Accutherm — Industrial & Mil-Spec and Marine-Duty Electric Heaters

An electric process heater converts electrical energy to heat by resistance heating (a/k/a Joule heating): current passes through a high-resistance alloy element — typically an 80/20 nickel-chrome coil — and the element's resistance dissipates that energy as heat. In an industrial heater the coil is embedded in compacted magnesium-oxide (MgO) insulation inside a metal sheath, which transfers the heat to the process while electrically isolating and protecting the coil. That sheathed element is then arranged for the job: bundled through a flange to heat a tank (immersion), built into a vessel that fluid flows through (circulation), or banked inside a duct to heat air (process air). Accutherm is the Aspeq Heating Group brand that builds these in hazardous-location, mil-spec, and marine-duty configurations for defense and shipboard service.

Any heater going into an area with flammable gas, vapor, or combustible dust must be rated for that classification — a standard heater is an ignition source. The rating is the Class, Division, and Group: Class I for flammable gases/vapors (oil and gas, petrochemical, solvents), Class II for combustible dusts (grain, sugar, coal, metal powders), Class III for fibers; Division 1 where the hazard is present in normal operation, Division 2 where it is present only under fault; and the Group narrows the specific gas or dust. The terminal enclosure, the sheath and shell temperatures (which must stay under the area's auto-ignition T-code rating), and the build all have to suit that rating. Accutherm's explosion-proof flange and pipe-thread immersion heaters are built specifically for this. Give us the area classification, the Group, and the T-code — never approximate a hazardous-area rating.

A hazardous-location heater is rated to the Class, Division, and Group of the area it serves. Accutherm builds explosion-proof flange and pipe-thread immersion heaters for Class I (flammable gases and vapors), Divisions 1 and 2, with explosion-proof terminal enclosures sized to the applicable gas Group, and the sheath and shell temperatures held under the area's auto-ignition T-code. The build, enclosure, and data-plate markings are specified to your exact classification — the heater is engineered and marked to suit rather than picked from a catalog. Because the marking has to match the certificate, share the full area classification, the gas Group, and the required T-code with the quote so the right approved build is specified from the start.

Marine and shipboard heaters are built for salt, humidity, vibration, and the documentation naval and offshore work require. Accutherm's marine line uses corrosion-resistant wetted parts (Monel for seawater, brine, and aggressive marine fuel/oil/water service), with marine-grade gasketing and bronze hardware; the explosion-proof and Monel options apply across the immersion and circulation builds. Defense work adds traceability and the documentation the governing spec requires. Identify the vessel or platform, the service fluid, and the governing spec up front, because marine and Navy qualification is matched to the specific application and documented.

Monel — a nickel-copper alloy — resists chloride pitting and crevice corrosion far better than stainless steel, which is exactly what seawater, brine, and salt-laden marine fuel and water service attack. That makes it the wetted-material of choice for over-the-side and circulation heaters in marine fuel, oil, and water systems, where a stainless sheath would pit and fail early. Monel costs more than stainless, so it is specified where the chloride exposure justifies it; for milder service the standard stainless or Incoloy sheath options apply. Tell us the fluid and its chloride content and the sheath is chosen to it.

Watt density is the heat output per unit of element surface area — watts per square inch. It is the single most important number in heater design, because it sets how hot the sheath runs, not just the process. Drive too much wattage through too little surface and the sheath overheats: it cokes and fouls viscous oils, scales hard-water service, damages heat-sensitive fluids, and shortens element life. The fix is to spread the same wattage over more element surface — a lower watt density. Clean water tolerates a high watt density; heat-transfer oils, fuels, and viscous or high-purity fluids need a low one; air heaters can run higher densities with good airflow. This is why most industrial heaters are engineered rather than pulled off a shelf — the watt density has to match the fluid.

The sheath is the element's only barrier against the process, so it is chosen for chemical compatibility and temperature, not cost alone. As a guide: steel for clean water and low-corrosion service; stainless steel (304) for general process work and mildly corrosive liquids; Incoloy for high-temperature service and harsher chemistry, including many oils and high-temp air; Monel for seawater, brine, and chloride-laden marine service; titanium for chlorides and aggressive plating and process solutions; copper where preferred. The wrong sheath corrodes or fails early; the right one lasts for years. Sheath choice and watt density together are what make an immersion or circulation heater fit its fluid.

Three mounting styles, three use cases. A flanged immersion heater bolts to a matching flange on a tank or vessel nozzle — the choice for larger heaters, higher pressures, ASME-code vessels, and hazardous-area service, where flange ratings and enclosure types matter. A screw-plug heater threads into an NPT coupling — compact and economical for smaller tanks and lower-pressure service. An over-the-side heater drops in through the top of an open tank with the element along the bottom or side — it can be added or removed without draining the tank, which suits marine fuel, oil, and water tanks. Match the style to the vessel: a nozzle and pressure rating point to flanged; a small threaded boss points to screw-plug; an open tank you cannot drain points to over-the-side.

The control method sets how precisely and how smoothly the heater delivers power. A contactor is a mechanical on/off switch — simplest and lowest cost. An SCR power controller is a solid-state device that proportions power continuously with zero-cross switching — tight temperature control, no moving parts, the choice for precise processes. A step controller / sequencer switches a large heater in stages so a big bank does not slam the line at once. For a classified area the control package itself has to suit the area: explosion-proof or purged enclosures, Ex-rated components, and a PID temperature loop with a thermocouple or RTD plus an independent high-limit safety. Accutherm supplies hazardous-location-rated control panels matched to the heater and the area classification.

Yes — industrial electric heating is a build-to-order business, and mil-spec and marine work especially so. Virtually every Accutherm product is engineered to the application: the kW, watt density, sheath material, flange and enclosure rating, hazardous-area marking, vessel, and control are specified per job. There is no published price list, and pricing is quote-only. Lead times typically run about 3 to 14 weeks depending on configuration and certification documentation. Because the heater is engineered, the more complete the data you provide up front — fluid, flow or volume, temperatures, pressure, area classification and T-code, governing marine/defense spec, voltage — the faster and tighter the quote.

A replacement has to match the application, not just the old part number. Capture from the failed unit and its vessel: the kW and voltage, the mounting (flange size and rating, or NPT thread), the sheath material and diameter, the immersed/heated length, the watt density, the terminal enclosure type and any hazardous-area marking, and the control scheme. For a hazardous-location or marine unit, also capture the certification markings on the data plate so the replacement carries the same approvals. Often the right move is to ask why it failed and re-specify, not merely duplicate. Send us the nameplate, a photo of the installation, and the failure symptom.

Kept within their design conditions, industrial heating elements last many years. The common failure causes are nearly all about heat that could not get away from the element fast enough: dry-firing or low fluid level (an immersion element energized without full coverage overheats in seconds), watt density too high for the fluid (coking, scaling, hot spots), fouling and scale that insulate the sheath, moisture ingress causing a ground fault, and contamination or chemical attack on the sheath — chloride pitting of an under-specified sheath in marine service is a classic case. Most are preventable with a proper watt-density spec, the right sheath (Monel for chlorides), a low-level cutoff, and a high-limit safety. An element that keeps failing is telling you the original specification was wrong.

The physics was established by James Prescott Joule in the 1840s: Joule's law showed that the heat a conductor gives off is proportional to its resistance and to the square of the current through it — which is why resistance heating is still called Joule heating. Turning that into a durable industrial device took the early 20th century, when nichrome resistance alloy (which holds up at high temperature without oxidizing away) and the magnesium-oxide-insulated, metal-sheathed tubular element were developed. That sheathed-element construction is still the basis of the immersion, circulation, and duct heaters in use today, including the hazardous-location and marine builds on this page.