Dangers of using superheated steam!

Thread Content

This post was last edited by goldliyang on 2017-3-14 at 14:49. During market inspections, it was found that many industrial manufacturers use the superheated steam supplied by power plants directly in their process heating equipment; this phenomenon has been observed in several companies recently. Superheated steam reduces heating efficiency and production efficiency, increasing energy consumption. Superheated steam must first be cooled down to its saturation temperature before it can release its latent heat; during this cooling process, the heat energy released by the superheated steam is very small compared to the latent heat. When the degree of superheating is high, this cooling process takes longer. Moreover, when the superheated steam on the surface of the heat exchanger releases heat and lowers the temperature, it is difficult for the superheated steam farther away from the heat exchange surface to release heat and cool down, as superheated steam has a conductivity as low as that of air, making it itself a poor conductor of heat. Therefore, although the temperature of superheated steam is higher than that of saturated steam under the same pressure, its heat transfer efficiency is much lower than that of saturated steam. The heat transfer coefficient of superheated steam varies depending on the process, but it is much lower than that of saturated steam. It is difficult to predict the heat transfer coefficient of superheated steam; generally speaking, the higher the degree of superheating of the steam, the lower its heat transfer coefficient. In most cases, for coiled tube heat exchangers, the heat transfer coefficient of superheated steam is 50–100 W/(m2·°C), whereas that of saturated steam is 1200 W/(m2·°C). Due to the low heating efficiency of superheated steam, production efficiency decreases. To ensure production volume, companies are inevitably forced to use more steam, resulting in energy waste. Using superheated steam shortens the equipment’s lifespan, increases the frequency of maintenance, downtime, and maintenance costs, and affects production. In heat exchangers, many process fluids form dry films and scale on the heat exchange surfaces; scaling occurs more rapidly at high temperatures, which hinders heat transfer and causes the walls of the heat exchanger to overheat, leading to its failure. Furthermore, the equipment is subjected to intermittent changes in temperature, with uneven temperatures at the top and bottom, as well as thermal expansion and contraction that generate stress. This makes the equipment prone to deformation, welds prone to cracking, and seals prone to leakage. When superheated steam is used, the heat exchange area of the heat exchanger needs to be larger. Overheated steam heating affects the equipment’s output, which in turn impacts production volume. To achieve the same production capacity, the heat exchange area of the heat exchange equipment needs to be increased. Based on practical experience, for every 2°C increase in superheat, the heat exchanger requires a 1% increase in its heat exchange area. During the heat release process of superheated steam, a temperature gradient exists, which affects product quality. The steam temperature varies greatly, resulting in a large temperature gradient; heat exchange is not a process that occurs at a relatively constant temperature. This affects the stability and uniformity of the temperature of the material being heated, thereby impacting the quality of the product. Apart from steam turbines, saturated steam should be used in almost all kinds of heating processes. Within the heat exchanger, the heat release of saturated steam occurs primarily through the release of latent heat associated with phase change, and the amount of latent heat released is much greater than that of sensible heat released due to temperature reduction. The heat transfer coefficient of saturated steam can be dozens of times higher than that of superheated steam. Therefore, for heating purposes, in order to rapidly turn into saturated steam during the heating process, superheated steam with a superheat of more than 10°C should not be used.