Are there any differences between positive-displacement heat exchangers and shell-and-tube heat exchangers?

Volume-type heat exchangers can be classified into three types based on the heat transfer mechanism: mixed-type, regenerative-type, and partition-type. Volumetric heat exchangers and mixed-type heat exchangers are heat exchangers that exchange heat through the direct contact and mixing of cold and hot fluids; they are also known as contact heat exchangers. Since the two fluids must be separated promptly after mixed heat exchange, this type of heat exchanger is suitable for heat exchange between gas and liquid fluids. For example, in the cooling towers used in chemical plants and power stations, hot water is sprayed from above downward, while cold air is drawn in from below. On the surface of the water film in the filler, as well as on the surfaces of droplets and mist, hot water and cold air come into contact with each other for heat exchange: the hot water is cooled and the cold air is heated, after which they are separated promptly due to the difference in density between the two fluids. Regenerative heat exchangers are types of heat exchangers that utilize the alternating flow of cold and hot fluids over the surface of the regenerative material (filling) located in the regeneration chamber, thereby enabling heat exchange; an example is the regeneration chamber used to preheat air beneath coke ovens. These types of heat exchangers are mainly used to recover and utilize the heat from high-temperature waste gases. Devices of a similar type designed to recover cold energy are called heat exchangers, and they are commonly used in air separation units. In a volumetric heat exchanger, also known as a surface heat exchanger, the cold and hot fluids are separated by a solid partition, and heat exchange takes place through this partition; these types of heat exchangers are the most widely used. Interwall heat exchangers in positive-displacement heat exchangers can be classified into tubular, plate-type, and other types depending on the structure of their heat transfer surfaces. Tubular heat exchangers use the surface of the tubes as the heat transfer surface, including coiled tube heat exchangers, shell-and-tube heat exchangers, and tubular shell heat exchangers, among others ； Plate-type heat exchangers use plates as the heat transfer surfaces, including plate heat exchangers, spiral plate heat exchangers, plate-fin heat exchangers, shell-and-plate heat exchangers, and umbrella plate heat exchangers, among others ； Other types of heat exchangers are designed to meet certain specific requirements, such as scraped surface heat exchangers, rotary disk heat exchangers, and air coolers. In positive-displacement heat exchangers, the relative flow direction of the fluid is generally either co-current or counter-current. During flow in the forward direction, the temperature difference between the two fluids is greatest at the inlet and gradually decreases along the heat transfer surface, reaching its minimum at the outlet. During counterflow, the temperature difference distribution between the two fluids along the heat transfer surface is relatively uniform. Under the condition that the inlet and outlet temperatures of the cold and hot fluids remain constant, and when neither fluid undergoes phase change, the average temperature difference is greatest in counterflow and smallest in co-flow. In volumetric heat exchangers, by using counterflow while achieving the same amount of heat transfer, the average temperature difference can be increased, thereby reducing the heat transfer area of the exchanger ； If the heat transfer area remains unchanged, using counterflow can reduce the consumption of the heating or cooling fluid. The former can save on equipment costs, while the latter can save on operating costs; therefore, counterflow heat exchange should be used as much as possible in design or production. In positive-displacement heat exchangers, when there is a phase change (boiling or condensation) in one or both of the cold and hot fluids, only the latent heat of vaporization is released or absorbed during this phase change; as a result, the temperature of the fluid itself does not change. Therefore, the inlet and outlet temperatures of the fluid are equal, and in such cases the temperature difference between the two fluids is independent of the direction in which the fluid flows. In addition to the two flow directions of co-current and counter-current, there are also flow directions such as cross-flow and mixed-flow. In the heat transfer process of positive-displacement heat exchangers, it is an important issue to reduce the thermal resistance in shell-and-tube heat exchangers in order to improve the heat transfer coefficient. The thermal resistance mainly stems from the thin layers of fluid adhering to the heat transfer surfaces on both sides of the partition (known as boundary layers), as well as the fouling layers that form on both sides of the walls during the operation of the heat exchanger; the thermal resistance of metal walls is relatively low. Volumetric heat exchangers increase the flow velocity and turbulence of the fluid, which can thin the boundary layer, reduce thermal resistance, and improve the heat transfer coefficient. However, increasing the fluid flow rate raises energy consumption; therefore, a reasonable balance must be struck between reducing thermal resistance and minimizing energy use during design. To reduce the thermal resistance of fouling, efforts can be made to delay its formation and to clean the heat transfer surfaces regularly. Volumetric heat exchangers are generally made of metal materials, with carbon steel and low-alloy steel being most commonly used for manufacturing medium- and low-pressure heat exchangers ； In addition to being used in various corrosion-resistant applications, austenitic stainless steel can also be employed as a material resistant to high and low temperatures ； Copper, aluminum, and their alloys are commonly used in the manufacture of low-temperature heat exchangers ； Nickel alloys are used under high-temperature conditions ； In addition to being used to manufacture gasket components, some non-metallic materials have begun to be employed in the production of corrosion-resistant heat exchangers made of non-metals, such as graphite heat exchangers, fluoroplastic heat exchangers, and glass heat exchangers.