Selecting the correct fin tube manufacturing method is one of the most consequential decisions in an air-cooled heat exchanger specification. Extruded, high-frequency welded, and crimped fin tubes are all helical fin configurations that look similar in a drawing, but they differ fundamentally in the nature of the fin-to-tube bond. The bond determines maximum service temperature, long-term thermal performance, fouling behaviour, and ultimately the life of the heat exchanger bundle. Specifying the wrong type can result in progressive thermal performance degradation that is expensive to diagnose and impossible to reverse without re-tubing the bundle.
ZC Steel Pipe manufactures HF-welded fin tubes with carbon steel, alloy steel (A213 T11/T22/T91), and extruded bimetallic fin tubes for EPC projects in refinery, gas processing, and petrochemical facilities across Africa, the Middle East, South America, and Southeast Asia.
Overview of the Three Manufacturing Methods
Each manufacturing method produces a different type of fin-to-tube joint:
| Property | Extruded (Bimetallic) | HFRW Welded | Crimped (Tension-Wound) |
|---|---|---|---|
| Bond type | Mechanical interference | Metallurgical fusion | Mechanical tension |
| Contact resistance | Very low | Zero | Moderate to high |
| Max tube-wall temp | ~200 °C | ~450 °C (steel fin) | ~120–150 °C |
| Fin material | Aluminum (standard) | Carbon/alloy/SS steel | Aluminum or steel |
| Base tube range | Carbon, alloy, SS | Carbon, alloy, SS | Carbon, alloy, SS |
| Manufacturing speed | Moderate | High | Very high |
| Relative cost | Moderate | Moderate–high | Low |
| API 661 compliant | Yes | Yes | Yes (limited service) |
Extruded Fin Tubes
Manufacturing Process
A carbon steel or alloy steel tube is cleaned and descaled, then fed into a co-extrusion press. An aluminum billet is loaded into the press die and forced around the tube under hydraulic pressure while the tube is rotated. The die geometry forms the helical fin profile simultaneously with the extrusion. As the aluminum exits the die, it is cold-worked against the tube surface under very high contact pressure, pressing it into the micro-roughness of the tube OD. The result is a bimetallic tube with an aluminum outer shell and integral helical fins.
Fin Root Bond
The fin-root bond is mechanical, created by the cold-work interference fit between the aluminum and the steel tube. In a new extruded tube, this produces very low thermal contact resistance — typically in the range of 0.00010–0.00020 m²·K/W. This is adequate for service temperatures below 200 °C where the differential thermal expansion between aluminum and steel remains small.
Service Temperature Limit
The fundamental limitation of extruded bimetallic fins is differential thermal expansion. Aluminum expands at roughly twice the rate of steel per degree of temperature rise. At tube-wall temperatures above 200 °C, repeated thermal cycling causes the aluminum sleeve to ratchet outward relative to the steel tube, permanently opening a gap at the fin root. This gap increases contact resistance and reduces effective thermal performance, sometimes by 20–40% compared with a new tube. Once the gap forms, it cannot be closed without re-tubing.
When to Specify Extruded Fin Tubes
Extruded bimetallic fin tubes are the standard choice for:
- Water and lube oil coolers where tube-wall temperatures remain below 150 °C
- Utility and instrument air coolers in non-refinery service
- Projects where aluminum fin material is specified for weight reduction
- Any service where tube-wall temperature does not exceed 200 °C in operation or during steam-out cleaning
For the complete dimensional requirements and peel strength testing criteria for extruded fin tubes, see the ASME Boiler Tube Spec Tables →
To convert between fin dimensions in inches and mm, use the Unit Converter →
High-Frequency Resistance Welded (HFRW) Fin Tubes
Manufacturing Process
A fin strip (typically 8–20 mm wide, 0.89–2.5 mm thick for carbon steel) is cold-formed to an L-shaped or T-shaped cross-section and fed through a guide die onto the rotating tube. As the fin strip contacts the tube surface, a high-frequency (usually 450–500 kHz) electrical current passes through the contact zone. The skin effect concentrates the current at the contact surface, generating intense local heating that melts the fin foot and tube surface metal in a zone approximately 0.2–0.5 mm deep. A pressure roll immediately behind the current contact point forges the molten metal together, expelling oxidised surface metal as a flash bead and forming a sound fusion weld.
The weld is continuous along the full length of the fin spiral. No filler metal is used. The entire bond zone is inspected by the continuity of the current flow — a gap or void breaks the current path and appears as an unmelted zone, which the operator can see immediately on the current monitor.
Fin Root Bond
HFRW produces a metallurgical fusion weld with zero thermal contact resistance at the fin root. Heat flows from the tube OD directly into the fin material without any interface resistance. This delivers the theoretical maximum fin efficiency for a given fin geometry, and performance does not degrade over time due to thermal cycling — unlike mechanical-bond fin types.
Service Temperature Capability
HFRW fin tubes with carbon steel fins can be used at tube-wall temperatures up to approximately 450 °C. With alloy steel fins (T11, T22, T91), the limit increases with the alloy composition. With stainless steel fins, HFRW fin tubes can serve at temperatures up to 600 °C or higher depending on the alloy. The base tube material, not the fin bond, becomes the limiting factor at elevated temperatures. Carbon steel base tubes to ASTM A179 are limited by oxidation and creep above 400–425 °C; A213 T11 or T22 alloy steel base tubes extend service to 510–550 °C.
When to Specify HFRW Fin Tubes
HFRW welded fin tubes are the standard choice for:
- Refinery and petrochemical air coolers per API 661 where tube-wall temperatures exceed 150 °C
- Gas plant trim coolers and product coolers
- Fired heater convection section air preheaters and economisers
- Any application where long-term thermal performance consistency is required
- Services with frequent steam-out or hot-oil cleaning where peak temperatures may briefly exceed 200 °C
Crimped (Tension-Wound) Fin Tubes
Manufacturing Process
A fin strip with an L-shaped foot (also called LL-type) is wound onto the tube at controlled tension using a powered winding head. The L-foot bears against the tube OD. At the end of each run, the fin strip is crimped (bent down) against the tube to lock the final wrap. No heat or welding is involved. The process is very fast and requires less capital equipment than extruded or HFRW manufacturing.
Fin Root Bond
The contact between the L-foot and the tube OD is maintained solely by the tension in the fin strip. This produces a finite contact resistance that is higher than extruded or HFRW bonds and varies with manufacturing tension, tube surface condition, and service history. At moderate temperatures (below 100 °C), newly installed crimped fin tubes can have contact resistances of approximately 0.0002–0.0006 m²·K/W. As temperature rises and thermal cycling occurs, residual tension in the fin strip relaxes, increasing contact resistance further.
Service Temperature Limit
Crimped aluminum fin tubes are generally limited to approximately 120 °C, beyond which thermal stress cycling causes noticeable fin strip tension relaxation. The TEMA manual and API 661 place explicit limits on the use of tension-wound fins in elevated-temperature service. In practice, many operators accept crimped fin tubes for cooling water, lube oil, and utility air coolers, but specify HFRW for process services.
When to Specify Crimped Fin Tubes
Crimped fin tubes may be suitable for:
- Low-temperature utility coolers (cooling water, compressed air after-coolers) below 100 °C
- Budget-constrained replacement bundles in non-critical utility service
- Installations in temperate climates where maximum ambient temperature is low
Crimped fin tubes are not recommended for:
- Refinery or petrochemical process air coolers
- Any service with repeated steam-out or thermal shock
- Offshore installations where corrosion of the L-foot/tube gap accelerates fin detachment
- Long-term unattended service where performance degradation cannot be easily detected
Side-by-Side Selection Guide
| Decision Factor | Choose Extruded | Choose HFRW Welded | Choose Crimped |
|---|---|---|---|
| Tube-wall temp > 200 °C | No | Yes | No |
| Tube-wall temp 100–200 °C | Marginal | Preferred | No |
| Tube-wall temp < 100 °C | Yes | Yes | Acceptable |
| Steam-out cleaning required | No | Yes | No |
| Long-term performance guarantee | Yes (below 150 °C) | Yes | No |
| Lowest first cost | No | No | Yes |
| Offshore or humid environment | Epoxy-coated only | Preferred | No |
| Alloy steel base tube (T11/T22) | Yes (aluminum fin) | Yes (steel fin) | Limited |
| API 661 refinery service | For low-temp only | Preferred | Limited |
Purchase Order Guidance
A purchase order for fin tubes must explicitly state the fin bonding method. Do not rely on the word "helical fin tube" alone — state one of: extruded bimetallic, HFRW welded, G-type embedded, or tension-wound. The bonding method directly determines the applicable temperature limit and testing requirements under API 661.
Procurement trap — steam-out temperature not included in the design basis: Many operators specify tube-wall temperature based on normal operating conditions but overlook the peak temperature during steam-out cleaning (typically 150–175 °C). An extruded fin tube rated for a 120 °C operating condition will experience fin-root bond degradation if steam-out cleaning is applied, especially if steam-out is repeated annually. Always check that the fin tube type can withstand the maximum temperature of all operating and cleaning conditions, not just the normal operating temperature.
Procurement trap — accepting crimped fins as equivalent to extruded: In some markets, suppliers substitute tension-wound crimped fin tubes when the specification says "low-fin" or "helical fin" without further qualification. Always review supplier's dimensional drawings and process capability documentation to confirm that the supplied fin type matches the specified bonding method before approving the drawing package.