Heat exchanger selection under large temperature difference conditions
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The operating conditions are as follows: hot side: 280°C (to be cooled to around 0°C), gas phase, 8 Bar pressure； Cold side: around -65°C, liquid phase, 4 Bar pressure ； The goal is to cool the thermal medium to 0°C without any phase change occurring. Customer B considered several options: enameled condensers, fluorine-lined heat exchangers (I didn’t quite understand what he meant by this), graphite heat exchangers, heat exchangers made of special materials, and silicon carbide heat exchangers. Option C was ruled out: 1) Enameled condensers: These can be immediately excluded. Enamel is a composite structure made of two materials; the coefficients of thermal expansion of the enamel and the base steel differ significantly. Under conditions of large temperature differences and pressure, the enamel layer is prone to cracking. Furthermore, with such high temperatures and pressures, it is difficult to find gaskets suitable for achieving sealing. 2) Fluorinated-coated heat exchangers (or possibly pure PTFE heat exchangers): Can the welding joints made of PTFE withstand such high temperatures? How to avoid the effects of heat and cold? Can the pressure-temperature combination be tolerated? 3) Graphite heat exchangers: The temperature difference between the inlet and outlet can reach 260°C, whereas in conventional graphite heat exchangers such a difference is only allowed to be a few degrees at most. Graphite is a composite material formed through impregnation; given such a large temperature difference and the different coefficients of thermal expansion of the two materials, what is the likelihood of them separating? 4) Heat exchangers made of special materials: First of all, regardless of corrosion resistance issues, with traditional tubular welded structures, the welds under high temperature differences are prone to failure; are U-tube and floating-head types viable alternatives? 5) Silicon carbide heat exchangers: They can definitely withstand high temperatures, and they’re also resistant to temperature fluctuations. This is because the tubes and tube sheets are relatively independent of each other, with no restraining forces acting on them. The issue lies in whether the material of the tubes themselves can endure such low temperatures. This is an interesting case; discussions are welcome
In such operating conditions, graphite heat exchangers are recommended, but not all types of graphite heat exchangers are suitable; for example, GH-type tubular graphite heat exchangers are not appropriate, as the pressure inside the tubes is high and the temperatures are high, which makes these tubular graphite heat exchangers prone to damage； The YK type round-block hole graphite heat exchanger is fully capable of handling such operating conditions. The block-hole graphite heat exchanger is used for cooling the gas in graphite hydrogen chloride synthesis furnaces; the gas temperature is 360 degrees in such applications. The temperature of the cooling water is around 10 degrees in winter and around 32 degrees in summer, and no problems arise with long-term use.
-Can graphite be used at 65°C? Won’t the composite separate at such a low temperature for graphite impregnation?
There is a substantial difference between impregnation and coating. The impregnation process for impregnated graphite involves thoroughly drying the porous, permeable graphite, placing it in an impregnation tank, and creating a vacuum to an extreme negative pressure (with a gauge pressure of -0.095 MPa in the low-altitude plains of East China), maintaining this condition for 2 hours. Resin is then introduced under these negative pressure conditions, ensuring that it covers the graphite by more than 100 mm. Once the vacuum is released, compressed air is introduced at a pressure of 0.6–1.0 MPa, and this pressure is maintained for more than 2 hours (usually throughout the night). The resin is subsequently removed, and after thorough draining, the pressure inside the impregnation tank is kept above 0.6 MPa. The temperature is increased according to a specified heating curve to promote the polymerization and curing of the resin. Repeat these steps three times in total, and the impregnation process for impermeable graphite is then complete. The penetration depth of the resin into the impregnated graphite must be 50 mm or more; microscopic examination reveals that the actual penetration depth can reach 100 mm or more. The resin that has penetrated into the graphite structure has been bonded to the graphite itself through polymerization and curing. Under special circumstances, the resin in the lower layers may separate, but the resin that has penetrated into the graphite structure will not peel away from the matrix like the surface resin, nor will this affect the impermeability of the graphite.
It is possible to consider using two heat exchangers in series. The first unit is cooled with desalinated water, using a water-gas co-current flow, with the generated steam used to recover thermal energy ； The second one is cooled by a refrigerant, with water and gas flowing in counterflow. This configuration not only recovers thermal energy but also reduces the energy consumption of the refrigerant.
I asked the clients about this; they want to use a single unit, but I’m not yet aware of the specific reason
Using multiple units with multi-stage cooling is definitely more reasonable
In terms of energy consumption and material selection, multiple heat exchangers with such a large temperature difference are superior! Material selection should be easier after staged cooling!
If the temperature difference is too large, it is better to use two-stage or even multi-stage heat exchange. Single-stage heat exchange can lead to various unforeseen problems and potential hazards, ranging from the design of the equipment and the choice of materials to the subsequent processing steps. Users can have wonderful ideas, but heat exchangers, their heat exchange efficiency, and their manufacturing are all practical issues. If users were truly divine like God, then let God handle such minor problems himself.
Haha, the user’s wish is great – it’s just to drive us to the brink of exhaustion