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This post was last edited by Fuleide on 2019-3-10 21:29. Chronic tumors lurking in steam equipment. As we all know, it’s warmer to wear shoes that are a bit larger in winter. The reason is that air provides good thermal insulation. The thermal conductivity of air is 0.025 W/(m·℃), that of water is 0.6 W/(m·℃), that of iron is 75 W/(m·℃), and that of copper is 390 W/(m·℃). Thanks to the excellent insulating properties of air, double-glazed windows, with air inserted between them, provide excellent heat retention. Most insulation materials, such as asbestos and glass fiber, contain a large number of tiny pores, and it is the air within these pores that plays the main role in insulation. However, the presence of non-condensable gases such as air in steam heating equipment can have serious consequences. Especially due to their hidden nature and persistence, they gradually reduce the equipment’s output capacity without being detected or noticed by most customers, eroding the equipment’s integrity – essentially acting as a \"chronic tumor\" for steam equipment. How exactly does air \"torment\" steam equipment? A. Reduction in the efficiency of heating equipment: Due to the extremely low thermal conductivity of air, the heat transfer efficiency of steam heat exchange equipment decreases when non-condensable gases such as air are present in it. The higher the air content, the lower the heat transfer efficiency. Non-condensable gases generally accumulate in larger quantities near the condensation surface of heat exchange equipment, and in smaller amounts at farther distances. Generally, for ease of understanding, we can consider these non-condensable gases as forming an air film on the inside of the heat exchanger. As mentioned above, due to the very low thermal conductivity of air, an air film 1 millimeter thick has a heat transfer effect equivalent to that of a copper wall 15.6 meters thick. The presence of an air film significantly reduces the efficiency of heat transfer from the steam side to the material side, which in turn **lowers the production efficiency of the heat exchange equipment**. B. Decrease in steam temperature and increase in steam consumption According to Dalton’s law of partial pressures, the total pressure of a gas mixture such as steam and air is equal to the sum of the partial pressures of each component gas occupying the entire space alone. For example, if the total pressure of the mixture of steam and air is 4 bar a (absolute pressure), and the volume of non-condensable gases such as air accounts for 25% of the total volume, then the partial pressure of air is 1 bar a, while the partial pressure of steam is 3 bar a; the pressure gauge will display 3 bar g (gauge pressure). If the steam temperature is inferred from the reading on the pressure gauge, it should be around 144°C; however, in reality the vapor pressure is only 2 barg, so the actual steam temperature is about 134°C, which is lower than the temperature corresponding to a pressure reading of 3 barg. The decrease in steam temperature also reduces the temperature gradient across the heat transfer surface, leading to a reduction in the heat exchange rate. As a result, operators have to increase the steam pressure to compensate for the problems caused by non-condensable gases, which in turn increases steam consumption. C. Causing corrosion in heat exchange equipment: When non-condensable gases such as air are present in the equipment, carbon dioxide and oxygen in them tend to dissolve in the condensate water, especially when the temperature of the condensate water is low, resulting in a greater amount of dissolution. Carbon dioxide dissolves in water to form carbonic acid, causing corrosion. Oxygen dissolved in water is also highly corrosive. Where do non-condensable gases such as air come from? Boiler feed water contains certain non-condensable gases such as air; when the water in the boiler vaporizes to form steam, these non-condensable gases enter the steam pipeline system and heat exchange equipment along with the steam. Ultimately, steam turns into condensed water after releasing its latent heat in the heat exchange equipment; however, these non-condensable gases cannot be condensed and remain accumulated within the equipment until measures are taken to remove them. Unless the boiler make-up water is completely demineralized and deoxygenated, the sodium carbonate salts remaining from the water treatment process will decompose to form carbon dioxide when heated in the boiler. In high-pressure boilers, although make-up water is deoxygenated through physical and chemical methods, the nitrogen present in the make-up water is often left untreated and eventually enters the heat exchange equipment. Even with the best physical and chemical treatments, the steam generated by boilers will inevitably contain non-condensable gases. When the equipment is started up, there is a lot of air in the pipes and within the equipment. During normal operation, even though the system is filled with steam, when the equipment stops running and the steam supply is cut off, the steam behind the shut-off valve condenses instantly, its volume decreasing to about one-thousandth of its original value. This creates a vacuum in the space, and air enters the system through valves, flange connections, and the ends of the equipment. This is especially true for heat exchange equipment equipped with vacuum breakers to facilitate drainage and prevent the equipment from deforming due to negative pressure; in such cases, the interior of the equipment becomes filled with air. I will share with you later on how to remove this “chronic tumor”. (This article is from “Fured Steam Energy Saving”)