Role of Dry cooling tower in Corrosion Resistance
This is a nuanced but critical aspect of cooling system design. The role of a dry cooling tower in corrosion resistance is fundamentally about eliminating the primary mechanisms that cause corrosion in wet cooling systems.
Here’s a detailed breakdown of its role and how it enhances corrosion resistance.
The Core Problem: Corrosion in Wet Cooling Towers
To understand the benefit of a dry cooler, we must first understand how wet (evaporative) cooling towers promote corrosion:
Oxygen Ingress: Wet towers are open to the atmosphere, continuously dissolving oxygen into the water. Oxygen is a primary driver of electrochemical corrosion (rust).
Concentration of Corrosive Salts: The evaporation process concentrates dissolved solids (chlorides, sulfates) in the water. These salts are highly aggressive and accelerate pitting and crevice corrosion.
Galvanic Corrosion: Wet systems often have multiple metals (copper tubes, steel frames, brass fittings, aluminum fans). The recirculating water, acting as an electrolyte, creates a perfect environment for galvanic corrosion between these dissimilar metals.
Microbiologically Influenced Corrosion (MIC): The warm, oxygenated, and nutrient-rich water in wet towers is a breeding ground for bacteria, algae, and fungi. Some bacteria, like Sulfate-Reducing Bacteria (SRB), produce corrosive byproducts that directly attack metal surfaces.
Chloride Attack: Airborne chlorides (from coastal areas or de-icing salts) are scrubbed from the air and concentrated in the water, which can rapidly degrade stainless steels and other passive alloys.
The Role of the Dry Cooling Tower: A Corrosion Mitigation Strategy
A dry cooling tower tackles these issues at their root by fundamentally changing the heat rejection process from an open, evaporative one to a closed, sensible one.
Corrosion Mechanism In a Wet Cooling Tower In a Dry Cooling Tower (Mitigation Role)
General & Pitting Corrosion High due to abundant oxygen and concentrated corrosive salts. Dramatically Reduced. The primary coolant loop is a closed, pressurized circuit. There is no continuous introduction of fresh oxygen or concentration of salts.
Galvanic Corrosion High risk due to the water acting as an electrolyte between dissimilar metals. Minimized. The closed loop can be filled with a properly inhibited fluid. The external fins (exposed to air) are typically a single material (aluminum or galvanized steel) and are dry, preventing galvanic currents.
Microbiologically Influenced Corrosion (MIC) Very high risk. Biofilms are common and difficult to control. Eliminated. The closed loop is a hostile environment for microbes: no sunlight, limited nutrients, and often contains biocidal inhibitors. The exterior is dry and inhospitable. Scale & Fouling Mineral scale (calcium carbonate) forms as water evaporates and concentrates. Scale creates crevices for under-deposit corrosion. Prevented. Without
evaporation, the chemistry of the closed-loop fluid remains stable. There is no mechanism for mineral scale to form on the internal heat transfer surfaces.
Chloride Stress Corrosion Cracking (CSCC) A severe risk for stainless steel components in wet towers, especially in coastal areas. Virtually Eliminated for the loop. The internal environment is controlled and chloride-free. External components can be specified with coatings or alloys resistant to the atmospheric environment.
Key Advantages for Corrosion Resistance
Closed-Loop Integrity: This is the single biggest factor. The system is sealed and pressurized. The same volume of treated fluid circulates indefinitely, preventing the introduction of new oxygen, contaminants, and salts.
Controlled Fluid Chemistry: The closed loop can be filled with a inhibited water-glycol mixture or specially formulated coolant. These fluids contain:
Oxygen Scavengers: To remove dissolved oxygen.
Corrosion Inhibitors: To form a protective film on metal surfaces.
pH Buffers: To maintain a non-corrosive alkaline pH.
Biocides: To prevent any potential microbial growth during stagnation.
Elimination of Evaporative Concentration: Since there is no water loss, there is no need for “blowdown” (draining concentrated water) or “make-up” (adding fresh water). The corrosive species never build up to dangerous levels. Simplified Maintenance and Monitoring: Corrosion control shifts from a constant battle (testing water, dosing chemicals, managing blowdown) to a periodic check. The fluid in the closed loop may only need analysis and replacement every few years.