Furnace Heating Elements – Design Considerations and Maintenance

Table of Contents

Introduction
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
Drying Out Procedure: Refractory Materials
Cycling
Helpful Practices and Suggestions
Conclusion
About Thermcraft

Introduction

This article discusses general issues relating to the use, care and maintenance factors relative to obtaining longevity in electric furnaces. The complexity of issues relating to resistance type heaters indicates the need for a universal guide as a starting point.

Electrical Lead Considerations

It is not just necessary to consider the type of electric heater and placement and wattage requirements, but it is also necessary to consider the different types of electrical leads used and the methods by which they exit and terminate the heated area. Certain considerations while selecting leads are listed below:

  • Temperature of lead area
  • Flexibility
  • Relative cost
  • Contaminants in the lead area
  • Abrasion resistance needed
  • Convenience to controls

Heating Element Leads and Power Connections

Certain norms to be followed with regards to electric connections are listed below:

  • The line voltage should be compatible with the heater's rated voltage.
  • Electric wiring to the heater must be installed in accordance with national and local electric codes.
  • Polarity should be observed always. Adjacent leads should always be connected to the same polarity. Failure to observe polarity may result in premature heater failure.

Lead Styles

Element leads are available in a wide variety of styles, but can normally be grouped into certain categories that include the following:

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

Single Conductor Leads

The single conductor concept is the most common and is mostly the standard form of supply for ceramic and vacuum formed fiber, heating elements.

Twisted Pair Leads

Twisted pair indicates a lead in which the element conductor is folded back on itself and then twisted together in a particular manner. This type of lead configuration is recommended where possible.

Rod Leads

Rod leads include fastening a heavier lead to the actual element. Typically a rod will be welded to the heating element conductor.

Pad or Bar Lead

The pad or bar lead is similar in nature to the rod concept only that either a flat bar is used or if the element uses "strip" instead of wire, the strip is often folded back on itself once or twice to expand the cross-sectional area. This style of lead is used with fiber based heating packages

Bending Radius

It must be possible to bend the lead wire from the heating elements as per customer’s requirements. The minimum bend radius of the wire must be four to eight times the wire diameter. This rule applies to both iron-chrome-aluminum alloys and nickel- chrome alloys. In really cold conditions iron-chrome-aluminum alloys may still break or crack on bending.

Brittleness

Traditional iron-chrome-aluminum materials become brittle on reaching a temperature of 950ºC and this happens immediately. The powder metal based alloys also become brittle on heating though this is more gradual and depends on temperature and time. It is important to cool these alloys to a color temperature above 500ºF so that they can be repositioned without incurring any mechanical damage. These are also brittle at low temperatures so if they need to be worked with it is better to have a temperature of around 70ºF or more. It is also important to note that on welding of these alloys, the nearby areas become brittle hence must be handled carefully.

Terminations

Proper terminations are crucial to a successful heating element application and if not performed appropriately will affect element life drastically. It is important to ensure that the bulk of the element lead wire is in close physical contact with the actual termination.

Lead Protection

Often it is desirable to provide a protective coating over the element leads. This may be required based on electrical or mechanical considerations. The selection of a protective shield for the leads must be done with great care. Generally use of self sticking tapes should b avoided as even the high temperature grades use organic based mastic/adhesive which can break down into carbon based substances. These may react with the wire causing embrittlement, corrosion and carbon infiltration. Insulation grades must be carefully examined. While handling refractory fiber based materials, an approved respirator should be worn especially if the heater has been at a high temperature for a long time and is being replaced.

Repairs

In larger rod-style elements it may be possible to repair a break. For iron-chrome-aluminum alloys, a similar operation is used except that the material should be heated to "red" color temperature before it is moved. This will allow bending of the conductor segments without causing any breakage.

Handling, Storage, Environmental Factors

The reason behind modern metallic based heating elements operating at temperatures up to 1400ºC for long time periods is that they form an outer protective oxide coating. In case there is any kind of surface contamination, the element will fail prematurely. Hence the element cleanliness needs to be maintained.

Element storage is again an important area of consideration. It is important to keep them weather-protected and stored in a dry, cool place preferably one preferably having a low humidity. It is also important to wear cotton gloves to protect exposed elements from body oil present on the hands. Smaller sizes are more prone to getting contaminated. The elements need to be always kept on a protective barrier avoiding direct contact to the shop floor or other contaminated areas.

After storage, heaters must be warmed to a minimum temperature of 68ºF before installation. 68ºF may be ideal as a minimum temperature, practically up to 100ºF is desirable. As ceramic based heaters can get damaged easily, one needs to be careful not to force fit them or drop them.

Vibrations

For places with high vibration, shock mounting is essential using standard shock mounting techniques. Too much of vibration may also impact wire connections.

Loading

A 20% maximum load reduction should be allowed for if a contactor is used instead of an SCR control. It needs to be noted that this SCR control is either a phase angle fired or variable time base fired zero cross over unit. Normally, a zero crossover unit is more desirable but actual application will determine the practical choice.

Drying Out Procedure: Embedded Elements

Before initially heating the furnace, it is necessary to see whether any embedding cement has broken loose from the ceramic heaters and if the heater wire is visible. Then application of embedding cement can be done as per instructions for patching heaters.

Drying Out Procedure: Refractory Materials

It is highly recommended to increase temperature gradually in order to dry the moisture from the refractory lining. It is suggested that the unit be heated to 200º F for one to two hours, then increased to 500º F and maintained at this temperature for four to six hours, open to air. Then the temperature should be increased at 150º F per hour thereafter until the normal operating temperature is achieved.

Cycling

The most efficient way to get long life is to utilize a large cross sectional area element with moderate watt loading, and never shut it off. The problem with cycling is that the oxide will either crack or spall off exposing the base material to more oxidation and eventual failure.

Helpful Practices and Suggestions

Some helpful practices while handling furnace hating elements are listed below:

  • The equipment needs to be maintained clean, especially surrounding the terminals, wiring enclosure, and heater itself, using a regular maintenance program.
  • Field wiring that can withstand the temperatures must be used. It is essential to avoid the use of wax, rubber, thermoplastic or impregnated insulated wire for high temperature heater applications.
  • Thermal insulation needs to be used wherever possible in order to reduce heat losses and cost of operation.

Conclusion

Furnace heating elements need to be maintained well in order to ensure that they serve their purpose and remain useful during their lifetime.

About Thermcraft

Thermcraft Inc. manufacturers custom industrial ovens, laboratory furnaces, and other furnace related supplies and is a recognized leader in high temperature heating equipment. Their state-of-the-art facility and design software will allow their staff to meet your requirements for any furnace or oven configuration that you may need.

Source: Thermcraft

For more information on this source please visit Thermcraft

Comments

  1. Prakash Bhandari Prakash Bhandari India says:

    Sirs,

    We want to design a coil in NiCr:8020 having following specifications.

    Coil OD-- 20 mm
    Wire Dia-- 11 SWG
    KW -- 4.33 Kw
    Voltage--440 V

    Please let us know the length of  wire required to make such a coil.

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