Various Heat Exchanger Cleaning Methods

I. Heat Exchanger Classification and Structural Characteristics

Heat exchangers are core equipment for achieving heat exchange in industrial production, widely used in chemical, petroleum, power, metallurgy, food, pharmaceutical, and HVAC industries. Based on structural form, heat exchangers commonly found in industrial applications are primarily classified into plate heat exchangers, shell and tube (tubular) heat exchangers, spiral plate heat exchangers, double-pipe heat exchangers, and air coolers. Due to their unique flow channel structures and heat transfer mechanisms, various heat exchanger types exhibit significant differences in scaling characteristics and cleaning methods. Selecting the correct cleaning solution must be based on a thorough understanding of the equipment structure.

Plate heat exchangers consist of a stack of press-formed corrugated thin metal plates assembled together, with gaskets forming narrow flow channels between plates (plate spacing typically 2 to 6 mm). Hot and cold fluids flow in counter-current on opposite sides of adjacent plates for heat exchange. Plate heat exchangers achieve heat transfer coefficients of 3,000 to 7,000 W/(m²·K), 3 to 5 times that of shell and tube types. Common plate materials include 304 stainless steel, 316L stainless steel, titanium, and Hastelloy. Gasket materials are primarily NBR (nitrile rubber, temperature resistance ≤130°C) and EPDM (ethylene propylene diene monomer, temperature resistance ≤150°C). Their disassemblable structure facilitates cleaning, but the narrow, tortuous flow channels also cause rapid flow capacity decline after scaling. Single-unit heat exchange area typically ranges from 0.5 to 2,000 m², with operating pressure generally not exceeding 2.5 MPa.

Shell and tube heat exchangers (tubular) are the most widely used heat exchanger type in industry, accounting for over 60% of all heat exchanger applications. The structure consists of a shell, heat exchange tube bundle, tube sheets, baffles, and heads. One fluid flows inside the tubes (tube side), while the other flows in the space between the shell and tube bundle (shell side). Tube outer diameters are typically 19 mm, 25 mm, or 38 mm, with wall thicknesses of 1.5 to 3 mm. Tube arrangement patterns include triangular and square pitch, with tube spacing 1.25 to 1.5 times the tube diameter. Shell and tube heat exchangers can withstand pressures above 10 MPa and temperatures exceeding 400°C, suitable for high-temperature, high-pressure conditions, with maximum heat exchange areas reaching 10,000 m². Tube-side cleaning is relatively convenient, but the shell side, due to the presence of baffles and support plates (typically 5 to 15 baffle plates), has numerous cleaning blind spots, making it the primary challenge in cleaning engineering.

Spiral plate heat exchangers are fabricated by coiling two parallel thin metal plates into a pair of concentric spiral channels, with each channel independently sealed. Fluids flow in counter-current through the spiral channels. Spiral channel width is typically 5 to 20 mm, with single channel lengths reaching tens to hundreds of meters. Their advantages include compact structure and self-cleaning capability (secondary flow generated by fluid in curved channels scours the wall surface), with heat transfer efficiency 1.5 to 2 times that of shell and tube types. However, spiral plate heat exchangers have a fully welded sealed structure and cannot be disassembled. Once severe internal scaling occurs, only chemical cleaning can be used, and the extra-long flow channels impose stringent requirements on cleaning pump head (typically 50 to 80 m) and flow rate (30 to 60 m³/h). Operating pressure generally does not exceed 2.5 MPa, with heat exchange areas typically 1 to 300 m².

Double-pipe heat exchangers consist of two tubes of different diameters concentrically assembled. One fluid flows in the inner tube, while the other flows in the annular gap between the inner and outer tubes. Inner tube outer diameters are typically 25 to 89 mm, outer tube inner diameters are 50 to 150 mm, with annular gap widths of approximately 10 to 30 mm. Their structure is simple and cleaning is relatively convenient, but heat exchange area is limited (single unit typically not exceeding 30 m²), and they are mostly used for low-flow or high-pressure applications. Chemical cleaning of the annular space requires special attention to prevent air pockets, ensuring the cleaning solution uniformly fills the entire annular region. Multiple double-pipe units can be connected in series to expand the total heat exchange area.

II. Common Heat Exchanger Scale Types and Causes

During heat exchanger operation, various deposits gradually form on heat transfer surfaces, collectively referred to as fouling. Understanding the types and causes of scale layers is the prerequisite for developing cleaning solutions. Common scale layers primarily include the following five categories:

Water Scale (Crystallization Scale): Calcium and magnesium ions in cooling water or heating media precipitate as temperature increases and solubility decreases, forming hard crystalline deposits of Calcium Carbonate (CaCO₃), Calcium Sulfate (CaSO₄), and Magnesium Silicate. The thermal conductivity of water scale is only 0.5 to 2.3 W/(m·K), far below carbon steel's 45 W/(m·K). Just 0.5 mm of water scale can reduce heat transfer efficiency by over 10%. In high-temperature zones above 80°C, CaSO₄ exhibits reverse solubility (decreasing solubility with increasing temperature), causing the scaling rate to accelerate dramatically.

Corrosion Product Scale: Carbon steel tube walls generate Iron Oxide (Fe₂O₃/Fe₃O₄) under the action of water and dissolved oxygen, while copper tubes generate Copper Oxide. These loose, porous corrosion layers not only add thermal resistance themselves but also adsorb suspended solids from water, forming multi-layer composite scale. In chloride-containing environments (Cl⁻>100 ppm), stainless steel may develop corrosion product accumulation within pits. The generation rate of corrosion product scale is closely related to water pH, dissolved oxygen concentration, and temperature — corrosion rate accelerates significantly when pH<6 or >9, with every 10°C temperature increase, and when dissolved oxygen exceeds 4 ppm.

Biofilm/Slime Scale: Bacteria, algae, and fungi in cooling water multiply rapidly at suitable temperatures (20 to 40°C), secreting polysaccharides that bind microbial cells and inorganic suspended particles into a gelatinous slime layer. Biofilm has extremely low thermal conductivity (approximately 0.5 W/(m·K)) and is acidic (metabolic products contain organic acids), causing under-deposit corrosion. Open-loop circulating cooling water systems are particularly prone to biofilm growth — taking a cooling water system with 10,000 m³/h circulation rate as an example, biofilm thickness can increase to 0.5 mm within one week during high-temperature summer periods.

Process Media Scale: In chemical and petroleum refining processes, heavy components in process media (such as tar, polymers, asphaltenes, resins, etc.) deposit and carbonize on heat transfer surfaces, forming dense organic scale layers. Heat exchangers in refinery crude distillation units, cokers, and ethylene crackers are high-incidence areas for process media scale. This type of scale has high hardness and strong adhesion, typically requiring combined cleaning processes of high-temperature alkaline boiling (NaOH 3% to 5%, 70 to 90°C, 12 to 24 hours) followed by acid cleaning for effective removal.

Particulate Deposition Scale: Suspended solids such as sand, catalyst powder, and rust particles carried in fluids settle and accumulate in low-velocity zones (such as the leeward side of shell-side baffles and tube sheet surfaces), forming particulate scale. Particulate scale formation is closely related to flow velocity — when tube-side velocity is below 0.5 m/s or shell-side velocity is below 0.2 m/s, the particle settling rate increases significantly. Although particulate scale itself is loose, it can serve as a growth substrate for other scale layers, accelerating composite scale formation.

III. Comparative Analysis of Various Cleaning Methods

For different heat exchanger types and scale characteristics, the industrial cleaning field has developed multiple technical approaches including chemical cleaning, high-pressure water jet cleaning, ultrasonic cleaning, mechanical cleaning, online sponge ball cleaning, and dry ice cleaning. Each method has its own pros and cons in terms of cleaning principle, applicable scope, removal effectiveness, and cost. In practical engineering, comprehensive selection is required based on equipment structure, scale properties, and safety and environmental requirements. The following provides a systematic comparison of mainstream cleaning methods across six dimensions:

IV. Targeted Cleaning Processes for Various Heat Exchanger Types

Plate Heat Exchanger Deep Cleaning Process: The cleaning effectiveness of plate heat exchangers highly depends on standardized operational procedures. Step 1: Before disassembly, number the plate corners and gasket positions sequentially and photograph the original stacking order — this is a critical step to prevent reassembly errors. Step 2: After disassembling plates one by one, use 500 to 800 bar HP water jet fan nozzles to flush the corrugated plate surfaces, maintaining a distance of over 300 mm between the nozzle and plate surface to prevent plate deformation. Step 3: Immerse plates in a cleaning tank containing 5% to 8% Sulfamic Acid + 0.3% BTA corrosion inhibitor at 40 to 50°C for 2 to 4 hours; for 304 stainless steel plates, substitute with 3% to 5% HNO₃ solution; for 316L stainless steel plates, 8% to 10% Sulfamic Acid can be used. Step 4: Thoroughly rinse with clean water to neutral (pH 6.5 to 7.5), individually rinsing each plate to ensure no residual acid in corrugated grooves. Step 5: Inspect gaskets plate by plate — replace any gaskets showing hardening (Shore hardness >80A), cracking, or permanent compression deformation >30%; use specialized adhesive to position and cure new gaskets for over 30 minutes. Step 6: Reassemble in numbered sequence, symmetrically and uniformly tightening clamping bolts in steps to design torque (typically 200 to 400 N·m); after tightening, measure the total plate pack length against the nameplate value with deviation not exceeding ±1%. If oil contamination is present, add an 80°C alkaline degreasing step (2% to 3% NaOH + 0.5% Surfactant, soaking for 4 to 6 hours) before Step 3 acid cleaning.

Shell and Tube Heat Exchanger Tube-Side and Shell-Side Stepwise Cleaning: Tube-side cleaning preferably uses HP water jetting for individual tube flushing, employing rotating tube cleaning nozzles at working pressure of 700 to 1,000 bar, flow rate of 40 to 60 L/min, self-advancing speed of approximately 0.3 to 0.8 m/s, effectively stripping tube wall water scale and rust scale. For severe scaling, chemical circulation softening can be performed first: seal one end of the tube-side outlet, inject 5% to 8% HCl + 0.3% Urotropine + 0.1% BTA composite inhibited cleaning solution, circulate at 50 to 60°C for 6 to 10 hours, maintaining flow velocity at 0.5 to 1.0 m/s for adequate scouring, then thoroughly flush residual scale debris with HP water. Shell-side cleaning is the greatest challenge in shell and tube heat exchanger cleaning — a temporary circulation loop must be constructed at the shell-side inlet and outlet flanges, using an acid-resistant pump to drive cleaning solution circulation within the shell-side space. The shell side has a large volume with many baffles, requiring circulation flow velocity to be increased above 1.0 m/s (corresponding to approximately 30 to 50 m³/h for DN200 shell side) to ensure the cleaning solution reaches all dead zones. For severely coked organic scale on the shell side, first perform alkaline boiling with 3% to 5% NaOH solution at 80 to 90°C for 12 to 24 hours, using saponification and dispersion to soften the coke layer, then drain the alkaline boil solution and proceed with acid cleaning, which can significantly improve cleaning effectiveness.

Spiral Plate Heat Exchanger Forced Circulation Cleaning: Since spiral plates have a fully welded non-disassemblable structure with flow channels reaching tens of meters in length, chemical cleaning is the only viable deep cleaning method. Before cleaning, accurately measure the pressure drop of both channels — during normal operation, pressure drop is typically 0.05 to 0.10 MPa; if one channel's pressure drop exceeds 0.20 MPa, this indicates severe blockage. It is recommended to use a composite formulation of 8% to 10% Sulfamic Acid + 3% Citric Acid + 0.5% BTA + 0.1% Surfactant, with forced circulation cleaning at 55 to 65°C for 8 to 12 hours. The cleaning pump outlet pressure must be maintained at 0.3 to 0.5 MPa to ensure the flow velocity at the far end of the long channel is no less than 0.3 m/s (preventing secondary deposition of solid scale debris within the channel). The circulation direction must be switched every 2 hours (alternating forward and reverse), utilizing the turbulence generated by flow direction changes to strip wall scale layers — each direction should complete no fewer than 3 cycles. The cleaning endpoint is indicated when the cleaning solution acid concentration and iron ion concentration (Fe³⁺<50 ppm with <5% change over 1 hour) show no further change for 1 continuous hour. For scale with high silicate content, 3% to 5% Ammonium Bifluoride can be added to the formulation to enhance complexation and dissolution of SiO₂. After cleaning, flush with deionized water until conductivity drops below 50 μS/cm, followed by alkaline neutralization and passivation treatment at pH 9 to 10 (NaNO₂ 1% to 2%, circulating for 4 hours).

Double-Pipe Heat Exchanger Annular Gap Cleaning: The annular gap between inner and outer tubes has a uniform cross-sectional area along its length, making chemical cleaning relatively straightforward. Connect the cleaning circuit to both ends of the inner tube and the annular gap inlet/outlet of the outer tube, with cleaning solution entering from the low point and exiting from the high point of the annular gap to facilitate air venting. Use 5% to 8% HCl + 0.3% Urotropine corrosion inhibitor for circulation cleaning, with flow velocity no less than 0.5 m/s, temperature 50 to 60°C, circulating for 4 to 8 hours. The annular gap volume is small, requiring minimal cleaning solution (typically only 10% to 20% of tube-side cleaning volume), making it economical. The most common problem in annular gap cleaning is air pockets — if air pockets exist in the annular gap, localized areas will not contact the cleaning solution. Installing an automatic air vent valve at the highest point effectively resolves this. For double-pipe heat exchangers with downstream U-bends connected in series, pay special attention to scale debris accumulation at the bottom of the bends; short-term flow velocity increase to 1.5 m/s for pulse flushing at the end of cleaning can be effective.

V. Heat Exchanger Cleaning Method Quick Selection Guide

VI. Typical Engineering Case Studies

Case 1: Chemical Plant Shell and Tube Heat Exchanger Shell-Side Coking Cleaning. This equipment was used for high-temperature thermal oil and process media heat exchange. After 2 years of operation, the shell side was severely coked, with inlet-outlet differential pressure rising from the normal 0.05 MPa to 0.25 MPa and heat exchange efficiency declining by over 40%. First, 5% NaOH solution was injected into the shell side for alkaline boiling at 85°C for 18 hours, after which a large amount of black coke particle suspension was discharged. Subsequently, the system was switched to an 8% HCl + 0.3% Urotropine + 0.1% Lan-826 corrosion inhibitor cleaning solution, circulating at 60°C for 10 hours, with circulation direction switched every 3 hours. After cleaning, shell-side differential pressure recovered to 0.06 MPa and the heat transfer coefficient recovered to 92% of new equipment. The corrosion rate of the carbon steel shell, tested by coupon method, was only 0.35 g/(m²·h), far below the national standard (≤6 g/(m²·h)).

Case 2: Food Factory Plate Heat Exchanger Composite Scale Cleaning. This stainless steel plate heat exchanger was used for milk preheating and cooling in the pasteurization process. After 6 months of operation, due to dual contamination from water-side scale and product-side milk stone, heat exchange efficiency declined by 35%. After plate disassembly, warm water at 50 to 60°C was first used to flush away loose milk stone, then plates were immersed in a cleaning tank containing 5% Sulfamic Acid + 0.3% BTA at 40°C for 3 hours to remove water scale. After plate cleaning, 0.5% to 1% HNO₃ solution was used for passivation treatment, forming a dense chromium oxide passive film to restore stainless steel corrosion resistance. All gaskets were replaced with food-grade EPDM gaskets, and after reassembly in original order, the hydrostatic test passed. After cleaning, heat exchange efficiency recovered to new equipment levels, and product microbiological indicators showed no abnormalities.

Case 3: Power Plant Spiral Plate Heat Exchanger Silicate Scale Cleaning. This spiral plate heat exchanger was used for geothermal water waste heat recovery, with a single channel length of approximately 45 m and channel width of 12 mm. Due to SiO₂ content in the geothermal water reaching as high as 120 ppm, after 14 months of operation, the A channel pressure drop rose from 0.08 MPa to 0.32 MPa (severe blockage), and flow velocity dropped from the design value of 1.2 m/s to 0.4 m/s. A composite formulation of 8% Sulfamic Acid + 3% Citric Acid + 5% Ammonium Bifluoride + 0.5% BTA was adopted, with forced circulation cleaning at 60°C for 16 hours (standard cleaning time extended due to extremely high silicate scale hardness), cleaning pump outlet pressure at 0.4 MPa, direction switched every 3 hours. After cleaning, channel pressure drop recovered to 0.10 MPa and flow velocity recovered to 1.1 m/s. Waste discharge analysis showed the cleaning solution dissolved approximately 2.3 kg of total SiO₂, verifying the efficient complexation and dissolution capability of NH₄HF₂ for silicate scale.

VII. Cleaning Strategy Selection Principles and Preventive Maintenance

Selecting the appropriate cleaning solution requires comprehensive consideration of six key factors: First is the heat exchanger type and disassemblability — this is the primary constraint determining cleaning method feasibility; second is scale category and thickness — XRD analysis of scale samples and hydrochloric acid solubility testing can accurately determine the optimal cleaning formulation; third is the chemical corrosion resistance of equipment materials — stainless steel, copper alloys, and titanium each have compatible inhibitor systems and temperature limits; fourth is on-site construction conditions — whether space is adequate, whether water source and 380V power supply are available, and whether waste liquid has compliant discharge channels; fifth is safety and environmental regulations — chemical waste liquids must be neutralized for compliant discharge (pH 6 to 9, COD<100 mg/L), and HP water operations must have safety warning zones established; sixth is economic cost — including direct costs (chemicals, labor, water/electricity) and indirect costs (downtime losses, disassembly/reassembly labor).

In many practical engineering cases, a single cleaning method struggles to achieve ideal results, and the optimal strategy often involves combining two or more technologies — for example, first chemically circulating to soften the scale layer, then using HP water jetting for thorough removal. This can shorten chemical cleaning time by 30% to 50%, reduce chemical consumption by over 40%, and simultaneously lower the required HP water pressure. From a long-term perspective, it is even more effective to establish a preventive maintenance system: For open-loop circulating cooling water systems, regularly dose scale inhibitors (organophosphonate type) and biocides (isothiazolinone type), control concentration cycles at 3 to 5 times, and test full water quality indicators monthly; for the process media side, set early warning thresholds based on differential pressure and temperature change trends — schedule cleaning when differential pressure rise exceeds 25% of design value or heat exchange efficiency decline exceeds 15%, rather than waiting until the equipment is completely blocked for emergency treatment. The investment in regular cleaning is far lower than the long-term energy waste caused by reduced heat exchange efficiency — taking a shell and tube heat exchanger with 500 m² heat exchange area as an example, the additional annual energy cost from 1 mm of water scale can reach ¥150,000 to ¥250,000, while the cost of one professional chemical cleaning is typically only 1/5 to 1/3 of that amount. Danyang Blue Star Anti-corrosion Cleaning Co., Ltd. possesses 20 years of industrial cleaning experience and can provide customized cleaning solutions based on the actual operating conditions of various heat exchangers, undertaking cleaning projects nationwide. Please call for consultation.

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