[Haichuan Talk] What is a bayonet boiler? Where is it mainly used?

Plug-in heat exchanger https://bbs.hcbbs.com/thread-193970-1-1.html (Source: Huahaichuanliu-Haichuan Chemical Forum Network)

Bayonet heater. This type of heat exchanger is commonly used in evaporation processes and can also be used in vertical or horizontal installations. Heat exchangers installed in evaporators can be constructed of glass or glass-lined.

Waste heat recovery can not only bring economic benefits in terms of energy, but also reduce environmental pollution； Waste heat boiler is an important equipment that uses waste heat in industrial production processes to produce steam. It is widely used in waste heat recovery in chemical or petrochemical industries such as coal gasification industry, ammonia industry, ethylene, sulfuric acid, and nitric acid industry. ； Waste heat boilers are installed in various industrial processes to recover waste heat and play a very important role in improving the thermal efficiency of the overall device, reducing environmental pollution and meeting certain process requirements. Traditional fixed tube plate waste heat boilers are equipped with thick tube plates in high temperature areas, which need to compensate for the relative elongation caused by temperature differences. The outer casing is restricted by the tube plate and cannot expand and contract freely, requiring an expansion compensation structure. The biased flow phenomenon in the water tank of the traditional bayonet tube type waste heat boiler prevents the feed water from being evenly distributed into the inner casing, resulting in a flow dead zone in the steam and water tank. The sealing structure of the waste heat boiler shell is complex, and the connection between the outer casing and the thick tube plate is prone to leakage. The plug at the bottom of the outer casing is prone to accumulation of debris, which worsens the flow of circulating water and causes poor cooling of the casing end and burnout.

The Hengli methanation project adopts the technology of Danish Topsoe Company. The core components of the project include two methanation waste heat boilers, one No. 1 steam superheater and two No. 2 steam superheaters. The methanation waste heat boiler product is large in size, with a length of more than 19 meters and a total weight of about 310 tons. Methanation waste heat boiler products of such a large size previously relied on foreign imports. The deep-hole welding of the flexible tube plate is a new structure, and the welding forming requirements are high and extremely difficult. The No. 1 steam superheater adopts a bayonet-type heat exchange tube structure and is currently the world's largest Topsoe technology steam superheater. The manufacturing precision of the heat exchange tubes is extremely high, the process is complex, the equipment is large in size, more than 21 meters in length, and the total weight is about 390 tons. It is extremely difficult to manufacture methanation waste heat boilers and steam superheater products with Topsoe structure. Topsoe has only authorized the manufacturing qualifications of two domestic companies, including Dongfang Boiler.

The advantages and disadvantages of the bayonet tube waste heat boiler are as follows. (l) Advantages: There is no thermal stress caused by temperature difference between the tube and shell, so it can be used in situations with large temperature difference and high pressure. ； From the connection structure between the tube plate and the shell, and the inner tube and the tube plate, it is relatively convenient to disassemble, assemble, clean and repair. ； Due to the vertical structure, it is conducive to the natural circulation of water during production. (2) Disadvantages: In terms of the structure of the inner and outer walls of the shell, it is relatively complex. ； This equipment has no sewage discharge facilities and has strict requirements on the quality of the boiler feed water. Otherwise, scale accumulation in the tube will cause heat transfer to deteriorate and cause the heat transfer tube to burst and be damaged.

The principle of steam generation by bayonet tube waste heat boiler is as follows. The high-temperature reforming gas of about 1000°C from the second-stage reformer enters the boiler through the gas distributor on the side of the furnace bottom. It is cooled to 482°C by water heat exchange with the baffles and heat transfer tubes and comes out from the upper end of the shell, as shown in Figure 14-2. The 286°C boiler feed water from the drum through the downcomer enters the high-pressure tube box from the top, flows from top to bottom into the inner tube inserted into the tube, and then flows out from the inner tube upward into the annulus of the inner and outer tubes. It flows together with the high-temperature process steam entering from the lower part of the shell and conducts heat transfer upward, causing the water to be continuously heated, evaporated, and vaporized into a mixture of steam and water. Due to the different density differences of water and steam, a heavy pressure head is formed between the downcomer and the riser to balance all the resistance losses of water and steam in the flow, so that the boiler can form a natural circulation of water without a power source. The driving force ρ of natural circulation can be expressed by the following formula: ρ = (H + h) (ρ water - ρ mixture) g where ρ water - the density of water in the downcomer ； ρmix - the density of the steam-water mixture in the riser ； h - effective length of bayonet tube ； H - the position difference between the steam drum and the boiler ； g - acceleration due to gravity. The driving force of natural circulation overcomes the frictional resistance of the fluid in the circulation path and establishes a normal circulation. As a result, the steam-water mixture returns to the steam drum through the riser pipe, and the steam-water separation is completed to generate 10.MPa high-pressure saturated steam.

In the 1970s, large quantities of imported chemical fertilizers used bayonet tube waste heat boilers.