Overlapped L Type Fin Tube Carbon LL Type Finned Tube for Heat Exchanger

Product Overview

Overlapped L type fin tube, also called LL type finned tube or LL-foot fin tube, is a tension-wound finned tube where L-shaped fins are helically wrapped around a carbon steel base tube and each fin foot overlaps the previous one. This creates a double “L” foot that completely jackets the outer surface of the tube.

For carbon LL type fin tubes, the base tube is typically seamless low- or medium-carbon steel (such as ASTM A179, A192, A210 or A106), while the fins are usually aluminum or copper strip. This construction combines the strength and pressure resistance of carbon steel with the high thermal conductivity and good corrosion behavior of non-ferrous fins, making it ideal for air-cooled and gas-side heat exchanger service.

Specifications

Typical design and manufacturing range for carbon steel LL type fin tubes (values are indicative and can be customized):

• Base tube material (carbon steel):

  • ASTM / ASME: A179, A192, A210 Gr A1 / Gr C, A106 Gr B

  • EN: EN 10216-2 grades such as P235GH / P265GH

  • JIS: G3461 STB340

  • GB / GOST: GB/T 5310, GOST 8734/8733

• Base tube outer diameter:

  • Approx. 16–51 mm (can be extended to ~73 mm depending on design)

• Base tube wall thickness:

  • Typically 2.0–6.0 mm for boiler and heat-exchanger service

• Fin material:

  • Aluminum alloys: 1060, 1070, 1100, 6063 series

  • Copper strip when higher thermal conductivity or better corrosion performance is required

• Fin geometry (LL type):

  • Fin height: up to ~16 mm

  • Fin thickness: approx. 0.4–0.6 mm (other thicknesses on request)

  • Fin pitch: ≥ 2.3 mm (typically 8–11 fins/inch, depending on duty)

  • Fin foot: overlapped “LL” foot fully covering the tube surface

• Tube length:

  • Usually up to 12–18 m (longer or U-bend tubes on request, subject to transport and handling limits)

• Typical design conditions:

  • Service: air, flue gas, clean or mildly corrosive gas streams

  • Working temperature:

    • With aluminum fins: typically up to about 170 °C on fin side

    • With copper fins or special designs: can be higher, subject to project rating and alloy selection

Key Features

• Full tube sheathing
  The overlapped LL foot design completely covers the carbon steel tube, providing a continuous metallic jacket and significantly improving external corrosion protection compared with simple L-footed fin tubes.

• High fin-to-tube contact pressure
  Fins are helically wound onto the tube under controlled tension, giving strong mechanical contact between fin foot and tube surface, which enhances heat transfer and minimizes thermal contact resistance.

• Improved service life in corrosive atmospheres
  Because the base tube is fully covered by overlapping fin feet, the tube wall is largely isolated from direct gas-side attack. LL fin tubes are widely used as a cost-effective alternative to extruded fins in mildly to moderately corrosive environments.

• Stable fins under thermal cycling
  The mechanical winding process and interlocking “LL” geometry reduce the risk of fin loosening or rattling under repeated heating and cooling, mechanical vibration, or flow-induced vibration.

• Good cleanability and low fouling tendency
  Smooth, continuous fins with uniform pitch provide low gas-side pressure drop and help limit dust accumulation, simplifying online or offline cleaning by air blowing, steam blowing or light brushing.

Our Advantages

Below are some of the typical questions end users and EPC contractors raise about LL type (overlapped L) carbon finned tubes—and how a well-engineered product answers them.

1. How does LL type compare with standard L type and KL type fin tubes?

• Concern: Which fin type provides the best combination of corrosion resistance, heat transfer and cost?

• Answer:

  • Standard L type provides good contact and basic corrosion shielding but leaves small exposed gaps between fin feet.

  • LL type overlaps the fin feet to completely enclose the tube, giving better corrosion resistance than L type and approaching extruded fins, at a significantly lower cost.

  • KL type combines L-foot fins with knurling on the base tube to further improve mechanical bonding and heat transfer, but it is typically more expensive and used when maximum performance is needed.

2. Will the fins stay tight during long-term operation?

• Concern: Fin loosening under thermal cycling and vibration can severely reduce heat transfer efficiency and cause noise.

• Answer:

  • LL fins are tension wound and mechanically locked by overlapping feet. The continuous wrap and controlled tension maintain strong grip on the tube, even under repeated heating and cooling.

  • Correct material pairing (carbon steel tube + aluminum or copper fins) also helps accommodate differential thermal expansion without cracking the fin foot.

3. Is LL type suitable for my gas temperature and environment?

• Concern: Users want assurance that aluminum or copper fins will not soften or corrode too quickly in service.

• Answer:

  • For aluminum fins, typical design gas-side temperatures for LL wound fins are around 150–170 °C, with higher limits possible depending on detailed design, alloy tempers and safety margins.

  • Copper fins can be used at somewhat higher temperatures and in certain corrosive atmospheres, but final selection should be based on the flue-gas composition (SOx, chlorides, moisture) and site experience.

  • The full tube coverage of LL feet provides extra protection for the carbon steel tube where external corrosion is a concern, for example in coastal or slightly acidic flue-gas environments.

4. How do I select fin height and fin pitch?

• Concern: Engineers must balance heat transfer area, pressure drop and fouling tendency.

• Answer:

  • Typical LL fin height is ≤16 mm with pitch ≥2.3 mm; higher fins and tighter pitch increase surface area but also raise pressure drop and fouling risk.

  • For dusty flue gases, slightly lower fin height and a more open pitch often give the best life-cycle performance.

  • A competent supplier can optimize fin geometry using the duty conditions (gas flow, temperature profile, allowable pressure drop and cleaning method).

5. Are the base tubes and fins compliant with international pressure-part standards?

• Concern: EPCs and owners need compatibility with ASTM, EN, JIS, GOST and GB codes.

• Answer:

  • Carbon steel base tubes are typically supplied to widely recognized pressure standards such as ASTM A179, A192, A210, A106; EN 10216-2; JIS G3461; GB/T 5310; and corresponding GOST specifications.

  • These standards clearly define chemical composition, mechanical properties and mandatory NDT / pressure tests, ensuring safe use in boilers, economizers and other heat-exchanger equipment.

Chemical Composition

Mechanical Properties

Applicable Standards

ASTM / ASME (USA):

• ASTM A179 / ASME SA179 – Seamless cold-drawn low-carbon steel heat-exchanger and condenser tubes.

• ASTM A192 / ASME SA192 – Seamless carbon steel boiler and superheater tubes for high-pressure service.

• ASTM A210 / ASME SA210 – Seamless medium-carbon steel boiler and superheater tubes (Grades A1 and C).

• ASTM A106 / ASME SA106 – Seamless carbon steel pipe for high-temperature service (commonly used as base tubes for finned coils).

EN (Europe):

• EN 10216-2 – Seamless steel tubes for pressure purposes, non-alloy and alloy steel tubes with specified elevated-temperature properties (grades P235GH, P265GH, etc.).

JIS (Japan):

• JIS G3461 – Carbon steel tubes for boiler and heat exchanger (e.g. STB340).

GB (China):

• GB/T 5310 – Seamless steel tubes and pipes for high-pressure boiler.

GOST (Russia / CIS):

• GOST 8734-75 / GOST 8733-74 – Seamless cold-formed steel tubes and associated technical requirements, widely used for boiler and heat-exchanger tubing.

LL finned tubes are usually supplied with plain bare ends, cut to length, and fully comply with the base tube standard for chemical composition, mechanical properties, hydrostatic testing and non-destructive examination. The finning process is applied on qualified tubes only.

Application Fields

General Application Areas

Carbon LL type (overlapped L) finned tubes are widely used in gas-side heat transfer equipment where a compact, corrosion-resistant extended surface is required, including:

• Power generation (thermal power plants, waste-to-energy)

• Petrochemical and refinery units

• Chemical and fertilizer plants

• Metallurgy and steelworks

• HVAC and industrial drying systems

• Waste heat recovery units and economizers

Typical Uses Within Each Field

• Power and boiler island:

  • Air pre-heaters, economizers, low-temperature heaters and gas-gas heat exchangers.

  • Flue-gas heat recovery behind boilers or gas turbines.

• Petrochemical and refinery:

  • Air-cooled heat exchangers (air coolers) in crude, vacuum, coker, FCC and hydrotreater units.

  • Gas coolers for hydrogen, fuel gas or flare gas systems.

• Chemical and fertilizer:

  • Process gas coolers and heaters in ammonia, methanol or syngas lines.

  • Waste-heat boilers and convection banks with flue-gas on shell side.

• Metals and high-temperature process industry:

  • Recuperators and heat-recovery coils in reheating furnaces and annealing lines.

  • Off-gas coolers and air heaters in sinter plants or blast-furnace gas systems.

• HVAC, drying and general industry:

  • Hot-air coils, steam coils and gas heaters for dryers and ovens.

  • Heat recovery ventilation coils and industrial air-heating batteries.

Q: Are you trading company or manufacturer ?

A: manufacturer,also can do trading.

Q: How long is your delivery time?

A: Generally speaking,it is 10-15 days if the goods are in stock,or it is 30-40 days if the goods are not in stock,

it is according to quantity.

Q: Do you provide samples? is it free or extra ?

A: Yes, we could offer the sample for free charge but need pay the cost of freight.

Q: What is your terms of payment ?

A: Payment<=2000USD, 100% in advance. Payment>=2000USD, 30% T/T in advance ,balance before shippment.

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