Top
top banner

 

 

6.Hot Dip Galvanizing Department

 

6.1.Main plants of Hot Dip Galvanizing department

6.1.1.Pre-Heating Dryer

This is a high-temperature dryer, where the hot air is used for a complete drying and a considerable preheating of items to be hot-dipped, in order to allow the fast dipping into the melted zinc. Before being hot-dipped, the drying and preheating of items allow to obtain some benefits, such as:

1) Shorter time of dipping and shorter dipping cycles = more cycles per day.

2) Shorter time of dipping and shorter dipping cycles = less zinc adhesion on items.

3) Less zinc spatter caused by humidity = less quantity of ashes.

4) Less heat shocks of items to be hot-dipped = less distortions and deformations of items.

5) Shorter times of flux sublimation = cleaner items' surface and less salt residues.

6) Less dangerous because of zinc spatter = less possibility of accident conditions.

7) No dripping in the furnace nearby area = clean and safe working conditions.


The efficient drying and preheating is mainly due to the huge air volume that is recycled within the drying chamber, so that the huge hot air flow is blowing over the items uniformly, and that provides a quick and efficient drying without using very high temperatures. The necessary hot air is produced with the help of a Heat Economizer. It includes an air-to-air exchanger to avoid the carbon dioxide exhalations into the environment.

The dryer is essentially a pit built in steel and refractory materials.

The self-moving doors are suitably insulated to limit the heat dispersion towards outside. To preserve the life of self-moving doors, all the five external surfaces (except the bottom directed towards the dryer) are coated with acid-proof paint.

Inside dimensions14,200 x 10,000 x 3,200 mm
Working temperature80 - 100C
Normal capacity 10 tons/hour
Peak capacity12 tons/hour
Heating by Heat Economizer
Given electric power15 HP

Two headers of air distribution built in steelwork: section bars and metal sheets to anchor to the plates embedded into reinforced concrete. Headers of housing for door rails, built in steel plates and metal sheets to anchor to striker plates of reinforced concrete, each of them including:

Two worm motor gears, each of them with a 1.5 HP electric motor, adjustable friction device for the setting of maximum efforts and application plate, overhanging assembled tooth wheel with a contrast oscillating spring connection to maintain the contact wheel/rack unchanged. Slide ways and trails for door wheels in plates and in section bars.

A series of limit switches of door stop, of armoured type, with an oscillating rod, complete with position regulation plates. A push-button panel for the door electric control in fibre glass with control acid-proof pushbuttons.

Two self-moving doors built in a rugged metal work in section bars and metal sheets, each of them including:
Two sliding wheels with flat seat to compensate the thermal expansion, complete with housing, shafts and bearings with lubricators. Two rack bars to move the doors.

Flexible lips in vulcanized rubber, complete with pressure plates, to limit the thermal losses.
An exhaust fan that removes the "surplus" air from inside the dryer, to compensate the extra volume of released waste gases, to keep the drying chamber in weak vacuum and to act as an exhauster of residual combustion products of the furnace in case of emergency or failure of the main exhauster fan, including:

A centrifugal electric fan, complete with a 5 HP electric motor, motor starter, steel stainless impeller, manual flu damper to regulate the flow rate and switch flu damper, to allow the natural draught in case of emergency.

A flu damper with vertical axis in the waste-gas ducts with a manual control to adjust the air flow to extract from the dryer, complete with rods, rotary ball bearing and position indicator.
A chimney for "surplus" air totally built in metal work with a total height of 14 m from ground level, complete with a support basement, a flexible joint of connection to the fan, anchor flasks to the shed and to the output with a noise-protective device Various components, including:

K-type thermocouples for temperature control in more points, each of them with two elements with a steel stainless tip. Sealings in vulcanized rubber and ceramic fibre.

Electric components, including:

A series of proximity limit switches for the automatic control. Control electric components, installed in the general control board.

Electric system, including:

Part of general control board as described elsewhere. Electronic control systems, installed and described in the general control board.

Materials and wiring diagrams between the control board and this group, including connector blocks, channels, cables, flexible hoses and accessories.


6.1.2.Zinc Containing Kettle

Metal sheet thickness50 mm
Inside dimensions14.00 x 2.00 x 3.00 (h) mtr
Total weight52,300 kg

The kettle is built in certified pure iron sheets with rounded edges, provided with a reinforcing upper band and lifting hooks. Controlled along the main welds with ultrasound. Not annealed against distention.


6.1.3.Hot Dip Galvanizing furnace


The Hot Dip Galvanizing furnace with the dual-fuel combustion and 12 high velocity "on/off" burners, insulation coating and independent support system against hydrostatic shock.
The special heat distribution inside the furnace provides the major temperature difference between the top and the bottom of melted zinc, with up to 5C difference during the production cycle, so that the lower bottom temperature allows to obtain some important benefits, such as:
1) Facilitates the dross settling out and limits the amount of floating dross.
2) Diminishes the zinc adhesion on the items that remain for a longer time inside the kettle, because of their position on the bottom.
3) Improves the coating in homogeneity between the two ends of those items that are very long and remain dipped into the melted zinc for different periods of time.

The hydrostatic shock that the zinc generates on kettle walls is inhibited by three layers of holding props. One of these layers is positioned immediately under the upper edge of the kettle to limit the superficial deformations of the kettle edge, while the other two are positioned in the lower part.

The shock that is absorbed by the holding props is transferred to the counterforts in metal work, partially embedded in the foundations. This peculiarity allows to keep the kettle and the furnace stable independently of containing action of the furnace metal sheet covering. These counterforts are supplied by us, but they should be embedded by the civil works company. The combustion system is divided into 4 different areas controlled independently. Each burner is regulated separately because each of them is provided with a ratio controller, and the fading of one or more burners doesn't make any difference for the regular work of the other ones.

The conditions of negative, neutral or positive pressure that should settle within the combustion chamber are kept automatically constant at all combustion states with a proportional flue damper, embedded into the waste-gas ducts and controlled by a pressure sensor.

Kettle inside dimensions 14.00 x 2.00 x 3.00 (h) mtr.
Working temperature 445/455C
Normal capacity 10 tons/hour
Peak capacity12 tons/hour
Thermal capacity2,760,000 Kcal/h
Type of burners "on/off" high-velocity burners
Areas for automatic regulation4, each consisting of 3 burners
Type of automatic regulation P.I.D.
Given electric power 35 HP
Furnace ledge towards the floor 550 mm

Structural works, including:

Rugged framework for external sustainment, self-supporting and shock-proof in metal sheets and metal section bars, composed of more elements to make easier the assembly and the transportation.
Upper covering plates of the furnace in metal sheet partially screwed inside the framework and partially welded to the zinc containing kettle. Supporting structure of the furnace room ceiling in section bars and plates, bound to the furnace structure. A series of holding metallic counterforts against the hydrostatic shock at the third layer, built in simple metal bar, counterfort's guide applied to the furnace frame, stop bush and support plate.


Combustion system with 12 burners, including:

Combustion air producing and regulating station, including:

Centrifugal electric fan with a 25 HP electric motor, direct-coupled, built in carbon steel sheet with an anti-noise box padded with rock wool. Vibration-damping joints and supports

A series of membrane pressure switches for the pressure control of combustion air. Feed piping of combustion air from air producing fans to the burners in flanged shafts with supports and accessories. 12 burners, each of them having capacity of 230,000 Kcal/hour, including:

Refractory burner block and connection plate in cast-steel.
Air and gas conveyor with injection pipe and micro-metric control valve.Diffuser and mixer in refractory stainless steel.
Throttle valve with a manual control for primary air setting.Solenoid, on-off and safety valve with a control of flow rate and opening speed of the shutter.

Ratio controller with membrane for the automatic stoichiometric balance of fuel on each burner.
Vibration-damping stub pipes and extra-fine manometer.
Flexible pipes in nylon and copper, fittings, terminals and accessories.

Field control electric boards for burners' functioning in a metallic box with a 400x200x150 mm transparent front part, each of them including: Terminal board of connection with plug fuses of solenoid valves. Screened and insulated cables with ceramic cap for the connection to the ignition spark plug of the ionization electrode.

Electronic instruments, ignition timers and flame control put near each burner. Additional switches and accessories.

An exhaust system of waste gases from the outside part of the furnace into the dryer placed in the proximity of (10 m), including: A flu damper with a servo mechanism and manual control, complete with connection flanges, servomotor stand, levers and accessories. A centrifugal electric fan with a vertical axis, plunged in the waste-gas duct, for the extraction of waste gases, built in metal work, with an impeller in stainless steel, insulated with rock wool box and support framework with a 5 HP electric motor.


Various components, including:

A series of four inspection wickets, placed on the furnace headers for the visual control of combustion and zinc kettle condition, complete with inspection windows in strained glass and with connections for checking the inside depression. Detector for accidental zinc leakage from the kettle caused by accidental drilling, consisting of containment wells built in metallic sheet, contact electrodes in stainless steel and a diaphragm in lead sheet.

Two square thermocouples, type K, Ni-CrN-'made, with an ARMCO steel case for zinc kettle. Two straight thermocouples in stainless steel, type K, Ni-CrNi, each of them with 2 elements for the temperature control of the external wall of the zinc containing kettle, complete with a pressure device provided with contrast springs. Two straight thermocouples in stainless steel, type K, Ni-CrNi, each of them with 2 elements for the temperature control of waste gases.


Hydraulic connections, including:

Materials and diagrams for the preparation and assembly of pipes, fittings, supports and accessories for the distribution of waste gas and combustion air, as far as the components supplied by us are concerned.
Materials and diagrams for the preparation and assembly of pipes, fittings, supports and accessories for the natural gas feeding from the gas reducing manifold to the furnace.


Electric system, including:

Part of general control board as described elsewhere. Electronic control systems, installed and described in the general control board. Materials and wiring diagrams for electric connections in this group including connector blocks, channels, cables, flexible hoses and accessories. Materials and wiring diagrams between the control board and this group including connector blocks, channels, cables, flexible hoses and accessories.


6.2.Service equipment of Hot Dip Galvanizing Department

6.2.1.Raised flooring for the underlying furnace room, supportbeams,metal sheets covering


It includes the structural works for furnace room pit and for emergency moulds' covering. The criterion of calculation for furnace room flooring takes into consideration the fact that this plane should normally carry also the weight of zinc packs.

Furnace room dimensions 20 x 10 m
Furnace room covered area to be determined
Emergency moulds covered area to be determined

Metal works, including:

A rugged self-supporting framework for furnace room flooring in striated metal sheets and section bars composed of more elements to make easier the assembly and the transportation.
A rugged self-supporting framework of emergency moulds covering in striated metal sheets and section bars composed of more elements to make easier the assembly and the transportation.


6.2.2.Exhaust gas ducts

Exhaust gas is taken out from the galvanizing furnace and is directed through flue ducts embedded in foundations to heat economizer (described elsewhere). From here the gas is sent to the heat generator, installed in the dryer, to be mixed or released, according to their residual temperature. The "surplus" fan also expels this exhaust gas in case the main in ground fan fails.

Exhaust gas temperature180 - 550C
Total exhaust gas ducts length 40 m

Materials for insulations and lining of exhaust gas ducts in reinforced concrete. Covering metal sheet of exhaust gas ducts, leant against the masonries to back up the covering insulation castings.

A series of manual flue dampers for the cutting off exhaust gas ducts, built in rugged metal work, plunged in the flue pipe, complete with a shaft, rod, control lever and accessories.


6.2.3.GasPressure Reducing Manifold

This unit is necessary to reduce the pressure of fuel from the supply unit up to the operating units and plants. This manifold is placed before our equipment and is provided with all on-off and safety devices according to the current regulations.

Max. charge pressure 0.4 Bar
Output pressure 60 (100) mBar
Max. flow rate300 Nm3/hour
Type of energy source Gas>8.300 Kcal/ Nm3.

Gas preparing and regulating manifold, including:

Pressure reducing device with an automatic block device. Double solenoid on-off and safety valve and with air supply valves. Cutting-off valves, pressure switches, manometers and accessories. Gas supply piping from gas reducing manifold till different burners, made in flanged shafts with supports and accessories.


Hydraulic connections, including:

Materials and diagrams for the preparation and assembly of pipes, fittings, supports and accessories for the distribution of fuel and of combustion air, as far as the components supplied by us are concerned. Materials for the preparation and assembly of pipes, fittings, supports and accessories for the distribution of natural gas among the different units.


Electric system, including:

Part of general control board as described elsewhere. Electronic controls, installed and described in the general control board of the department. Materials for electric connections in this group including connector blocks, channels, cables, flexible hoses and accessories. Materials for electric connections between the control board and this group including connector blocks, channels, cables, flexible hoses and accessories.


6.2.4.Transfer Cart

Empty racks transportation from the end of the line to the load stations, or from one aisle to another, is provided by the transfer cart. It is possible to carry up to 4 racks at a time. The cart starts in a manual way, and every time the flashing light and the buzzer are activated. Each truck is made of steel section bars, protected by a yellow acid-proof paint and is equipped with a 5 HP motor. The transition is provided by metal wheels sliding along the runways and is regulated by an inverter (integrated in the truck) which controls accelerations and brakeage.