What is a Gilled Tube?
A gilled tube, also known as a finned tube, is a heat exchange element that increases heat transfer efficiency by expanding the heating surface. It is primarily used in heat exchange equipment such as waste heat boilers, economizers, and condensers, and is widely used in the steel, power generation, and chemical industries. Its core function is to enhance heat dissipation or absorption by increasing the surface area of the fins, utilizing air turbulence to improve heat transfer efficiency and reduce soot accumulation.
Gilled Tube Structure and Types
Basic Structure: Consists of a base tube and symmetrically welded fins. The fins are connected to the base tube using processes such as flash resistance welding, requiring a weld penetration rate exceeding 99% and a fin pitch tolerance of ±0.2mm.
Common Gilled Tube Types:
H-Type Finned Tube: Features an in-line design, offering excellent wear resistance, low soot accumulation, and a service life of over 10 years.
Spiral/T-Type: A spiral ring or T-shaped tunnel structure is created on the surface of the plain tube using a rolling process.
Gilled Tube Working Principle
By increasing the surface area of the fins to expand the contact area with the high-temperature flue gas, heat energy is transferred to the cooling medium (such as water) through conduction and convection, while protecting the furnace wall from overheating. For example, finned tube water-cooled walls can achieve a heat transfer efficiency of over 65% for waste heat boilers, saving up to 8,680 tons of coal annually for a single project.
Gilled Tube Manufacturing Process
Material: The base tube is typically made of heat-resistant steel or stainless steel, while the fins are typically made of carbon steel or alloy.
Process Requirements: The weld width must be greater than the fin thickness, the heat-affected zone depth of the steel tube must be ≤0.5mm, and the high-frequency welding speed must be able to reach 50-150m/min.
I. Gilled Tube High-Frequency Welding Process
High-frequency current heating melts the contact surface between the fin and the base tube, achieving a metallurgical bond. This process boasts a weld rate exceeding 99%, and the fin pitch tolerance can be controlled within ±0.2mm. It is suitable for materials such as stainless steel and carbon steel.
High-frequency welding speeds can reach 50-150m/min, resulting in high production efficiency. However, the depth of the heat-affected zone (HAZ) must be strictly controlled (≤0.5mm) to avoid deformation of the base tube.
II. Gilled Tube Rolling Process
Cold Rolling: The base tube and fins are integrated using roller pressure at room temperature. Fin height is typically ≤10mm, resulting in a smooth, high-precision surface. It is suitable for plastic materials such as copper and aluminum.
Hot Rolling: Steel is processed under high-temperature conditions, allowing for taller fins (such as spiral fins). However, subsequent scale treatment is required. This process is primarily used in high-temperature applications such as industrial boilers.
III. Gilled Tube mounting processes
There are three methods: expansion welding, welding, and mechanical joining.
Expansion welding: uses an interference fit to secure the fins. This method is low-cost but offers weaker connection strength.
Brazing: Uses a low-melting-point brazing filler metal to join dissimilar materials (such as copper and aluminum). This method provides excellent sealing and is suitable for refrigeration equipment.
Mechanical joining: Fastens with bolts or rivets. This method facilitates maintenance but increases pressure loss.
IV.Gilled Tube Winding Process
A metal strip is pressed into an L-shape and then wound onto the surface of a base tube. This allows for the production of complex fin structures such as serrated and spoke-shaped fins. This method reduces the number of steps and improves material utilization. For example, smooth L-shaped fins are tightly wound and compressed using a wheel mechanism to form a stable heat transfer surface.
V. Gilled Tube Surface Treatment Processes
These include galvanizing, spray coating, and oxidation to enhance corrosion resistance. For example, when the zinc coating thickness reaches 85μm, carbon steel finned tubes can achieve a lifespan of 8-12 years in heating environments. Stainless steel (such as 316L) is over three times more durable than carbon steel in corrosive environments.
VI. Special Process for H-Type Finned Tubes
Two rectangular fins are welded to the base tube using resistance welding to form an “H” shape. This increases the heat transfer area by 2-10 times compared to plain tubes, and offers excellent wear resistance, making it suitable for high-dust environments.
Finned Tube Materials
The material selection for finned tubes should be considered based on a combination of operating environment (temperature, corrosion, pressure, etc.) and cost. The following are the mainstream materials and their characteristics:
I. Carbon Steel Finned Tubes
Features: Low cost (unit price is 1/3-1/2 that of stainless steel), good processability, and a thermal conductivity of approximately 45W/(m・K). However, corrosion resistance is poor, requiring regular anti-corrosion maintenance.
Applicable Applications: Medium to low temperatures (≤400°C) and low-corrosion conditions, such as conventional coal-fired boilers (pressure Force ≤ 1.6 MPa)
Lifespan: 5-8 years, prone to rust in high-sulfur flue gas environments
II. Stainless Steel Finned Tubes
304 Stainless Steel
Features: Contains 18% chromium and 8% nickel, resistant to weak acid and alkali corrosion, thermal conductivity 16.3 W/(m·K), temperature resistance ≤ 400°C
Applications: Civilian HVAC, low-pressure hot water systems (such as central air conditioning heat exchange coils)
316L Stainless Steel Finned Tubes
Features : Molybdenum is added to resist chloride ion corrosion (such as seawater and strong acids), with a temperature resistance of up to 450°C and a thermal conductivity of 15.1 W/(m・K)
Applications: Highly corrosive environments such as chemical and marine engineering, with a service life of 12-15 years.
III. Copper Finned Tubes
Advantages: Optimal thermal conductivity (approximately 400 W/(m・K)), corrosion resistance, but high cost.
Weaknesses: Susceptible to oxidation at high temperatures, mostly used in refrigeration equipment or precision heat exchange.
IV. Aluminum Finned Tubes
Advantages: Lightweight, good thermal conductivity (approximately 237 W/(m・K)), low price, but susceptible to oxidation at high temperatures.
Applications: Low-temperature applications with limited budgets (such as cooling electronic equipment).
5. Titanium and Titanium Alloy Finned Tubes
Features: Resistant to strong corrosion (such as acids, alkalis, and seawater) and high temperatures, but extremely expensive, primarily used in specialized industries.
Gilled Tube Applications
Industrial Applications: Waste heat recovery, boiler water walls, and heat exchange in chemical equipment.
Advantages: Compact structure, low dust accumulation, and high heat transfer efficiency, particularly suitable for highly dusty or corrosive environments.