Let’s discuss the classification of boilers

Boilers can be classified from different perspectives. 1) Classified by purpose, they can be divided into power station boilers, industrial boilers, domestic boilers, locomotive boilers, and ship boilers, etc. 2) Classified by capacity, they can be divided into large boilers (with a evaporation rate greater than 100 t/h), medium-sized boilers (with an evaporation rate of 20 t/h to 100 t/h), and small boilers (with an evaporation rate less than 20 t/h). 3) Classified by steam pressure, they can be divided into low-pressure boilers (P≤2.50MPa), medium-pressure boilers (2.50MPa＜P≤4.0MPa), sub-high-pressure boilers (3.82MPa＜P＜10MPa), high-pressure boilers (10MPa≤P＜14MPa), ultra-high-pressure boilers (14MPa≤P＜17MPa), sub-critical boilers (17MPa≤P＜22MPa), and super-critical boilers (P≥22MPa, that is, above the critical pressure). 4) Classified by fuel type and energy source, they can be divided into coal-fired boilers, oil-fired boilers, gas-fired boilers, biomass fuel-fired boilers (firewood, sugarcane, rice husks, coconut shells, household waste, industrial waste, etc.), nuclear power boilers, and waste heat boilers (exhaust heat boilers), etc. 5) Classified by the flow pattern of the medium, they can be divided into natural circulation boilers, forced circulation boilers, once-through boilers, etc. 6) Classified by the structure of the boiler itself, they can be divided into shell boilers (fire-tube boilers), water-tube boilers, fire-and-water-tube boilers, heat-pipe boilers, vacuum phase-change boilers, etc. 7) Classified by the heat carrier, they can be divided into steam boilers, hot water boilers, dual-purpose steam and water boilers, and boilers using organic heat carriers, etc. 8) Classified by the combustion method of the fuel in the boiler, they can be divided into layer-fired boilers (which are further divided into fixed grates and mechanical grates: chain grates, vibrating grates, lift-grate boilers, reciprocating grates, coal-throwing grate boilers, etc.), chamber-fired boilers, and fluidized-bed boilers. 9) Classified by factory type, they can be divided into bulk boilers, assembled boilers, and fully assembled boilers. Power station boilers are generally water-tube boilers with high pressure (above medium pressure), large capacity (above medium size), and room combustion; they can be further divided into many types. Industrial boilers generally have low pressure (2.45 MPa and below) and small capacity (65 t/h and below); they mostly use a layer combustion method. There are a wide variety of structural forms, combustion methods, and types of combustion equipment (see Table 3-1). It is mainly used for steam in industrial production and heating. Table: Types of Industrial Boilers – Classification Methods
Boiler types: Classified by boiler structure
Shell-type: Vertical horizontal tube type, vertical curved tube type, vertical straight tube type, vertical horizontal fire tube type, horizontal internal combustion backfire tube type, etc.
Tube-type: Single-drum vertical type, single-drum horizontal type, double-drum vertical type, double-drum horizontal type, vertical and horizontal drum type, forced circulation type, etc.
Water-tube and fire-tube type: Horizontal quick-install type

Classified by fuel equipment: Fixed grate, movable hand-cranked grate, chain grate, reversing grate, coal feeder, vibrating grate, bottom-fed grate, fluidized bed furnace, semi-fluidized bed furnace, reciprocating push grate, chamber combustion furnace, cyclone furnace, etc.
Classified by fuel type: Anthracite, lean coal, bituminous coal, low-quality bituminous coal, lignite, oil, gas, firewood, sugarcane bagasse, rice husk, coal gangue, special fuels, waste heat, etc.
Classified by delivery format: Quick-install type, assembled type, bulk type
Classified by heating medium: Steam, hot water, and other media

Basic parameters of boilers and common terms 1. Basic parameters of boilers There are many technical parameters for boilers, but the main parameters that reflect the technical characteristics of a boiler are as follows: (1) Capacity The capacity of a boiler, also known as its output, is a fundamental characteristic parameter of the boiler. Steam boilers are expressed in terms of evaporation rate, while hot water boilers and organic heat carrier boilers are expressed in terms of heat power. 1) Evaporation rate: The amount of steam generated per hour by a steam boiler during continuous operation over an extended period is referred to as the boiler’s evaporation rate, denoted by the symbol “D”. The unit of measurement is tons per hour (t/h). The term evaporation rate commonly referred to is the boiler’s \"rated evaporation rate\", that is, the maximum amount of evaporation that must be produced per unit of time when the boiler operates continuously under its rated conditions (rated steam pressure, temperature, and feedwater temperature). The evaporation rate indicated on the boiler’s nameplate is the rated evaporation rate. 2) Thermal power: When a hot water boiler or a boiler using an organic heat carrier operates continuously for an extended period, the effective amount of heat carried away by the water per hour, at the rated outlet temperature, pressure, and rated water circulation rate, is referred to as the boiler’s rated thermal power. This value is denoted by the symbol “Q”, and its unit is megawatt (MW). A hot water boiler generates 0.7 MW of heat, which is roughly equivalent to the heat produced by a steam boiler that generates 1 t/h of steam. The capacity parameter series for boilers in our country have been incorporated into **standards**. (2) Pressure: The operating pressure (gauge pressure) at the boiler outlet (drum or superheater), usually denoted by the symbol “P”, is measured in megapascals (MPa). The pressure indicated on the boiler nameplate and design documents is the boiler’s rated operating pressure. For boilers equipped with superheaters, it refers to the steam pressure at the outlet of the superheater ； For boilers without superheaters, it refers to the steam pressure inside the boiler drum ； For hot water boilers, it refers to the pressure of hot water at the inlet of the outlet valve. The boiler pressure mentioned in everyday conversation refers to gauge pressure. Both the pressure values listed in the steam thermodynamic properties table and those used in thermal calculations are absolute pressures. The boiler furnace and flue are usually under negative pressure. (3) Temperature: The temperature indicated on the boiler nameplate refers to the temperature of the medium at the boiler outlet, also known as the rated temperature. For steam boilers without superheaters, the rated temperature refers to the saturation steam temperature of the boiler at the rated pressure ； For steam boilers equipped with superheaters, the rated temperature refers to the steam temperature at the outlet of the superheater ； For hot water boilers, the rated temperature refers to the temperature of the hot water at the boiler outlet. 2. Common Terms and Basic Concepts in Boilers (1) Heated Surfaces: Heated surfaces refer to the metal walls that separate the combustion products from the water vapor medium. The heating surface transfers the heat from the combustion products to the water vapor medium and withstands the pressure of this medium; it is subjected to both heat and pressure, operating under extremely harsh conditions, and constitutes the fundamental structure for the operation of a boiler. Such as the furnace shell of a boiler, the water wall tubes, and the lower abdominal wall of the boiler drum. (2) Flame and flue gas: When fuel burns in a boiler, gaseous combustion products are generated. The heat released by fuel combustion first heats the gas combustion products themselves, raising their temperature to very high levels. Then, these combustion products transfer the heat to the water vapor medium through the heated surfaces, while their own temperature continues to drop. Combustion products with a temperature above 900°C are usually referred to as flames, or high-temperature flue gases ； Combustion products with a temperature of 900°C or lower are called flue gas. (3) Radiant heating surface: A heating surface that absorbs heat from combustion products through radiant heat transfer. It generally refers to the heating surfaces inside the furnace that can absorb radiant heat and are in direct contact with the flame, such as the walls of water-cooled tubes and the walls of the furnace chamber. (4) Convective heating surface: A heating surface that absorbs heat from high-temperature flue gas through convective heat transfer. It generally refers to the heated surfaces subjected to flue gas erosion, such as the walls of flue tubes and the walls of convective tube banks. (5) Furnace and flue: The space in a boiler that is made of bricks or metal and used for burning fuel is called the furnace, or combustion chamber. The furnace chamber is the place where fuel burns and where radiation heat exchange occurs between the flame and the heated surfaces. After leaving the furnace, the combustion products flow through the flue toward the chimney. Heating surfaces are also typically installed in the flue; as the flue gas flows through it, heat is transferred to the water vapor medium via these heating surfaces. Therefore, the flue is the passage for flue gas flow and further heat exchange with the heated surfaces. Generally, the temperature of the combustion products decreases gradually from the furnace chamber to the flue; inside the furnace chamber there is a flame, but once it exits the furnace chamber and enters the flue, its temperature drops rapidly as it turns into flue gas. (6) Fuel consumption: The amount of fuel consumed by a boiler per unit of time is referred to as fuel consumption. (7) Waste discharge volume: During waste discharge, the flow rate of the wastewater is referred to as the waste discharge volume. (8) Steam quality: The degree of purity of steam is indicated by its quality. Typically, saturated steam contains more or less trace amounts of saturated moisture, and steam with a moisture content exceeding the standard requirements is generally considered to be of poor quality. (9) Boiler thermal efficiency: The percentage of the total heat supplied to the boiler that is absorbed by the boiler’s water and steam medium is known as the boiler’s thermal efficiency, denoted by the symbol η. Thermal efficiency is the main economic indicator used to evaluate boilers. Its formula is: η = (heat output/heat input) × 100%. For steam boilers, the thermal efficiency is given by η = … × 100%; for hot water boilers, it is η = … × 100%. (10) Water tubes, flue tubes, and fire tubes: Both water tubes and flue tubes are convection heating tubes in boilers. The difference is that in water-tube boilers, the flue gas flows outside the tubes (such as in a convective tube bank), whereas in fire-tube boilers, the flue gas flows inside the tubes. There are two types of fire tubes: those with a larger diameter are generally referred to as the furnace chamber, in which grates are installed; they constitute the main radiation heating surface in shell-and-tube boilers. Fire tubes with a smaller diameter are also called smoke tubes. (11) Water-tube boiler: A boiler in which water and steam flow inside the tubes, while flue gas flows outside the tubes is called a water-tube boiler. (12) Shell boiler: A boiler in which the heating surfaces are mainly arranged inside the shell is called a shell boiler. It includes horizontal shell boilers, vertical shell boilers, etc. (13) Horizontal shell boiler: A boiler in which the longitudinal axis of the shell is parallel to the ground, the fuel burns inside the furnace chamber or in an external combustion chamber, and the flue gases flow into the flue tubes is called a horizontal shell boiler. (14) Vertical boilers: Boilers whose longitudinal axis of the boiler shell is perpendicular to the ground are called vertical boilers. (15) Steam boilers – Boilers used to generate steam are called steam boilers, and they are generally used for industrial purposes. (16) Hot water boilers – Boilers used to generate hot water are called hot water boilers, and they are generally used for heating purposes. (17) Natural circulation boiler: A boiler in which the water circulation is achieved by the density difference between the water in the downcomer and the steam-water mixture in the upcomer is called a natural circulation boiler. The lower the operating pressure of the boiler, the greater the density difference, and the more reliable the circulation. (18) Forced circulation boiler: A boiler in which the circulation of water inside the boiler is achieved primarily through the head pressure of a circulation pump, in addition to relying on the density difference between water and the water-vapor mixture, is called a forced circulation boiler. (19) Direct-flow boiler: A boiler in which a mixture of water and steam, as well as steam itself, passes through all the heating surfaces once under the pressure of the feed water pump is called a direct-flow boiler. It has only interconnected heating surfaces, with no drum.

The original poster can find a professional book on boilers; it provides detailed information on all the categories. You can post a book on boiler principles, or on boiler design and calculation on the forum; it’s worth taking a look at – it will definitely help improve your knowledge.

Thank you to everyone above; I will organize the information provided. I’m actively looking for textbooks at the moment.:)

There are quite a number of textbooks, such as Principles of Boilers (there are likely many versions, and all of them can be used as introductory materials)

Classified by use: power station boilers, industrial boilers, boilers for vehicles and ships, domestic boilers, etc； Classified by fuel: coal-fired boilers, oil-fired boilers, gas-fired boilers, etc ； Classified by outlet steam pressure: low-pressure boilers (P〈2.5MPa), medium-pressure boilers (2.5〈P〈4.0MPa), high-pressure boilers (4.0〈P≤10MPa), ultra-high-pressure boilers (10〈P≤13.7MPa), subcritical boilers (13.7〈P≤16.7MPa), and supercritical boilers (P≥22MPa). Power station boilers are used in power plants, while industrial boilers are commonly used in factories other than power plants. Boilers for vehicles and ships are mainly used on ships.

First, focus on your own field of study, gain familiarity with it before expanding the scope – that might be more practical!

I also support that book on boiler principles

Some time ago, I came across a project: it was related to the use of sludge as fuel in circulating fluidized bed boilers. It could be considered a new challenge in the field of fuels

Industrial boilers, power station boilers. What we usually come into contact with are industrial boilers. Power station boilers are located in thermal power plants.

Personally, I think the principle behind Erchen’s boilers is excellent; the same principle applies whether it’s a large boiler or a small one. There are three cycles: water (steam), air (flue gas), and fuel, along with heat transfer between these various media