I. Project Background and Production Line Baseline Specifications

This case study involves a natural gas-fired, direct-heating, environment-friendly hot-dip galvanizing production line destined for the South American market, with steel structural parts as the primary processing target. The zinc kettle has internal dimensions of 4.7 meters in length, 1.0 meter in width, and 1.8 meters in depth. These dimensions directly determine the rated zinc capacity of 50 tons and constrain the maximum hourly output to approximately 2 tons. The entire production line is modularly designed to meet ocean freight export standards, with a total weight of approximately 159.7 tons, corresponding to a shipping arrangement of six 40-foot high-cube containers plus one 20-foot general-purpose container.

The electrical system must accommodate the local South American grid conditions – 220V/60Hz three-phase power. This frequency differs from the domestic 380V/50Hz standard, requiring all motors, variable frequency drives, and PLC control units to be re-calibrated to 60Hz to ensure that timing, rotational speed, and communication protocols match the local power supply characteristics.

II. Heating System and Gas Supply Parameters

The heating energy source is piped natural gas. The technical specification imposes clear limits on gas quality: the calorific value must fall within the range of 8,400 to 8,600 multiplied by 4.18 kJ/m³, the inlet pressure must be maintained between 100 and 200 kPa, and the gas node pressure is controlled between 90 and 80 kPa. These boundary conditions directly influence burner selection.

The combustion section is equipped with four high-speed pulse burners, model TJ100. Each burner has an instantaneous gas consumption of approximately 28 m³/h, bringing the total instantaneous gas consumption to 112 m³/h. To ensure continuous production without interruption from gas supply fluctuations, the gas pipeline adopts a dual-main-inlet configuration with one unit in service and one on standby, allowing online switching. The ignition system supports both automatic and manual high-low dual modes – manual fine-tuning is available during commissioning, while automatic tracking is engaged during normal production. The furnace operates under a micro-positive pressure combustion mode, designed to prevent cold air backflow and maintain thermal stability within the furnace.

III. Process Tank Materials and Full-Line Layout

All pre-treatment and post-treatment tanks share unified external dimensions of 4.7 meters in length, 1.0 meter in width, and 1.8 meters in depth, consistent with the zinc kettle dimensions. This design decision simplifies crane path planning, allowing hoist travel and positioning points to be reused within a single coordinate framework. The tanks are constructed with steel structural frames and 5mm steel plates as the base material, lined with fiberglass-reinforced plastic (FRP) anti-corrosion layers to resist chemical attack from acids, alkalis, and fluxing salts.

The pickling section comprises four enclosed pickling tanks. The immersion time per tank is adjustable between 6 and 30 minutes, depending on the surface oxidation condition of the workpieces. The pickling room operates under negative pressure, with ground rail carts used for material loading and unloading, ensuring that acid mist does not diffuse into the workshop during operation. After pickling, workpieces proceed sequentially to two rinsing tanks for overflow rinsing, then enter the fluxing tank. There is one fluxing tank, with the operating temperature maintained between 50 and 70°C and pH controlled at approximately 5. An automatic iron removal device limits the ferrous ion concentration to within 1 mg/L to ensure flux bath activity. The cooling section includes one cooling tank, paired with a fiberglass-reinforced plastic cooling tower for circulating water cooling. The heat released during cooling is directed to the acid preheating system, achieving complementary thermal integration. The passivation section also consists of one tank, employing a chrome-free passivation process to meet export environmental requirements.

IV. Acid Mist and Zinc Fume Treatment System Configuration

The enclosed pickling room, combined with negative-pressure extraction, directs acid mist to the treatment system. The acid mist first passes through a dilution stage – the dilution water is also used for preparing fresh acid, a design that reduces alkali consumption – and then enters the acid mist absorption neutralization tower. The neutralization tower is equipped with an automatic dosing device and is driven by a 22 kW variable-frequency induced draft fan. pH detection probes are installed at both the inlet and outlet to monitor pH changes before and after neutralization in real time, providing a basis for dosing adjustments.

Zinc fume treatment begins above the zinc kettle, using a double-sided suction capture hood that covers both lateral sides of the kettle width. The captured zinc fumes pass sequentially through a bag filter for particulate removal and then through a water-curtain deodorization stage to remove odorous components, finally being discharged by a 37 kW variable-frequency induced draft fan. The fan is started 2 to 3 minutes before the workpieces are lowered into the kettle, to establish sufficient suction negative pressure during the initial phase of heavy zinc fume generation.

V. Cascaded Waste Heat Utilization System (Core Technology Module)

The most significant engineering feature of this production line is its five-stage cascaded waste heat recovery from zinc kettle combustion flue gas, with temperatures progressively decreasing from approximately 500°C at the furnace outlet to near ambient conditions. The following sections describe each utilization level in descending temperature order.

Stage 1: Fluxing liquid heating. The flue gas exits the furnace at approximately 500°C and first enters a stainless steel heat exchanger, where it heats circulating water to approximately 90°C. This hot water is then sent to a titanium alloy heat exchange coil inside the fluxing tank, raising the fluxing liquid from ambient temperature to the process setpoint of 50 to 70°C. Titanium alloy is a mandatory material choice here, as the fluxing solution contains zinc chloride and ammonium chloride, which are corrosive to ordinary stainless steel.

Stage 2: Workpiece drying. After the first-stage heat exchange, the flue gas temperature drops to approximately 300°C and then enters the drying tank section. The heat exchange pipe used here is a spiral tube with an outer diameter of 630 mm and a wall thickness of 8 mm. The larger surface area facilitates waste heat release. A fan simultaneously blows the hot air from near the pipe surface onto the workpieces, preheating them before they enter the zinc kettle, thereby reducing thermal losses from the zinc bath.

Stage 3: Combustion air preheating (patented). As the flue gas continues to cool to approximately 200°C, it enters the combustion air preheater – a device for which a patent has been obtained. The flue gas heats the combustion air required by the burners from ambient temperature to 50 to 150°C. This preheated combustion air, when delivered to the furnace, increases the theoretical combustion temperature. The design estimates a gas saving of 2 to 3 m³ per ton of galvanized parts – actual savings depend on site-specific operating conditions. However, the qualitative effect of each 100°C rise in preheated air temperature on flame propagation speed is well established in combustion science as positively contributing to thermal efficiency.

Stage 4: Sludge drying (patented). Above the flue gas duct of the combustion air heat exchanger, a 316L stainless steel sludge drying tank measuring 6,000 mm in length, 750 mm in width, and 375 mm in height is installed, utilizing flue gas waste heat at approximately 150 to 200°C. This device is also a patented technology. It processes filter press sludge with an initial moisture content of approximately 80%, drying it down to approximately 10% moisture content, thereby significantly reducing off-site disposal weight.

Stage 5: Pot-edge waste heat recovery and workshop heating. The surface thermal radiation at the zinc kettle edges, where temperatures reach approximately 200°C, is recovered by a stainless steel double-layer coil. The recovered heat is used to heat circulating water for purposes such as bathing, acid preheating, or workshop space heating. The low-grade waste heat remaining before final discharge to the chimney can also be directly released into the workshop through steel pipes, though burn protection measures must be implemented.

VI. Lifting, Logistics, and Automation Control

Workpiece lifting employs a 3-ton plus 3-ton dual-hoist configuration. Four sets of hoists of this specification are installed in the pickling area, mounted on the exterior of the pickling room roof, with hooks extending into the room for operation via wireless remote control. Workpieces are first manually attached to large racks using a lifting frame, raised to a fixed height, then transported by ground rail cart into the pickling room. Overhead cranes inside the room lift the large racks into the acid tanks, sequentially processing through each tank station.

The logistics solution provides two automation options for the buyer: one is a gantry electric hoist with remote control operation; the other is a ring-rail electric hoist with RGV carts, which can control immersion time and galvanizing temperature via PLC programs. The programmed control approach prevents operators from extending immersion time based on individual experience, thereby avoiding excessively thick zinc coatings that waste material and increase costs.

VII. Electrical Load Distribution and Control Logic

The total installed electrical capacity of the entire production line is approximately 205 kW. The acid mist induced draft fan is rated at 22 kW, and the zinc fume induced draft fan is rated at 37 kW – both are equipped with variable frequency drives to accommodate varying extraction demands under different operating conditions. The overhead crane motor is rated at 15 kW, the iron removal equipment at 15 kW, the air compressor at 7.5 kW, the three spray pumps at 5.5 kW each, and the hoist motors total 30 kW. A 20 kW reserve is allocated for future expansion or temporary loads.

Zinc bath temperature control employs a 3-stage PID regulation loop, with temperature signals acquired via thermocouples and displayed on a large screen. The target control range is 438 to 450°C. The system automatically adjusts burner output based on the deviation between measured temperature and setpoint, eliminating the need for frequent manual intervention.