Some issues regarding the manufacturing process of glass-lined tubular heat exchangers and graphite heat exchangers at Wuxi Weiyee

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1. Tube finish rate: Glass-lined heat exchange tubes can be either internally or externally lined. For tubes with internal glass lining, the fluid flows through the tube side, where it is cooled or heated; this type of tube is suitable for liquid substances. The fast flow rate of the fluid through the tube side creates a falling film effect on the inner wall of the tube, which facilitates rapid cooling or evaporation of the liquid. The standard sizes for tubes with internal glass lining are generally DN32*2100, DN50*2100, DN50*3100, or custom sizes can also be manufactured. Materials with external glass lining are generally used in the shell side; the shell side requires glass lining treatment. The outer wall of the tubes with external glass lining is also coated with glass lining. Cooling water flows through the tube side, and this setup is suitable for cooling gaseous materials. With the materials flowing in the shell side, there is ample space for condensate to accumulate, making it easy to remove it. The standard dimensions for tubes with external glass lining are usually Φ*57*3100, or longer lengths can be produced as custom orders. Enameled steel tubes are all produced using the cold enameling process: a bottom coat is applied on one side, while two to three top coats are applied; the thickness of the enamel is controlled between 0.8 and 1 mm. Well-type furnaces are used, along with a spiral-shaped heating method that allows for gradual temperature increase. Enameled heat exchange tubes do not deform easily and maintain good verticality. Since there are no flanges or chamfers in their structure, there is no risk of the enamel cracking due to stress. After the enameling process is complete, these tubes undergo two periodic tests using electric spark testing, with current pulse testing being employed as well. Another test is carried out before assembly, to ensure that 100% of the assembled heat exchange tubes are free from any conductive issues. 2. What is the machining accuracy of the tube sheet, and in particular, how can the thickness and accuracy of the glass lining be ensured? Traditional glass-lined tubular heat exchangers use fixed or split-type glass-lined tube sheets, but this approach is not viable. The reason is that when there are many holes in the tube sheet, stress levels increase, and the chamfered areas are prone to ceramic failure. As a result, traditional glass-lined tubular heat exchangers almost always use large-diameter glass-lined tubes, which reduces the number of holes in the tube sheet and thus lowers the stress levels. However, this approach is unsuitable for applications requiring large-area heat exchange; under pressure, the ceramic layer still fails, and if the tube sheet is damaged, the entire heat exchanger becomes unusable, thereby increasing operating costs. Our company has abandoned the traditional glass-lined tube sheet design; instead, we use a **patented structure featuring PFA tube sheets that are molded integrally with a steel lining. PFA is heated and molded onto steel plates with a thickness of 40 mm, resulting in a lining thickness of 4–5 mm. Such tube sheets can withstand materials at high temperatures of up to 250°C, as well as high pressures and vacuum conditions. Since the molding is done by opening the mold, the lining thickness is determined by the thickness of the mold, ensuring high precision. 3. Sealing method between the tube sheet and the tubes, as well as material of the seals: Wuxi Weiyee Chemical’s new glass-lined tubular heat exchangers are detachable types that utilize a double-tube-sheet sealing system. When cooling water is used in the shell side, the first tube sheet is made of carbon steel; each tube sheet contains a packing box. The glass-lined heat exchange tubes have threads on their outer sides, and rubber composite O-rings are placed inside the packing boxes. After the glass-lined tubes are installed, packing is added and carbon steel nuts are screwed on to achieve sealing. This mechanism is similar to that of hydraulic solid-sealing. Once all seals are in place, a pressure test is conducted on the shell side; such heat exchangers can withstand pressures of up to 0–1.0 MPa, which is sufficient for most cooling or heating applications available on the market. After the pressure testing is completed, the second steel-lined PFA tube sheet is installed; a packing box is also placed inside this tube sheet. The PFA material is molded into a single unit, and fluororubber composite O-rings or tetrafluoroethylene composite packing are used in the packing box, depending on the actual operating conditions of the material. Pure PFA nuts, which have been molded, are then screwed in to achieve sealing. Once the main body of the heat exchanger is assembled, end caps are attached to both ends, followed by pressure testing on the tube side – it can withstand pressures ranging from -0.1 to 1.0 MPa. When the material flows through the shell side, using externally lined glass tubes, the direction of the anti-corrosion coating on the tube sheet is opposite to that in heat exchangers with internally lined glass tubes; the principle of sealing remains the same. The advantages of this structure are that it overcomes the drawback of traditional glass-lined shell-and-tube heat exchangers, where the glass lining can burst and become unusable; moreover, it is detachable, easy to clean, and easy to maintain, thereby significantly reducing the operating costs for manufacturers. 4. Can glass-lined tubes have both the tube side and the shell side made of glass lining? Sure, it’s necessary to specify the purpose for doing this – is the cold source corrosive? In such a case, anti-corrosion design is required. 5. The operating temperature and pressure range for your products: Operating temperature –50°C to 250°C; Operating pressure –0.1 MPA to 1.0 MPA. 6. In terms of price, they are slightly more expensive than graphite heat exchangers. The exact price depends on the operating conditions and the total heat exchange area. However, their service life and warranty period are much longer. Compared to graphite heat exchangers, tube-type heat exchangers offer higher heat exchange efficiency due to their structural advantages; taking maintenance costs into account, they provide good cost-performance! Disadvantages of graphite heat exchangers: 1. Poor resistance to high temperatures. Graphite blocks are generally made of impregnated graphite, which contains resin; as a result, their maximum temperature tolerance is low, and they tend to get damaged at higher temperatures. 2. Poor pressure resistance: The structure of graphite heat exchangers is generally block-and-hole type. Due to this structural flaw, when the heat exchange area increases, these exchangers are prone to leakage under high positive pressures or high vacuums. Once a leak occurs, operations must be halted for maintenance and new seals installed; all of these are maintenance costs. Although the unit price might be low, the overall cost considering maintenance makes it uneconomical to use such exchangers. 3. It is not resistant to high levels of corrosion and organic solvents. Due to the manufacturing process of graphite heat exchangers, the graphite used is modified graphite that contains resin; when exposed to highly corrosive organic solvents, its service life is very short, and stockpiling such graphite is even necessary to maintain production. 4. Low heat exchange efficiency: In block-hole graphite heat exchangers, the thermal conductivity of the graphite material itself is not an issue; however, due to their block-hole structure with recirculation flow, the convective heat transfer coefficients between the cold and hot sources are very low. There are dead zones as well, and some areas where fluid flows do not receive sufficient cooling, resulting in low utilization of the heat exchange surface area. 5. Incomplete discharge of material: When there is pressure during the cooling process, the material can be discharged along an S-shaped path normally. However, once production stops, due to structural reasons, the remaining material loses its pressure; after cooling, it tends to accumulate and clog inside the heat exchanger. Over time, a crystalline film forms on the surface of the graphite, which reduces the heat exchange efficiency of the graphite heat exchanger. The only solution is to wait until maintenance time to clean it out piece by piece, which increases maintenance costs. 6. Post-use graphite treatment is difficult; once graphite heat exchangers are damaged and become unusable, it is hard to recycle them, and environmental regulations cannot be met.