Structure of tubular heating furnaces and refractory materials used

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Inside a lined, enclosed chamber, a large number of interconnected high-quality or alloy seamless steel tubes are placed. The heating medium flows through these tubes at high speeds; fuel burns within the enclosed chamber, generating hot flue gases. These hot flue gases transfer heat to the medium being heated through radiation, convection, and conduction, thereby raising the temperature of the medium to the level specified by the production process or enabling a certain degree of chemical reaction to take place； These types of equipment are collectively referred to as tubular heating furnaces. The tubular heating furnace we are discussing today is a process heating furnace used in oil refining and petrochemical production facilities, commonly referred to as petrochemical process heating furnaces. http://www.znhcl.com/uedits/c029a71236c5454cabf957f3dc755142.jpg A tubular heating furnace used in petrochemical processes consists of a radiation chamber, a convection chamber, a waste heat recovery unit, a burner, an air supply system, and a smoke exhaust system. 1. Radiation chamber: The radiation chamber is the part that plays a key role in radiant heat transfer within a heating furnace. Since it is the location where the flame is directly present, it is the part of the heating furnace with the highest temperature. Careful consideration must be given to the composition, strength, thickness, and mechanical structure of the materials used there. Functionally, 70–80% of the total thermal load of the furnace is absorbed in the radiation chamber. In furnaces where fluids undergo reactive conversion and cracking, reactions and cracking generally take place entirely within the radiation chamber. It is therefore no exaggeration to say that the quality of the radiation chamber determines the quality of the heating furnace. 2. Convection chamber: The convection chamber is the part of the heating furnace that plays a key role in convective heat transfer. It features closely arranged tube bundles, and this structure enables the flue gas to move at higher speeds through the tubes, thereby facilitating effective convective heat transfer. Generally, the convection chamber absorbs 20–30% of the total heat load of the furnace. The convection chamber can be located above the radiation chamber ； It can also be placed on the ground and connected to the furnace body via a flue, but the principle remains the same. Nipple tubes or finned tubes can be used in the convection chamber. 3. Waste heat recovery device: A waste heat recovery device is a system used to recover the heat from the flue gas exiting the convection chamber; or, in the absence of a convection chamber, from the flue gas exiting the radiation chamber. The waste heat recovery measures currently adopted by refineries at home and abroad fall into roughly the following categories: a. From the perspective of the overall process flow and plant balance, cold feed – hot oil for air preheating. b. From the perspective of improving and simplifying the circulation system, there is a heat carrier circulation – hot oil to preheat the air. Including open-loop and closed-loop heat carrier cycles. c. Considering the addition of auxiliary recycling equipment, there are ① steel tube type ； ②Glass tube type ； ③rotary ； ④Cast iron pipe type ； ⑤vortex generator type ； ⑥Heat pipe type ； ⑦Plate-type air preheaters, etc. d. From the perspective of energy savings in combination with other equipment, there is ① a waste heat boiler ； ②Cogeneration. 4. Burners are one of the key components of a heating furnace. The quality of their technical performance, as well as their suitability to the furnace’s operational requirements, structure, and heat transfer characteristics, have a direct impact on the furnace’s operation, energy consumption, and environmental performance. Choosing a burner whose technical performance matches the process requirements of the heating furnace, its structural design, and heat transfer characteristics is of great importance for ensuring the efficient operation of the heating furnace, as well as its long-term, safe, stable, and optimal performance. 5. Ventilation system: The function of a ventilation system is to introduce air for combustion while expelling smoke outside the furnace. There are two types: natural ventilation and forced ventilation. The former relies on the ventilation power of the chimney ； The latter uses an exhaust fan to forcefully draw out the smoke. Generally, when the pressure loss inside the furnace is low, natural ventilation through a chimney is used. The chimney is installed at the top of the furnace and integrated with it; its height should be such as to overcome the pressure loss inside the furnace and ensure sufficient ventilation. Heating furnaces with complex structures, those experiencing high pressure losses inside, or those equipped with waste heat recovery systems often use induced draft fans to achieve forced ventilation. 6. Furnace tubes and elbows: Furnace tubes and elbows are the most important components in tubular heating furnaces, accounting for 40% of the total steel used in such furnaces and 60% of the total investment cost. To withstand pressure, corrosion, and direct heat from flames, and to ensure the successful completion of processing tasks as well as production safety, the furnace tubes are designed in accordance with the following standards: The length and total number of furnace tubes are determined based on the heat load, thermal efficiency of the tubular furnace, and its structural design. The pipe diameter and number of tube passes are determined based on flow rate, allowable pressure loss, and residence time. The wall thickness is determined by internal pressure, thermal stress, static loads, wall temperature, material strength (including high-temperature creep and brittleness), and corrosion margin. The properties of a material are determined by its allowable temperature, high-temperature strength (including creep strength), heat resistance, corrosion resistance, and brittleness. Carbon steel furnace tubes are used when the furnace tube temperature is below 500°C ； Low-alloy steel furnace tubes (T9~T22) are used when the furnace tube temperature is between 500℃ and 700℃ ； Stainless steel furnace tubes (304, 316) are used when the furnace tube temperature is between 700°C and 800°C ； Use 309 furnace tubes when the tube temperature is between 800°C and 900°C ； High-alloy steel centrifugally cast furnace tubes (HK-40) are used when the furnace tube temperature exceeds 900°C. The radiant tubes of the tubular heating furnace are made of plain tubes, while the convection chamber uses nailhead tubes or finned tubes. There are two methods for connecting furnace tubes to elbows: welding and expansion jointing. When welding is used, the elbows are sharp bends formed by pressing furnace tubes, with 180° elbows being the most common; cast elbows are less frequently used. When expansion jointing is used, cast elbow fittings with plugs are employed; by removing the plugs, mechanical cleaning of the inside of the pipe can be carried out. The downside is that they are more expensive, as they are made from the same material as the furnace tubes. In heating furnaces where the oil to be heated is light, making coking of the furnace tubes less likely, or those equipped with online coking removal technology, elbow bends with sharp curves can be used throughout. In heating furnaces where the oil to be heated is heavy, leading to a higher risk of coking in the furnace tubes and where no online coking removal technology is available, cast elbow bends can be used in the radiation chamber section. 7. Pipe supports and guide frames: The pipe supports bear the weight of the heating pipes inside the furnace and transfer this weight to the framework of the heating furnace. The requirements for a tube support are: excellent resistance to oxidation and corrosion, high strength at high temperatures, and the ability to remain freely expandable and contractable structurally as the temperature of the heating tube changes. For the material of the tube supports, 25Cr—12Ni steel or 25Cr—20Ni steel is used in the radiation chamber and the lower part of the convection chamber. Dense-furnaced cast iron “HR” and 5Cr alloy steel are used in the upper part of the convection chamber. The problem with using high-chromium steel is high-temperature brittleness, as well as corrosion caused by vanadium and sulfur in the fuel. In such situations, the bracket should be designed with a replaceable structure; high-temperature brittleness should be suppressed using ferrite, and corrosion caused by vanadium can be addressed through a cast protective layer. Generally, knife-edge type tube supports are used in the radiation chamber, while orifice plate type tube supports are used in the convection chamber; the pipe rack merely serves to guide the tubes. 8. Refractory materials: Refractory materials are essential for heating furnaces, and they can be roughly divided into shaped materials and unshaped materials. a. Shaped refractory materials refer to heavy refractory bricks, light refractory bricks, and insulating bricks, etc. Heavy refractory bricks are characterized by their high refractoriness, and they are used in the combustion channels of burners as well as in the lining on the fire-facing side of heating furnaces with high temperatures. Lightweight refractory bricks possess both refractoriness and thermal insulation properties, and are commonly used for the side walls and roof of furnaces. Insulation bricks have very low heat resistance, and are mainly used for lining the furnace bottom and side walls in small quantities. Users in need of refractory products such as refractory bricks can visit the Refractory Materials Website at http://www.znhcl.com. Manufacturers across the country supply directly, and Alipay transactions are supported; goods are available in stock with immediate payment. b. Amorphous refractory materials generally refer to amorphous refractory materials that are made by mixing aggregates and cement with water and are constructed in a manner similar to concrete; they are simply known as castables. They are divided into castables that prioritize fire resistance along with certain thermal insulation properties (heavy castables) and insulating castables (lightweight ones); the methods of application include pouring, coating, and spraying. For tubular heating furnaces, plastic castables with high fire resistance and certain thermal insulation properties are used for the furnace roof, side walls, and bottom. For the linings of flues and chimneys, as well as the inner linings of their side walls, insulating plastic castables are used. Ceramic fiber blocks, rock wool blankets, and ceramic fiber spraying also belong to non-shaped refractory materials. 9. The furnace frame, side walls, and platform constitute the steel structure that bears all the loads of the heating furnace; the material used is ordinary carbon steel, with channel steel and I-beam being commonly employed. The side panels are the steel plates on the outer surface of the furnace wall; their function is to protect against rain and dust, as well as to prevent unnecessary air from entering the furnace. Based on their function, they can be divided into two categories: those that help bear part of the structural load of the furnace, and those that serve only as a covering. Platforms and ladders are provided for operation and maintenance, and are used for daily operations, maintenance, and inspections.