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Heating Elements Design Considerations

Designing heating elements

Heating elements sound very simple and straightforward, but there are, many different factors that engineers have to consider in designing them. There are roughly 20 – 30 different factors that affect the performance of a typical heating element, including obvious things like the voltage and current, the length and diameter of the element, the type of material, and the operating temperature. There are also specific factors you need to consider for each different type of element. For example, with a coiled heating element made of round wire, the diameter of the wire and the form of the coils (diameter, length, pitch, stretch, and so on) are among the things that critically affect the performance. With a ribbon heating element, the ribbon thickness and width, surface area, and weight all have to be factored in.

And that's only part of the story, because a heating element doesn't work in isolation: you have to consider how it will fit into a bigger appliance and how it will behave during use, when it's used in different ways. How, for example, will your element be supported inside its appliance by insulators? How big and thick will they need to be and will that affect the size of the appliance you're making? For example, think about the different kinds of heating elements you'd need in a soldering iron, the size of a pen, and a large convector heater. If you have an element "draped" between supporting insulators, what will happen to it as it gets hotter? Will it sag too much and will that cause problems? Do you need more insulators to stop that happening, or do you need to change the material or the element's dimensions? If you're designing something like an electric fire with multiple heating elements close together, what will happen when they're used individually and in combination? If you're designing a heating element that has air blown past it like in a convector heater or a hair dryer, can you generate enough airflow to stop the element overheating and dramatically shortening its life? All these factors have to be balanced against one another to make a product that's effective, economical, durable, and safe.

Heating Element Design

The following calculations give a guide to selecting an electrical resistance wire heating element for your application

Heating Element Design Calculations

Here is an introduction to electrical resistance of tape and wire heating elemets, a calculation of element resistance and a temperature-resistance table.

To perform as a heating element the tape or wire must resist the flow of electricity. This resistance converts the electrical energy into heat which is related to the electrical resistivity of the metal, and is defined as the resistance of a unit length of unit cross-sectional area. The linear resistance of a length of tape or wire may be calculated from its electrical resistivity.

Where:

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

As a heating element, tape offers a large surface area and therefore, a greater effective heat radiation in a preferred direction making it ideal for many industrial applications such as injection mould band heaters.

An important characteristic of these electrical resistance alloys is their resistance to heat and corrosion, which is due to the formation of oxide surface layers that retard further reaction with the oxygen in air. When selecting the alloy operating temperature, the material and atmosphere with which it comes into contact must be considered. As there are so many types of applications, variables in element design and different operating conditions the following equations for element design are given as a guide only.

Electrical Resistance at Operating Temperature

With very few exceptions the resistance of a metal will change with temperature, which must be allowed for when designing an element. As the resistance of an element is calculated at operating temperature, the resistance of the element at room temperature must be found. To obtain the elements resistance at room temperature, divide the resistance at operating temperature by the temperature resistance factor shown below:

Where :

R = Rt / F

Surface Area Loading

It is possible to design a heating element in a variety of sizes all of which would in theory give the desired wattage load or power density dissipated per unit area. However, it is essential that the load on the surface of the heating element is not too high as the transfer of heat by conduction, convection or radiation from the element may not be rapid enough to prevent it over-heating and failing prematurely.

The suggested surface loading range for the type of appliance and heating element are shown below – but this may need to be lower for a heating element working with more frequent operating cycles, or at nearly its maximum operating temperature, or in harsh atmospheres.

heres.

Appliance Element Type Suggested Surface Loading
Range (W/cm²)
Fire Spiral Element in Free Air 4.5 – 6.0
Fire Pencil Bar 6.0 – 9.5
Band Heater Mica-Wound Element 4.0 – 5.5
Toaster Mica-Wound Element 3.0 – 4.0
Convector Spiral Element 3.5 – 4.5
Storage Heater Spiral Element 1.5 – 2.5
Fan Heater Spiral Element 9.0 – 15.0
Oven Element Tubular
Sheathed Element
8.0 – 12.0
Grill Element 15.0 – 20.0
Hotplate 17.0 – 22.0
Water Immersion Heater 25.0 – 35.0
Kettle Element 35.0 – 50.0

Designing a Round Wire Element

Where:

Here is how the design calculations are done:

1.  Calculate the wire diameter and length required, operating at a maximum temperature of C°C, the total resistance of the element at operating temperature (Rt) will be:

Rt = V² / W

2.  Using specific heating element alloy wire, find the Temperature Resistance Factor at C°C operating temprature as F thus the total resistance of the element at 20°C (R) will be:

Rt = Rt / F

3.  Knowing the dimensions of the heating element type, the length of wire that may be wound round it may be estimated. Thus, the resistance required per metre of wire will be:

A = R / L

4.  Find the heating element wire of standard wire diameter which has a resistance per metre which is closest to A.

5.  To verify the actual wire length (L):

L = R / A

A change in heating element wire length may mean adding or subtracting the pitch of the wire to achieve the total resistance value required.

6.  To verify the surface area loading (S):

S = W / (l x d x 31.416)

This surface area loading should fall within the range shown in the table above for heating element type noting that a higher value gives a hotter element. The surface area loading can be higher or lower if it is considered the heat transfer be better or worse, or depending upon the importance of the heating elements life.

If your calculated surface area loading is too high or low you should re-calculate changing one or more of the following:

Coiled or Spiral Elements

Wire heating elements formed into a coil allow a suitable length of wire to be accommodated in a relatively short space, and also absorb the effects of thermal expansion. When forming the coil care must be taken not to damage the wire by nicking or abrasion. Cleanliness of the heating element is also important. The maximum and minimum recommended ratios of inside-coil diameter to wire diameter are 6:1 and 3:1. The length of the close wound coil may be found using the equation below.

Where:

X = L x d x 1000 / π x (D + d)

When this close wound coil is stretched the stretch should be about 3:1 as closer winding will result in hotter coils.

Apart from accidental damage the service life of a heating element may be shortened by localised burn-outs (hot spots). This may be caused by change to the wire's cross section (e.g. nicks, stretching, kinks), or by shielding an area where the heating element cannot dissipate its heat freely, or by poor supporting points or terminations.

Designing a Tape Element

The method for designing a tape heating element is similar to that used in designing a round wire heating element.

Where:

Here is how the design calculations for the tape heating element are done:

1.  To calculate the tape size and length required for a specific heating element in heater, operating at a maximum temperature of C°C, the total resistance of the element at operating temperature (Rt) will be:

Rt = V² / W

2.  Using specific heating element alloy wire, find the Temperature Resistance Factor at C°C operating temprature as F thus the total resistance of the element at 20°C (R) will be:

Rt = Rt / F

3.  Knowing the dimensions of the heater, the length of the tape that may be wound round it may be estimated. Thus, the resistance required per metre of tape will be:

A = R / L

4.  Find a heating element tape of standard size of b mm x t mm having a standard resistance per metre of stock size which is near to A ohms/m.

5.  To verify the actual tape length (L)

L = R / A

A change in tape length may mean altering the pitch of the tape to achieve the total resistance value required.

6.  To verify the surface area loading (S):

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

If your calculated surface area loading is too high or low as per the table above, you should re-calculate changing one or more of the following:

– The tape length and size

Practical Design Considerations

This article discusses general issues relating to the use, care and maintenance factors relative to obtaining longevity in electric heaters and 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 heating element 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:

Heating Element Leads and Power Connections

Certain norms to be followed with regards to electric connections to electric heating elemets in heaters are listed below:

Lead Styles

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

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 element 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 950°C 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 500°F 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 70°F 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 1400°C 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 68°F before installation. 68°F may be ideal as a minimum temperature, practically up to 100°F 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 200°F for one to two hours, then increased to 500°F and maintained at this temperature for four to six hours, open to air. Then the temperature should be increased at 150°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 heating elements are listed below:

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

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