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Kanthal Globar SDSilicon carbide heating elements

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Contents

Introduction 4

Product rangeKanthal Globar SD heating elements 5

Kanthal Globar SD elements 5

Kanthal Globar SD multi-leg elements 6

Applications 8

Installation methods 10

Kanthal Globar SD rod element installation 10

Kanthal Globar SD multi-leg element installation 11

Vertical mounting 11

Horizontal mounting 13

Element spacing all element types 14

Electrical characteristics 14

Electrical specications 15

Element perormance 16

Element loading 16

Start up procedure 16

Operating temperature 16

Efect o atmosphere 18

Glazes and coatings 19

Continuous or intermittent operation 19

Power supplies 20

Element connections 20

Matching o resistance value 21

Voltage reserve 21

Power supply equipment 21

1 Variable output transormer 22

2 Thyristor (SCR) unit 232.1 Phase-angle ring 24

Power monitoring phase-angle 25

2.2 Fast-cycle ring 26

Power monitoring ast-cycle 27

3 Transormer + thyristor (SCR) 27

4 Direct-on-line 27

Ordering 29

Rod type elements 29

Multi-leg elements 29Accessories 30

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4

Introduction

Kanthal is the heating technology brand withinSandvik; the worlds leading manuacturer osilicon carbide (SiC) heating elements.

Kanthal Globar SD (SiC) elements are manuactured in recrystal-lized alpha silicon carbide, and have been designed to maximizeperormance in the widest range o high temperature equipment.

Kanthal Globar SD elements are used in applications ranging intemperature rom below 600C (1110F) up to 1600C (2910F)

in both air and controlled atmospheres, although the type o atmo-sphere used will determine the maximum recommended elementtemperature.

Kanthal Globar SD elements may be mounted either vertically orhorizontally, and as the material remains rigid, even at the maximumoperating temperatures, no special supports are required.

Kanthal Globar SD elements will also accept signicantly higherelectrical loadings than metallic elements while maintaining superiorperormance in both continuous and intermittent heat processes.

These actors can result in considerable savings in urnace construc-tion costs and maintenance is greatly simplied. Elements can nor-mally be replaced quickly, even when the urnace is hot, to minimizedowntime.

Kanthal Globar SD elements are available in the orm o round sec-tion rods or tubes, in diameters rom 10 mm 55 mm (0.39 2.17 in).Standard sizes are available to replace all previous ranges o Kanthalsilicon carbide rod elements, such as Globar LL, Silit ED and Kanthalhot rods and also in special sizes and resistances, to replace elementssupplied by other manuacturers.

In addition to simple rod elements, Kanthal Globar SD is availablealso in multi-leg derivatives, using 2, 3 or 4 legs, to allow all the elec-trical connections to be made at one end o the element, and simpliyinstallation.

EATURES

Can be used in applicationsranging rom 600C up to1600C (1110 2910F)

Accepts signicantly higherelectrical loadings thanmetallic elements

Considerable savings inurnace construction cost

Maintenance is greatly

simplied Available in multi-leg

derivatives

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A, in 3/8 1/2 9/16 5/8 3/4 1 1 1/4 1 1/2 1 3/4 2 1/8

B max. 13.8 19.7 23.6 23.6 33.5 33.5 49.2 57.1 94.5 94.5

L max. 25.6 37.4 39.4 45.3 57.1 62.3 78.7 88.6 129.9 129.9

A, mm 10 12 14 16 20 25 32 38 45 55

B max. 350 500 600 600 850 850 1250 1450 2400 2400

L max. 650 950 1000 1150 1450 1600 2000 2250 3300 3300

L

Test volts/ampsAluminum sprayed A

B

Product rangeKanthal Globar SD heating elements

Kanthal Globar SD elements

Kanthal Globar SD heating elements are made romhigh purity alpha silicon carbide grains, that areextruded in the orm o rods or tubes, beore beingbonded together by a process o re crystallization,at temperatures o over 2500C (4530F). Thering process ensures the creation o rods withstrong uniorm bonds between adjacent grains, andthe particle size distribution is closely controlledto ensure optimum density and resistance to theprocess atmosphere.

Kanthal Globar SD elements have the conventionalcentral hot zone and two low resistivity cold ends.Elements with unequal cold end lengths can also besupplied i required.

Kanthal Globar SD elements can be supplied in1-piece or 3-piece construction, according to thedemands o the application. One-piece elementseature a joint ree construction, where the cold

ends are ormed by lling the pore structure with a

low resistivity silicon alloy. 3-piece elements eaturespecial low resistance cold ends that are joined tothe hot zone using a reliable and strong reaction-bonding technique.

As the resistivity o the hot zone material is consider-ably higher than that o the cold ends, the majorityo the heat is generated in the hot zone, when pow-er is applied. The cold ends, which pass throughthe urnace lining, remain relatively cool, and aresprayed over a short length at the end with aluminum,

to orm a low resistance contact or the aluminumterminal braids.

Standard elements are available in a range o diam-eters between 10 and 55 mm (3/8 and 2 1/8 in) asdetailed in Fig. 1. All o the elements are tubular,apart rom 10 mm (3/8 in) diameter elements,which are manuactured in the orm o solid rods.

Standard dimensions or Kanthal Globar SD elements

Fig. 1 Standard dimensions or Kanthal Globar SD elements.

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6

A mm

16

in

5/8

mm

20

in

3/4

mm

20

in

3/4

mm

25

in

1

mm

32

in

1 1/4

mm

38

in

1 1/2

mm

45

in

1 3/4

B max. 500 19.7 800 31.5 800 31.5 850 33.5 1400 55.1 1400 55.1 1400 55.1

C max. 400 15.7 500 19.7 500 19.7 550 21.7 625 24.6 625 24.6 625 24.6

D 35 1.38 38 1.50 52 2.05 52 2.05 64 2.52 89 3.50 108 4.25

E 45 1.77 45 1.77 45 1.77 45 1.77 45 1.77 75 2.95 75 2.95

F 25 0.98 32 1.26 32 1.26 38 1.50 44 1.73 54 2.13 54 2.13

G (U) 60 2.36 65 2.56 79 3.11 87 3.43 102 4.02 145 5.71 165 6.50

G (CU) 35 1.38 38 1.50 52 2.05 52 2.05 64 2.52 89 3.50 108 4.25

G (W) 95 3.74 106 4.17 134 5.28 139 5.47 166 6.54 234 9.21 273 10.7

Kanthal Globar SD multi-leg elements

Kanthal Globar SD multi-leg elements are manu-actured rom 2, 3 or 4 silicon carbide legs o exactlythe same material and manuacture as standardKanthal Globar SD elements, each leg being divided

into two clearly dened lengths o hot zone andcold end. All legs in any one element are matchedin resistance value, to ensure that the load is evenlydistributed, and are joined using silicon carbidebridges in a special process, which bonds the com-ponents into a monolithic structure o alpha siliconcarbide. There are no jointing cements or weldswhich might limit the temperature capabilities othe nished element. The connecting bridges do notorm part o the hot zone and can be used as endsupports i required.

The type U element, manuactured rom 2 legs, is asingle-phase unit, although groups o elements maybe connected to orm a balanced 3-phase load.

The type CU is similar to the type U, but has ashortened bridge or applications where only lim-ited space is available.

The type W element, with 3 legs, orms an eectivestar connected 3-phase element.

The type M element, with 4-legs, can replace twotype U elements, reducing the number o terminalconnections, and holes through the urnace lining.The 4 legs may have identical heated lengths, orthe inner pair may have shorter or longer hot zones

than the outer, to provide non-uniorm heat distri-bution, where required.

Note that the bridge depth F is not included inthe hot zone length. Full details o preerred sizesand resistance values may be obtained rom theelement size list, and non-standard elements willalso be considered on request. Elements havingnon-standard leg center distances (D) and supportslot positions (E), to match competitors multi-legelements, can be supplied i required.

Standard dimensions or Kanthal Globar SD multi-leg elements

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MSpecial order only

Aluminum sprayed A

B

C

F

U CU W

E

D

G G G

Fig. 2 Standard dimensions or Kanthal Globar SD multi-leg elements.

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Applications

All Kanthal Globar SD rod and multi-leg elementsmay be mounted vertically or horizontally as shownin Fig. 3. The single ended connection o multi-legelements makes them ideally suited to various

applications where standard Kanthal Globar SDrod elements cannot conveniently be used, such aswhere access would be dicult, where the urnacespan is too wide to allow conventional elements tobe t, or in any case where single ended connec-tions are essential.

Kanthal Globar SD elements are used in a widevariety o urnace applications, rom small labora-tory urnaces to large industrial heating processes,in dierent atmospheres and temperature ranges.The elements allow great reedom in urnace design

which, combined with simple installation and longoperating lie, makes them the preerred choicein many applications including: glass, ceramics,electronics and metal industries, and also or re-search and development. Examples o some typicalurnaces where Kanthal Globar SD elements arethe natural choice are illustrated below.

Fig. 3 Typical urnace applications.

Continuousurnaces

Aluminum melting

and holding urnaces

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Low pressure die

casting urnaces

Rotary hearthurnaces

Elevator urnaces

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Pattern element A Lmm in mm in mm in

SL10 10 3/8 18 0.71 75 2.95

SL14 12 & 14 1/2 & 5/9 26 1.02 75 2.95

SL20 16 & 20 5/8 & 3/4 31 1.22 75 2.95

SL29 25 1 43 1.69 75 2.95

SL32 32 1 1/4 47 1.85 75 2.95

SL38 38 1 1/2 56 2.20 75 2.95

SL45 45 1 3/4 62 2.44 75 2.95

SL55 55 2 1/8 70 2.76 150 5.91

L

A

Ceramic

fber

washer

Note that the hot zone

must not extend into the

element holes

Ceramic fber pad

Kanthal Globar SD rod element installation

Although silicon carbide is rigid and sel support-ing, it has a airly low impact strength, and caremust be taken when unpacking and handling theelements so that they are not subjected to mechani-cal shock. Elements should always be supported intwo hands.

It is important to ensure that the elements are notrestricted in any way and are ree to move radially,as well as axially, in their support holes. Elementholes must be in line, and the whole alignment

should be checked by passing a straight bar, o thesame diameter as the support holes, right throughthe urnace, beore tting the elements.

Under no circumstances should the element hotzones be allowed to enter the element supportholes as this will lead to localized overheating andpremature ailure.

Special lead-in sleeves are available or mountingall diameters o Kanthal Globar SD elements and

these are detailed in Fig. 4 and in the accessoriessize list. Sleeves should be t rom the outside othe urnace, in holes bored to a diameter that willensure a loose t, and the sleeves should never becemented into position. In general, the sleeves willbe ar shorter than the thickness o the urnace

Installation methods

Fig. 4 Standard sleeve patterns and installation.

Fig. 5 Installation without sleeves.

Pattern standard dimensions

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Element connections must be kept reasonably cooland any terminal guards t should be well venti-lated. Where sealed terminal boxes are essential,or example in controlled atmosphere urnaces,then the ollowing procedures should be ollowed:

Increase the ree length o cold end outside theurnace to 4 5 times the element diameter

Ensure that the surace area o the terminalcovers is sucient to ensure adequate cooling.It may be necessary to provide nned covers toincrease the surace area

Use type D terminal clamps, and larger sectionconnecting braids than normal

Introduce a proportion o the process gas via theterminal boxes, to assist in cooling

In very severe cases, some kind o orced coolingmay be required

Where Kanthal Globar SD elements are to bemounted vertically, a support o electrically insu-lating, heat-resistant material should be providedbelow the terminal end. Alternatively, elementso 16 mm (5/8 in) diameter and above may besupplied to special order with support slots andwashers to allow them to be suspended rom thetop o the urnace, in a similar manner to vertically

mounted type U elements (Fig. 6).

Kanthal Globar SD multi-leg element installation

All multi-leg elements must be unpacked andhandled with care, using both hands or support,and ensuring no stress is placed on the elementlegs. Any transit straps t across the legs should beremoved, just beore installation.

Vertical mounting

Multi-leg elements are usually mounted vertically

and suspended through pre-drilled reractory sup-port blocks shaped to t corresponding holes in theurnace roo. Elements and blocks may be pre-assembled and placed in position as required. Theprecise shape and size o the blocks will depend onthe element dimensions and the construction o theurnace roo.

Elements are usually supported using specially de-signed support washers as detailed in Fig. 6, butmay be supported on a suitable ceramic t below

the bridge, i required.

insulation, but the clearance holes should be drilledright through to the hot ace, as shown in Fig. 4, toprevent any contact between the elements and thelining, as this can lead to adhesion, and prematureailure.

I sleeves are not to be used, or example in brick-lined urnaces, then the elements may be t throughholes about 4 5 mm (1/6 1/5 in) larger than theelement diameter or element sizes up to 20 mm(3/4 in), and about 8 10 mm (1/3 in) larger orelements rom 25 55 mm (1 2 1/8 in) diameter.Even larger holes may be required where there isa possibility o volatiles condensing in the elementholes or on to the cold ends, or where the urnacelining is exceptionally thick.

Elements should always be centralized by support-ing each cold end on a small pad o ceramic mate-rial, to prevent contact between the element coldends, and the lead-in holes. Under no circumstancesshould the cold ends be wrapped with ceramic beror other insulation, as this will lead to restrictionand possible premature ailure. To prevent radia-tion on to the terminal connections and minimizeheat losses, a fexible ceramic ber washer may bet over each terminal end and positioned so that

it rests against the outer ace o the urnace (Fig. 5).

In all cases, the element ends should extend beyondthe sleeve fange, or the outer ace o the berwasher, by a distance o approximately 2 3 timesthe element diameter.

The use o aluminum braid is recommended ormaking terminal connections as it does not suerrom progressive oxidation and is suciently fatand sot to allow a good connection to be made.

Braids are normally attached using type H springclips, which are easy to install and require no tools.Where space is restricted, type C clips may be used,and these require a special tool or installation.Both H and C clips rely on spring steel to maintaina good contact, and must be maintained at a tem-perature below about 250C (480F). For highertemperatures, types G and D clamps may be used.These are clamped to the elements using stainlesssteel set screws, and high temperature lubricantshould be applied to the threads beore installation,

to prevent seizing, and later diculty in tighteningand removal. Screws should be retightened ater 24hours use.

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2

Note that the

hot zone must

not extend into

the element

holes

Support

washer

assembly

Insulating

board

Ceramic fber

location block

Support

washer

Locking

washer

Precautions must be taken to prevent the terminalsrom overheating, especially i the elements areoperating at the higher end o their temperaturerange or i the roo lining is relatively thin. In mostcases a 25 mm (1 in) thick pad o ceramic ber

t below the support washers will be sucient,providing the terminal boxes are adequately venti-lated. In more severe cases it may be necessary tomount the elements through a separate supportplate, t about 75 100 mm (3 4 in) above theurnace casing, to allow a ree fow o air or processgas over the exposed cold ends. Alternatively, ele-ments can be supplied with support slots or holesin a non-standard position, to allow or a greaterree length o terminal.

Support blocks must be manuactured rom areractory material capable o withstanding themaximum element temperature (which may beconsiderably higher than the urnace temperature)and should have a suciently high electrical resis-tivity to prevent conduction between adjacent legsat that temperature. Generally, a high alumina, low

Fig. 6 Typical vertical installation.

iron reractory or vacuum-ormed ceramic bermaterial will be most suitable.

It is essential to ensure that the element mountingholes are parallel, and accurately drilled at the

correct center distance. The hole diameter shouldgenerally be about 6 mm (1/4 in) greater thanthat o the element legs or elements up to 20 mm(3/4 in) diameter, and 9 mm (1/3 in) greater or thelarger sizes.

Sandvik supplies special LB ceramic ber locationblocks to t the most common sizes o U and CUelements. It should be noted that the fange on thelocation blocks is not designed to carry the mass othe elements, and a separate hard, strong insulating

board, cut with clearance holes or the element legs,must be t above the fange, to transer the load onto the urnace casing.

The LB location blocks are available in 3 dierenttemperature ratings o 1250C (2280F), 1400C(2550F) and 1500C (2730F), which should

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Ceramic tube to

support legs

Pivot pin

bonded

to bridge

Alumina

tube pivot

correspond to the maximum expected elementtemperature.

Under no circumstances should the element hotzones be allowed to enter the element support

holes as this will lead to localized overheating andpremature ailure.

Elements may also be mounted vertically with theterminals at the base and should be located on anelectrically insulating, heat resistant support tbelow the terminal end. Long elements supportedin this way may need a stabilizing support or hookt at the bridge end.

Type M elements have wider cold end spacings than

the 2 and 3-legged elements, and it may be preer-able to t the elements rom inside the urnace, us-ing SL sleeves to isolate the cold ends or the liningand casing o the urnace.

Horizontal mounting

Special precautions are required where multi-legelements are to be installed horizontally, especiallywith long elements, and the element cold ends mustbe supported on a hard smooth surace in sucha way that they are ree to move laterally during

heating and cooling. An alumina tube, installedwith its axis at 90 to the legs axes is a suitablesupport. Holes through the urnace insulationshould be in the orm o slots, or clearance holes,so that the legs are supported only by the ceramictube, and do not contact the insulation. Elementsshould never be cantilevered, and the bridge shouldalways be supported. It is important to ensure thatthe bridge support is level, and co-planar with theceramic tube supporting the element legs, other-wise one o the legs may not be ully supported

and may ail prematurely.

As it is oten impossible to ensure that the supportsare exactly in line, sel-aligning systems have beendeveloped by Kanthal, and special elements can besupplied with a pivot pin attached to the bridge,to allow it to rotate reely and prevent mechanicalstressing o the legs. Alternatively, standard elementbridges can be supported on a length o aluminatube, which will allow ree rotation o the bridge,relative to the element legs (Fig. 7). Fig. 7 Typical horizontal installation methods.

Horizontal elements should generally be installedwith the legs in the same horizontal plane. Installa-tion with leg centers in the same vertical plane ispossible only with short elements, and is not gen-erally recommended, due to diculty in ensuring

that both legs are equally supported.

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A

D

A

C

A

B A

Element spacing all element types

Elements should be spaced at a minimum o 2diameters between centers, but 2.5 3 times the di-ameter is preerred. There should be 1 diametersbetween element centers and the reractory lining,and a distance o at least 2 diameters should beallowed between the element centers and the prod-ucts being red. It may be necessary to increase thisi uniormity o heating is required, especially i thedistance between adjacent elements is large (Fig. 8).Where multi-leg elements are being used, it is ad-visable to increase the minimum clearance betweenelements slightly, to ensure at least 15 mm (5/8 in)clearance between adjacent bridges.

Fig. 8 Recommended element spacings.

A = (1.5 D) = Minimum spacing between element center and

any adjacent reractory

B = (2 D) = Minimum spacing between adjacent element

centers

C = (2 D) = Minimum spacing between element centers and

hearth plates or work

D = Element diameter

Note:I under hearth heating is to be used then the hearth plates

should be as thin as possible and have a thermal conductivity

o at least 14 W/mK.

It may be necessary to limit the power output to prevent

overheating.

Electrical characteristics

Kanthal silicon carbide elements have a muchhigher resistivity than metallic elements and can beoperated at higher surace loadings (i.e. W/cm2(W/in2) o the hot zone surace area).

The resistivity/temperature characteristic is shownin Fig. 9.

Kanthal Globar SD elements have a high and vari-able resistivity at room temperature, but this allswith increasing temperature, reaching a minimumat about 700C (1290F). At element temperaturesabove 700C (1290F), resistivity increases withrising temperature.

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5

300

280

260

240

220

200

180

160

140

120

100

80

0 200 400 600 800 1000 1200 1400 1600C

32 390 750 1110 1470 1830 2190 2550 2910F

Element temperature

% o nominal resistance

Minute variations in the quantities o minor impu-rities in the material have a disproportionate eecton the cold resistance value, as indicated by thedotted curves, and room temperature resistancemeasurements give no indication o the resistance

at working temperature.

Resistance measurements should always be carriedout at a constant temperature at or above 1000C(1830F), and the value determined by dividing thevoltage across each element by the current passingthrough it.

Electrical specications

The nominal resistance values detailed in the ele-ment size lists are based on an element temperature

o 1000C (1830F), as illustrated in Fig. 9. Theresistance values are subject to a tolerance o 15%and although the nominal value is generally usedwhen calculating the voltage range o the power

supply, the negative resistance tolerance must beconsidered when calculating the maximum currentdrawn (see page 22).

Kanthal Globar SD elements are marked with a

standard test voltage, calculated to raise an elemento nominal resistance to a temperature o 1000C(1830F) in ree air. The test current value at thisvoltage is also marked on each element, and shouldbe used when matching element resistances. Thetest voltage is or calibration purposes only, andthe maximum recommended voltage with newelements will not normally exceed 80% o the testvoltage.

The size lists detail the range o preerred element

sizes, but non-standard elements, including KanthalGlobar SD elements with oset hot zones, can alsobe supplied.

Resistivity/temperature characteristic curve

Fig. 9 Typical R/T curve.

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Element perormance

All silicon carbide elements increase in resistanceduring their lie in operation, and the rate at whichthis occurs is aected by the ollowing actors:

Furnace temperature

Element surace loading in W/cm2 (W/in2)

Atmosphere surrounding the elements

Mode o operation continuous or intermittent

Operating practices and power control methodsused

As a general guide, Kanthal Globar SD elementsmay increase in resistance at a rate o about 5 6%per 1000 hours operating continuously in clean airat a temperature o 1400C (2550F) and at about3% per 1000 hours use at 1000C (1830F). Itshould be noted that small changes in operatingconditions can alter these rates considerably.

Element loading

Silicon carbide elements do not have a specicrating in watts, and the rated power is a unctiono the required temperature, the atmosphere inwhich the elements will be used, and the operatingcycle. Expressed in W/cm2 (W/in2), the suraceloading is derived by dividing the power rom each

element, by the surace area o the hot zone section,which is detailed in the element size lists, or can becalculated by:

D L,

where D is the outer diameter o the element in cm(in), and L is the hot zone length in cm (in).

As the element temperature is directly proportionalto the surace loading applied, the lowest power

loading consistent with the urnace design shouldbe used or optimum element lie, and this is usu-ally in the range 3 8 W/cm2 (19 52 W/in2).

Fig. 10 illustrates the relationship between urnacetemperature, element surace loading and elementtemperature.

For example:At a urnace temperature o 1400C (2550F) andan element loading o 5 W/cm2 (32 W/in2) the ele-

ment temperature would be 1450C (2640F), as

indicated by the red line. At a urnace temperatureo 1100C (2010F) and a loading o 6 W/cm2(39 W/in2) the element temperature would be about1200C (2190F) as indicated by the brown line.

The curve shows maximum recommended elementloadings or elements operating in air. These valuesmay be used as a guide, but or maximum elementlie a lower loading should be used wherever pos-sible. A lower loading may also be required whereelements are to be operated in reducing or otherprocess atmospheres, to maintain element tempera-tures within the limits detailed in the table, page18. The minimum recommended surace loading is3 W/cm2 (19 W/in2), although lower values are pos-sible, where the power supply has sucient voltage

available to overcome the high cold resistance othe elements.

Start up procedure

Rapid heating is detrimental to all ceramic materi-als, and although Kanthal Globar SD and multi-leg elements are particularly resistant to thermalshock, care must be taken when heating up romcold to limit the applied voltage, and hence theelement heating rate. The test voltage marked onevery element must never be exceeded, but in

general, it will be benecial to limit the appliedvoltage to a lower value, calculated rom the elementdesign power, and the nominal element resistance:

V= WR

Where W is the design power o the element inwatts, and R is the nominal element resistance.

Operating temperature

Kanthal Globar SD elements may be used in air at

urnace temperatures up to maximum o 1600C(2910F), but the use o other atmospheres mayreduce this limit considerably. There is no lowerlimit to the operating temperature o the equip-ment, but the element surace loading should beset to achieve an element temperature o at least900C (1650F).

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Furnace temperature, C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 W/cm2

6 13 19 26 32 39 45 52 58 64 71 77 84 90 97 103 110 116 W/in2

Surace loading

1600

1550

1500

1450

1400

1350

1300

1250

1200

1150

1100

1050

1000

950

900

850

800

750700

600

500

Furnace temperature, F

2910

2820

2730

2640

2550

2460

2370

2280

2190

2100

2010

1920

1830

1740

1650

1560

1470

13801290

1110

930

Maximum load Consult manuacturer

1600

1550

1500

1450Elementtemperature

1400

1350

1300

1250

1200Elementtemperature1150

1100

1050

1000

950

900

850800750700

600500

Fig. 10 Surace loading chart.

Recommended element loadings or Kanthal Globar SD operated in air

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Atmosphere Maximum element temperature CommentsC

Clean dry air 1625 2950 1500C (2730F) or 3-piece construction

Pure oxygen 1500 2730 Faster oxidation than in air

Nitrogen 1350 2460 Forms silicon nitride at >1350C (2460F)

Dry hydrogen 1200 2190 Oxidizes in wet hydrogen

Dry exothermic gas 1400 2550 Very dependent on composition

Dry endothermic gas 1250 2280 Very dependent on composition

Vacuum 1200 2190 Generally or short term use only

Maximum element temperature in various atmospheres

Efect o atmospheres

Although clean, dry air is the preerred atmospherein which to use Kanthal Globar SD elements, manyother process gases may be used. Silicon carbideoxidizes readily in air, but the product o oxidation,silicon dioxide, orms a stable amorphous silicalm over the silicon carbide grains, thus retardingthe later rate o oxidation, which is limited bydiusion o oxygen through the silica layer. Thispassive oxidation requires a ree oxygen contento at least 1%, and at lower oxygen levels, activeoxidation may occur due to the inability to orm astable silica lm.

Active oxidation also occurs in water vapor, as thesilica ormed is likely to be crystalline and non-protective. Elements exposed to water vapor, evenor a short period o time, will be seriously dam-aged, and will never revert to normal oxidation,even in a dry air atmosphere. Furnaces should bethoroughly dried beore the elements are installed,but i it is essential to use the elements or drying,then the urnace must be very well ventilated andno build up o steam must be allowed to occur.Glazed Kanthal Globar SD elements should alwaysbe specied where water vapor is likely, and or atleast the rst set o elements in brick lined or cast

reractory lined urnaces, where some evolution osteam is inevitable during the rst heating.

Kanthal Globar SD elements are also used success-ully in neutral or reducing atmospheres, althougha lower temperature limit is normally imposed.

In pure dry hydrogen or example, the maximum

element temperature at which a useul lie may beobtained is about 1200C (2190F). With lowerpercentages o hydrogen however, the maximumtemperature may be raised and elements are success-ully operated at temperatures o over 1300C(2270F) in commercial reducing atmospheressuch as exothermic and endothermic gases. Inpure nitrogen there is likely to be some reactioncausing the ormation o silicon nitride at urnacetemperatures over 1300C (2370F). Pure oxygenatmosphere can be used, with only a slight increase

in oxidation rate, but aluminum braid connectionsmay react exothermically with the atmosphere,unless maintained in very good condition. Thetable below shows maximum suggested elementtemperature limits or various process gases.

Other process volatiles may also adversely aectelement lie by attacking either the silicon carbideor the protective silica coating. Alkali vapors,halogens, metal oxides and halides are particularlyreactive and precautions must be taken to minimize

any attack, by adequate ventilation o the urnacechamber, or by minimizing the transer o aggres-

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9

sive media to the elements. Ventilation will alsoassist in preventing condensation o volatiles inthe element location holes, where they may causesticking and subsequent element breakage due torestriction. Although it is possible in most cases to

reduce volatile attack to an acceptable level, it maybe necessary in severe cases to sheathe the elementsin suitable reractory or metallic tubes. The useo sheaths can result in a considerable increase inelement temperature, particularly i ceramic sheathmaterials having relatively poor emissivity andthermal conductivity values are used, and it may benecessary to reduce the element loading to preventoverheating.

Where the atmosphere contains hydrocarbons,

deposition o electrically conductive carbon canoccur on the urnace walls, inside the pore structureo the reractories, and also on the elements them-selves, causing tracking o the electrical supply. Thisproblem can be minimized by strict control o thecarbon potential o the gas, but it may be necessaryto open the urnace to air periodically to burn othe carbon deposits. Note that carbon deposited incooler areas o the urnace cannot be removed bythis method, i the local temperature is insucientto cause the carbon to burn.

Glazes and coatings

Special glazes and surace treatments have beendeveloped which can extend element lie in variousoperating conditions, particularly where chemicalattack is a problem. Details o these will be providedon request.

Continuous or intermittent operation

Kanthal Globar SD heating elements are suitableor both continuous and intermittent operation,but at temperatures o 1400C (2550F) or abovecontinuous operation is recommended or maxi-mum service lie. Volumetric changes, which rupturethe protective silica coating i allowed to cool belowabout 900C (1650F), leave the silicon carbideexposed to urther oxidation and consequent re-sistance increase. It is preerable to idle high tem-perature urnaces at about 900C (1650F) duringshort periods o non-production, where practical.

At temperatures below 1400C (2550F), theamount o silica ormed is relatively small and theeect o cyclic operation will be minimal, especiallyi the cycle time is short. An economical elementlie may also be obtained under intermittent opera-tion at temperatures above 1400C (2550F) butKanthal Globar SG elements are normally recom-

mended or these applications.

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Fig. 11 Series/parallel connection .

Power supplies

Alternative method o series/parallel connection, not recommended

Preerred method o series/parallel connection

Parallel connection is ideal, as any small variations

in resistance value will tend to balance with use,whereas with series connection, the variation willtend to increase, resulting in a reduced element lie.

As Kanthal Globar SD elements increase in resis-tance airly slowly however, the eect o any im-balance is small, and up to our elements may beconnected in series, providing that they are wellmatched in resistance value. At urnace tempera-tures above 1400C (2550F) it is recommendedthat the number o series connected elements

should be limited to two.

A series-parallel combination is usually an eectivecompromise, and in this case, the series groupsshould be connected in parallel. Elements shouldnever be connected with parallel groups connectedin series, as ailure o one element will result inoverloading o the remaining elements in thatgroup.

It is important to install, connect and control theelements in the recommended way, to ensure opti-mum lie.

Element connections

Kanthal Globar SD elements may be connected inparallel, series or combinations o the two.

Kanthal Globar SD elements can be considered

as simple resistive loads and the normal electricallaws apply: i.e. where V = volts; I = amperes; W =watts; R = resistance in

V = IR = WR =W

I

W = VI = IR =V

R

I =V

=W

=W

R R V

R =

V

=

V

=

W

I W I

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2

3-phase connections may include star (wye) ordelta. Where a star connection is to be used, a4-wire supply is recommended, to ensure that thephase voltages are balanced, irrespective o thephase resistances. I a 3-wire star connection must

be used, then the phase resistances must always beclosely matched.

Matching o resistance value

It is recommended that any series connected ele-ments be selected within a resistance range o 5%o each other, and elements connected in parallelmay have a wider range o 10%.

I any element ails or is broken ater only a shortperiod in use it can usually be replaced with a

new element; preerably one rom the higher endo the resistance tolerance (low amp value). I theelements have been in use or a considerable timehowever, the entire group should be replaced other-wise an excessive load will all on either the new orthe old elements, resulting in premature ailure.

It is good practice to divide the total number oelements in the urnace into relatively small controlgroups, to simpliy matching at a later date. Forexample, a urnace t with 48 elements, will be

ar more fexible i the elements are divided into8 groups o 6 than with 3 groups o 16 elements,and matching o element resistances will be greatlysimplied.

When a group o elements has been replaced, it isessential to ensure that the voltage output o thepower supply equipment is reduced to the correctvalue beore switching on, as overloading o ele-ments, even or a very short period, can cause irre-parable damage. The old elements may be retained

or later use with others which have been in useor a similar period o time. I possible, voltage andcurrent readings should be taken rom each elementbeore removal and the increased resistance valuemarked on the terminal to assist in matching at alater date.

It is important to note that the resistance values oelements at room temperature give no indicationo their resistance at operating temperature andresistance measurements should always be taken at

a constant temperature above 1000C (1830F).

Dierent types o heating elements should never beoperated in the same electrical circuit, as variationsin rates o resistance increase will cause overload-ing o one type or the other depending on themethod o connection.

Voltage reserve

To compensate or the increase in element resis-tance which occurs with use, a variable voltagepower supply is usually provided. The amounto voltage reserve required will depend upon theelements rate o resistance increase and the lieexpected, but is usually in the order o 50 100%o the voltage required to give ull power with newelements.

For example, i 125 V is required to give ullpower with new elements, then a voltage range o125 250 V will be required to give 100% voltagereserve, and a range o 125 187.5 V to give 50%reserve.

Where elements are to be operating or long peri-ods at temperatures o about 1400C (2550F) orabove, or where the urnace conditions are suchthat an excessively high rate o resistance increasewill occur at a lower temperature, then allowance

should be made or 100% voltage reserve. Con-versely, i the element temperature is very low, orthe urnace only inrequently used, a voltage reserveo 50% or less may be ound sucient.

Power supply equipment

A variable voltage power supply is usually pro-vided to enable the design power to be maintainedthroughout the lie o the elements. The type oequipment used may have an eect on elementperormance and it is important that the correct

selection procedures are adhered to i the best ele-ment lie is to be obtained. Various types o powersupply can be used:

1. Variable output transormer2. Thyristor unit (SCR)

a. Phase-angle ringb. Cycle-proportioning

3. Combined thyristor (SCR)/transormer system4. Direct-on-line connection

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Generally speaking, tapped transormers provideonly on-o control, unless used in combinationwith thyristors. Although robust, and insensitive toshort term overloading, the transormer is consid-ered to be heavy, bulky and relatively expensive

in most cases. Thyristor control oers a ar morecompact solution, with the option o step lessvariation o power, and 3-term, accurate tempera-ture control, but will require an over-rated supply,and may cause disturbance in the supply lines. It isnot usually practical to provide a voltage reserve omore than 50% using thyristor control alone, andwhere a large voltage reserve is required, in combi-nation with 3-term control, the best, although mostexpensive, option is to combine the control benetso the thyristor, with the large voltage span o a

transormer.

Thyristor control alone is common in low temper-ature urnaces, laboratory urnaces, and otherequipment where the rate o change o elementresistance is likely to be low. For high temperaturecontinuous industrial urnaces, where a large volt-age reserve is required as well as accurate tempera-ture control, then the additional costs o the com-bined transormer/thyristor system can usually bejustied in terms o perormance.

1 Variable output transormer

Innitely variable transormers are occasionallyused to power small laboratory and experimentalunits but are usually too expensive or largerurnaces, where multi-tapped transormers withstepped outputs are more economic. The maximumvoltage step on a tapped transormer should neverexceed 7% o the initial, ull power voltage (= WR),where W is the urnace design power and R is thenetwork resistance based on the nominal resis-

tance o the elements) and the element resistancetolerance must always be taken into account whencalculating the maximum secondary current ratingo the transormer (= W

Rmin).

e.g. i a urnace is to be rated at 5 kW and t withKanthal GlobarSD elements having a networkresistance o 2 (15%) then the transormer spec-ication might be calculated as ollows:

Nominal ull power voltage == WR = 5000 2 = 100 V

Voltage steps must not be greater than7% of 100 V = 7 V

Minimum network resistance == 2 -15% = 1.7

Maximum secondary current == 5000

1.7= 54 A

Minimum voltage required == 5000 1.7 = 9 V

Assuming that 100% voltage reserve is required

then the specication could be as ollows:

Input: Single-phase, to suit supply

Output: Variable rom 92 V to 197 V in 15 stepso 7 V (= 4 coarse 4 ne tappings)

Rating: 5 kVA rom 92 V upwards (maximumsecondary current 54 A)

(I) Where a tapped transormer is to be used then

an allowance should be included in the urnacedesign power or the reduction in power output,which will occur between tap changes. With a 7%voltage step or example, the power will all byabout 12.5% beore the voltage can be adjusted tothe next setting.

(II) A ew taps below 92 V may be included orlower powers i required.

(III) I an ammeter is to be used, then it should be

installed in the primary circuit where the voltage isconstant; the current reading will then give a trueindication o the power dissipated by the elements,irrespective o the secondary voltage setting.

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Firing pulses

Thyristor (SCR)

stack

Heating elements

A.C. supply

Thermocouple

Fig. 12 Thyristor (SCR) unit.

2 Thyristor (SCR) unit

A thyristor is a semi-conductor switching devicewhich can control the average output to the ele-ments by switching the mains supply on and overy rapidly. Each thyristor will conduct in onlyone direction and or control o a.c. loads thethyristors are installed in pairs, connected in inverseparallel. The thyristors are switched by a series opulses ed rom a suitable driver unit or tempera-ture controller.

Thyristors can be simple devices, but are oten pro-vided with closed loop eedback, to compensate orvariations in supply voltage, load characteristics,etc. Typical eedback modes include current control(I2 eedback), voltage control (V2 eedback) andtrue power control (VI eedback). In general, onlyV2 eedback is suitable or use with silicon carbideelements, although there are some exceptions.

Temperature controller

and driver unit with

manual output control

Current control tends to increase the power to theelements as they increase in resistance, and bothpower and current control can cause serious dam-age to the elements when starting rom cold, whenthe element resistance is high. When presented with

this high load resistance, both current and powercontrol are likely to result in the application omaximum voltage to the elements, and i the avail-able voltage is higher than the elements can with-stand, then serious damage may occur. With voltagecontrol, the delivery o power will be controlledby the resistance/temperature characteristics o theelement, starting with a relatively low power, andgradually increasing as the elements heat up.

The output characteristics o the thyristor are

governed by the ring method used and the twoprincipal types are as ollows:

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New elements Old elements

O On OnO

Fig. 13 Phase-angle fring.

2.1 Phase-angle ring

Each thyristor is triggered once in every hal cycleo the a.c. supply, conduction ceasing at the endo the hal cycle as the current alls to zero. In thisway, the sinusoidal supply waveorm is chopped,resulting in a reduction in the RMS output voltageo the thyristor stack. For control o silicon carbideelements, a manual voltage limit control must beprovided to vary the conduction angle o the thyris-tors and thus compensate or resistance increase othe elements.

Phase-angle red units may also be t with a currentlimiting device, which may protect the thyristorsrom accidental overload, by preventing the currentoutput rom exceeding a pre-set value, irrespectiveo the voltage setting. This current limit must notbe used to control the power input to the elements,as the power input (= I2R) will gradually rise withincreasing element resistance, resulting in progres-sive overloading o the elements, and possible dam-age when starting rom cold.

As phase-angle red thyristors give smooth, stepless control o the applied voltage, they are ideallysuited or use with silicon carbide elements. How-ever, they can cause both radio requency interer-

ence and supply waveorm distortion, and consid-eration should be given to these actors when se-lecting the system to be used. The use o phase-angle ring is discouraged in many countries, orseverely limited by local regulations.

As there is no current fow or part o the supplycycle, phase-angle controlled loads will give riseto an apparently poor power actor, even with apurely resistive load, and this can lead to problemson large installations, especially i the ring angle

is small. Generally, the starting voltage with newelements should not be less than 60% o the supplyvoltage, to minimize this eect.

Supply cables must be rated at the RMS currentdrawn by the elements (= power thyristor outputvoltage and notpower supply voltage). This meansthat the power supply will be rated higher thanthe urnace design power, and the more voltagereserve is provided, the higher will be the requiredover-rating.

In 3-phase installations, generation o third har-monics will give rise to cumulative distortion othe supply waveorm and may result in excessivelyhigh neutral currents o up to twice the line currentin 3-phase, 4-wire, star-connected loads. Neutralcables must be adequately rated to carry thisexcess current. Although 3-wire star connectioncan be used to eliminate this problem, this canresult in voltage imbalances between phases, espe-cially where elements are nor perectly matched in

resistance.

To minimize supply distortion in large installations,the use o a 6-wire, open-delta arrangement isrecommended. The thyristors are required to carry

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Line 1

Line 2

Line 3

Linecurrent

Line

voltage

Stack 1

Stack 2

Stack 3

Phase current

Load

Line 1

Line 2

Line 3

Stack 1

Stack 2

Stack 3

Load

Linecurrent

Line

voltage

Line 1

Line 2

Line 3

Line current = phase current

Star point

Balanced 3phase load

Stack 1

Stack 2

Stack 3

Line

voltage

Line 1

Line 2

Line 3

Line current = phase current

3 phase load

Neutral line

Stack 1

Stack 2

Stack 3

Line

voltage

only the phase current and not the line current aswould be required in a closed delta installation,leading to a reduction in cost, and phases can becontrolled independently, leading to more fexibil-ity o control.

Calculation o the voltage output and current rat-ing o phase-angle red thyristor units is carriedout in the same way as or a transormer, takingaccount o the element resistance tolerance whencalculating the maximum current drawn.

Fig. 16 Open delta: 6-wire.

Fig. 14 Phase-angle fring: 3-wire star. Fig. 15 Phase-angle fring: 4-wire star.

Fig. 17 Closed delta: 3-wire.

3-phase, 3-wire, star-connection (phase-angle fring)Stack voltage rating = line voltage = phase voltage

Stack current rating = line current = phase currentNote: phase resistances and loads must be balanced

3-phase, 4-wire, star-connection (phase-angle fring)Stack voltage rating = phase voltage = line voltage

Stack current rating = line current = phase currentNeutral cable rating = line current 2 or saety

3-phase, open delta connectionStack voltage rating = line voltageStack current rating = phase current (= line current)

3-phase, closed delta connectionStack voltage rating = line voltage

Stack current rating = line current = phase current

Power monitoring phase-angle

Most voltmeters and ammeters will not indicatea true RMS reading rom phase-angle controlledloads, and great care must be taken to ensure thatthe elements are not inadvertently overloaded.Some o the digital meters currently available willrespond accurately to non-sinusoidal waveorms,but the manuacturer should be consulted to ensurethat a suitable instrument is supplied. True RMS,hall-eect meters, with a crest actor o 7 or above,is recommended or an accurate response.

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OOn

OOn

2.2 ast-cycle ring

The thyristors are triggered at the beginning oa mains cycle and remain conductive or one ormore complete cycles beore being switched o orone or more cycles. This operation is continuously

repeated, thus limiting the eective input to theelements. A manual control must be provided tovary the on/o ratio, and thus compensate or thegradual resistance increase o the elements.

Although the average power input to the elementsmay be within the normal recommended limits orstart-up, temperature and atmosphere, each ullcycle o the ull mains voltage may cause loadingso several times this value to occur, and this canresult in increased rates o resistance increase and

premature, i not immediate, element ailure. Forthis reason, it is necessary to connect the elementsin such a way that the element loading during ullcycles o the supply voltage does not exceed15 W/cm2 (97 W/in2).

To minimize the eect o the on burst, the timebase o the ring cycle must be as short as possibleand preerably less than 30 cycles o a 50 Hz supply(i.e. 50% power = 15 cycles on + 15 cycles o).

Slow-cycle thyristors generally have cycle times oseveral seconds and are not suitable or direct control

o silicon carbide elements. They may be used onthe secondary side o tapped transormers how-ever, in place o conventional electro-mechanicalcontactors.

The optimum type o burst-red thyristor or usewith silicon carbide elements is the single-cycleburst-ring type, where the required output isalways reached over the minimum possible numbero complete cycles (i.e. 50% power = 1 cycle on +1 cycle o).

The required voltage rating or the thyristors willbe the same or higher than the supply voltage, butthe current rating o the unit must be determinedby dividing the RMS supply voltage by the mini-

mum network resistance.

i.e.current ratingo thyristor

supply voltageminimum resistance

=

The rating o the thyristor will thereore be muchhigher than that o an equivalent phase-angle redunit.

To calculate the required output rom the thyris-tors, the power at the supply voltage must be cal-culated, and the on/o ratio o the thyristor limitedto whatever percentage will provide the requireddesign power.

Fig. 18 Fast-cycle fring.

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e.g. i the supply voltage is 200 V, and the resis-tance o the element network is 4 , then the powerat 200 V will be 200

4= 10000 W. I the required

power is 5 kW, then the thyristors must be set togive an output o 5000

10000= 50%

Resistive loads controlled by ast-cycle thyristorshave a unity power actor and, as only ull mainscycles are delivered to the load, there is no cumula-tive distortion o the mains supply. With heavyloads however, voltage drop may cause fickeringlights and also aect sensitive equipment.

Power monitoring ast-cycle

It is dicult to obtain voltmeters and ammeterswhich will respond accurately to ast-cycle con-

trolled loads, and as most meters will indicate areading considerably less than the actual thyristoroutput, great care must be taken to ensure that theelements are not inadvertently overloaded. Therequired output settings can be calculated, asabove, based on the known resistance o the ele-ments. I there is any doubt about the requiredsetting, then the control should be set initially ata very low value and adjusted as required untilsucient power is available to raise the urnaceto temperature. The control should be let at this

setting until, due to resistance increase o the ele-ments, insucient power is available to maintainthe urnace at temperature (or reach temperaturei the urnace is in intermittent use). The controlsetting should then be increased slightly until su-cient power is again available.

3 Transormer + thyristor (SCR)

Due to the limitations imposed on the use o thyris-tor control, outlined in (A) and (B) it is sometimesnot possible to provide an adequate voltage reserve,

and it may be necessary to adopt a combined thyris-tor/tapped transormer power supply to ensurean adequate element lie. Thyristors can be ton either the primary or secondary side o thetransormer, although precautions are especiallyrequired or primary connection. The transormerscan be t with only 2 or 3 tappings in most cases,as the intermediate steps can be accommodatedby the thyristors. I the use o a combined systemis being considered, then both transormer and

thyristor manuacturers should be inormed at thedesign stage, to ensure compatibility o the equip-ment supplied. Kanthal can provide suggestionsor the design o the equipment, on receipt o theelement details, required power and temperature,

and the local supply voltage.

4 Direct-on-line

Networks o elements may be connected directly tothe mains voltage providing that the network resis-tance is suciently high to prevent element over-loading. The network should be designed so thatthe initial power output is higher than the urnacerating to provide a power reserve and compen-sate or element resistance increase. Although thecapital cost o a variable voltage supply is saved by

this method, only a minimal power reserve can beprovided, and in addition, a larger number o ele-ments may be required in the urnace to dissipatethe initial excess power.

It may be possible to gain an extra power reserveby modiying the element connections ater theelements have aged (e.g. rom 2 parallel brancheso 3 elements in series to 3 parallel branches o 2elements in series; or rom 2 elements in series con-nected in delta to 2 elements in parallel connected

in star). Because o its inherent drawbacks, direct-on-line connection is normally limited to relativelylow temperature applications (under 1100C(2010F)) or inrequently used cyclic urnaces upto 1300C (2370F).

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L

A

B

A

B

C

U CU W

E

D

The minimum inormation required when ordering

Kanthal Globar SD elements is as ollows:

Rod type elements

Element type (Kanthal Globar SD)

Diameter, mm ( A)

Hot zone length, mm (B)

Overall length, mm (L)

Nominal resistance,

Ordering

Multi-leg elements

Element type (Kanthal Globar SD-U/CU/W)

Diameter, mm ( A)

Hot zone length, mm (B)

Cold end length, mm (C)

Leg center distance, mm (D)

Slot position* (E)

Nominal resistance,

*Note: Standard slot position will be supplied un-less specied otherwise (see Fig. 2).

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Accessories

Accessories available rom Sandvik (ull size list on request).

LB block

SL sleeve

CT2 (clip-tool, large)

CT1 (clip-tool, small)

HC clip

G clamp

CC clip

D clamp

D1 braid D2 braid

S1 braid S2 braid

Q1 braid Q2 braid

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Sandvik Group

The Sandvik Group is a global high technology enterprise with 47,000 employees in 130 countries. Sandviks operations are concentratedon three core businesses: Sandvik Tooling, Sandvik Mining and Construction and Sandvik Materials Technology areas in which the groupholds leading global positions in selected niches.

Sandvik Materials Technology

Sandvik Materials Technology is a world-leading manuacturer o high value-added products in advanced stainless steels and special alloys,and o medical implants, steel belt-based systems and industrial heating solutions.

Kanthal is a Sandvik owned brand, under which world class heating technology products and solutions are ofered. Sandvik, Kanthal andGlobar are trademarks owned by Sandvik Intellectual Property AB.

Quality management

Sandvik Materials Technology has quality management systems approved by internationally recognized organizations. We hold, or example,the ASME Quality Systems Cer ticate as a materials organization, approval to ISO 9001, ISO/TS 16949, ISO 17025, and PED 97/23/EC, aswell as product approvals rom TV, JIS and Lloyds Register.

Environment, health and saety

Environmental awareness, health and saety are integral parts o our business and are at the oreront o all activities within our operation.We hold ISO 14001 and OHSAS 18001 approvals.

Disclaimer

Recommendations are or guidance only, and the suitability o a material or specic application can be conrmed only when we know theactual service conditions. Continuous development may necessitate changes in technical data without notice.

This printed matter is only valid or Sandvik material. Other material, covering the same international specications, does not necessarilycomply with the mechanical and corrosion properties presented in this printed matter.

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