Boiler Basics

2022-03-22 07:31:40

A boiler is a device that uses the heat energy released by the combustion of fuel or other forms of heat energy to heat the working medium (water or other fluid) to certain parameters (pressure, temperature). It consists of two parts, the pot and the furnace. The boiler refers to the heating surface system that absorbs heat and transmits it to the working medium, and the furnace is a combustion device that converts the chemical energy of the fuel into thermal energy.

The function of the boiler is to generate steam or hot water with specified characteristics through thermal energy conversion. The physical quantities that characterize the physical properties or states of steam or hot water are called state parameters, generally referred to as parameters. There are six thermal state parameters, the most commonly used ones are pressure and temperature.

Terminology:

Rated steam pressure: refers to the steam pressure at the boiler outlet that must be guaranteed for long-term continuous operation within the specified feed water pressure and load range, that is, the pressure indicated on the boiler nameplate. The unit is MPa. There are 6 pressure levels in my country's industrial boiler parameter series, namely 0.4, 0.7, 1.0, 1.25, 1.6 and 2.5MPa (gauge pressure).

Rated water outlet pressure: the pressure is maintained by the circulating pump at the boiler outlet under the condition of the rated circulating water volume of the hot water boiler. The unit is also MPa. In China, the water pressure level specified by the parameters of the hot water boiler is the same as that of the steam boiler.

Rated evaporation: the maximum evaporation of the steam boiler under rated parameters (pressure, temperature), rated feed water temperature, using design fuel and ensuring boiler efficiency and continuous operation. Its unit is ton/hour (t/h) or kilogram/hour (kg/h) industrial boiler standard parameter series are 0.1, 0.2, 0.5, 1, 2, 4, 6, 8, 10, 15, 20, 35 and 65t/h, a total of 13 evaporation levels.

Rated thermal power: the maximum heat supply that the hot water boiler should guarantee during long-term continuous operation under the condition of rated return water temperature, rated return water pressure and rated circulating water volume. Its unit is megawatt, or MW. The standard parameter series of hot water boilers are 0.1, 0.2, 0.35, 0.7, 1.4, 2.8, 4.2, 7.0, 10.5, 14.0, 29.0, 46.0, 58.0 and 116.0MW, with a total of 14 thermal power levels.

Rated feed water temperature: refers to the feed water temperature that the steam boiler should guarantee within the specified load range. The feed water temperature for industrial boilers is 20°C, 60°C and 105°C.

Boiler efficiency: the boiler thermal efficiency. It refers to the percentage of the effective utilization heat Q of the steam or hot water output by the boiler and the heat Qr released by the complete combustion of the fuel entering the boiler at the same time, usually represented by the symbol "η". That is, η=Q/Qr×100%, the thermal efficiency of oil-fired gas boiler products, the industry standard of the Ministry of Machinery Industry stipulates that oil-fired boilers should ensure 80% to 85%, and gas boilers should ensure 82% to 87%. High efficiency is required for boilers with large capacity.

Definition of boiler type:

Drum: A cylindrical pressure vessel used in a water tube boiler to form a water circulation loop, separates steam and water and stores water and steam. It consists of a head and a cylinder. It is an important part of the water tube boiler.

Shell: A cylindrical pressure vessel that accommodates steam and water and heats the heating surface in a shell-type boiler, consisting of a tube sheet (head) and a cylinder.

Furnace: Also known as combustion chamber. In the boiler, the fuel and air are mixed, burned, and the high-temperature flame and flue gas conduct heat transfer to the heating surface.

Furnace gallbladder: Also known as fire gallbladder. The hearth or combustion chamber in a shell boiler.

Heating surface: The metal wall used by the boiler to transfer heat.

Radiation heating surface: The heating surface that absorbs heat from the exothermic medium and transfers it to the heat absorbing medium mainly by means of radiative heat exchange. Such as the water wall of the furnace of the water tube boiler, the furnace of the shell boiler and so on.

Convective heating surface: The heating surface absorbs heat from the flue gas and transfers it to the heat-absorbing medium mainly by convection heat transfer. Such as the convection tube bundle of the water tube boiler, the smoke tube of the shell boiler, etc.

Boiler type distribution:

Shell-type boilers are divided into:

1. The axis is perpendicular to the ground, which is called vertical, which is represented by the Chinese pinyin letter "L".

2. The axis is parallel to the ground, which is called horizontal, and is represented by the Chinese pinyin letter "W".

According to the positional relationship between the combustion chamber (furnace) and the boiler shell, it is divided into:

1. The combustion chamber is outside the pot shell, which is called external combustion and is represented by the Chinese pinyin letter "W".

2. The combustion chamber is inside the pot shell, which is called internal combustion and is represented by the Chinese pinyin letter "N".

Water tube boilers are divided into: natural circulation boilers according to the water circulation mode. If the boiler has only one drum, it is called a single drum, denoted by "D"; if the boiler has two drums, it is called a double drum, denoted by "S".

According to the relationship between the longitudinal axis of the drum and the flow direction of the combustion flame and flue gas, it is divided into:

1. The drum is parallel to the flue gas flow direction, which is called vertical type, which is represented by "Z".

2. If the drum is vertical to the flue gas flow, it is called horizontal type, which is represented by "H".

Boiler form code:

Shell boiler overall type code:

The overall type code of the water tube boiler:

The model of the boiler is to combine the overall type code and parameters in a prescribed way to indicate the type and main performance and specifications of the boiler product.

All models are divided into three parts, connected by a short horizontal line in the middle. In each part, "△" is the code of Chinese pinyin, and "X" is the parameter represented by Arabic numerals.

Fuel type code, the provisions of JB/T1626 are: diesel-Yc; heavy oil-Yz; coke oven gas-Qj; natural gas-Qt; liquefied petroleum gas-Qy.

Analysis of vertical shell boiler:

Although the boiler itself has straight water tube bundles, its combustion chamber is inside the shell, and the tube bundles are arranged between the upper and lower drums, which can also be regarded as inside the shell, so it is included in the shell boiler. The boiler body is composed of the head, the upper and lower drums, the upper and lower tube plates, the furnace gallbladder, the U-shaped foot ring, the central large-diameter descending pipe and the ascending pipe bundle. The high-temperature flue gas generated by the fuel (gas) in the combustion chamber enters the tube bundle through the throat, and rotates 360° around the central large-diameter downpipe (the periphery is the smoke box) to dissipate heat, and then is discharged to the smoke box. Because the flue gas scours the tube bundle laterally, it has better heat transfer effect than the flue tube of the countersunk head boiler. Because the upper and lower shells have manholes, it is more convenient to check and clean the tube sheet. The working pressure of this kind of boiler generally does not exceed 1.0MPa, and the output does not exceed 2000kg/h.

**Analysis of horizontal shell boiler:**
Compared with the water tube boiler of the same capacity, this kind of boiler has a simple structure and is easy to form a quick installation form. The boiler body is mainly composed of furnace gallbladder, smoke transfer chamber, smoke pipe and boiler shell. The inclined tie rods are arranged between the boiler shell and the tube sheets according to the strength requirements, and the straight tie rods are arranged between the tube sheets and the tube sheets. The most commonly used are the center backfire structure and the three return structure.
The flue gas flow of the horizontal internal combustion boiler is as follows: the fuel is burned in the furnace, and the high-temperature flue gas produced enters the flue pipe convection and releases heat after radiation heat exchange, and finally is discharged from the boiler. Therefore, the flue gas must be rotated to enter the smoke pipe. Usually, the revolving part of the flue gas from the furnace to the second return flue pipe is called the flue turning chamber. The turning part from the second return to the third return is called the front smoke box (room).
The horizontal internal combustion shell boiler is suitable for oil and gas because of its wavy furnace. Therefore, it is more suitable as a medium-sized oil and gas furnace and is now widely used. Horizontal internal combustion fuel (gas) shell boilers are generally divided into two types: dry-back three-pass and wet-back three-pass.
**Horizontal vacuum boiler structure:**

Differences between dry back boilers and wet back boilers:
Wet-back boiler means that the smoke-turning chamber of the boiler is moved into the drum, and the tube sheet, back plate and cylinder of the smoke-turning chamber are surrounded by boiler water, while the dry-back type means that the flue gas return space is surrounded by refractory materials. completed.
**Other differences:**
1. There is a smoke return type behind the wet-back boiler, while the dry-back vertical boiler does not, and the horizontal type has it.
2. The wet-back boiler is a horizontal three-pass combustion cycle, the flue gas flushes the furnace and the threaded flue pipe in turn, and then is discharged into the atmosphere from the rear flue chamber through the chimney. Most of the dry-back types are vertical two-return combustion cycles.
3. The second inlet of the dry boiler is easy to be burned and exploded, and the wet back type will not because it is soaked in water.
4. The dry back type is mostly vertical wet type, while the wet back type is mostly horizontal.

Previous: No Prev Post

Next: Read Next Post

Back