Models

Shunt Tap Operation

The number of shunts required is usually determined by the length of the uniform temperature zone and the temperature variation allowable.  By placing a suitable length of resistance wire from one shunt tap to another, a parallel circuit is created, reducing the amount of current through the windings at this point.  The amount of current bypassed will depend on the length of resistance wire used.  A low resistance shunt causes a relatively large drop in temperature in the corresponding zone inside the furnace.  Conversely, a high resistance shunt affects the internal temperature very little.  Using a shunt decreases the overall resistance of that zone causing a decrease in the total resistance of the entire heating element.  This means a slightly greater current will flow through the entire furnace so that while the shunted zone will be coded, the remaining zones will be slightly heated.  Therefore, the temperature of the required flat zone must be somewhat less than the maximum rating of the furnace. See the temperature profile capability curves to the right.  Resistance shunt wire and resistance data between shunts are supplied with each furnace.

Features

• VERSATILITY – A wide range of flat, variable, or stepped temperature adjustments is possible with shunt taps provided as standard equipment on all single zone Marshall tubular furnaces.
• OPERATING TEMPERATURES FROM 1100°C TO 1700°C
• UNIFORMITY – Flat temperature zones within ±1°C can be achieved.
• SINGLE ZONE OR OPTIONAL MULTI-ZONE FLEXIBILITY – The furnaces operate in either a vertical or horizontal position. Stands and brackets are available to accommodate most installation requirements.
• RUGGED DESIGN – Rugged design assures years of dependable, trouble-free service. The durable chrome shell is standard on all Marshall furnaces.
• SUPPORT STANDS – All Marshall™ furnaces are equipped with aluminum mounting blocks for bench mounting or easy attachment to Marshall™ support stands and existing testing machines. The two optional stand types offered are the fixed horizontal support stand and the universal positioning stand.
• FULL LENGTH TEMPERATURE MONITORING – A ¼” I.D. thermocouple well, easily accessible from either furnace end and runs parallel to the heating element for full length temperature probing.
• WIDE RANGE OF SIZES – Standard furnaces are available In lengths from 12″ to 36″ in 4″ increments and bore diameters from 1½” to 7″ I.D.
• HIGH PURITY CERAMIC MUFFLES – Series 1000, 1100, and 1300 furnace muffles are constructed of mullite, and these furnaces are optionally available with recrystallized high purity alumina muffles. Series 1400 and 1500 furnaces have these higher purity alumina muffles as standard equipment. Series 1200 furnaces have exposed element windings.
• CONVENIENCE – No exposed electrical terminals; cooler shell temperatures.
• REBUILD PROGRAM – Furnaces can be restored to virtually new condition. Includes new heating element, electrical connections and insulation.

Options

• VACUUM OR ATMOSPHERE CAPABILITY – Furnaces are available with flanged Inconel or ceramic retorts for either vacuum or gas operation. Appropriate pumping ports and viewing windows can be supplied. The normal vacuum capability at temperature is in the 10-5 Torr range. Ceramic retorts are recommended for specimen temperatures above 1200°C.
• MULTI-ZONE WINDING – Multi-zone furnaces with or without shunts can be supplied if control of two or more zones is desired. Shunt tap spacing in each zone can be located as desired, depending on the type of temperature profile required.
• THERMOCOUPLE PORTS – Ports, with or without thermocouples, installed transverse to the furnace axis may be used for monitoring specimen or element temperature. Unless muffle penetration is specified, thermocouples are installed in a closed refractory tube butting against the outside muffle wall. For longest element life, this is the preferred control thermocouple location.
• VARIABLE SHUNT TAPS – Shunt tap spacing can be varied as desired to a minimum of 1″ on all except the 1200 series where the minimum spacing is 2″.
• VIEWING WINDOWS – Viewing windows ½” in diameter (Series 1200 windows are ½” wide by ¼” high) can be installed in any position except on a vacuum/atmosphere retort. On retorts, windows can be installed in end flanges.
• SUPPORT STANDS – The two optional stand types offered are the fixed horizontal support stand and the universal positioning stand.
• COOLING – Air or water cooling coils can be provided when extremely steep temperature gradients are necessary.
• THERMOCOUPLE WELLS – Additional wells for thermocouples can be supplied adjacent to the well normally supplied or at 90° increments.
• MUFFLE TUBE – High purity recrystallized alumina muffles are available for the 1000. 1100 and 1300 Series furnaces. These impervious tubes are recommended for use in applications involving toxic or corrosive gases and for quartz work tubes at temperatures where devitrification can result Irom contamination by the standard mullite muffle. These high purity muffles are standard on the 1400 and 1500 series furnaces.