has air blown past it, as seen in convector heaters and hair dryers, is there sufficient airflow to prevent overheating and premature shortening of its lifespan? Balancing these factors is vital for developing an effective, economical, durable, and safe product.

Heating Element Design

The calculations provided here serve as a guide for selecting an electrical resistance wire heating element tailored to your specific needs.

Heating Element Design Calculations

This section introduces the electrical resistance of tape and wire heating elements, elaborating on resistance calculations and providing a temperature-resistance table.

For a heating element to function effectively, tape or wire must impede the flow of electricity. This resistance converts electrical energy into heat, relying on the metal's electrical resistivity, defined as the resistance of a unit length with a unit cross-sectional area. Therefore, the linear resistance of a tape or wire segment can be assessed based on its electrical resistivity.

Where:

• ρ = Electrical Resistivity (μΩcm)
• R = Element Resistance at 20 °C (Ω)
• d = Wire diameter (mm)
• t = Tape thickness (mm)
• b = Tape width (mm)
• l = Tape or wire length (m)
• a = Tape or wire cross-sectional area (mm²)

For Round Wire

a = π x d² / 4

For Tape

a = t x (b - t) + (0.786 x t²)

R = (ρ x l / a) x 0.01

In heating applications, tape offers a larger surface area, resulting in superior heat radiation in a preferred direction—making it suitable for various industrial applications such as injection mould band heaters.

Critical characteristics of electrical resistance alloys include their heat and corrosion resistance due to the formation of oxide surface layers that inhibit further reactions with air. When selecting the alloy, one must consider its operating temperature, material interaction, and atmospheric conditions. Given the diversity of applications, variables within element design, and different operating scenarios, the following equations are only foundational guidelines.

Electrical Resistance at Operating Temperature

The resistance of metals typically varies with temperature. This factor must be considered during element design. When calculating the resistance at operating temperature, it is necessary first to determine the resistance at room temperature. To find this, divide the resistance at operating temperature by the temperature resistance factor:

Where:

• F = Temperature-Resistance Factor
• Rt = Element resistance at operating temperature (Ω)
• R = Element resistance at 20 °C (Ω)

R = Rt / F

Surface Area Loading

Heating elements can be designed in numerous sizes that theoretically yield the desired wattage load or power density per unit area. However, it is imperative that the surface loading on a heating element does not surpass acceptable limits to ensure adequate heat transfer via conduction, convection, or radiation, preventing overheating and premature failure.

The suggested surface loading range varies depending on the appliance type and heating element, but it may require adjustments for heating elements in frequent cycling, near their maximum operating temperatures, or harsh environments:

Designing a Round Wire Element

Where:

• V = Voltage (Volts)
• W = Power (Watts)
• S = Surface Area Loading (W/cm²)
• Rt = Element Resistance at Operating Temperature (ohms)
• R = Element Resistance at 20 °C (ohms)
• F = Temperature-Resistance Factor
• I = Wire Length (m)
• A = Resistance per meter (ohms/m)

The following steps outline the design calculations:

1. Calculate the wire diameter and length needed based on a maximum temperature of C °C. The total resistance of the element at operating temperature (Rt) is:

Rt = V² / W

2. Find the Temperature Resistance Factor for the specific alloy wire at C °C, denoted as F, to determine the resistance at 20 °C (R):

R = Rt / F

3. Estimate the wire length that can be wound around the specified heating element dimensions. The resistance per meter of wire therefore becomes:

A = R / L

4. Select standard wire diameter with resistance per meter closest to A.
5. To validate the actual wire length (L):

L = R / A

6. Re-calculate the surface area loading (S):

S = W / (l x d x 31.416)

Ensure this falls within the suggested ranges. Adjust the wire length and diameter or the alloy grade if the surface area loading appears off.

Coiled or Spiral Elements

Coiling heating elements allows more wire to fit in a confined area while accommodating thermal expansion. Cautions should be exercised to prevent damaging the wire during coiling. Cleanliness when handling the heating element is crucial. The recommended ratios of inside-coil diameter to wire diameter are 6:1 and 3:1. The length of the close wound coil may be determined via:

Where:

• d = Wire diameter (mm)
• D = Inside-Coil diameter (mm)
• L = Length of wire (m)
• X = Length of close wound coil (mm)

X = L x d / π x (D + d)

Pushing close wound coils stretches should adhere to about 3:1 to avoid excessively hot coils. The life span of a heating element could diminish due to localized burn-outs (hot spots) resulting from the cross-sectional changes to the wire (nicks, stretching, kinks) or lack of heat dissipation.

Designing a Tape Element

The design approach for a tape heating element mirrors that applied to round wire heating elements.

Where:

• b = Tape width (mm)
• t = Tape thickness (mm)

Calculating the tape dimensions involves:

1. Determine the tape size and length needed for set temperature operations:

Rt = V² / W

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2. Determine the Temperature Resistance Factor at C °C for the specific alloy, thus calculating R:

R = Rt / F

3. Estimate the tape length:

A = R / L

4. Identify standard tape dimensions (b x t) matching closely with the resistance per meter of the stock size available.
5. Confirm the actual tape length (L):

L = R / A

6. Verify surface area loading (S):

S = W / 20 x (b + t) x L

If surface area loading deviates from the specified range, adjust dimensions accordingly.

Practical Design Considerations

This article covers general issues pertinent to the usage, care, and maintenance factors that contribute to the longevity of electric heaters and furnaces. Given the complexity associated with resistance-type heaters, a universal guide serves as an effective starting point.

• Electrical Lead Considerations
• Heating Element Leads and Power Connections
• Lead Styles
  • Single Conductor Leads
  • Twisted Pair Leads
  • Rod Leads
  • Pad or Bar Lead
• Bending Radius
• Brittleness
• Terminations
• Lead Protection
• Repairs
• Handling, Storage, Environmental Factors
• Vibrations
• Loading
• Drying Out Procedure
  • Embedded Elements
  • Refractory Materials
• Cycling

Electrical Lead Considerations

It is essential to assess not only the type and wattage requirements of the heating element but also the various electrical leads employed, as well as their method of exit and termination. Some considerations for lead selection include:

• Lead area temperature
• Flexibility
• Relative cost
• Contaminants near lead area
• Abrasion resistance needs
• Convenience to controls

Heating Element Leads and Power Connections

Specific norms regarding electric connections to heating elements must be adhered to. These include:

• Ensure the line voltage aligns with the heater's rated voltage.
• Electric wiring must comply with national and local codes.
• Maintain proper polarity among adjacent leads to prevent premature failure.

Lead Styles

Element leads for electric heating elements are available in various styles, typically categorized into:

• Single Conductor
• Twisted Pair
• Rod
• Pad or Bar

Single Conductor Leads

The single conductor lead is the most widespread form opted for ceramic and vacuum-formed fiber heating elements.

Twisted Pair Leads

This configuration involves folding back and twisting the element conductor, recommended wherever possible.

Rod Leads

Rod leads feature a heavier lead affixed to the element, often through welding.

Pad or Bar Lead

Similar to rod leads, a flat bar may be employed, or a strip can be used, accommodating the specific heating element package required.

Bending Radius

The lead wire must allow for customer-specific bending requirements. The minimum bend radius should be four to eight times the wire diameter for both iron-chrome-aluminum and nickel-chrome alloys. Notably, iron-chrome-aluminum alloys are prone to breakage in extremely cold conditions.

Brittleness

Traditional iron-chrome-aluminum alloys become brittle at approximately 950 °C. This transition occurs abruptly. Powder-metal-based alloys also exhibit brittleness upon heating at varying rates influenced by temperature and exposure duration. Cooling these alloys above 500 °F is vital to prevent mechanical damage during repositioning. Handling at moderate temperatures of around 70 °F is recommended to avoid low-temperature brittleness. Welded areas may also demonstrate brittleness and must be handled with care.

Terminations

Correct terminations are vital for a successful heating element application, affecting the element's effective lifespan. Ensuring that the majority of the lead wire makes close contact with the termination point is crucial.

Lead Protection

Providing a protective coating over element leads is often beneficial for mechanical or electrical safeguards. Careful selection of protective shields is necessary. Self-sticking tapes should generally be avoided due to potential reactions with high-temperature adhesive components that can lead to embrittlement and corrosion.

Helpful Practices and Suggestions

To maintain furnace heating elements effectively, consider the following practices:

• Maintain cleanliness around terminal areas, applying a regular maintenance program.
• Utilize field wiring that can withstand high temperatures, sidestepping wax, rubber, or thermoplastic-insulated wires.
• Implement thermal insulation wherever feasible to reduce heat losses and operational costs.

Caring for furnace heating elements will ensure they consistently serve their intended purpose throughout their operational life.

If you're interested in exploring Resistive Heating Elements, reach out for expert consultations!

Make sure to consider every factor when selecting a heating element, as it plays a critical role in your application’s performance and reliability. If you are in the process of choosing a heater for your next project, Datec Engineers can assist you. Discover how we can support you in designing an ideal heating solution.