Boiler Classification

Steam Boiler

In the first half of the 18th century, steam engines used in British coal mines, including Watt's early steam engines, operated at steam pressures equal to atmospheric pressure. In the latter half of the 18th century, steam pressures exceeding atmospheric pressure were adopted. By the 19th century, the common steam pressure reached approximately 0.8 MPa. Correspondingly, the earliest steam boilers were large-diameter vertical cylindrical shells that held water, later switching to horizontal shells, with fires burning beneath the shell.

Double-flue boiler

As boilers grew larger, fire tubes were added inside the shell to increase the heating surface area. Fire is burned at the front end of the tubes, and the flue gases exit from the back, passing through brick-lined flues to the chimney, thereby heating the exterior of the shell. This type of boiler is known as a fire tube boiler. Initially, only one fire tube was installed, referred to as a single fire tube boiler or Cornish boiler. Later, two fire tubes were added, known as a double fire tube boiler or Lancashire boiler.

Horizontal External Combustion Reheat Pipe Boiler

Around 1830, after mastering the production and expansion technology of high-quality steel pipes, fire-tube boilers emerged. Some fire tubes were installed inside the boiler shell, forming the main heating surface of the boiler, with the flame (flue gas) flowing through the tubes. A large number of fire tubes were installed below the water level line of the boiler shell, known as horizontal external fire-tube boilers. They have a lower metal consumption but require extensive masonry.

Pot drum

In the mid-19th century, the water-tube boiler emerged. The heating surface was the water pipes outside the boiler shell, replacing the boiler shell itself and the firebox and fire tubes inside. The increased heating surface area and steam pressure of the boiler were no longer limited by the diameter of the boiler shell, which was beneficial for enhancing the evaporation capacity and steam pressure of the boiler. The cylindrical boiler shell in this type of boiler was then renamed to the drum, or commonly referred to as the steam drum. Early water-tube boilers used only straight pipes, and the pressure and capacity of these straight-pipe boilers were both limited.

In the early 20th century, steam turbines began to develop, requiring boilers with higher capacity and steam parameters. The straight tube boilers were no longer sufficient. With advancements in manufacturing techniques and water treatment, bent tube boilers emerged. Initially, they were multi-drum designs. As water-cooled walls, superheaters, and economizers were adopted, and improvements were made to the steam-water separation elements within the drums, the number of drums gradually decreased, saving metal and also benefiting the boiler's pressure, temperature, capacity, and efficiency.

Safety

Gas boilers differ from other gas appliances due to their unique installation location, high gas consumption, and higher safety requirements. Therefore, in North America, all gas fireplace products must undergo rigorous safety and environmental protection standards before being launched into the market. These standards are continuously updated with the development of the products. Each type of fireplace product has specific safety testing standards. For instance, the testing standards for balanced gas fireplaces and flue gas fireplaces are different, as are those for decorative gas fireplaces and heating gas fireplaces. Consequently, North American balanced gas fireplace products should have certifications such as Ansi Z21.88 or CSA2.33, indicating that the product's safety and environmental protection meet the standards, and users can use them with confidence.

Auxiliary Boiler

The earlier fire-tube boilers, water-tube boilers, and firebox boilers were all types of natural circulation boilers. Due to varying heating conditions in the ascending and descending pipes, density differences were created, resulting in natural flow. While developing natural circulation boilers, the use of once-through boilers began in the 1930s, and auxiliary circulation boilers were introduced in the 1940s.

Forced circulation boiler

The auxiliary circulating boiler, also known as a forced circulation boiler, is an evolution of the natural circulation boiler. A circulating pump is added to the downcomer system to enhance the water circulation of the evaporative heating surface. In a once-through boiler, there is no drum. The feedwater is supplied to the economizer by the feedwater pump, passes through the evaporative heating surfaces like the water-cooled walls and superheater, and becomes superheated steam that is sent to the steam turbine. All the flow resistances in the various sections are overcome by the feedwater pump.

Composite circulating boiler

Following World War II, both types of boilers experienced rapid development due to the demand for high-temperature, high-pressure, and large-capacity power generation units at the time. The aim of developing these boilers was to minimize or eliminate the need for a drum, allowing for the use of small-diameter tubes as heating surfaces and providing more flexibility in their arrangement. With advancements in automatic control and water treatment technologies, they became increasingly mature. At supercritical pressure, once-through boilers were the only feasible option. In the 1970s, the largest single unit capacity was a 27 MPa pressure with a 1300 MW power generation unit. Subsequently, a hybrid circulation boiler was developed, which combined auxiliary circulation boilers and once-through boilers.

Gas boiler maintenance involves its own routine, not just the boiler itself but also the auxiliary equipment and accessories. Taking care of a single gas boiler unit is a complex matter, as the boiler's structure is intricate. Only regular normal use and thorough maintenance can ensure a longer lifespan for the boiler. Compared to more complex boiler types like circulating fluidized bed boilers and power plant boilers, the maintenance of gas hot water and gas steam boilers is relatively easier due to the simpler structure of the boiler itself, but it is still much more challenging than standard machinery.

1. Cleanliness:

Open the burner door, remove the rear shell panel, and unscrew the boiler's side cleaning port screws. Clean the gas pipeline with a wire brush. Clean the combustion chamber with a soft-bristled brush. Remove the residue inside the boiler, tighten the cleaning port screws, close the burner door, and check if the seal ring is intact.

2. Antifreeze

If the boiler is installed in an area with temperatures below 0°C, the following protective measures must be taken: If the boiler room is outdoors, lower the indoor temperature to 15°C or 18°C to keep the boiler running all day. If the room will be unoccupied for an extended period, drain the water from the boiler.

3. Cathodic Protection

Boiler components are made of various materials, such as cast iron, copper, and aluminum. Cathodic protection measures must be taken to prevent electrochemical corrosion.

4. Wastewater Discharge

Bottom drain of gas boilers can prevent smoke condensation. Smoke condensation occurs due to water content in the flue gas (diesel or natural gas) and low flue gas temperature. Burning 1kg of diesel produces 1.04kg of water. Some of this water is exhausted through the chimney, some is absorbed by the chimney walls, and the remainder settles at the bottom of the chimney. The amount of water produced is affected by factors such as flue gas temperature (efficiency reaches 91% with flue gas temperature at 170℃), the total surface area of the chimney, the material of the chimney, and the insulation of the chimney.

5. Burner Maintenance:

Maintenance of gas boilers and burners is best conducted after the heating season, rather than at the start of it. This is because combustion products can corrode the boiler and accumulate carbon deposits, so it's crucial not to allow these products to linger in the boiler for too long.

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