Product Overview
A two-pass finned-tubular element is a finished resistance element built on the Aspeq tubular platform: a nickel-chromium coil in compacted magnesium oxide inside a metal sheath, with a corrugated fin helically wound on the sheath to extend the heat-transfer surface for air and gas. Its defining feature is the two-pass geometry — a U-bent coil that brings both terminals out the same end, folding the heated length into a compact bundle and letting the element be permanently mounted without the flexible expansion connection a one-pass straight element requires. Choose it when you need high air-heating output in a short element, wired and mounted from one side.
Related Accutherm element & air-heating options
Finned-Tubular Marine Heaters — marine-duty finned elements in copper-nickel / Monel for salt-air air and space heating
Tubular & Finned Elements — the full OEM element line — straight, hairpin, finned, one- or two-pass
Custom-Formed Elements — elements bent and engineered to a customer vessel or fixture profile
Process Air / Duct Heaters — a finished, ducted air heater assembly instead of a bare element
Key Features & Benefits
- Both leads at one end — mount it permanently — the return-bend geometry puts both electrical connections on a single accessible face, so the element bolts in with factory hardware and gets wired from one side. It drops the loose, flexible expansion connection a straight one-pass element needs, simplifying the mount and the wiring run.
- Folded heated length packs more kW into a short element — the return-bend geometry doubles the heated path back on itself, so a compact straight bundle delivers the wattage a much longer one-pass element would need. The answer when the duct or fixture is short on length but the heat load is high.
- Finned surface heats air without cooking the sheath — the helically wound fin multiplies the heat-transfer area, so the same wattage runs cooler at the metal and the element lasts longer in convection and duct service. Gentler surface temperatures for a given heat load than a bare element delivers.
- Built and listed as a finished element — it ships as a complete UL Recognized resistance element — coil, MgO, sheath, fin, seals, and terminals — ready to drop into your equipment, with military-standard certification available. Designed to be incorporated into OEM equipment, not assembled in the field.
Specifications
- Operating principle
- Electric resistance (Joule) heating — AC current passes through a high-grade nickel-chromium resistance coil precisely centered in a heavy-gauge metal tube filled with compacted granular magnesium oxide. Rolling reduces the assembly diameter and compacts the MgO for rapid coil-to-sheath heat transfer and high dielectric strength. A helically wound fin extends the surface so the element dumps its heat into moving air or gas at a lower sheath temperature than a bare element.
- Two-pass coil & termination geometry
- Two-pass construction. A U-bent coil inside the sheath brings both terminals out the same end; the opposite end is sealed by welding or silver soldering. That folds the heated length into a compact straight bundle and — unlike a one-pass straight element — removes the need for a flexible electrical connection at the far end to absorb thermal expansion, so the element can be permanently mounted with factory-furnished hardware.
- Fin construction & density
- A corrugated metal fin is helically wound on the sheath to extend the heat-transfer surface for air and gas heating. Depending on sheath and fin material the fins are mechanically attached or furnace-brazed, then the assembly passes through a computer-controlled atmosphere furnace. Standard fin density is five fins per inch; eight fins per inch is available when specified for lower-sheath-temperature service.
- Element construction
- Nickel-chromium resistance coil in compacted magnesium-oxide insulation inside a metal sheath; cold pins welded to the coil at each end. The two-pass coil is offered in 0.430″ and 0.475″ O.D. sheath sizes (finned or unfinned). Bends are recompacted to restore MgO density after forming, preventing the localized overheating that shortens coil life on high-watt-density builds.
- Sheath materials & max sheath temperature
- Selected to the service temperature and atmosphere, with the published maximum sheath temperature in parentheses. On the FINNED two-pass element: steel / copper-plated steel (750°F), 304 / 316 / 321 stainless steel (1400°F), and Monel (900°F). Copper (350°F), Incoloy 800 (1700°F), and Inconel 600 (1800°F) are bare-tubular (unfinned) two-pass options, not finned-element options. Stainless sheaths are the usual choice for finned air heaters.
- Sheath diameter & wall
- Two-pass sheath O.D. 0.430″ (standard wall 0.028″) or 0.475″ (standard wall 0.035″), per Table I of the catalog. Rolling holds a round cross-section and consistent diameter — important where the element is inserted into a machined hole.
- Sheath / heated length
- Sheath length is built to the wattage and watt density the application allows. For the two-pass sizes the catalog lists sheath lengths from a 6″ minimum up to a 79″ maximum on 0.430″ and 170″ on 0.475″; sheath- and heated-length tolerance is 1% of overall length (minimum 1/8″ and 1/2″ respectively).
- Cold (unheated) section length
- An unheated (cold) section at the terminal end keeps heat away from the seals and field wiring. Cold ends run from a 1.5″ minimum up to a 16″ maximum on the two-pass sizes, specified to the terminal enclosure and ambient.
- Watt density
- Watt density (W/in² of sheath surface) is matched to the air/gas temperature, the airflow, and the sheath material — it is the single most important factor in element life. The finned surface lets a given wattage run at a lower watt density and sheath temperature than a bare element. Set by the catalog family-of-curves and the watt-density equation (W = element wattage ÷ (π × diameter × heated length)) at quote time; specific values are configured to the duty.
- Terminals
- Standard termination is a threaded stainless-steel terminal welded onto the cold pin, with stainless nuts and washers for field wiring; the terminal must be held against rotation while it is wired. Threaded-terminal torque is specified by thread size (for example 20 in-lb on a #10-32 standard terminal, 30 in-lb on a 1/4-28 hermetic terminal).
- Moisture seals
- Element ends are sealed against moisture, which rapidly degrades the magnesium-oxide insulation. The seal is selected to the maximum temperature the limiting controls allow; a hermetic seal is available where an absolute moisture barrier is required. Consult the factory on seal temperature limits.
- Voltage / current rating
- Wound to the service voltage and rating. The two-pass sizes are rated to 240 V on 0.430″ and 300 V on 0.475″ (the 600 V ceiling is the one-pass column), with a maximum current draw of 15 A on 0.430″ and 30 A on 0.475″ per the catalog electrical table; do not operate above the marked rating.
- Mounting / installation
- Permanently mounted with factory-furnished hardware (the single-end termination is what makes that practical), or inserted into clamped, drilled, or sandwiched fixtures for surface and platen heating. In forced-air service distribute airflow evenly; the catalog gives the support-spacing and expansion rules.
- Approvals & listings
- Most elements are UL Recognized under UL 1030 (Indeeco/Aspeq file E78533) and CSA approved; elements are dielectric tested per UL 1030 before shipment. Certification to military standards MIL-H-22577 Rev C and MIL-PRF-22594 Rev C and CE marking are available on request. Quote the exact listing required.
- Build & lead time
- Custom build-to-order — no published price list, quote-only. Lead time depends on sheath alloy, fin material, length, terminations, and any certification or hydrostatic testing specified; consult the factory for the current schedule.
Common Applications
- Forced- and natural-convection air heating in ducts, ovens, and process enclosures
- Compact high-output air / gas heating in tight duct runs and small enclosures
- Drying, curing, and space / comfort heating that needs extended fin surface
- OEM build-in heat for equipment that needs both leads on one accessible side
- Freeze protection and process-air pre-heat in convection airstreams
Fit limit: finned elements are for moving low-pressure, low-temperature air and gas. For service above roughly 400°F, for compressed gases, or where airborne lint or straw can catch in the fins, choose an unfinned tubular element; for heating a liquid in a tank or a flowing stream in a pipe, the immersion and circulation builds are the answer.
Design & Selection Considerations
- Watt density is the make-or-break spec — set it to the airflow — too high a W/in² overheats the sheath and burns the coil out; too low drives the price up. The finned surface buys headroom, but only if the air actually moves across it. Use the input form to give us the duty conditions and we work backward to a safe density. This is the No. 1 reason finned elements fail early.
- Fins help low-temperature air — bare elements win above the limit — finned elements are preferred for low-pressure, low-temperature air and natural convection; for service above roughly 400°F, for compressed gases, or where airborne lint or straw can lodge between the fins, an unfinned element is the right call. Match fin choice to the air, not just to the kW.
- Pick the fin density for the sheath temperature — five fins per inch is standard; eight fins per inch is offered only for lower-sheath-temperature service, because tighter fins trap heat and need cooler running. Denser fins are not automatically better — they shift the sheath-temperature math.
- Match the sheath alloy to the temperature and the atmosphere — each sheath material carries a published maximum sheath temperature (copper 350°F up through Inconel 600 at 1800°F); the alloy also has to survive the gas chemistry, not just the temperature. Spec the sheath to the hottest surface it will see and the atmosphere it sits in.
- Two-pass simplifies expansion — one-pass still needs slip room — because two-pass terminations are fixed at one end, the element grows toward the sealed end; a straight one-pass element instead needs one end free to slip through a support and a flexible lead. Either way the element expands about 1/8″ per foot of heated length — leave for it. Constrain a heated element rigidly at both ends and it buckles.
- Megger and bake out after storage — the magnesium-oxide insulation is hygroscopic, so an element off the shelf can read low insulation resistance. Megger with a 500 VDC meter; if it reads below 1 megohm, bake at 250°F (or energize at reduced voltage in air) until it recovers before putting it in service. A “bad” new element is usually just damp MgO.
- Protect the seals and respect the film temperature — keep terminals clear of drippings, condensation, and fumes — outdoor service needs a moisture-resistant housing — and on any liquid duty hold the maximum film temperature the fluid tolerates. The seal and the film temperature are what set the element’s real service life.
To spec the right Accutherm two-pass finned-tubular element:
Use the input form to send your air or gas temperature rise (ΔT) and maximum air temperature, the airflow rate and duct or fixture dimensions, the available voltage and phase, the sheath material the atmosphere requires, and any certification (UL, CSA, MIL-spec) you need — and we’ll spec the right Accutherm two-pass finned-tubular element for your equipment.
Electric Heating Application Sheet ›Talk to an engineer directly — Scott Prater, Principal · 917-580-0878 · scott@pratertechnical.com
Specifications compiled by Prater Technical Partners from Aspeq Heating Group product datasheets.