Corrosion Resistant Air Preheater Lined Tubes
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Solution Overview
Problem
Tubular air preheaters in thermal power stations face significant dew point corrosion due to cold ambient air cooling the metal heat exchange tubes below the acid dew point, leading to premature tube failure and increased maintenance costs, with existing solutions either consuming more energy or using materials with low thermal conductivity that crack under thermal expansion.
Innovation Solution
A double-wall carbon steel tubing structure with inner liners maintains the metal temperature above the dew point of corrosive acids by using high thermal conductivity materials and an air pocket to protect the inner liners from rapid heat loss, reducing corrosion and fly ash fouling without compromising thermal efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If carbon steel tubes are used for heat exchange, then thermal efficiency is improved due to high thermal conductivity, but the tubes suffer from dew point corrosion when temperature drops below acid saturation point
Solution Approach 1:
The patent applies composite materials by combining carbon steel (for high thermal conductivity) with a protective lining material (for corrosion resistance). The lined tube structure allows the carbon steel to maintain thermal efficiency while the lining protects against dew point corrosion when the tube temperature drops below the acid saturation point.
Solution Approach 2:
The protective lining acts as an intermediary between the corrosive flue gas environment and the carbon steel tube. This intermediate layer prevents direct contact between the corrosive chemicals and the metal surface, eliminating dew point corrosion while allowing the carbon steel to maintain its high thermal conductivity for efficient heat exchange.
2Duration of action of stationary object
If stainless steel tubes are used to resist corrosion, then tube life is improved, but thermal efficiency decreases due to low thermal conductivity causing tubes to sweat and trap fly ash
Solution Approach 1:
Instead of using stainless steel alone, the patent creates a composite structure with carbon steel (high thermal conductivity) as the base material and a protective lining (corrosion resistance) as the surface layer. This composite approach achieves both high thermal efficiency and corrosion resistance, eliminating the need to sacrifice thermal performance for durability.
Solution Approach 2:
The patent applies local quality by providing corrosion protection only where needed - on the inner surface of the tube that contacts the flue gas. The carbon steel bulk material maintains its high thermal conductivity throughout, ensuring overall thermal efficiency is not compromised while the localized lining provides durability.
3Strength
If stainless steel tubes are used for corrosion resistance, then tube durability is improved, but the tubes crack under thermal expansion due to low thermal conductivity
Solution Approach 1:
The composite structure combines carbon steel with high thermal conductivity and appropriate thermal expansion properties with a protective lining. This allows the tube to handle thermal expansion stresses without cracking while the lining provides corrosion resistance, achieving both durability and thermal compatibility.
Solution Approach 2:
The protective lining is applied locally to the inner surface where corrosion occurs, while the bulk carbon steel material handles the thermal expansion and mechanical stresses. This localized approach allows each material to perform its optimal function without compromising the other.
Data Source
AI summary
A dew point corrosion resistant heat exchanging system having a plurality of hollow heat transferring tubes through which cooler ambient air or hot combustion product gases flow. The other of the air or gas flows across the outer surfaces of the tubes, and heat is transferred from the hot gases to the ambient air, thus heating the air. A portion of the tubes includes an inner liner forming an air pocket chamber between the liner and the outer wall of the tube. The air pocket chamber provides heat transfer advantages that maintain the tubes at a temperature above the dew point of the gases in the system, thus inhibiting corrosion of the tubes.