Why Brass with Copper Fins?

A finned tube consists of two materials: the base tube and the fins. Their physical properties and chemical compatibility determine the reliability of the final product.

Why Brass Base Tubes Pair with Copper Fins?

This combination comes down to three factors: thermal conductivity matching, thermal expansion consistency, and electrochemical compatibility.

Thermal conductivity

The heat transfer path goes from the fluid inside the tube, through the brass base tube, to the fin root, and then to the fin tip. Brass has a thermal conductivity of 100–120 W/m·K. For a base tube wall thickness of 0.5–1.5 mm, the thermal resistance contributed by the tube itself is minimal. The fins are made of C12200 copper, which has a thermal conductivity of about 380 W/m·K, allowing heat to spread quickly from the root to the tip once it reaches the fin. This “moderate conductivity in the tube, high conductivity in the fin” approach strikes a practical balance between cost and thermal performance.

Coefficient of thermal expansion

Brass has a linear expansion coefficient of roughly 18–19 × 10⁻⁶/K, while C12200 is about 16.5–17.5 × 10⁻⁶/K. These values are close enough that under thermal cycling, the joint between tube and fin sees relatively low stress, reducing the risk of fatigue cracking.

Electrochemical compatibility

When dissimilar metals come into contact in a humid environment, galvanic corrosion can occur. Brass and pure copper sit close to each other on the galvanic series, with a potential difference of less than 0.1 V, so galvanic corrosion is negligible. If copper fins were paired with a carbon steel tube, or aluminum fins with a copper tube, the potential difference could exceed 0.5 V, and corrosion would accelerate significantly.

Brass Base Tube vs. Copper-Nickel Tube

Copper-nickel alloys (such as B10 or B30) are commonly used as base tube materials in marine applications, but they are not a universal choice. The table below summarizes the differences.

Parameter	Brass Base Tube	Copper-Nickel Base Tube (90/10 or 70/30 Cu-Ni)
Thermal conductivity	100–120 W/m·K	30–50 W/m·K
Cost	Lower	Higher (nickel content 10%–30%)
Seawater corrosion resistance	Moderate; dezincification risk	Excellent; good erosion-corrosion resistance
Stress corrosion resistance	Sensitive to ammonia	Excellent
Fabricability	Good; easy to bend and expand	Poor; work hardens readily
Typical applications	Fresh water, air, refrigerants, oil	Seawater, sulfur-containing media, high temperature and pressure

For fresh water cooling, air conditioning and refrigeration, compressed air, and similar conventional applications, brass offers better cost-effectiveness. For direct seawater cooling or environments with sulfides, copper-nickel is the necessary choice.

Manufacturing Process of Oval Finned Tube Brass + C12200 Fins

The connection between the brass base tube and the copper fins is achieved by brazing, not high-frequency welding. High-frequency welding is typically used for joining steel strip to steel tube; for reliable copper-to-copper joints, furnace brazing or induction brazing is the standard approach.

How to Weld Tube and Fins?

C12200 copper fins are pre-wound or pre-assembled onto the oval brass base tube. Brazing filler metal—usually a silver-based alloy (BAg) or a copper-phosphorus alloy (BCuP)—is placed at the interface between the fins and the tube. The assembly is then loaded into a controlled-atmosphere furnace (with nitrogen, hydrogen, or dissociated ammonia) and heated to a temperature above the melting point of the filler metal (600–800°C) but below the solidus temperature of both the base tube and the fins. The molten filler metal is drawn into the narrow gap between the fin and the tube by capillary action, and upon cooling, forms a continuous metallurgical bond.

Advantages of Brazing

Key Quality Control Points

Three factors are critical to achieving a sound brazed joint:

• Gap control: The clearance between the fin and the base tube should be kept between 0.05 and 0.15 mm. If the gap is too small, the filler metal cannot flow in; if too large, voids may form.
• Atmosphere protection: A reducing atmosphere such as pure hydrogen or dissociated ammonia is essential to prevent oxidation of the copper surfaces. Oxides prevent the filler metal from wetting the surfaces, leading to poor bonding.
• Filler metal selection: Copper-phosphorus filler metals (e.g., BCuP-5) contain phosphorus, which can reduce surface oxides and are well suited for copper-to-copper joints. Silver-based filler metals (BAg) offer better flow characteristics and are preferred for complex geometries.

Comprehensive Performance Specifications

Parameter	Range / Value	Remarks
Overall heat transfer coefficient (air side)	40–60 W/m²·K (natural convection) 80–120 W/m²·K (forced air)	Oval shape reduces air-side pressure drop by 30–50% compared to round tubes, with a 15–25% increase in heat transfer coefficient at the same airflow
Fin efficiency	0.85–0.92 (fin height 12–20 mm)	High thermal conductivity of copper minimizes temperature drop from fin root to tip; efficiency is higher than for aluminum fins (typically 0.7–0.8)
Contact thermal resistance	≤ 5 × 10⁻⁵ m²·K/W	Brazed metallurgical bond results in negligible contact resistance
Pressure capability	Working pressure ≤ 4.0 MPa for 0.8–1.2 mm wall thickness	Oval tubes have slightly lower pressure ratings than round tubes of the same wall thickness; a major/minor axis ratio around 2:1 is adequate for most applications
Tensile strength (base tube)	Annealed: ≥ 290 MPa Half-hard: ≥ 340 MPa	Meets strength requirements for HVAC, refrigeration, and general industrial heat exchangers
Corrosion life	Fresh water / air: 15–20 years Coastal industrial environment: 8–12 years	Brass is susceptible to dezincification; arsenic-containing brass (inhibited) can improve corrosion resistance
Weight per unit area	30–40% heavier than copper–aluminum composite fin tubes	All-copper construction adds weight but provides longer service life and higher scrap value at end of life

Media and Temperature Conditions

Tube-Side Media (Inside the Base Tube)

Media Type	Suitability	Limitations
Fresh water / cooling water	Suitable	pH 6.5–8.5; flow velocity recommended ≤ 2 m/s
Chilled water / hot water	Suitable	0–120°C
Refrigerants (R22, R134a, R410a, etc.)	Suitable	Compatible with refrigerants and lubricating oils
Compressed air	Suitable	No restrictions
Lubricating oil / hydraulic oil	Suitable	No restrictions
Seawater	Not recommended	Brass is prone to dezincification and pitting in seawater; use copper-nickel instead
Ammonia-containing media	Not recommended	Ammonia causes stress corrosion cracking in brass
Acidic media (pH < 5)	Not recommended	Copper and copper alloys have poor resistance to strong acids

Shell-Side Media (Outside the Fins)

Media Type	Suitability	Limitations
Dry air	Suitable	Maximum 200°C
Humid air / dusty air	Suitable	Increase fin spacing to prevent fouling
Sulfur-containing flue gas	Not suitable	Sulfur compounds aggressively corrode copper
Marine atmosphere	Use with caution	Recommend epoxy coating or switch to pure copper fins

Temperature Limitations

Parameter	Limit	Reason
Maximum continuous operating temperature	200°C	Above 200°C, copper-phosphorus filler metal may soften; brass base tube may undergo stress relaxation
Minimum operating temperature	–196°C	Copper and copper alloys have no ductile-to-brittle transition at low temperatures; suitable for cryogenic applications such as LNG
Thermal cycling range	ΔT ≤ 150°C	Exceeding this range requires evaluation of thermal fatigue; however, this material combination has good thermal expansion compatibility and resists fatigue better than dissimilar-metal alternatives
Flue gas temperature (short-term)	≤ 250°C	Only for non-corrosive flue gases; requires silver-based high-temperature filler metal