Electric heating element (tubular heater)

01

Choose the sheath material for the medium and temperature

Tempco's tubular heater catalogue gives maximum sheath temperatures for its standard sheath materials: Incoloy 840 and Incoloy 800 at 1600 °F (871 °C), 316, 304 and 321 stainless steel at 1200 °F (649 °C), low-carbon steel at 750 °F (399 °C) and copper at 350 °F (177 °C), with Inconel 600 (1800 °F, 982 °C) and Incoloy 825 (1100 °F, 593 °C) for limited diameters. It notes that maximum sheath temperature refers to the sheath material, and that the temperature the heated material can safely take must also be considered. [1]

The catalogue links materials to uses: Incoloy 840 for air heating, Incoloy 800 for air heating and immersion in potable water, 316 stainless where chlorides cause pitting and in deionised water, 304 stainless in food, medical and chemical heating, steel for oils, asphalt, tar, wax, molten salt and heat-transfer media, and copper for clean water. It advises that the supplier of the gas or liquid is the best source for chemical compatibility with the sheath. [1]

Editorial note: sheath temperature in service depends on watt density and on how well the medium carries heat away; ask the supplier for the surface load used in the design and its basis.

OMEGA's tubular heater application guidelines give suggested watt densities by medium: low-viscosity oil heated to 180 °F (82 °C) with a steel sheath at 23 W/in², medium-viscosity oil at 15 W/in² and high-viscosity oil at 6.5 W/in²; water solutions and mildly corrosive solutions to 200 °F (93 °C) with 304 stainless or Incoloy sheaths at 50 W/in²; moving air at 9 ft/s up to 500 °F with Incoloy at 40 W/in², and still air in ovens at 700 °F at 30 W/in². Its range runs from 3 to 53 W/in² for elements of 75–10,000 W up to 172 in long. [3]

The same guide says pressurised liquids should not be heated above the element's maximum sheath temperature minus 100 °F, that the heated section must stay immersed whenever energised, and that watt density should be 20–30% lower in a vacuum because no air carries the heat away. Editorial calculation: 23 W/in² is about 3.6 W/cm² and 50 W/in² about 7.8 W/cm² (÷ 6.45). [3]

Give the supplier the medium heated and its operating temperature, flow or circulation, sheath material, required power and voltage, phase and connection, element diameter, sheath and heated length or vessel geometry, form (straight, bent, flanged bundle, screw plug), mounting, termination type and enclosure, sensor or thermostat needs and approvals.

02

Set voltage and current limits, lengths, terminations and finish

Tempco's electrical limits by element diameter are 250 V and 15 A for 0.260 in (6.6 mm), 480 V and 30 A for 0.315 in (8.0 mm), 600 V and 30 A for 0.375 in (9.5 mm), and 600 V and 40 A for 0.430, 0.475 and 0.625 in (10.9–15.9 mm), with sheath lengths from 11 in (279 mm) to 200 in (5,080 mm) or 255 in (6,477 mm) for the larger diameters. Length tolerances grow with length, for example ±1/8 in sheath and ±1/2 in heated length for 20–50 in elements, with a minimum unheated length of 1-1/4 in at each end. [1]

Standard terminations include a plain pin (pin diameters from 0.091 in for 0.260 in elements to 0.160 in for 0.625 in), a threaded stud with ceramic or mica insulator, a nickel-plated steel lug welded straight or at 90°, and 1/4 in quick-connect terminals. The standard surface is black chrome oxide from annealing before forming; bright annealing in dissociated ammonia gives a clean metallic finish. [2]

Editorial calculation: the 0.375 in element's limits of 600 V and 30 A bound a single element at about 18 kW (600 × 30); actual rating is set by resistance, length and permitted watt density. [1]

03

Check the maker's watt density limit for your heated medium

Watlow's tubular application guide includes a Supplemental Applications Chart that gives, for each heated material, a maximum operating temperature and a maximum watt density and sheath material for its round WATROD and flat FIREBAR elements. For vegetable oil it cites WATROD at 30 W/in² (4.6 W/cm²) or FIREBAR at 40 W/in² (6.2 W/cm²) with a 304 stainless steel sheath. [4]

The guide states performance capabilities of sheath temperatures up to 1800 °F (983 °C), watt densities up to 120 W/in² (18.6 W/cm²) and process assembly ratings up to 1000 psi. When choosing by watt density it asks buyers to consider liquid viscosity at start-up and at process temperature, operating temperature and chemical composition. [4]

Editorial note: for clean water Watlow's chart allows 60 W/in² on WATROD elements, while OMEGA's guideline above gives 50 W/in² for water solutions; makers set different limits, so ask the supplier which limit its design uses. Editorial calculation: static heat transfer oil at 600 °F is listed at 10 W/in² against 15 W/in² when circulating, so the same power needs 1.5 times the heated sheath area in a static bath (15 ÷ 10). [4] [3]

04

Trace the parts of a tubular heating element

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your heater

A heating element specification needs values for your equipment. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the medium, power and geometry data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Medium: [liquid, air or gas; temperature; flow]
Power: [kW; voltage; phase; connection]
Sheath: [material]
Geometry: [diameter; sheath and heated length; form]
Mounting: [flange, screw plug or bracket]
Terminals: [type; enclosure]
Controls: [thermostat; sensor]
Volume: [quantity]

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