The role and application of graphite heat exchanger

What is the main function of a graphite heat exchanger? As for the graphite heat exchanger, its function is closely related to its use, so let's examine them together.

Graphite heat exchangers are widely used. Radiators used in daily life, condensers in steam turbine devices, and oil coolers on space rockets are all examples of graphite heat exchangers. They are also extensively utilized in the chemical, petroleum, electric power, and atomic energy industries. Their primary function is to ensure the specific temperature required by the medium in the process, and they are also one of the main devices for improving energy utilization efficiency.

A graphite heat exchanger can be a standalone device, such as a heater, cooler, or condenser; it can also be a component of a processing equipment, such as the heat exchanger in an ammonia synthesis tower.

Due to limitations in manufacturing technology and scientific advancements, early heat exchangers could only adopt simple structures, featuring small heat transfer areas, large sizes, bulky volumes, and cumbersome designs, such as coil heat exchangers. With the development of manufacturing technology, the shell-and-tube heat exchanger gradually emerged. It not only boasts a larger heat transfer area per unit volume but also exhibits superior heat transfer efficiency. For a long time, it has been a typical heat exchanger in various industries.

A hybrid heat exchanger, also known as a contact heat exchanger, is a type of heat exchanger that performs heat exchange through direct contact and mixing of hot and cold fluids. After the two fluids are mixed and exchange heat, the graphite heat exchanger needs to be separated in a timely manner. This type of heat exchanger is suitable for heat exchange between gases and liquids. For example, in cooling towers used in chemical plants and power plants, hot water is sprayed from the top to the bottom, while cold air is inhaled from the bottom to the top, on the surface of the water film filled with fillers or the surface of the container. Water droplets and air droplets, hot water and cold air, come into contact with each other to exchange heat. The hot water is cooled, the cold air is heated, and then separated in a timely manner through the density difference between the two fluids.

A regenerative heat exchanger is a heat exchanger that utilizes alternating flows of cold and hot fluids across the surface of a thermal storage medium (valuable material) within a regenerator to exchange heat, such as the preheated air storage chamber beneath a coke oven. This type of heat exchanger is primarily used for recovering and utilizing the heat from high-temperature exhaust gases. A similar device used for recovering cold air is called a cold storage, which is mainly employed in air separation equipment.

The cold and hot fluids of a divided heat exchanger are separated by a solid partition plate and exchange heat through the plate. Therefore, it is also known as a surface heat exchanger. This type of heat exchanger is widely used.

Based on the structure of the heat transfer surface, split-wall heat exchangers can be classified into tubular, plate, and other types. Tubular heat exchangers utilize the surface of the tube as the heat transfer surface, including coil heat exchangers, double-tube heat exchangers, and shell-and-tube heat exchangers. Plate heat exchangers use plate surfaces as the heat transfer surface, including other types of heat exchangers such as spiral plate heat exchangers, plate-fin heat exchangers, plate-shell heat exchangers, and umbrella plate heat exchangers, which are designed to meet specific conditions and special requirements. Examples include scraper heat exchangers, rotary disc heat exchangers, and air coolers.

The relative flow direction of fluids in a heat exchanger typically falls into two categories: forward flow and counterflow. In forward flow, the temperature difference between the two fluids at the inlet is relatively large, gradually decreasing along the heat transfer surface until the temperature difference at the outlet is smaller. In counterflow, the temperature difference between the two fluids along the heat transfer surface is more evenly distributed.

Under the same heat transfer conditions, using counterflow will increase the average temperature difference. This reduces the heat transfer area of the heat exchanger; if the heat transfer area remains unchanged, the consumption of heating or cooling fluid can be reduced when using counterflow. The former can save equipment costs, while the latter can save operating costs. Therefore, counterflow heat exchange should be used as much as possible in design or production.

When one or both of the cold and hot fluids undergo phase change (boiling or condensation), the temperature of the fluid itself remains unchanged because only latent heat of vaporization is released or absorbed during the phase change process. Therefore, if the inlet and outlet temperatures of the fluid are equal, the temperature difference between the two fluids is independent of the choice of fluid flow direction. Besides downstream flow and counterflow, there are also other flow directions, such as crossflow and zigzag flow.