Cast iron plate air preheater

The plate-fin cast iron air preheater is a new type of air preheater developed by taking advantage of the erosion resistance and corrosion resistance of cast iron, combined with the advantages of plate-fin heat exchangers. It boasts advantages such as a long service life, low resistance, and a compact design, enabling reliable operation over a wide range of flue gas temperatures (e.g., 650°C to 140°C) for an extended period of time (typically 10 years). Suitable for flue gas waste heat recovery systems in petrochemical tubular heating furnaces and various types of boilers. Main performance and features 1. Physical properties of cast iron: a) The metal matrix of cast iron contains a large amount of free carbon ； b) The cast iron matrix usually contains a pearlitic structure, which gives the cast iron materials mechanical properties similar to those of steel ； c) Free carbon exists in the form of very thin plates or flakes. d) Free carbon is inert to most corrosive media. e) Cast iron exhibits excellent corrosion resistance over a wide range of operating conditions. Initially, its surface will show slight rusting, but afterward, almost all of the rust is absorbed by the carbon flakes. The corrosion resistance of cast iron is incomparable to that of steel. In the chemical industry, there are many examples of the durability of cast iron. 2. Structural features a) The plate-fin cast iron air preheater consists of multiple plate-fin cast iron heat transfer tubes combined in various ways to form a single-tube or multi-tube assembly. b) Each plate-fin cast iron heat transfer tube consists of a cast iron plate-fin tube, a fixed-end support, and a sliding-end support. The space between the fixed support and the heat transfer tube, as well as between the sliding support and the heat transfer tube, is sealed with packing, which is compressed by compression strips. As shown in the figure: c) It has a compact structure and occupies little space. d) Light weight: Under the same heat load, the plate-fin cast iron air preheater is more than 30% lighter than the tube bundle air preheater, and is comparable to or slightly heavier than the heat pipe air preheater. e) Modular structure, suitable for assembly and transportation. 3. Thermal performance: a) High heat transfer coefficient; the heat transfer coefficient calculated for flat plates can reach 50–60 W/m2·℃, which is unmatched by conventional tubular air preheaters and heat pipe air preheaters. b) It has good airtightness and a low air leakage rate. c) Under the same operating conditions, cast iron air preheaters have a longer lifespan than carbon steel air preheaters. It usually can last for over 10 years. 4. Material and manufacturing process: a) While ensuring a certain heat exchange area, the fin density of the cast iron heat transfer tubes can be adjusted according to the temperature of the flue gas from the user. b) Ductile iron shall be used exclusively for cast iron heat transfer tubes. c) To further enhance wear resistance and resistance to corrosion caused by flue gas acid dew points, the low-temperature flue gas contact surface of the heat transfer tubes can be lined with enamel or coated with other anti-corrosion coatings. d) For single-tube and integral pressure testing, the gas leakage rate must not exceed 4% within half an hour. e) Low price. The main problems with existing air preheaters are as follows: In the flue gas waste heat recovery systems of petrochemical tubular heaters, the common issues with current steel tube-type and heat pipe-type air preheaters are that when the flue gas temperature is low (for example, below 160°C), the wall temperature at the flue gas outlet can easily drop below the acid dew point temperature of the flue gas, resulting in severe corrosion caused by low-temperature acid dew points. As a result, their service life is generally short, sometimes only a few months. Furthermore, ordinary steel-heat exchanger tubes are prone to burst and fail when the flue gas temperature is above 350°C. The typical lifespan of ordinary steel-heat exchanger tubes is about 3 years. In the flue gas waste heat recovery systems for various types of boilers, since boiler flue gas is a corrosive polluted gas containing large amounts of ash particles, soot, and sulfuric acid vapor, the common problem with existing steel tube air preheaters is severe wear and corrosion as well as a short service life. The common issues with conventional steel-water heat exchanger fin-type air preheaters include ash accumulation on the fins, ongoing corrosion, high costs, and tube failure at high temperatures. In recent years, steel-tube air preheaters and heat-pipe air preheaters have begun to use enamel technology on the surface of their flue gas-side tube bundles to address the issue of acid dew point corrosion caused by low-temperature flue gas. Enameled tubes have advantages such as wear resistance, corrosion resistance, and a smooth surface that prevents dust accumulation, which helps to address the problem of corrosion that often affects steel air preheaters. However, if the enamel tube has defects such as pinholes or chipped enamel, corrosion is still inevitable.

Only heat pipes can achieve a heat transfer coefficient of 50–60 W/m2·°C; cast iron plates have a thickness of at least 6 MM and also have a low thermal conductivity. Please read some materials on heat transfer

This guy has studied these topics, but his understanding isn’t deep enough; first of all? ? Does corrosion occur when the flue gas temperature is below 160 degrees? ? This statement is definitely incorrect. Even when the fuel is residues or heavy oil and other flue gases with a high sulfur content, the dew point temperature of such flue gases is usually around 160 degrees, so dew point corrosion does not occur. Alternatively, by modifying the heat exchange structure of the heat pipe, it is also possible to prevent corrosion. Second: But your claim that the tube will burst at temperatures above 350 degrees is one-sided as well; heat pipes can be used in flue gases at 1000 degrees. It is the temperature inside the tube that determines whether it will burst, not the temperature of the flue gases. The tube will explode only when the pressure inside it exceeds the pressure limit that the tube can withstand. Third: Does the heat pipe have a lifespan of only three years? ? Where did you see it? ? During the initial phase of heat pipe promotion, they were manufactured by the Institute of Chemical Engineering at Nanjing University of Technology, with a service life of over 8 years. Later, as industrial production began, many companies started manufacturing them; in pursuit of cost efficiency, many steps in the manufacturing process were omitted, which resulted in a significant reduction in the performance and lifespan of these heat pipes. However, the lifespan of heat pipes manufactured by reputable manufacturers is guaranteed to be 8–10 years. However, during use, heat pipes experience a decline in thermal efficiency, which generally becomes apparent after three years, with an efficiency reduction of 3 to 5%. I’m just giving a brief explanation, haha.

I’d like to add a few more points. Cast iron plate heat exchangers are resistant to acid dew point corrosion in terms of materials, but this phenomenon still occurs; as a result, the equipment tends to accumulate dust, requiring frequent cleaning with water. Moreover, cleaning can only be done by water, which makes daily maintenance of such equipment rather troublesome. In my opinion, if heat pipe heat exchangers are used, the problem can be solved at a technical level, preventing acid dew point corrosion from occurring. This also makes it less necessary to clean the equipment regularly. If there is a lot of dust, a dust cleaning device can be installed to enable online cleaning, allowing the equipment to operate effectively and stably over the long term. Heat pipes do not have the problems of lifespan and corrosion mentioned by others; it’s that some manufacturers lack the capability to produce them properly! Hehe

A heat transfer coefficient of 20–25 W/m2·°C is considered good for a cast-iron plate preheater. Dust accumulation in the fin channels leads to a further reduction in the thermal coefficient; therefore, under the same design conditions, the size and weight increase.

In the corrosion test conducted by the Corrosion Laboratory of the Shanghai Institute of Materials on domestically produced two-way double-fin cast plate-type air preheaters, the corrosion rate under conditions of 70°C, 50 wt% H2SO4 solution, and the original as-cast surface was 5.57 mg/cm2·h. At this rate of corrosion, a 10mm thick cast iron plate with a surface area of 1 cm2 weighs about 0.0785 kg; it should be completely corroded after around 14,000 hours. Assuming 8,000 hours of operation per year, this means it will take less than two years. Typically, cast iron plate heat exchangers are used in the tail flue, where the exhaust gas temperature is around 130–150°C, and the air inlet temperature is about 20°C. The wall temperature at the air inlet side of the preheater should be below the dew point temperature. Considering the characteristics of dew point corrosion as well as the composition of the exhaust gases, the sulfuric acid that forms is concentrated at first and then becomes diluted; there will always be a temperature range in which sulfuric acid with a concentration of 50 wt% exists. As a result, after two years, it is possible for localized perforations to occur in the cast iron plates. As far as I know, foreign cast-iron plate heat exchangers can last for 30 years because a glass-tube air preheater is installed after their cast-iron plate preheaters. Moreover, the fins on these foreign cast-iron plates are lower at the front and higher at the back, which adjusts the fin ratio on the flue gas side and the air side at low temperatures, allowing the temperature of the plates to be closer to that of the flue gas side and thus minimizing the risk of dew formation. In China, due to issues with manufacturing processes, the fin height on each plate is usually consistent, and cast-iron plate preheaters typically employ a cross-mixed flow pattern (single cross). In principle, it is impossible to avoid the dew point temperature in such cases. The high heat transfer coefficient also needs to be reconsidered: at the same flow rate, that of the cast iron plate is likely to be slightly higher than that of the turbulence generator, as there are many fins on both the flue gas side and the air side, whereas the turbulence generator does not have such fins. But when comparing it with heat pipes, there’s no point in making such a comparison at all. A look at the formula for calculating the heat transfer coefficient makes this clear – the heat transfer coefficient inside a heat pipe is many times higher than that outside, to the point where it can be practically ignored. In contrast, the heat transfer coefficients on the flue gas side and the air side of the cast iron plate are quite similar; taking the reciprocal of these values gives a value that is at most half of the heat transfer coefficient of just one of those sides. Having cast-iron plates at the back of a furnace that burns heavy oil is very problematic; dust accumulation is inevitable, and it cannot be resolved by increasing the flow rate of the smoke gases. Even if this can provide some relief, the fan pressure required must be much higher, which in turn increases electricity consumption significantly. The above are merely personal opinions; my knowledge is limited, so I hope for your guidance!

Plate-type air preheaters are also gradually being used for air preheating; information on their advantages, disadvantages, and related technologies can be found at http://bbs.hcbbs.com/thread-344019-1-1.html.

I have a few suggestions. Does a heat pipe heat exchanger corrode below 160 degrees? General corrosion is related to the wall temperature of the tube; the most effective way to deal with this issue is by raising the wall temperature. In normal designs, the exhaust temperature of heat tubes that use coal as fuel is set at 140 degrees, while low-temperature heat tubes can achieve a recovery temperature of 120 degrees. If the fuel is something like heavy oil, which has a high sulfur content and a relatively high dew point temperature, it can normally be reduced to 140 degrees. The service life is not as short as 3 years as you claim; perhaps you encountered products made by small manufacturers. Nowadays, heat pipes have been commercialized, and in pursuit of profits, many manufacturers skip various processing steps or are simply unaware of such steps, which results in heat pipes having a short lifespan. Nanjing University of Technology was the first in China to conduct research on heat pipes; the heat pipes they developed at that time could have a lifespan of up to ten years. Generally, heat pipes can last for over 5 years, with a normal service life of more than 8 years. It’s not the case that the tube will burst just because the temperature exceeds 350 degrees; tube failure is determined by the temperature inside the tube, not by the temperature of the flue gas. We have ones that reach 1000 degrees, as well as heat pipes. Hehe

Our company’s 1 million-ton natural gas methanol plant uses plate-type air preheaters imported from the United States. The pressure drop and temperature values do not meet the design specifications. When the plant was put into operation in July of this year, at 70% load, the pressure drop on the air side reached 2100 Pa (the design value being 1100 Pa), while the pressure drop on the flue gas side reached 2000 Pa (the design value being 1500 Pa). The induced draft fan reached its maximum speed at 75% load, and it is no longer possible to increase the load any further. The flue gas exit temperature is above 240 degrees. Could everyone please help analyze this? Thank you