What factors affect the size of the fouling thermal resistance of stainless steel heat exchange tube

The fouling thermal resistance of stainless steel heat exchange tubes (referring to the thermal resistance generated by the fouling layer deposited on the tube wall surface, measured in m²・K/W) is a key indicator for evaluating the degree of fouling on the heat exchange tubes. Its magnitude directly affects the heat transfer efficiency (the greater the fouling thermal resistance, the smaller the heat transfer coefficient K). The formation and accumulation of fouling thermal resistance are influenced by multiple factors in combination, mainly falling into four categories: fluid characteristics, operating conditions, the inherent characteristics of the heat exchange tubes, and maintenance measures. The specific details are as follows: 

I. Fluid Characteristics: Determining the "Source" and "Nature" of Scale Formation

The composition, state, and chemical properties of the fluid are the fundamental factors that determine the type and deposition rate of scale formation (such as crystalline scale, corrosive scale, biological scale, etc.).

1. Content of impurities and dissolved substances in the fluid

Suspended particles: Suspended particles such as sand, dust, metal debris, fibers, etc. in the fluid will settle due to gravity or decrease in flow velocity and adhere to the pipe wall, forming particulate scale. For example, untreated river water or industrial wastewater (containing a large amount of sand) flowing through stainless steel heat exchange tubes will have a much faster particle deposition rate than pure water, resulting in a rapid increase in fouling thermal resistance.

Dissolved salts: Salts such as calcium, magnesium, silicon, and sulfate (such as CaCO₃ and MgCO₃ in hard water) that are dissolved in the fluid are prone to crystallize and precipitate when the temperature changes (such as when the solubility decreases during heating) or when evaporated and concentrated, forming crystalline scale (scale deposits). For example, calcium and magnesium ions in boiler feed water crystallize on the surface of high-temperature heat exchange tubes, and the 0.1mm thick scale has a thermal resistance of 0.0002-0.0005 m²・K/W (approximately 10-20 times that of the stainless steel pipe wall thermal resistance).

Organic substances and colloids: Organic substances such as oils, proteins, humus, or colloidal particles (such as lignin in paper black liquor) will form viscous scale due to adsorption and polymerization on the pipe wall. For example, sugar solutions and dairy fluids in food processing, organic substances are prone to deposit on the pipe wall, and such scale has an extremely low thermal conductivity (λ ≈ 0.1-0.3 W/(m・K)), resulting in a greater thermal resistance.

Corrosive substances: Corrosive components such as acids, bases, chloride ions, etc. in the fluid will react chemically with the stainless steel pipe wall (even if stainless steel has strong corrosion resistance, long-term contact with high-concentration corrosive media may still cause local corrosion), generating corrosion products (such as iron oxide, chromates), forming corrosive scale. For example, seawater with Cl⁻ concentration exceeding 500 ppm may cause local pitting corrosion of stainless steel, and the deposition of corrosion products increases the fouling thermal resistance. 

II. Operating Conditions: Determining the "Deposition Rate" and "Stability" of Scale

The actual operating parameters of the heat exchange tubes (temperature, flow rate, time, etc.) directly affect the formation speed and accumulation degree of scale, and are the "dynamic drivers" of the change in scale thermal resistance.

1. Temperature and Temperature Difference

Tube wall temperature: High temperatures accelerate the crystallization of salts (such as CaCO₃, whose solubility drops sharply above 60°C) and the decomposition and polymerization of organic substances (such as fats, which carbonize above 100°C), promoting scale deposition. For example, when the tube wall temperature exceeds 80°C, the deposition rate of scale is 3-5 times that at normal temperature.

Heat transfer temperature difference (Δt): The greater the temperature difference between the cold and hot flows, the more significant the temperature gradient between the tube wall and the fluid, which is likely to cause local supersaturation of the fluid near the tube wall (for example, when heated, the fluid near the tube wall reaches the crystallization temperature first), accelerating the formation of crystalline scale. For instance, in a working condition with a temperature difference of 50°C, the increase in scale thermal resistance is 2-3 times faster than that in a condition with a temperature difference of 20°C.

Phase change influence: If the fluid undergoes phase change during heat exchange (such as steam condensation, liquid boiling), it will exacerbate scale deposition. For example, during steam condensation, trace impurities (such as silicates) will be concentrated due to water evaporation and form a hard deposit layer on the tube wall; during liquid boiling, the "disturbance" of bubbles detaching from the tube wall may carry impurities and form scale.

2. Fluid Flow Rate

Flow rate is a key parameter affecting scale deposition, and it influences the scale thermal resistance through the "scouring effect" and "residence time" dual effects:

Low flow rate (＜1m/s, water medium): The fluid turbulence is weak, and particle impurities are prone to settle due to gravity and adhere to the tube wall. Moreover, the low flow rate makes it difficult for scale to be washed away, resulting in a fast deposition rate. For example, at a flow rate of 0.5m/s, the growth rate of scale thermal resistance is 4-5 times that at a flow rate of 2m/s.

High flow rate (＞3m/s, water medium): The turbulence is intense, and the scouring effect can inhibit scale deposition, but an excessively high flow rate will increase fluid resistance (energy consumption increases) and may cause tube wall wear (especially for particle-containing fluids), resulting in the formation of new scale due to the debris produced by wear.

Flow rate fluctuation: Frequent changes in flow rate (such as pump start-stop, flow regulation) will disrupt the stability of the fluid flow pattern, causing loose deposited scale to fall off, while promoting the attachment of new impurities, leading to an increase in the fluctuation of scale thermal resistance.

3. Operating Time and Cycle

The scale thermal resistance increases in a "first fast then slow" trend with operating time: In the initial stage, the tube wall surface is clean, and impurities are prone to adhere, causing the thermal resistance to rise rapidly; in the later stage, the scale layer gradually becomes dense, new impurities are difficult to embed, and the growth rate slows down (until reaching "dynamic equilibrium", where the deposition and detachment rates are equal). For example, for a stainless steel heat exchange tube operating continuously for 1 month, the scale thermal resistance may reach 0.0003-0.0008 m²・K/W; after 6 months of operation, it may increase to 0.001-0.002 m²・K/W (specifically depending on the working conditions). 

III. Characteristics of Heat Exchange Tubes: Impact on the "Difficulty of Adhesion"

The material, surface condition, and structural design of stainless steel heat exchange tubes determine whether dirt is easily adhered and accumulated, which is a "fundamental factor" affecting the thermal resistance of dirt.

1. Surface Finish

The surface roughness (Ra value) of stainless steel heat exchange tubes directly affects the ability of dirt to adhere:

High finish surface (Ra ≤ 0.8 μm, such as polished treatment): The surface is smooth, the contact area between impurities and the tube wall is small, the adhesion is weak, and dirt is easily washed off by the fluid, resulting in a low dirt thermal resistance. For example, the dirt thermal resistance of polished 316L stainless steel tubes is 30%-50% lower than that of un-polished tubes (Ra = 3.2 μm).

Rough surface (Ra ≥ 1.6 μm, such as rolled without treatment, welding marks): The surface is uneven, prone to "vortex dead zones", impurities deposit in the recesses, and the rough surface provides more "adhesion points", making it difficult to remove dirt, and the thermal resistance is greater.

2. Material and Surface Treatment

The corrosion resistance and surface energy (affecting hydrophilicity / hydrophobicity) of stainless steel affect the type of dirt and deposition speed:

Corrosion resistance difference: 316L stainless steel (containing Mo) has stronger resistance to chloride ion corrosion than 304 stainless steel. In high-salt environments, 304 tubes have more corrosion deposits due to local corrosion, resulting in a higher dirt thermal resistance.

Surface modification treatment: Through passivation (forming a dense oxide film), coating (such as polytetrafluoroethylene coating), or electrolytic polishing, the surface energy can be reduced, the hydrophobicity can be enhanced, and dirt adhesion can be reduced. For example, the dirt (such as algae, bacterial films) attachment amount on stainless steel tubes after passivation treatment is 40%-60% less than that of untreated tubes.

3. Tube Type and Structure

Different tube types have different dirt deposition characteristics due to different flow patterns:

Special-shaped tubes (wavy tubes, spiral groove tubes): The fluid flow in the tubes is turbulent (Re is high), with strong turbulence and large disturbances, making dirt less likely to deposit, and the dirt thermal resistance is 20%-40% lower than that of straight tubes. For example, the wavy structure of the wave-shaped tubes causes the fluid to generate vortices, continuously washing the tube wall, inhibiting dirt accumulation.

Straight tubes: The flow is stable. If the flow velocity is not uniform (such as near the tube plate), a local "dead zone" is likely to form, resulting in concentrated dirt deposition, and the local dirt thermal resistance is much higher than that in the middle of the tube body. 

IV. Maintenance and Pre-treatment Measures: The "Control Effect" on Fouling Thermal Resistance

The quality of fluid pre-treatment and the frequency of maintenance cleaning of heat exchange tubes directly affect the "initial formation" and "late removal" of fouling, and are "human intervention factors" for reducing fouling thermal resistance.

1. Fluid Pre-treatment

Pre-treating the fluid entering the heat exchange system can reduce impurity content and lower fouling thermal resistance at the source:

Filtering: Removing suspended particles (such as sand, fibers) through filters and sedimentation tanks can reduce particulate fouling. For example, the fouling thermal resistance of heat exchange tubes after filtering river water with a 50μm filter is more than 60% lower than that of unfiltered water.

Softening treatment: Softening high-hardness water (with high calcium and magnesium ion content) using methods such as ion exchange and reverse osmosis can reduce the formation of crystalline fouling (scale). For example, the fouling thermal resistance of water after softening (total hardness <50mg/L) on the heat exchange tube surface is only 1/5 - 1/3 of that of unsoftened water.

Bactericidal and algal control: Adding bactericides (such as chlorine preparations, ozone) to fluids containing microorganisms (such as circulating cooling water) can inhibit bacterial and algal reproduction and reduce biological fouling. For example, a circulating water system that is regularly sterilized has a biological fouling thermal resistance that is 50% - 70% lower than that of untreated systems.

2. Cleaning Frequency and Method

Regular cleaning can remove deposited fouling and restore the performance of heat exchange tubes:

Cleaning cycle: Unregularly cleaned heat exchange tubes have fouling thermal resistance that continuously increases over time; while cleaning once every 1-3 months (depending on operating conditions) can control the fouling thermal resistance at a lower level (such as <0.0005 m²·K/W).

Cleaning method: Chemical cleaning (acid washing, alkaline washing) can effectively remove scale and corrosion scale, but excessive cleaning may damage the surface of stainless steel (such as acid washing that is not done properly causing the passivation film to be destroyed); physical cleaning (high-pressure water flushing, mechanical scraping) is suitable for loose fouling and has little damage to the tube wall, but is difficult to remove dense fouling. For example, a combined method of chemical cleaning and high-pressure water flushing has a 20%-30% higher descaling efficiency than a single method, and the reduction in fouling thermal resistance is more significant. 

Summary

The size of the fouling thermal resistance of the stainless steel heat exchange tubes is the result of the combined effects of fluid characteristics (type and content of impurities), operating conditions (temperature, flow rate, time), heat exchange tube characteristics (surface condition, structure), and maintenance measures (pre-treatment and cleaning). Among them, the impurity content in the fluid, temperature/flow rate control, and surface smoothness are the most significant core factors. In practical applications, by optimizing fluid pre-treatment (reducing impurities), designing a reasonable flow rate (1-2 m/s for water medium), selecting high-polish stainless steel tubes (such as polished 316L), and conducting regular cleaning, the fouling thermal resistance can be effectively controlled (typically, it should be kept below 0.0001-0.0005 m²・K/W), ensuring the long-term stability of heat exchange efficiency.