L-foot
The cheapest wrapped fin. The foot shields most of the tube surface from the atmosphere. Contact can loosen after many thermal cycles, so it suits steady, low-temperature service such as lube-oil and water coolers.
Heat transfer guide
Aluminium Finned Tubes for Industrial Heat Exchangers
An aluminium finned tube is a base tube, usually steel, stainless steel, copper or aluminium, with aluminium fins wound, embedded or extruded on its outside to multiply the surface in contact with air. The fin type sets the maximum operating temperature, the corrosion protection of the base tube and the cost. This guide compares the main types, from L-foot wrapped fins to extruded bimetallic tubes, and lists what to specify when you order.
- Fin material
- 1060 / 1100 aluminium
- Fin types
- Extruded, L, LL, KL, G
- Temperature
- Up to about 400 °C (G-type)
- Key standard
- API 661 · ASTM B404
What finned tubes do and why aluminium fins are used
In an air-cooled heat exchanger the process fluid flows inside the tubes and ambient air is blown across the outside. Because the heat transfer coefficient on the air side is many times lower than on the liquid side, the air side controls the size of the exchanger. Adding fins raises the outside surface by a factor of roughly 10 to 25, so far fewer tubes and a smaller plot area are needed for the same duty.
Aluminium is the default fin material for this job. It conducts heat about four times better than carbon steel, weighs about a third as much, forms easily into thin helical fins and resists atmospheric corrosion. A typical fin is made from 1060 or 1100 strip about 0.4 mm thick, wound around the tube under tension at 7 to 11 fins per inch. The base tube is chosen for the process fluid: carbon steel for hydrocarbons and cooling water, stainless steel or duplex for corrosive streams, copper alloys or titanium for special duties, and aluminium for low-pressure, weight-sensitive coolers.
The weak point of any finned tube is the joint between fin and tube. Air gaps or loosening at that interface add contact resistance, and that resistance grows as the tube heats and cools and the two metals expand at different rates. The fin types below differ mainly in how they make and keep that contact.
Main types of aluminium finned tube compared
The temperatures below are the limits commonly applied in industry for aluminium fins on steel tubes, in line with the practice reflected in API 661. Always confirm limits with the exchanger designer for your process conditions.
| Type | How the fin is attached | Approx. max. temperature | Base-tube corrosion protection | Relative cost |
|---|---|---|---|---|
| L-foot (wrapped) | Strip formed with an L-shaped foot and tension-wound so feet butt together | 150 °C | Good: feet cover the tube | Low |
| LL (overlapped L) | As L-foot, but each foot overlaps the next to seal the tube completely | 180 °C | Very good | Low–medium |
| KL (knurled L) | Tube surface knurled, then L-foot fin wound and pressed into the knurl | 260 °C | Very good | Medium |
| G-type (embedded) | Helical groove machined in the tube; fin wound into it and locked by peening back the groove | 400 °C | Limited: tube exposed between fins | Medium |
| Extruded (bimetallic) | Aluminium sleeve over the base tube, fins rolled out of the sleeve wall under high pressure | About 285 – 300 °C | Excellent: tube fully sheathed | High |
| Crimped | Fin strip gathered (crimped) at its root as it is wound, without a foot | About 270 °C | Limited | Low |
| Integral low fin (mono-aluminium) | Fins rolled directly from the wall of a thick aluminium tube | Set by aluminium alloy | Not applicable: one metal | Medium |
Aluminium fins lose strength and can creep at elevated temperature, so the type limits are set by fin-to-tube contact rather than by melting. For very hot flue gas, steel or stainless fins attached by high-frequency or laser welding are used instead of aluminium.
L, LL and KL wrapped fin tubes
Wrapped fins are made on a finning machine that forms a foot on aluminium strip and winds it helically onto the rotating base tube. The tension of the strip holds the fin in contact. They are the most economical aluminium fin tubes and dominate refinery and petrochemical air coolers working at moderate temperatures.
LL (double L)
Overlapping feet seal the tube fully, so it performs better than plain L-foot in humid or mildly corrosive air. It is often chosen as a lower-cost alternative to extruded fins.
KL (knurled foot)
The knurled tube surface bites into the foot, increasing contact area and resisting loosening. This raises the temperature limit to about 260 °C and improves long-term thermal performance.
Typical wrapped-fin geometry for refinery coolers is a 25.4 mm (1 in) base tube with fins 12.7 or 15.9 mm high (giving 50.8 or 57.2 mm fin diameter), about 0.4 mm thick, at 7 to 11 fins per inch. Larger fins increase surface per metre but also raise air-side pressure drop and fan power.
Embedded G-type and extruded bimetallic tubes
G-type (embedded) fin. A helical groove is cut into the base tube, the aluminium strip is wound into it and the displaced tube metal is rolled back to lock the fin. The mechanical bond is very strong and stays tight at high temperature, which is why G-fin tubes are used for steam condensers, hot gas coolers and compressor after-coolers up to about 400 °C. The base tube needs extra wall thickness for the groove (commonly at least about 2.1 mm), and the tube surface between fins is exposed, so G-fin is less suited to corrosive atmospheres.
Extruded (bimetallic) fin. A thick-walled aluminium sleeve, usually 1060 or 1100, is slid over the base tube and the fins are rolled out of the sleeve wall in one operation. The aluminium is pressed tight against the tube along its full length, giving excellent heat transfer and complete protection of the base tube from the atmosphere. Extruded fins are stiff enough to tolerate high-pressure water cleaning, which makes them the preferred choice in coastal, offshore and chemically aggressive sites despite the higher cost.
Typical extruded fin tube range
- Base tube OD
- 12.7 – 38.1 mm (1/2 – 1 1/2 in)
- Base tube material
- Carbon steel, stainless steel, duplex, copper, copper-nickel, titanium
- Fin material
- 1060 or 1100 aluminium
- Fin height
- About 10 – 16 mm
- Fin thickness
- About 0.3 – 0.65 mm at the root tapering towards the tip
- Fin density
- 5 – 11 FPI (about 197 – 433 fins per metre)
- Tube length
- Up to about 15 m, with plain (unfinned) ends to suit tube sheets
Ranges are typical industry values; the exact geometry is agreed for each order.
Crimped, low-fin, longitudinal and welded fin tubes
Crimped fin
Strip is crimped at the root so it can be wound tightly without a foot. Simple and robust, used for compressor coolers, oil coolers and hydraulic power-unit radiators.
Integral low fin (ASTM B404)
Fins are rolled directly out of a seamless aluminium tube, so there is no contact resistance at all. Alloys include 1060, 3003, Alclad 3003, 5052, 5454 and 6061. Used in light condensers, heat-pipe recovery units and weight-critical coolers.
Longitudinal fin
Straight fins run along the tube axis, usually welded U-channels in steel, with some aluminium designs. Used in double-pipe and hairpin exchangers, tank heaters and gas heaters where the shell-side fluid flows lengthwise.
High-frequency welded fin
Steel or stainless strip welded to the tube by high-frequency resistance heating, solid or serrated. Used for boiler economisers and heat-recovery steam generators where temperatures exceed aluminium limits.
Laser welded fin
A continuous laser weld joins the fin to the tube, including some aluminium fin on stainless tube combinations, for low contact resistance in compact coolers.
Finned coils and special shapes
Finned tubes bent into U-tubes or serpentine coils, oval tubes with rectangular plate fins, and studded tubes for fired heaters.
How much surface do fins add?
The table calculates the outside surface of a 25.4 mm base tube with 0.4 mm aluminium fins at two common fin diameters. It includes both fin faces, the fin tip and the tube surface between fins, and compares the total with a bare tube (0.080 m² per metre). Fin mass is for the fin discs only, at 2.70 g/cm³.
| Tube OD / fin OD (mm) | Fins per inch | Outside area (m² per m) | Area vs bare tube | Aluminium fin mass (kg per m) |
|---|---|---|---|---|
| 25.4 / 50.8 | 7 (276 per m) | 0.93 | 11.6 × | 0.45 |
| 25.4 / 50.8 | 9 (354 per m) | 1.17 | 14.6 × | 0.58 |
| 25.4 / 50.8 | 10 (394 per m) | 1.29 | 16.2 × | 0.65 |
| 25.4 / 50.8 | 11 (433 per m) | 1.41 | 17.7 × | 0.71 |
| 25.4 / 57.2 | 7 (276 per m) | 1.23 | 15.4 × | 0.61 |
| 25.4 / 57.2 | 9 (354 per m) | 1.56 | 19.5 × | 0.79 |
| 25.4 / 57.2 | 10 (394 per m) | 1.72 | 21.6 × | 0.88 |
| 25.4 / 57.2 | 11 (433 per m) | 1.88 | 23.6 × | 0.96 |
Doubling fin density does not double heat transfer, because fin efficiency falls as fins get taller and the air between closely spaced fins slows down. Air coolers in dusty or fouling service often use 7 to 8 FPI so the fins can be cleaned; clean-air services use 10 to 11 FPI to save plot area.
Standards and specifications for finned tubes
| Standard | Scope |
|---|---|
| API 661 / ISO 13706 | Air-cooled heat exchangers for petroleum, petrochemical and natural gas industries: fin types, fin-to-tube bond, temperature limits and testing |
| ASTM B404 / B404M | Seamless aluminium and aluminium-alloy condenser and heat-exchanger tubes with integral fins |
| ASTM B234 | Aluminium-alloy drawn seamless tubes for condensers and heat exchangers (bare aluminium base tubes) |
| ASTM A179, A214, A213, A249 | Common carbon and stainless steel base tubes for heat exchangers |
| ASME Section VIII | Pressure-vessel code governing the tube bundle and headers |
| EN 10204 3.1 | Inspection certificate for base tube and fin strip material |
API 661 is the reference most buyers cite for refinery and gas-plant coolers. It requires the fin type and maximum process temperature to be compatible, sets minimum fin thickness, and calls for bond testing on extruded and embedded fins. Typical inspection includes fin height, pitch and thickness checks, fin-to-tube bond or pull-off tests, hydrostatic testing of the base tube, and visual checks for split or leaning fins.
How to choose the right finned tube
Start from the maximum temperature the fins will see, then narrow the choice by atmosphere, cleaning method and budget. A practical sequence used by many exchanger designers is:
- Check temperatureUp to about 150 °C any wrapped type works; 150 – 260 °C needs LL, KL or extruded; 260 – 400 °C points to G-type embedded fins; above that, aluminium fins are no longer suitable.
- Check the atmosphereIn coastal, offshore, fertiliser or chemical plants the base tube must be shielded. Extruded fins give the best protection, LL and KL fins are next; G-type leaves steel exposed between fins.
- Check cleaningIf the bundle will be cleaned with high-pressure water or steam, extruded fins resist bending far better than thin wound fins.
- Check thermal cyclingFrequent start-stop or swinging process temperatures loosen tension-wound fins. KL, G-type and extruded fins keep their contact much longer.
- Balance costFor mild, steady duties L or LL fins give the lowest installed cost; the extra price of extruded or G-type tubes is repaid where downtime or re-tubing would be expensive.
Where aluminium finned tubes are used
- Oil, gas and petrochemical: air-cooled heat exchangers (fin-fan coolers) for overhead condensers, product coolers and gas compression
- Power generation: air-cooled condensers, lube-oil and generator coolers, closed-loop water coolers
- Chemical and process plants: process coolers and heaters, often with extruded fins for corrosion protection
- HVAC and refrigeration: heating and cooling coils, ammonia and refrigerant condensers
- Heat recovery: heat-pipe recovery units and exhaust air recovery in ventilation systems
- Machinery: compressor inter- and after-coolers, hydraulic oil coolers, transformer and inverter coolers
- Marine and offshore: topside coolers where extruded fins protect steel tubes from salt-laden air
Information needed to order finned tubes
- Process conditionsFluid in the tube, design pressure and temperature inside, maximum air-side temperature, and the surrounding atmosphere (inland, coastal, industrial).
- Base tubeMaterial and specification, outside diameter, wall thickness and length.
- FinsFin type (L, LL, KL, G, extruded, crimped), fin material, height, thickness and density (FPI or fins per metre), solid or serrated.
- Ends and extrasLength of plain ends at each end, U-bending, tube-end preparation, and testing such as hydrotest or bond test.
- Quantity and deliveryNumber of tubes, documentation (EN 10204 3.1), packing in wooden crates with fin protection, and delivery terms (EXW, FOB, CIF or DAP).
References and standards
- API Standard 661 / ISO 13706 – Petroleum, petrochemical and natural gas industries: air-cooled heat exchangers
- ASTM B404/B404M – Seamless Aluminum and Aluminum-Alloy Condenser and Heat-Exchanger Tubes with Integral Fins
- ASTM B234 – Aluminum and Aluminum-Alloy Drawn Seamless Tubes for Condensers and Heat Exchangers
- ASME Boiler and Pressure Vessel Code, Section VIII – Rules for Construction of Pressure Vessels
- Tubular Exchanger Manufacturers Association (TEMA) – Standards of the TEMA
Aluminium (aluminum) finned tubes: frequently asked questions
Which finned tube type is best for high temperatures?
Among aluminium-fin types, the G-type embedded fin handles the highest temperature, about 400 °C, because the fin is mechanically locked into a groove in the tube. Extruded fins reach about 285 – 300 °C and KL fins about 260 °C. Above that, steel fins welded to the tube are used.
What is the difference between extruded and L-foot finned tubes?
An L-foot fin is a separate strip wound onto the tube and held by tension, so it is cheap but can loosen over time. An extruded fin is rolled out of an aluminium sleeve pressed onto the tube, giving a tight bond, full corrosion protection of the base tube and resistance to high-pressure cleaning, at a higher price.
Which aluminium alloy is used for the fins?
Most wound and extruded fins use 1060 or 1100 commercially pure aluminium, which combines high thermal conductivity with the ductility needed to form fins. Integral low-fin tubes to ASTM B404 are also made from 3003, 5052, 5454 and 6061.
How many fins per inch should I specify?
Air coolers commonly use 7 to 11 fins per inch. Lower densities are easier to clean in dusty or fouling environments; higher densities give more surface per metre but raise air-side pressure drop and fan power.
Are finned tubes suitable for coastal or offshore sites?
Yes, but choose the fin type carefully. Extruded fins or LL fins cover the base tube and protect it from salt-laden air. G-type fins leave the tube exposed between fins and are usually avoided in aggressive atmospheres unless the tube itself is corrosion-resistant.
Can finned tubes be supplied U-bent with plain ends?
Yes. Tubes can be supplied with unfinned ends of the required length for tube sheets, and bent into U-tubes or coils as specified, with documentation and protective crating.
Need finned tubes or fin strip?
Send the process conditions, base tube, fin type and geometry, lengths and quantity. Metline will review the specification and quote delivery to your site.
