materials used in dry cooling towers
materials used in dry cooling towers, explaining why they are chosen and how they differ from wet cooling towers.
Core Principle First
A dry cooling tower rejects waste heat directly to the atmosphere using convective heat transfer through a finned-tube heat exchanger (like a car radiator), unlike a wet tower which uses evaporative cooling. This fundamental difference dictates the material choices, with a heavy emphasis on durability and corrosion resistance for the external air-side surfaces.
The materials can be divided into two main categories: those for the heat exchanger core and those for the overall structure and components.
1. Heat Exchanger Core (The Most Critical Part)
The heat exchanger is the heart of the dry cooling tower. It’s made of finned tubes to maximize surface area for heat transfer.
A. Tubes
The tubes carry the process fluid (hot water or steam) that needs to be cooled.
Carbon Steel: The most common and economical choice for lower-temperature applications with water. It’s strong but requires protective coatings to prevent corrosion.
Stainless Steel (e.g., 304, 316): Used for more corrosive fluids, higher temperatures, or in environments with high salinity or pollution (coastal or industrial areas). It offers excellent corrosion resistance but is more expensive.
Carbon Steel with Internal Lining: For highly corrosive process water, carbon steel tubes can be lined with epoxy or other protective polymers.
Copper or Copper-Nickel Alloys: Less common in large industrial dry coolers, but sometimes used in smaller HVAC or specific industrial applications for their excellent thermal conductivity.
B. Fins
The fins are attached to the outside of the tubes to dramatically increase the surface area exposed to the cooling air.
Aluminum: The dominant material for fins. It is lightweight, has excellent thermal conductivity, and forms a protective oxide layer that resists atmospheric corrosion. It’s also highly malleable, making it ideal for the mechanical process of being wound or pressed onto the tubes.
Copper: Used in some specialized applications where maximum heat transfer is critical, but it is heavier and more expensive than aluminum.
Galvanized Steel: Sometimes used in corrosive environments where aluminum might not be sufficient, but its thermal conductivity is lower than aluminum.
C. Fin-to-Tube Bond
The efficiency of the heat exchanger depends on a perfect thermal connection between the fin and the tube. The main methods are:
Mechanically Expanded: The tube is expanded hydraulically or mechanically inside the fin collar, creating a tight mechanical bond. This is very common.
Hot-Dip Galvanizing (HDG): After mechanical assembly, the entire bundle can be dipped in molten zinc. This creates a strong metallurgical bond and provides a robust, corrosion-resistant coating for both the tubes and fins. This is ideal for harsh environments.
L-Bundle Construction: Individual finned tubes are welded to headers. This allows for easier replacement of single tubes.
2. Structural and Casing Materials
This encompasses the frame, casing, fan stacks, and other supporting parts.
Hot-Dip Galvanized Steel (HDG): This is the standard and most widely used material for the structure and casing. The galvanizing process provides a thick, durable, sacrificial zinc coating that protects the underlying steel from rust for decades, even when exposed to rain and varying weather conditions.
Aluminum: Used for louvres, fan blades, and sometimes for entire casings in highly corrosive environments (e.g., near the ocean) where even galvanized steel might corrode over time. It is lightweight and corrosion-resistant but more expensive.
Fiberglass Reinforced Plastic (FRP): Commonly used for the fan cylinders (or stacks) and sometimes for casings and louvres. It is:
Corrosion-proof: Ideal for the moist, warm environment directly around the fan.
Lightweight: Reduces stress on the motor and structure.
Strong and Durable.
Stainless Steel: Used for fasteners (bolts, nuts), components in direct contact with corrosive exhaust, or for critical structural elements in extremely aggressive environments. It’s a premium choice due to cost.
Concrete: Used for the support structure of very large, direct dry cooling systems (common in power plants). The massive heat exchanger bundles are often mounted on a large concrete plenum.
3. Other Components
Fans: Large, slow-rotating axial fans are used to draw air across the heat exchanger.
Blades: Typically made from aluminum or FRP for their balance of strength, weight, and corrosion resistance.
Drives and Motors:
Motors: Standard electric motors, often with weatherproof (NEMA 3R or IP54/55) enclosures.
Gearboxes/Drives: Precision-made from hardened steels and lubricated with oil for long service life. For variable speed, Variable Frequency Drives (VFDs) are used.
Louvres: Usually made from galvanized steel or aluminum, they help control airflow and prevent recirculation of hot exhaust air.
Comparison with Wet Cooling Tower Materials
Component Dry Cooling Tower Wet Cooling Tower Reason for Difference
Heat Transfer Surface Finned Tubes (Steel/Al) Bare PVC Splash Fill or Film Fill Dry towers need conductive fins for air cooling. Wet towers use cheap, high-surface-area fill for water-air contact.
Cold Water Basin Not Required Reinforced Concrete or Coated Steel Dry towers are a closed-loop system; no water collection is needed.
Structure & Casing HDG Steel, Aluminum Wood (Redwood, Fir), FRP, Concrete Wet tower environment is constantly saturated, requiring materials immune to water and biological decay.
Corrosion Protection Galvanizing, Aluminum Epoxy Coatings, Stainless Steel Dry tower environment is less aggressive (no constant water exposure), so durable coatings suffice.
Summary: Key Material Selection Drivers
Corrosion Resistance: The primary driver, especially for components exposed to the outside air, rain, and potential pollutants.
Thermal Conductivity: Critical for the heat exchanger fins and tubes to maximize efficiency.
Strength and Durability: The structure must support heavy heat exchanger bundles and withstand wind and weather for 20-30 years.
Cost: A constant balance between initial capital cost and long-term maintenance/lifetime cost.
Weight: Impacts the structural design and foundation requirements.
Because of these factors, Hot-Dip Galvanized Steel for the structure and Aluminum-Finned Carbon Steel Tubes for the heat exchanger represent the most common and cost-effective material combination for industrial dry cooling towers.