1 The influence of raw materials (impermeable graphite) on the performance of round hole graphite heat exchanger

Impermeable graphite has the advantages of good corrosion resistance, high thermal conductivity, and higher operating temperature. However, the use of graphite raw materials (particle size, impregnant, degree of graphitization, etc.) affects the service life and safety of the equipment.

1.1 Particle size

The larger the particle size of the graphite base material, the smaller the material bulk density and the lower the mechanical strength. The larger the holes between the particles, the more through holes may be exposed when drilling holes, and the porosity increases. In order to achieve the required compactness, the amount of impregnating agent will increase, which will affect the quality of impregnation and the thermal conductivity of impermeable graphite. The greater the density, the greater the mechanical strength. Therefore, if the base material with high bulk density (fine particles, grade B) is selected for immersion heat treatment, the quality of the equipment processed and produced has high stability and safety, and the use effect is good.

1.2 Impregnant

The impregnant fills the gaps in the graphite base material, and the type of impregnant will directly affect the corrosion resistance of impermeable graphite. The amount of immersion will affect the temperature and corrosion resistance of impermeable graphite. The impregnation vacuum is large and the pressure is high, which can reduce the number of impregnations, and the heat exchange effect is better than the traditional impregnation process (three-pass impregnation, three-pass heat treatment).

Because the corrosion resistance of the impregnating resin is much lower than that of the graphite itself, the more the immersion amount, the worse the corrosion resistance and the worse the temperature difference. Therefore, it is generally required that the immersion amount should not exceed 10%. Phenolic resin impregnated impermeable graphite can meet the requirements of this type of equipment. Impermeable graphite materials are treated at a medium temperature of 180∽300℃, which can increase the stability of the impregnating resin and extend the service life of the equipment.

1.3 Degree of graphitization

Graphite materials can generally be divided into two categories: semi-graphitized electrodes (regenerated electrodes) and graphitized electrodes. The thermal conductivity of the regenerative electrode is lower than that of the graphitized electrode, so it is not suitable for use as a thermal conductive material. The thermal conductivity of the graphitized electrode is λ=100∽130w/m·k or higher, and the resistance to rapid temperature difference is better than that of the regenerative electrode. The degree of graphitization directly affects the heat exchange effect of the equipment, and all foreign countries are fine-particle chemical special graphitized materials. my country's newly promulgated national standard "Graphite Pressure Vessel" recommends the use of Grade B graphite materials close to foreign standards.

2 The structure and characteristics of round hole type graphite heat exchanger

The round hole type graphite heat exchanger is a kind of graphite heat exchanger with more advanced and superior performance. The cylindrical heat exchange block adopts standard unit blocks, which has high structural strength. This structure does not use cementing agent but adopts PTFE O-ring sealing medium. It is equipped with a pressure spring as an automatic compensation mechanism for thermal expansion and contraction. Short passages, additional redistribution chambers to improve the turbulence effect, etc. Therefore, this heat exchanger has the characteristics of high structural strength, strong temperature and pressure resistance, good impact resistance, high heat transfer efficiency, long service life and easy maintenance. This series of products are widely used as heaters, coolers and condensers.

Technical characteristics: Design temperature: -20～200℃ (can be increased to 320℃)

Design pressure: normal pressure～0.6MPa (this type of product with a pressure of 0.6～2.4MPa can also be designed and manufactured according to user needs) Specifications: 3～400 square meters

3 Heat transfer calculation of graphite heat exchanger

The application of graphite equipment can be divided into two forms of liquid-liquid and liquid-gas heat exchange. The flow directions of the two fluids in the round-hole graphite heat exchanger are cross-flow and simple baffle. That is: the longitudinal direction flows in one direction, and the transverse direction (cooling water or frozen salt water) and the longitudinal direction are perpendicular to the longitudinal direction and flow in one direction first, and then fold back to flow in the opposite direction, so that the baffle movement is repeated.

The calculation of the average temperature must be multiplied by the correction coefficient εΔt, εΔt>0.8 is reasonable, otherwise the process parameters need to be adjusted.

For the calculation of the heat transfer coefficient of the graphite heat exchanger, please refer to the data in Table 2 and Table 3; the scale coefficient αd (w/m2·k) of the medium is shown in Table 4.

Experience value of heat transfer coefficient of graphite heat exchanger Table 2

No. Processing material Average temperature ℃ Heat transfer

Form Heat transfer coefficient K w/(m2·k) Remarks

1 Hydrogen chloride gas cooling 100∽40 cooling 34.89∽46.52 plate type

2 Hydrogen chloride condensation (97% HC1 water cooling) 75∽40 condensation 34.89∽174.45 tube type

3 Hydrogen chloride condensation (99% HC1 brine cooling) 40∽-15 condensation 17.45∽34.89 tube and tube type

4 Hydrochloric acid cooling (21%HC1) 130∽25 cooling 581.5∽930.4 tube, block hole type

5 Hydrochloric acid cooling (31%HC1) 80∽40 cooling 348.9∽465.2 plate type

6-tube gas cooler 34.89∽58.15