The Features and Options of the Marshall Tubular Furnaces Series 1000 to 1500 from Thermcraft

Thermcraft offers rugged and reliable Marshall™ Tubular Furnaces (Figure 1) with operating temperature of 1100-1700°C. The company offers a host of tubular furnaces with increasing operating temperature range, including Series 1000, 1100, 1200, 1300, 1400 and 1500. All these furnaces provide fast delivery at a competitive price. This article discusses the features and options of these Thermcraft products.

Marshall™ Tubular Furnace

Figure 1. Marshall™ Tubular Furnace

Furnace Temperature/Flat Zone Selection

Bore diameter-to-heated length ratio influences the length of uniform temperature zone. The heated length must be longer than the length of the flat zone to achieve better uniformity in the flat zone. Table 1 lists the recommended overall lengths to achieve the required uniformity in the flat zone.

Table 1. Flat zone/furnace length selection table.

UNIFORMITY REQUIRED OVERALL FURNACE LENGTH
±6° C (10° F) Flat Zone Length + 2 x Bore Diameter + 4"*
±3° C ( 5° F) Flat Zone Length + 3 x Bore Diameter + 4"*
±1° C ( 2° F) Flat Zone Length + 4 x Bore Diameter + 4"*

* For Series 1400 and 1500 with bore ID of 2½" or greater, add 8 inches.

The operating temperature range and the elements used in different Marshall Tubular Furnace lines are given in Table 2.

SERIES TEMPERATURE ELEMENT
1000 To 1100° C (2000°F) Heavy gauge nickel-chromium resistance wire*
1100 To 1200° C (2200°F) Iron-chromium-aluminum resistance wire*
1200 To 1300° C (2370 °F) Internally exposed iron-chromium-aluminum resistance wire*
1300 To 1425° C (2600 °F) Platinum 20% rhodium wire
1400 To 1540° C (2800°F) Four-strand platinum 40% rhodium wire**
1500 To 1700° C (3100°F) Nine-strand platinum 40% rhodium wire***

* For optimum element life, it is essential to operate resistance heated elements at temperatures less than maximum rated temperatures.
** Rated at 1700°C maximum by wire manufacturer; derated to account for deviations between element and work temperature, and to allow margin for inappropriate TC placement and operation.
*** Rated to 1750° to 1800° C maximum by wire manufacturer.

Shunt Tap Operation

The allowable temperature variation and the uniform temperature zone length are typically used to determine the required number of shunts. A parallel circuit is formed by mounting a resistance wire of appropriate length from one shunt tap to another, thereby lowering the current level that passes through the windings at this point.

The amount of resistance wire used influences the amount of current bypassed. Very high temperature drop will be caused by a low resistance shunt in the corresponding zone within the furnace, whereas the impact on the internal temperature by a high resistance shunt is very low. Marshall shunt tap is shown in Figure 2.

The overall resistance of a particular zone is lowered through the use of a shunt, thereby lowering the total resistance of the whole heating element. This leads to the passage of a slightly greater current through the entire furnace. Hence, coding the shunted zone will cause mild heating of the remaining zones. For that reason, the temperature at the required flat zone needs to be lower than the maximum rating specified for the furnace. Each furnace is supplied with resistance shunt wire and resistance data between shunts.

Features and Options of Marshall Tubular Furnaces

The following are the key features of Marshall Tubular Furnaces:

  • Rugged Design - Rugged design provides reliable, trouble-free service, with durable chrome shell as a standard element for all furnaces.
  • Versatility – Shunt taps enable a variety of flat, variable, or stepped temperature adjustments and are offered as a standard component for all single zone Marshall tubular furnaces.
  • Single Zone or Optional Multi-Zone Flexibility – Furnace operation can be either in horizontal or vertical position. Stands and brackets are offered to meet most installation requirements.
  • Operating Temperatures – Range from 1100° TO 1700° C
  • Wide Range of Sizes – Standard furnaces are available in bore diameters from 1.5" to 7" I.D. and in the length range of 12-36" in increments of 4".
  • Uniformity – It is possible to achieve flat temperature zones within ± 1°
  • High Purity Ceramic Muffles – Mullite is used to construct Series 1000, 1100, and 1300 furnace muffles, whereas Series 1400 and 1500 furnaces are equipped with recrystallized high purity alumina muffles. These higher purity muffles are available as option for Series 1000, 1100, and 1300 furnaces. Series 1200 furnaces feature exposed element windings.
  • Full Length Temperature Monitoring. – A 0.25 I.D. thermocouple well that can accessed easily from either end of the furnace runs in parallel direction to the heating element to probe the temperature across the entire length.
  • Convenience – Absence of cooler shell temperatures and exposed electrical terminals.
  • Rebuild Program – It is possible to rebuild furnaces to virtually new condition. The process includes new heating element, insulation and electrical connections.
  • Support Stands – All furnaces feature aluminum mounting blocks for bench installation or easy integration into existing testing machines and Marshall™ support stands, which are classified into fixed horizontal support stand and the universal positioning stand.

The following are the options of Marshall Tubular Furnaces:

  • Muffle Tube – High purity recrystallized alumina muffles are impervious tubes and are used in the 1000, 1100 and 1300 Series furnaces. They are useful in applications where corrosive or toxic gases are involved or for quartz work tubes operating at temperatures causing devitrification due to contamination by the standard mullite muffle. These high purity muffles are standard equipment in the 1400 and 1500 series furnaces.
  • Variable Shunt Taps – It is possible to vary shunt tap spacing down to 1" in all furnaces other than the 1200 series which has the minimum spacing of 2".
  • Vacuum or Atmosphere Capability – For gas or vacuum operation, furnaces are equipped with ceramic retorts or flanged Inconel. It is possible to supply appropriate pumping ports and viewing windows. The normal vacuum capability is in the range of 10-5 Torr at temperature. For temperatures beyond 1200°C, it is recommended to use ceramic retorts.
  • Thermocouple Wells – It is possible to supply extra wells for thermocouples alongside the well normally supplied or at 90° increments.
  • Thermocouple Ports – Element temperature or sample is monitored by ports with or without thermocouples that are installed transverse to the furnace axis. Thermocouples are typically mounted in a closed refractory tube butting against the exterior muffle wall.
  • Multi-Zone Winding – To achieve control of two or more zones, it is possible to supply multi-zone furnaces with or without shunts. It is possible to locate shunt tap spacing in each zone as desired based on the requirement of the type of temperature profile.
  • Viewing Windows – It is possible to install viewing windows with 0.5" of diameter (Series 1200 windows are 0.5" wide by 0.25" high) in any position other than on a vacuum/atmosphere retort, where windows can be mounted in end flanges.
  • Cooling – It is possible to provide water or air cooling to achieve extremely steep temperature gradients.

About Thermcraft

Thermcraft Inc. manufacturer's Custom Industrial Ovens, Laboratory Furnaces, and other furnace related supplies is a recognized leader in high temperature heating equipment. Their state-of-the-art facility and design software will allow their staff to meet your requirements for any furnace or oven configuration that you may need.

They have a worldwide network of sales representatives and engineers that can help you with any of your heating equipment requirements. Along with custom lab ovens, industrial furnaces and ceramic heaters, they offer a wide range of related products; from relatively simple items such as insulation, thermocouples, and heating elements to more complex, full-scale heat treatment equipment such as control panels with LED digital displays. Customized to fit your needs is always their goal.

This information has been sourced, reviewed and adapted from materials provided by Thermcraft.

For more information on this source, please visit Thermcraft.

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