Metal heating element

About heating elements

Heating elements convert electrical energy into heat energy. They can be made in many shapes and from numerous materials, but in kilns for glass and pottery they will take one general form: as stretched spirals of high temperature resistance wire of varying thickness and coil diameter.

Fig 1

They can be carried in grooves in the hotface of kiln walls and sometimes of roof door and floor. Shown are elements wound as a small diameter coil and supported in grooved brick walls in a pottery kiln.

Fig 2

They can be wound as larger coils and supported on refractory tubes clear of kiln walls or roof. Shown are the elements in the roof of a Riley FS-3 glass kiln.

Fig 28-03  to come.

Sometimes the element-on-tube can be recessed flush into the roof insulation.

The wire. 

Heating elements are made in Australia almost exclusively from a high temperature alloy called Kanthal A1. Apart from being able to withstand very high temperatures, Kanthal wire has three main characteristics which impact on its performance and durability:

• It creeps. Moves around, the coils bunch up or contract in length.
• It becomes very soft when heated to working temperature so that it cannot support any sort of load; not even its own weight.
• With use, it becomes extremely brittle at room temperature and can be easily broken; unlike new wire which can be coiled and bent without difficulty.

When fired to pottery temperatures this brittleness will develop quite rapidly, but takes longer to develop when taken only to glass temperatures. None the less a used element in a glass kiln will soon become too brittle to withstand the stress of pushing back int a corner from which it has contracted without the risk of it breaking.

A recent introduction is Kanthal APM, which is a more expensive material with a lower creep factor. This means that it will creep and 'walk about' to a lesser degree than Kanthal A1, not that it won't do it at all. However demand is too low here for it to be available to local kiln builders and for cost reasons it is unlikely to be used in imported kilns.

Spiral elements are usually wound as a close spaced coil and then stretched to whatever length is required.Creep causes the element in use to move around and shrink in length. On tubes it will bunch up in places and stretch out in others. In grooves, as well as bunching and shrinking in length, the coils may also lean over and fall one on another, as shown in fig 1 above. This image is from a pottery kiln where temperatures are higher and deterioration is generally less severe at glass temperatures.

Creep     

Creep occurs mainly when the wire is hot. The incidence will be least when an element is mounted horizontally, as in grooves in walls or on tubes on roofs of top-hat kilns; and will be greatest when mounted so that they are, in all or part, on a slope. This makes it so much easier for the coils to slide down the slope.

The problem can be minimised with elements on tubes in hinged lift-up lids by mounting them parallel to the hinge pins. Mounting them at right angles to the hinge pins should be avoided as the coils will slide down the tubes and most of the coils will congregate at the lower side of the lid. They will gradually do this when cold; but much more rapidly when hot. Incorrect orientation in a clamshell lid can cause all the elements to bunch to the hinge side and result in uneven heating of the glass.

The effect of creep when the elements are laying horizontal will be for random bunching and stretching over short lengths, so that overall distribution of heat from a number of elements won’t be all that different from when the coil spacing was uniform.

Little can be done about creep with continuous spiral elements in lids on top loading kilns, where the coils lie at all sorts of angles to the horizontal when the lid is raised.

Variable pitch elements              

Pitch here refers to the distance between adjacent coils in an element. The term refers to varying the space between the coils to vary the amount of heat inserted from point to point along its length. This will sometimes be seen with tube mounted elements across roofs, where there will be a concentration of coils at each end of an element. This is designed to apply more heat at the walls; to compensate for the heat lost through those surfaces.

Considering the penchant for wire elements to creep, this initial provision is usually of dubious value; especially considering that the design inserts extra heat at the walls where the elements terminate but makes no similar provision for the other walls of the kiln. Somewhat of a token gesture.

Oxide coating           

A new element will usually be shiny, but will rapidly develop a dull grey or blackish oxide coating. This coating has slight electrical resistance and prevents the coils shorting out should they bunch up or lean and touch one another as they age; this because the voltage between coils will rarely exceed 1Volt.

Nevertheless, if wishing to make an electrical connection to oxidised wire, be sure to first clean it down to a bright surface.

Do NOT make bolted or clamped connections inside a kiln. TIG (Tungsten Inert Gas) welding is the only safe way to do so. Unfortunately the high temperature produced immediately makes the wire brittle so such work should be done only by specialists in element design.

Power loading    

This refers to the amount of power which is dissipated per unit area of the surface of the wire; usually stated in Watts per square centimetre.The greater the power loading, the higher will the element temperature go above kiln temperature. Obviously the element must be hotter than the kiln temperature; otherwise it could not radiate any energy at all.

Kanthal element wire will melt slightly above 1400°C (2550°F).

Only a low power density can be used with elements in a pottery kiln designed to operate to 1300°C (2370°F) if the wire is not to be put at risk of burning out. In a kiln for kilnforming, where a design limit of about 900°C (1650^oF) is usually used, a much higher power density can be allowed for in the design.  

In practice the power density is up to about two Watts per sq. cm. for pottery kilns and about four for kilnforming. Thus, much more powerful elements can be substituted for the original ones in the slots of a pottery kiln if it is to be used in future at the lower temperature. Reference is made to this in 22. Evaluating a pre-owned kiln.

Element old age.  

Like everything else, elements deteriorate with use. Initially bright and shiny, then dull, it's surface becomes ever more pitted and eroded. This is the normal effect of high temperatures on metals. The greater the number of firings and the longer the time at temperature the greater will be this loss of surface material.

With loss of mass comes a reduction in diameter of the wire and consequent increase in electrical resistance.

Increased electrical resistance allows less current to flow, reducing the heat input to the kiln and increasing the time it will take to get to soak. Effectively, over time, an element of say 25Amps will carry only 24Amps, then 23Amps - 22 etc. This will be a very gradual process, extending over hundreds or even thousands of firings. It will be insidious and in some cases of little consequence; but in others the replacement of elements will have visible benefits. .

A large commercial Riley Top Hat kiln has recently had its elements replaced after 10 years of use and thousands of firings. The owner reported an obviously faster heat-up with same size elements.

Elsewhere in this site has been described the procedure for plotting the heating curve of a kiln. If, at some time in the future the test is repeated, comparing the two test results will show the effect of element ageing or deterioration. The ambient temperature and kiln load will affect the result of the test so, if doing this be sure to do the second test under similar conditions to the first.

Embedded elements

This is where the element is encased in an electrically insulating material to hold it in position.

It is commonly done deliberately to make what are called “muffle” furnaces; small diameter coiled elements wrapped around a mould and encased in a refractory castable material such as a ceramic fibre and colloidal silica mix similar to fibreboard composition. In this case the watt loading of the element will be substantially reduced from what it would be in the open air if premature failure is to be avoided 

Whilst this has no direct application to kilnforming, a similar situation can occur accidentally when material can get into an element groove in a kiln wall or floor and completely or partly cover a section of element.

This can have two effects;

• The insulating effect of the material can cause the embedded section of element to overheat and melt.
• The composition of the molten puddle may allow current flow to be maintained so long as the mass remains molten.

Thus, when glass sand or some mould material gets into a groove, the element may overheat and the mess may melt, but the current will continue to flow until the molten mass cools down; after which it may no longer do so. The firing when the accident occurred may not be interrupted but a subsequent firing may not be possible.

Not only will the element need replacing but the molten mass will have adhered itself tenaciously to the brickwork and require considerable effort to remove. Some rebuilding of the element groove may be necessary as, if the molten material was originally conductive, then it could again become so if reheated and could then short out a section of the element. Best to chip out all evidence of the glassy mass and replace with clean refractory.

Elements in grooves in walls

Elements require continuous support

Fig 4

Fig 4 shows a very well used brick lined pottery kiln. It's maintenance has been poor and broken lips of grooves have not been repaired, so gravity has done the inevitable; the element has spilled out of the groove on its way to the floor. A piece of ceramic fibre has been used to prevent it shorting to the element below. Great maintenance.

Again, this is a pottery kiln but does show what can happen.

Fig 5

In multi sided kilns, shrinking in length caused by creep can pull the element out of the corners or angles. The kiln user needs to check regularly to see that the coils are where they should be.

Retaining elements in grooves

Fig 6

Numerous techniques have been used to hold the coils in place so that they cannot rise up out of the groove, such as using IFB wedges in the corners, but they crumble with age and become ineffective. Ceramic pieces or wire pins can be inserted near the corners; some are shown in Fig 6. The ceramic pieces are 8mm diameter 2 hole thermocouple insulators, cut with a diamond saw to 25mm length. They can be bought from thermocouple makers and some instrument suppliers. The wire pin can be made from new element wire of about 16 to 18 B&S gauge or from high temperature nickel chrome resistance wire as used for fusing into glass as a hanging loop. 

Fig 7

Fig 7 shows one of the ceramic tubes in position. An 8mm diameter hole is drilled in the IFB so that the tube will just touch the element and prevent it lifting up. The hole should be placed as close to the corner as is practical;. Drill at a very slow speed if the element is already in place as there is the risk that the drill flute will catch on the element and pull it out and damage it.

Fig 8

The anchor pin is used in a similar position. Cut the wire about 50mm long and bend about 5mm of one end at right angles so it can be gripped with long nose pliers. The pin is pushed in as close to the corner as practical and at an angle so that it seats on top of two coils, as shown by the red coloured pin in the mock-up in Fig 8. Elements can be wound left or right hand, so angle the pin to suit.

Fig 9

Owners of existing kilns are more likely to find the element already pulling out of the groove. Because the wire becomes brittle with use it is usually better to secure it where it is than to try to force it back to where it came from. At glass temperatures an unsupported element of the length shown is unlikely to sag very much so it can usually be pinned in the positions marked using the method shown in Fig 8. Remember, the objective now is to stop any more of the element rising up out of the groove than has already done so.

Element in groove in lid

This method of mounting elements is used almost exclusively in American top loading glass kilns, although the Melbourne builder Tetlow is now making pottery kilns of a similar style..

When originally wound, the coil is like a spring, so that when it is stretched its tension will hold it against the side of the groove. The groove circles continuously so that the coil is normally pulling against one side or the other. However, with use, the tension can be lost and the tendency of the wire to move about, creep stretch – behave like a live thing – can allow it to drop out of the groove.

This is not helped by most of the element being suspended at significant angles to the horizontal when the lid is raised; especially if held there when the element is hot.

Users should inspect them regularly and secure them in place wherever necessary. This can be done using bent pins as shown earlier. There may be many pins already in position.

Fig 10A

Fig 10B,

Fig. 10A shows a representation of the conventional groove shape used in most top loader lids.The opening is made wide enough to allow the coil to enter and it is then partly supported by the lip at the edge of the groove. This is effective as long as tension is pulling the coil against the side of the groove

A more effective shape is shown in Fig 10B. Here the opening is too narrow for the element to fall out. The element must be threaded in from the end of the groove instead of dropping it in all along the slot. As this takes longer to do, and adds to the cost, it is not widely done.

It is possible that the elements in some American top loaders are now made of Kanthal APM wire. If this is the case, then owners of those kilns needing replacement elements should consider seeking them through their original kiln supplier. This because they may last a bit longer than those made locally before they fall out.

I don't know personally whether this is the case, and nor can I advise how to identify which alloy you have. However, as APM is more expensive than A1 it is unlikely that it is used.

Replacing top loader lid elements

A more sensible alternative is to have made locally a replacement element which will not fall out. Two or three layers of plastic shrink sleeving on a standard mandrel should increase the coil outside diameter sufficiently so it will rest on the lips of the groove. Measure the width of the slot opening and also the maximum width of the groove, then order the element with an outside diameter mid way between the two.

Fig 11

Prepare the lid to accept the larger diameter element by grinding away the lip overhang on the OUTER side of each major change in direction of the slot, as shown in fig 11. This can be done with a small mounted point in a power drill or similar tool. Mark on the element the points which should coincide with the widened sections of the groove then feed the element into the groove, starting at the centre and working toward each end and the termination.

If any pins are needed it will be only at the widened sections but, as the element will normally be under tension and pulling toward the inner side of the loop where the lip remains, there should be little need.

Cementing elements in place

In an emergency the element can be cemented in place using very small blobs of special cement. At glass temperatures a mixture of colloidal silica and kaolin can be used as described in 61X Adhesives & mortars elsewhere in this site. The blob should be placed in the bottom or back of the groove, be of a fairly stiff consistency and of a mass that will barely grip the wire then it is pressed into it.

Too much mortar could cause the wire to overheat.

Elements on tubes

Elements to be mounted clear of the kiln lining are supported on one of two possible types of refractory material;

• Thin wall high alumina tubes having OD/ID (outside /inside diameter) of 12/8mm, 15/11mm or 20/15mm. Most common lengths are 1000mm or 1500mm.
• Extruded silimanite type material. Most common size available is 25/13mm with length to about 750mm.

The support tube reaches the same temperature as the element, which can be considerably higher than the indicated kiln temperature, so must be well supported at reasonable intervals to prevent sagging.

Fig 12

The tubes shown in Fig 12 are 1500mm long 12/8mm diameter, with a centre as well as secure end support. These elements and tubes will last indefinitely without giving trouble.

Fig 13

On the other hand, the same size tube installed without adequate end and intermediate support will quickly sag. This encourages the element to creep as it slides downhill; and also has an adverse effect on the end security of both tube and element.

Sagging element support tubes.

Sometimes, even with adequate support, a tube will show signs of sagging. This can be easily overcome by turning the tube 180 degrees so that it arches upward instead of downward. It is more likely to occur at pottery temperatures than in glass kilns; but who knows when the kiln god may visit. Simply grasp the tube with a pair of long nose pliers and turn.

Tube support

Top-hat or clamshell hoods in glass kilns will invariably have a CF lining and irrespective of the size of the kiln, the tubes must always be supported at the ends.

Inertia places a downward load on the tubes when the hood ceases its downward travel. If they were supported in round holes in the CF, then over time the holes would become elongated and the ends of the tubes would drop. To prevent this, something more solid is needed. In Riley kilns, pieces of IFB are cemented into the fibre lining with a hole to accept the end of the tube. One has been highlighted in Fig 12.

The method of lining construction used in Riley kilns is called stackbond and the embedded IFB block is an effective tube support where stackbond is used. IFB blocks are less effective on the alternative 'wallpaper' lining method, but ceramic cuplocks can there be cemented to the CF hotface layer. There’s more detail on stackbond and wallpaper construction in 12. Ceramic fibre linings.

Sometimes, element design calls for support tubes up to 25mm diameter. Holes to this size can be drilled in the IFB blocks. Unfortunately the hole in the cuplocks currently available will accept only small diameter tubes.   

Stackbond lining construction also allows the use of an ceramic intermediate tube hanger developed by Riley Glass Kilns which does not require any attachment to the shell; thus allowing positioning as installation proceeds without prior planning of location. This item is also seen in Fig 12.

Element configuration

 Elements on tube below the roof can be arranged in one of two ways;

• On a single tube with one end terminated through each side wall, with a termination duct on both sides, or
• As in a hairpin shape, on two tubes and with both tails terminated on the same wall.

The form chosen will depend largely on the kiln dimensions and on the power rating of the element. In large kilns up to about 1400mm wide and with reasonably low power per square metre of hearth area the hairpin arrangement will usually be most suitable. This arrangement is shown in Fig 12

Fig 14A & 14B                                           

Fig 15

To prevent the  element from creeping , the looped end should be anchored in place by a wire hook as shown in Fig 15. Element wire of about 12 to 14B&S is ideal.

Accommodating tube expansion when hot. 

Like most materials, element support tubes expand when heated. They will also shrink again as they cool. Allowance must be made for this movement by allowing a sufficient length of penetration into the support at each end to accommodate all the shrinking to take place at either end. It is impossible to predict which end will move. About 20mm of support is generally sufficient

Joining tubes

There is a limit to the length of tubes available and a design may sometimes require them to be joined end-to-end. There are two possible ways of doing this; with a joiner inside the holes in the tubes, or on the outside.

There is good reason for rejecting the inside joiner option. The bore of the high alumina tubes most commonly used are only 8 or 11mm.  Any ceramic material of this size is flimsy and easily snapped if the tubes bend or go out of alignment. At other times a piece will be snapped out of the tube itself.

Fig 16

One method shown in Fig 16 uses a short length of alumina tube  which is a loose fit on the support tubes to keep the ends properly aligned. A blob of mortar is put inside and allowed to set before the joiner is installed. A section of the element is uncoiled so that it will straddle the joiner. The reason for the mortar is that should the element creep it could slide the joiner along the tube and leave one tube end unsupported. There have been instances where the weight of an unsupported tube has uncoiled the hot element wire and allowed the tube to sag down onto the glass.

A single hanger can be placed at the join to support both tube ends at the same time.

Fig 17 (to come)

Fig 17 shows an element installation in a clamshell kiln. The construction is much more robust than that necessary for a top-hat kiln; because of the need to withstand severe stress should the hood be slammed down too hard. The tubes are 30mm diameter, with a thick wall. Distance between supports is short. The hangars themselves are massive.

All this because refractory materials are brittle and can easily crack if over stressed; and closing a hinged hood with too much vigour could certainly do that.

Element Tail

This is the portion of the element from the end of the coiled section to the outside where the electrical connection is made. The tail must carry the same current as the coil itself and must also pass through the insulated kiln wall where the heat cannot be dissipated as easily as in the kiln.

The tail is often thickened with additional strands of wire to better dissipate the heat; often with one or two additional strands which are twisted tightly together.

Fig 18
Less frequently, the tail can be a length of larger diameter Kanthal rod, threaded at one end and with a hole drilled in the other end to accept the end of the element wire, as shown in Fig 18. A die is used to compress or ‘crimp’ the rod onto the wire to hold it in place and a good electrical connection is made by TIG welding through the hole in the side of the rod.

Element terminations

to come

Replacement elements

A simple coiled element consists essentially of a length of wire of the appropriate gauge, wound into a coil and stretched to the appropriate length.

The expertise lies in selecting the wire to be used, and in having the facilities to produce the coil of the correct diameter. There are professional element makers throughout Australia who can supply elements to meet most applications.

Mitigating against the handyman doing this is the difficulty and high cost of obtaining the small quantity of wire needed. Kanthal wire is quite expensive and minimum quantity restrictions apply.

CAUTION:Electricity can kill. Element replacement should be done by an electrician or someone knowledgeable about electricity. Unplug the kiln or MAKE SURE that the power is off before tinkering.