【Haichuan Illustrated Chemical Equipment】Series Posts – Working Principle of Superheaters

Thread Content

This post was last edited by HaiChuanLaoYu on 2026-5-2 09:15. Get to know equipment, understand it, and make good use of it. [HaiChuan’s Illustrated Guide to Chemical Equipment] series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196. Everyone is welcome to participate in the discussions. ----------------------------------------------------------- 1 What is a superheater? The core function of a superheater is to further heat saturated steam, while keeping the pressure constant, so that it becomes superheated steam at a specific temperature. For thermal power plants, increasing the initial temperature and pressure of the steam is the most direct way to improve the overall cycle efficiency. Data shows that for every 14–15°C increase in the temperature of superheated steam, the steam consumption of the entire system can be reduced by 1%. Furthermore, in specific industrial processes such as those in the chemical industry, high-temperature superheated steam is also an essential production medium. The realization of all this relies primarily on efficient heat conduction and convection mechanisms within the superheater. 2 Classification of superheaters: 1. Based on their structure and design, superheaters can be divided into vertical and horizontal types. 1) Vertical superheaters are the most commonly used type in industrial boilers. It is installed in a vertically suspended manner; such superheaters are less prone to dust accumulation. The coiled tubes do not require any additional lifting devices, and the steel frame that supports the header can be fixed to the furnace wall or to a steel frame outside the flue, allowing it to expand freely. Its drawback is the difficulty in draining water; before each startup, steam pressure is required to facilitate drainage. If the water is not completely drained, water remains inside the pipes during shutdown, which can lead to corrosion. 2) The horizontal superheater is installed horizontally; its advantage is that drainage is easy and scaling is less likely to occur ； The disadvantages are that dust tends to accumulate between the pipes, lifting is difficult, and the hanging parts and brackets can overheat and get damaged when exposed to high temperatures. 2. Based on the heat transfer method, they can be divided into convective, radiant, and semi-radiant types. 1) The convective superheater obtains heat in the flue gas duct behind the furnace outlet, through the scouring of the superheater by the flow of flue gas. 2) The radial superheater is located at the upper part of the furnace where the flame temperature exceeds 1000°C; it obtains heat primarily from high-temperature radiation. 3) The semi-radiant superheater is located at the furnace exit, receiving heat both from high-temperature radiation and from flue gas convection. Radial and semi-radial superheaters are commonly used in boilers with high parameters and large capacity. 3. Based on the flow direction of steam and flue gas, they can be classified into three types: co-current, counter-current, and mixed-flow. 1) Arranged in the downstream direction. As shown in Figure (b). In this arrangement, the steam and flue gas flow in the same direction, resulting in a small average temperature difference; therefore, its heat exchange efficiency is lower than that of a counterflow arrangement, and a larger heat transfer area is required. Since the steam inlet is located in the area of high-temperature flue gas, if the steam contains a large amount of water, salt deposits are likely to form inside the tubes at the inlet, thereby raising the tube wall temperature. This arrangement often increases the risk of burning out the tubes. 2) Counterflow arrangement, as shown in Figure (a). The flow of steam and smoke in these arrangements is in opposite directions. In terms of heat transfer efficiency, this arrangement provides the best heat exchange, enabling the greatest average temperature difference to be achieved. Compared to flowing with the current, the heat transfer area required to achieve the same superheating temperature is reduced, saving steel. However, in the first few rows of tubes, there is high-temperature steam on the inside and high-temperature flue gas on the outside; this not only makes them prone to overheating and damage but also leads to coking between the tubes. Therefore, for the first few rows of tubes, when the wall temperature of the superheated tube exceeds 480°C ； Alloy steel pipes must be used. 3) Mixed-flow arrangement, as shown in Figure (c). In this arrangement, the steam inlets and outlets are both located in the middle of the flue gas. The steam is first heated by the low-temperature flue gas, and then directed to the front where it is heated by the high-temperature gas. In terms of the flow direction of steam and smoke, it is first against the flow and then with the flow ； In terms of the process of steam being heated, it goes from low to high. Since the steam inlet is located in the low-temperature section of the flue gas, even if salt deposits accumulate inside the pipes at the inlet, the risk of tube wall overheating is much lower compared to when the steam flows in the same direction. Among the three aforementioned arrangements, the mixed-flow type is the best; it takes advantage of the strengths of both co-current and counter-current arrangements while avoiding their weaknesses, thereby improving the reliability of operation of the superheater and ensuring a large average temperature difference. 3. Accessories to be installed on the superheater: 1. A steam trap should be installed at the outlet of the superheater. Superheaters equipped with inlet header devices should also have steam traps. 2. The superheater outlet header should be equipped with a safety valve, a pressure gauge, temperature measurement instruments, and an exhaust valve.