3. Research needs

The continuous galvanizing process is coating the steel sheet with zinc. This coating contains a significant amount of energy. Not all of the energy used in galvanizing is converted to useful heat (useful heat is the amount of heat that is transferred to the steel strip during its movement inside the furnace), but the rest of the heat is lost in the form of losses. The energy used in different types of products is different because the process parameters are also different. Therefore, it is difficult for galvanizers to maintain a constant heat cycle in their work process.

For example, the amount of energy consumed and losses when galvanizing a steel strip 0.0028 ft (0.00086 m) thick, 5 ft (1.524 m) wide at a line speed of 450 ft / min (138 m / min) at 1700oF furnace temperature ( 927 ° C) with a steel strip 0.0042 ft (0.00128 m) wide, 4 ft (1.22 m) wide, line speed 400 ft / min (122 m / min) at 2200oF (1204oC). Research efforts can quantify this energy consumption based on products and process parameters.

The energy provided to the furnace is absorbed by the various elements of the furnace. It is difficult for galvanizers to identify this transferred energy, although they know the total amount of energy supplied to the furnace. By knowing the amount of energy transferred or the percentage of heat that enters the various elements of the furnace, galvanizers can identify areas that need improvement to work with their furnace, as efficiently as possible.

This situation in the galvanizing industry can be improved with a model that can meet these requirements by creating differences in energy consumption to change the parameters of the product process and calculating the amount of heat applied to different elements of the furnace by the method of heat balance.

1.3 The importance of research

Many studies on energy consumption have been studied by various galvanized lines, but relatively few studies have been conducted to study the exact energy.

Heat balance analysis is able to distinguish the heat supplied to the furnace as useful heat, and the heat lost in this process determines the amount of heat input to the various elements.

Furnace losses are shown in the figure below.

Furnace losses

The considered losses are very important in terms of heat balance. It is assumed that the total heat entering the furnace is dispersed as useful heat and losses as shown in the formula.

The following inputs and heat losses vary according to the product being produced.

Formula

Gas combustion energy (amount of gas & gas combustion) formula

For example, the difference in heat or energy consumption for two products with different process and product parameters is shown below.

S.No Product A Product B
1 Steel strip width – 4.3 ft Steel strip width – 4.8 ft
2 Steel strip thickness (Gauge) – 0.0021 ft Steel strip thickness (Gauge) – 0.0024 ft
3 Line speed – 423 ft/min Line speed – 405 ft/min
4 Furnace zone temperatures – 1650 oF Furnace zone temperatures – 1750 oF

Product and process parameters for two products:

Heat Input: Product A – NG input – 35,600 ft3/hr, Air – 372,353 ft3/hr

: Product B – NG input – 40,000 ft3/hr, Air – 432,500 ft3/hr

Product A:

Heat carried away by steel strip – 17.57 MMBtu/hr

Wall losses – 8.2 MMBtu/hr

Stack loss – 6.8 MMBtu/hr

Cooling water loss – 1.7 MMBtu/hr

Opening Loss – 0.0027 MMBtu/hr

Unaccounted loss – 0.82 MMBtu/hr

Total Heat – 35 MMBtu/hr

Product B:

Heat carried away by steel strip – 19.25 MMBtu/hr

Wall losses – 9.2 MMBtu/hr

Stack loss – 7.2 MMBtu/hr

Cooling water loss – 2 MMBtu/hr

Opening Loss – 0.0055 MMBtu/hr

Unaccounted loss – 1.5 MMBtu/hr

Total Heat – 40 MMBtu/hr

4. The coating formula

The coating process is performed in the same furnace for the various processes and product parameters tabulated above.

As shown above, heat balance determines the amount of heat applied to the various elements of the furnace. It is also obvious that comparing products with cost and energy based is possible using this method.

It also helps galvanizers do what to analyze and decide. Helps them compare current and improved conditions. From the example above, product A has a significant amount of heat released through the stack that can be reduced by controlled combustion. So an area for improvement has been identified. The percentage of oxygen in the stack is one of the criteria that affects the heat passing through the stack. Galvanizing can work on improving by replacing burners with oxygen fuel burners or adjusting the fuel-to-air ratio. After making the necessary changes to their process, the galvanizers can enter new values, obtain the model, and conclude for the modified conditions.

A detailed description of galvanizing is discussed in Section 1.4.

1.4 Galvanizing process

The galvanizing process consists of four basic elements:

  • Surface preparation
  • galvanization
  • turn off
  • Inspection
1.1.4 Surface preparation

Surface preparation is an important step in the application of any coating. In most cases, the coating deteriorates before the end of its useful life due to improper or insufficient surface preparation. The surface preparation stage in the galvanizing process has its own internal control tool that will not react with a surface of steel that is not completely clean. When removing steel from the melt, any damage or inadequacy in surface preparation will be immediately apparent as the untreated areas remain uncoated.

On-site painting or other on-farm corrosion protection systems may involve the use of various subcontractors and / or working groups to prepare the surface and apply the coating. This can lead to problems in coordinating activities, leading to costly and time-consuming delays, errors and disputes over liability and financial responsibility. Surface preparation for galvanizing typically involves three steps: Burning cleaning, acid pickling, and flux.

1.1.1.4 Burning cleaning

Alkaline hot solution is often used to remove organic contaminants such as dirt, paint, grease and oil markings from the metal surface. Epoxies, vinyl, asphalt or welding slag must be removed before galvanizing by blasting sand, blasting sand or other mechanical methods.

2.1.1.4 acid washing

Scale and rust are usually removed from the steel surface with acids in a dilute solution of hot sulfuric acid or hydrochloric acid at ambient temperature.

3.1.1.4 Charging

Flux is the final stage of surface preparation in the galvanizing process. Eliminates oxide flux and prevents the formation of subsequent oxides on the surface of the metal before galvanizing and binds zinc to the surface of steel or iron. The method of flux application depends on whether the particular galvanizing plant uses a wet or dry galvanizing process.

In the dry galvanizing process, steel or ferrous materials are immersed in aqueous ammonium chloride solution or pre-fluxed. The material is then thoroughly dried before being immersed in the melt. In the wet galvanizing process, a blanket of ammonium chloride floats on the liquid on top of the melt. Iron or steel to be galvanized passes through the flux inside the melt.

2.1.4 Galvanizing

At this stage, the material is completely immersed in a bath consisting of pure melt. The bath temperature is maintained at about 850 degrees Fahrenheit (454 degrees Celsius).

The products are immersed in the bath long enough to reach the bath temperature. The products are gently removed from the galvanized bath and the excess amount of zinc is removed by blowing air at a certain pressure with the help of an air knife.

The chemical reactions that lead to the formation and structure of the galvanized coating continue after the products leave the bath until they are close to the bath temperature. The products are cooled immediately after leaving the bath in ambient water or air, and the chemical reaction stops after cooling.

3.1.4 Shutdown

This process solidifies the zinc surface to ensure working with it. It also prevents the alloy reaction in the case of reactive steels, which continues below the melting temperature of zinc. Silent water usually contains an inactive chemical that delays the formation of white rust (wet storage stain) until the newly activated surface has formed a stable, protective carbonate layer on the base.

There are two methods of galvanizing, hot galvanizing and continuous galvanizing. In immersion galvanizing, the iron components to be galvanized are held by a overhead crane and immersed in tanks containing various liquids to prepare the surface, respectively, before immersing them in the final melting tub. Immersion galvanizing is done for steel products such as rods, channels, small and medium-sized machine components, steel plates, bolts, and other items that can be hung with wire.

On the other hand, continuous galvanizing is composed of galvanized sheet and continuous galvanizing includes galvanized sheet products of different measurements. The sheet steel strip is continuously fed through an efficiency loop and passes through several sections, and is coated with Zn / Zn alloy before being rewound. This process is done without interruption for weeks. Hence it is called continuous galvanizing. The continuous and modern galvanizing process was invented by Sendzimir more than half a century ago.

4.1.4 Inspection

The two properties of galvanized coating that are closely examined after galvanizing are the thickness of the coating and the appearance of the coating. The thickness of the coating is controlled by adjusting the amount.

Various types of physical and laboratory tests may be performed to determine the thickness, uniformity, adhesion, and appearance of pressure in air knives.

A detailed description of the various sections of the continuous galvanized line is discussed in the following sections.

1.5 Continuous galvanizing

The figure below shows a true image of a continuous galvanized line.

Continuous galvanized line for covering steel sheets

01 Decoiling 06 Air Knives
02 Welding 07 Galvanneal Furnace
03 Entry Loop Car 08 Levelling
04 Annealing Furnace 09 Cutter
05 Zinc Bath 10 Coiling

The equipment mentioned above consumes either electricity or natural gas. The main devices consuming electricity are large motors, which are used in these equipments and resistance or induction coils used for pre-melting and main zinc pots. In the case of natural gas, it is the largest consumer in this country.

۱.۱.۵ Decoiling

This is the initial stage of the galvanized line in which the steel sheets are loaded without coating. There are usually two decoilers. When a roll is about to end, the end of that roll is welded to the end of the second roll. Then, as the second loop opens, the new roll is held in place for unloading instead of the first. This helps keep the process going.

2.1.5 Welding

The end of the boring decorative roll is welded to the end of the new roll with the help of a seam welder. During this time, the line is fed through the steel sheet collected from the inlet ring.

3.1.5 Entry loop

In the event of a change in the decoupling rolls, the ring entry machine is located at the end of the input and its purpose is to maintain the continuity of the process. Covers the time delay caused by the seam welder. The ring machine consists of a series of zigzag rolls through which the steel sheet moves. Steel sheet inventory is stored in the battery by increasing the distance between successive rolls. Each time the battery is discharged, the rolls begin to close, thus releasing the steel sheet that passes through them. The ring machine is capable of storing steel sheets with a maximum length of 1000 feet.

4.1.5 Baking oven

Annealing is the process of heating a substance to a high temperature and then cooling it to soften the substance. The baking oven usually has 4 parts:

1) pre-heat section,

2) non-oxidizing part,

3) heating section and

4) Jet cooling section (controlled cooling).

The furnace is the largest equipment consuming natural gas in galvanized installations. It is discussed in detail in various sections.

A) Pre-heating section: This section generally includes burners that fire directly at the tape to remove impurities from the tape surface.

To) Non-oxidizing part: The non-oxidizing part of the baking oven heats the strip in the oxidizing atmosphere. The set point temperature in this section is between 2000ºF to 2450 ºF and varies depending on the type of steel. The furnace atmosphere is mainly composed of a gas mixture of 15% hydrogen and 85% nitrogen. Nitrogen is used to maintain a positive pressure inside the furnace and hydrogen atmosphere to prevent the surface of the strip from oxidizing.

C) Heating section: The heating unit usually has a setting point of 1500ºF to 2200ºF. Again, different for different types of steel. The heating section of the furnace helps maintain the temperature of the strip in the oxidizing atmosphere.

D) Controlled cooling section (jet cooling): The controlled cooling section of the furnace uses heat exchangers with cooling water and fans to permanently lower the steel temperature. The steel sheet in the jet cooling section cools to about 860 degrees Fahrenheit. The controlled cooling section is sometimes provided with electric heating elements, necessary to increase the temperature of the bar.

5.1.5 Zinc bath (molten metal vase)

The galvanized line usually consists of two pots, the pre-melting pot and the main pot. Zinc and other alloy metals are mixed in suitable compounds in the pot before melting and then transferred to the main pot with the help of channels. The snout, which is a kind of transfer of steel strip from the furnace to the pot, is immersed in the main pot. The main pot also has a dishwasher and stabilizing rolls in which they are immersed and through which the steel sheet of the snout passes. The pot is heated before heating and the main contents of the pot are heated by thermal elements such as inductors and natural gas burners. The heating method is different for different companies.

A typical galvanized bath is kept inside the main pot at a temperature of about 842 degrees Fahrenheit to 878 degrees Fahrenheit (450 degrees Celsius to 470 degrees Celsius). The temperature varies depending on the product produced. Continuous galvanized tubs usually contain a small amount of Al, often less than 0.3% to reduce the reaction between the Zn smelting alloy and the coated steel.

If extra corrosion coverage is required, the Al content in the bath can be up to 55%, but usually less than 1% is maintained for the desired level. After more than a decade of intensive research and development, it is now possible to define the optimal aluminum content of a bath cover.

About product and vase specifications: About 0.136% for galvanizing, 0.18% for galvanizing for the construction market and 0.25% for automotive exposed applications. The bath inside the pot is kept in the molten state even in case of breakdown (when there is no production).

6.1.5 Air Knives

The steel strip passes through the air knives after the coating process in the zinc pot. Use pneumatic knives to blow up the extra steel strip cover. The thickness of the coating is done according to the specifications by adjusting the pressure in the air knives.

7.1.5 Galvanic furnace

The galvanizing process is slightly different from the galvanizing process. The variation in the production process is that, to produce a galvanic coating, the strip taken out of the coating bath is further heated by passing it through a furnace. By heating to approximately 1000 to 1050 degrees Fahrenheit (538 to 565 degrees Celsius) and holding the strip at this temperature for a certain period of time, the zinc coating alloys with the iron by spreading it between the molten iron and the iron from the steel strip. The result is that the final product has a coating that is approximately an alloy of 90% zinc and 10% iron. The final concentration of iron depends on the heating cycle because the diffusion is a function of the time / temperature cycle.

Galvanic furnace is not necessary in all galvanized installations. It may not be available in facilities where galvanic products are not produced. Galvanneal is used in the automotive industry to improve production performance on models that use lighter and stronger steel grades. The advantage of galvanic coating is the improvement of spot welding ability and improved adhesion of the coating.

8.1.5 Leveling

Finally, the hot-dipped coated sheet can be rolled continuously at the outlet of any plant, creating stress. In this way, high quality and smooth surface can be produced. Before being placed in the ready-to-transport coils, the surface is chemically passive or lubricated to protect the steel strip from temporary corrosion and friction oxidation.

9.1.5 coil

As a final process, the steel strip is lubricated, screwed and coiled to be transported. The cooling system wraps the strip out of the processes. This is the final setting in a galvanized line.

1.6 furnaces

The galvanized line is composed of various components, of which the stove and the pot are the two main consumers of energy. This project focuses on energy saving opportunities in the furnace sector. The components of the furnace are listed as follows.

1.1.6 Furnace components

A hot coating line has two furnaces, one is a sintering furnace in which the steel strip is heated to a high temperature and cooled before coating, and the second is a galvanic furnace in which the steel strip is inserted after the zinc bath coating. Both furnaces are kept in the temperature range of 1400oF to 2200oF (760oC to 1204oC) depending on the products produced.

Baking ovens are usually divided into four sections:

1) Preheating section,

2) heating section,

3) Holder and

4) Cooling part.

The steel sheet enters from the beginning of the furnace through the part before heating and then passes through the heating part because the temperature is maintained at a high level. The steel sheet passes through the holder before it exits the cooling section, where the temperature is relatively lower than the heating sections. Finally, the tape enters the cooling sections or jet cooling sections where the tape cools at the temperature of the pot bath. This bar is inserted into the bathroom with the help of a snout. Hydrogen-reducing atmosphere, nitrogen gas is stored in the furnace up to the snout. The galvanic oven has the same characteristics of the annealing oven, except that there is no cooling part in the galvanic oven.

2.1.6 Furnace changes

The characteristics of furnaces used in galvanizing facilities vary based on the products produced and the technology used. The furnaces used may have radiant tube sections, direct hot sections or induction coils. In the radiant tube section, the strip is heated by the radiant heat from the radiant tubes inside the furnace. Direct hot furnaces are also called direct combustion furnaces.

Where the flame enters the furnace area directly. Induction heating is mostly used in galvanic furnaces in which the heating of the strip is achieved by passing it through many alternating magnetic fields.

1.2.2.6 Direct off furnace

Direct fuel furnaces are unique components in the process of producing coated steels. These furnaces are designed to create a uniform heating environment for the steel strip before the coating operation. High-speed burners are installed in a stepped manner along both sides of the furnaces, creating excellent temperature uniformity, as shown in the figure below.

Schematic of a direct stove

As air is injected tangentially into the stream, natural gas enters the burner along the central axis. In this way, a rotation is created to improve the fuel-air mixture for the combustion preparation. In this way, a rotation is created to improve the fuel-air mixture for the combustion preparation. This is a continuous process with a new tape being welded to the tail end of the previous tape.

2.2.1.6 Radiation tube furnace

Radiant tube furnaces are operated with reducing atmosphere and are heated by radiant tubes with natural gas. The heating of the strip is done almost entirely by radiation. The steel strip passes through a row of burner tubes above the strip and below the strips. Depending on the installation, these burner tubes are fired at a specific temperature. Heat is radiated from these tubes and absorbed by the tape. Some heating is also provided by radiation from the furnace walls after the furnace has been operating for a period of time. The amount or time of heating of the strip in this furnace depends on the temperature of the pipe. The higher the tube temperature, the faster the heating rate. The schematic of the traditional radiant tube burner section is shown in the figure below.

Schematic of the traditional radiant burner section [12 NOx high in the case of radiant tube furnaces

As shown in the figure above, combustion takes place at one end of the burner and flue gas escapes from the other end. The combustion air is preheated by the flue gas with the help of an electric plug. Heat transfer to the strip in a radiant tube furnace is not the same as in a fire chamber where combustion takes place in a large chamber in which heat can be transferred simultaneously to the entire inventory. The non-uniform or constant combination of combustion gases, alternating reducing and oxidizing atmospheres, is harmful to shiny metal pipes, and very hot gas pockets form high NOx in the form of radiant tube furnaces.

3.2.1.6 Induction furnace

The concept of induction heating is more widely used in galvanic furnaces than in kilns. The purpose of the galvanic furnace is to provide an alloy of iron and zinc to the strip, known as the “galvanic product”. The temperature at which a suitable alloy occurs is between 1000 degrees Fahrenheit to 1050 degrees Fahrenheit (538 degrees Celsius to 565 degrees Celsius). To prepare a high quality galvanic product, it is important to control the temperature of the tape in this range and then cool it.

The reason for using induction furnace on conventional heat of gas and radiant pipes is that induction heating is more efficient than the other two. In the case of normal gas heating, it is difficult to control the temperature of the strip and cool it quickly. In addition, the exhaust gases prevent the tape from cooling rapidly. Similarly, in the case of radiant heating, the tape must remain in the furnace for a long time to penetrate the zinc coating (which is highly reflective), which can lead to temperature control problems.

The basic principle of induction heating is quite simple. Alternating current passes through an electric coil. A magnetic field is generated that varies with the amount of current. This field is concentrated inside the coil. The steel strip passes through the coil, eddy currents are induced inside the coil and flow in the opposite direction of the current of the coil. Heating is caused by electrical resistance to eddy currents caused by the tape.

In any type of heating section, the length of time the strip takes to reach a certain temperature is very important. Factors that affect these criteria are:

It takes longer than a narrow strip, and for the same width for other widths, it takes a heavier-than-light sensor to heat up to the same temperature.