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Straight & Formed Tubular Heaters

Tubular heaters are the main heating source in most applications where electric heat is required. They are highly adaptable to the requirements of many applications. Tubular heaters can be used in their straight form or can be bent into various shapes. They can be used in free air, clamped to a surface, embedded, or cast into metals. Tubular heaters can provide heat up to 1500°F.

Bucan tubular heaters use 80% Nickel 20% Chromium high grade coiled resistance wire as a heating core. This core is welded at both ends to pins that provide a cold section that varies in length depending on the application requirements. The coil-pin assembly is precisely centred inside a heavy gauge, oversized metal tube, and embedded inside a 96% pure, high-grade MgO insulating medium. This assembly is then compacted through a roll-reducing process that reduces the outside tube diameter to its final size and transforms the MgO matrix into a rock-hard solid that acts as an excellent heat transferring medium, as well as an electrical insulation with high dielectric strength. Finally, heaters are annealed inside a high-temperature furnace to eliminate internal stresses accumulated during the cold-forming roll-reducing process to make them soft. Heating elements are then formed into special shapes or supplied in their straight form. Proper electrical terminations are added to the final product.

Straight Form Tubular Heater

Applications of Tubular heaters

  • FORMING MACHINES
  • HEATING MOLDS & PLATENS
  • IMMERSION INTO LIQUIDS
  • RADIANT & CONVECTION HEATING
  • EMBEDDED OR CAST INTO METAL
Tubular diameter (inches) Maximum voltage Maximum amps Minimum Ohms per heated length (inches) Maximum Ohms per heated length (inches) Minimum sheath length (inches) Maximum sheath length (inches)
0.260 240 15 0.1 17 11 240
0.315 300 30 0.06 20 11 240
0.375 600 30 0.05 20 11 240
0.430 600 40 0.05 20 11 240
0.475 600 40 0.05 20 11 240
 

Overall length (inches) 11-20 21-40 41-70 71-100 101-140 141-170 171-200 201+
Tolerance in sheath length (+/- in) 0.1 0.125 0.16 0.19 0.22 0.25 0.375 0.5
Tolerance in heated length (+/- in) 0.25 0.5 0.9 1.130 1.4 1.65 2 2.38
Min. unheated length (inches) 1 1.25 1.5 1.625 1.75 2.25 2.25 2.5

The two most critical factors that affect the durability of a tubular heater are:

  • Sheath material
  • Watt density

The corrosivity of the medium and its operating temperature are critical in determining the sheath material type. The table below lists various sheath materials, maximum allowable temperatures and mediums within which they are recommended to operate.

The watt density determines the temperature that a heating element sheath will attain within specific application conditions.

Sheath Material Maximum Sheath Temperature Applications
Copper 350°F Immersion into water and non-corrosive low viscosity liquids
Steel 750°F Oil, wax, asphalt, cast in aluminum or iron
Stainless Steel 304-316 1200°F Corrosive liquids, food industry, sterilizers
Incoloy 1500°F Air, corrosive liquids, clamped to surfaces
 

The watt density is determined with the following formula:

Factors to be considered when selecting watt densities

  • Application temperature
  • Application conditions
  • The maximum recommended temperature for the selected sheath material (table shown above).
  • The maximum watt density recommended for the material being heated. The table below shows some popular materials with their maximum recommended operational temperatures and watt densities.
  • In the case of possible scale or sludge formation, heater elements should run at lower watt densities.
  • In clamp-on applications, graph 1 (see below) shows the relationship between the watt density of the heating elements, the required operating temperature, and the maximum targeted sheath temperature.
  • When heating gases, the speed of the incoming gas and its outlet temperature should be considered in watt density calculations. Graphs 2, 3, 4 and 5 (seen below) show the relationship between the flow rate of air, its outlet temperature, the sheath temperature of the heating element selected and its corresponding watt density.
  • When operating in vacuum, the watt density should be 20% to 30% lower. Because of the absence of air, heaters in vacuum mostly conduct heat through radiation.

Maximum Watt Density Ratings for Various Solutions

Solution Maximum Watts/in2 Max Operating Temperature (°F)
Acetic acid 40 180
Chromic acid 40 180
Citric acid 23 180
Nitric acid 20-25 167
Phosphoric acid 25-28 180
Alkaline solutions 40 212
Asphalt, tar 4-10 200-500
Bunker C fuel oil 10 160
Caustic soda 2% 45 210
Caustic soda 10% 25 210
Caustic soda 75% 10 180
Ethylene glycol 30 300
Fuel oil pre-heating 9 180
Gasoline 20 300
Machine oil, SAE 30 18 250
Mineral oil 16-26 200-400
Molasses 4-5 100
Heat transfer oils 12-20 500-650
Vegetable oil 30-50 400
Degreasing solution 23 275
Hydraulic oil 12-15 100
Sodium phosphate 40 212
Trichlorethylene 23 150
Clean water 55-80 212
Deionized water 60 212
Demineralized water 60 212

Crimped Fitting

Part # *Material Used on Thread Size Dim. A Dim. B
THF26C-B53 Brass 0.260" 1/2 - 20 17/32" 3/4"
THF31C-B53 Brass 0.315" 1/2 - 20 17/32" 3/4"
THF31C-B75 Brass 0.315" 5/8 - 18 3/4" 1"
THF37C-B75 Brass 0.375" 5/8 - 18 3/4" 1"
THF43C-B75 Brass 0.430" 5/8 - 18 3/4" 1"
THF43C-B87 Brass 0.430" 5/8 - 18 7/8" 1"

*Fittings with different materials are available

Welded Fitting

Part # *Material Used on Thread Size Dim. A Dim. B
THF26W-S53 SS304 0.260" 1/2 - 20 17/32" 3/4"
THF31W-S53 SS304 0.315" 1/2 - 20 17/32" 3/4"
THF31W-S75 SS304 0.315" 5/8 - 18 3/4" 1"
THF37W-S75 SS304 0.375" 5/8 - 18 3/4" 1"
THF43W-S75 SS304 0.430" 5/8 - 18 3/4" 1"
THF43W-S87 SS304 0.430" 5/8 - 18 7/8" 1"

*Fittings with different materials are available

Brazed Fitting

Part # *Material Used on Thread Size Dim. A Dim. B
THF26B-B53 Brass 0.260" 1/2 - 20 17/32" 3/4"
THF31B-B53 Brass 0.315" 1/2 - 20 17/32" 3/4"
THF31B-B75 Brass 0.315" 5/8 - 18 3/4" 1"
THF37B-B75 Brass 0.375" 5/8 - 18 3/4" 1"
THF43B-B75 Brass 0.430" 5/8 - 18 3/4" 1"
THF43B-B87 Brass 0.430" 5/8 - 18 7/8" 1"

*Fittings with different materials are available

"C" Clamp

Part # *Material "C"
C15 SS304 1.5"
C20 SS304 2"

Mounting Bracket

Part # *Material Fig. #
MB1000 Steel 1
MB2000 Steel 2

Termination Styles for Straight and Formed Tubular Heaters

Bending

Annealed tubular heaters can be bent. The inside radius of the bend should not be less than the recommended radii shown in the table below. For optimum results, bending should start from the center of a tubular heater and gradually move towards the ends. Care should be taken to ensure that the connection between the cold pin and the coil does not fall in the bent area. A minimum of 1/2” clearance should separate this connection from the bend. The following sketch provides the necessary guidelines.

Sheath Diameter (inches) Minimum Factory Bend Radius (inches) Minimum Field Bend Radius (inches)
0.260 5/16 3/4
0.315 5/16 1
0.375 3/8 1 5/8
0.430 1/2 1 5/8
0.475 5/8 2

*For smaller bending radii please consult our factory

Standard Bending Formations

Re-compaction

During the process of bending tubular heaters, the rock-hard MgO insulating material forms cracks, specially on sharp bends. These cracks and fractures are weak points that lead to overheating and failure in dielectric strength. This problem becomes more emphasized in high-watt or high-temperature conditions. In order to re-establish compactness and prevent failure, recompressing elements at bent locations becomes necessary.

Mounting Tips of Tubular Heaters

  • Tubular heaters expand when heated. At least 1% of element length should be considered as expansion and adequate clearance included in total design.
  • When a tubular heater is attached to a surface, the middle clamp screws should be tightened completely. However, the end clamp screws should be tightened enough to hold the heater down and allow for expansion at the same time. This procedure will prevent the tubular heater from getting detached from the surface during the heating cycle.
  • When tubular heaters are placed in grooves, the groove depth should be less than the heater diameter by 0.008”- 0.010”, in order to ensure proper clamping.
  • Insulating materials (if used) should never be in direct contact with heaters. An air gap should separate the heater sheath from the insulating material.
  • Tubular heater electric terminals should not be placed in vacuum or heated zones.

Moisture Resisting Seals

The MgO insulating medium inside a tubular heater is highly hygroscopic and can absorb moisture from its terminal ends. Moisture resisting seals are barriers that resist or stop moisture and contamination.

Silicone Resin

This seal is a silicone-based resin that is applied to tubular heater terminal ends. The seal penetrates a short length of the MgO insulation and transforms it into a moisture and contamination resistant medium suitable for temperatures below 390°F.

RTV Seal

This is a silicone room temperature vulcanizing seal that can resist moisture and contamination for up to 450°F.

Epoxy Seal

This is a liquid resin which is thermally cured to reach solid state. This moisture barrier is adequate for temperatures up to 250°F.