Is a waste heat boiler a heat exchanger?

A waste heat boiler, also known as a excess heat boiler, is a heat exchanger that uses high-temperature streams from the production process as a heat source to generate steam. It serves both as a cooler for these high-temperature streams in the process flow and as a device that utilizes waste heat to produce steam. In ethylene plants, for example, the pyrolysis gas coming out of the tubular furnace (with temperatures as high as 800–900°C) is directly fed into a quench waste heat boiler, where its temperature is reduced to 350–600°C within 0.1 seconds. This not only prevents a decrease in ethylene yield due to excessive pyrolysis but also enables the production of high-pressure steam at pressures above 10 MPa, which plays an important role in improving the thermal efficiency and economic benefits of the entire plant. Currently, waste heat boilers are widely used in many production processes, such as those in the nitric acid, sulfuric acid, synthetic ammonia, and petroleum refining industries, to enable full or partial self-sufficiency in power and thermal energy required for these processes. 　Classification 　Based on their structural design, waste heat boilers can be divided into shell-and-tube types and flue-type types. The structure of shell-and-tube waste heat boilers differs little from that of shell-and-tube exchangers, and they also come in fixed-plate, floating-head, and U-tube types. The structure of a flue-type waste heat boiler is similar to that of a conventional boiler; it has a furnace built from refractory bricks, and a tube bank is installed inside the furnace. High-temperature gases pass through the furnace, heating the water flowing within the tube bank and causing it to vaporize. 　　Depending on whether the high-temperature gas passes through inside or outside the tubes, waste heat boilers are divided into fire-tube waste heat boilers and water-tube waste heat boilers. A fire-tube waste heat boiler features high-temperature gases flowing within the furnace tubes. Boiling water flows from the drum through the downcomers to the bottom of the boiler, where it is heated on the tube walls and turns into a mixture of steam and water, which then returns to the drum via the upcomers. The fire-tube waste heat boiler has a simple structure; since the shell bears the steam pressure, it is suitable for producing steam at low pressures. A water-tube waste heat boiler features boiling water flowing inside the tubes, with hot gas flowing outside them. Since the boiler shell does not bear steam pressure, it is suitable for producing high-pressure steam. Based on the flow pattern of the boiling water, tube waste heat boilers can be further divided into: ① natural circulation type, where the driving force for the flow of boiling water within the tubes is the density difference between the water in the downcomer and the steam-water mixture in the upcomer. To ensure a certain circulation volume, the steam drum must have sufficient installation height. ②Forced circulation type: a circulation pump is installed in the downward piping to increase the flow rate of the boiling fluid within the pipes, thereby preventing bubble formation from impairing heat transfer. Due to the many disadvantages of this type of boiler in terms of operation, maintenance, and investment, it is used only in special circumstances. Features Compared with ordinary boilers, the main features of waste heat boilers are: ① High temperature and pressure; for example, in the production of synthetic ammonia, the waste heat boiler located after the heavy oil vaporizer has an inlet temperature of 1350°C and a pressure of 3.2–4.0 MPa ； ②Quenching: In certain industrial processes such as the cracking of petroleum hydrocarbons to produce ethylene, it is necessary to cool the gas as quickly as possible in order to reduce side reactions; therefore, waste heat boilers must have a high heat transfer rate ； ③Strictly control the gas outlet temperature; when high-temperature gases (such as pyrolysis gases) are cooled, if the temperature drops below the dew point of the high-boiling-point components, these components will condense and precipitate out. If it contains corrosive gases such as SO3, acid mist will be formed as well, which can corrode equipment ； ④Due to the high dust content, and the fact that some high-temperature gases contain dust (for example, the flue gas at the outlet of the boiling furnace in sulfuric acid plants contains about 200 g/m3 of dust, while petroleum cracking gases contain particles such as carbon black), it is necessary to take into full account the risks of dust blockage and abrasive wear in the design ； ⑤Low pressure drop – the pressure drop of gases must be kept within the limits permitted by the process conditions; ⑥ Sealing: Some high-temperature gases are flammable, explosive, or toxic, and sealing often poses a challenge in the design of waste heat boilers.

A waste heat boiler, also known as a residual heat boiler, is merely a heat exchanger for gas-water/steam, but it lacks the combustion process found in a conventional boiler. Recovering waste heat can improve the thermal efficiency of the cycle.

Waste heat boiler has two meanings: waste heat, and boiler. So its definition is: a boiler that uses waste heat to produce steam. There are two types: for waste heat utilization and for chemical production. The former is not much different from a regular boiler; the only difference is the fuel. The latter is what is often mentioned: no fuel is burned; instead, the waste heat from the production process is utilized to cool the process gases, and at the same time, the waste heat from these gases is used to generate steam. It is a device in the production process that connects the preceding and subsequent processes, which is different from ordinary ones. Theoretically, it should be a heat exchanger.

From the perspective of heat transfer, it is a heat exchanger