Thread Content
Heat transfer in heat exchangers is a discipline that studies the heat exchange between two media at different temperatures; according to the law of conservation of energy, heat transfer is also a form of energy balance. Thermal equilibrium is an important part of heat transfer theory; it mainly includes the energy transferred from a high-temperature medium to a low-temperature medium, plus the heat losses of both mediums. In other words, the heat transferred from the high-temperature medium to the low-temperature medium + the heat loss of the high-temperature medium = the heat absorbed by the low-temperature medium + the heat loss of the low-temperature medium. The process of heat transfer generally occurs in three ways: conduction, convection, and radiation. In heat exchangers, these three methods often occur simultaneously; however, depending on the circumstances, one method tends to dominate. In industrial heat exchangers, heat is transferred from the hot medium to the cold medium through the wall – that is, the heat first passes to the wall and then from there to the cold medium. This process involves both heat convection and heat conduction, and this constitutes the basic theory of heat transfer in heat exchangers. The theoretical basis for the heat transfer calculation methods in heat exchangers stems from the quantification of heat transfer. Currently, the heat transfer calculation methods commonly used are the Kern method and the Bell method. The Cohen method was developed in the 1950s; it treats the heat exchanger as a whole, taking into account not only heat transfer but also issues such as flow, temperature distribution, fouling, and structure within the calculation methodology, and it also incorporates two-phase flow theory into this approach. The Belfa method is a heat transfer calculation technique that was developed in the early 1960s by introducing flow path correction factors based on extensive experiments; it is a semi-analytical approach that enables more accurate calculations of heat transfer in the shell side of heat exchangers. Both calculation methods are commonly used in heat transfer calculations; however, the Bell method is currently more frequently employed for such purposes. Recently, a flow path analysis method has been developed in China. Its advantage lies in its ability to determine the relationship between the structure of the shell side and the pressure drop when the conditions of various flow paths change. This enables the calculation of the flow distribution among different flow paths, allowing designers to analyze problems more effectively and take appropriate measures to improve the efficiency of the heat exchanger. The heat transfer calculation methods for heat exchangers are relatively complex, and they can be divided into two main categories: heat transfer without phase change and heat transfer with phase change. Heat transfer with phase change is further divided into condensation heat transfer and boiling heat transfer. The appropriate calculation formula should be selected based on the properties of the fluid and the design of the heat exchanger, in order to determine a reasonable and economical heat transfer area and pressure drop; only then can the optimal choice of heat exchanger be made. The heat transfer and pressure drop calculations for air-cooled heat exchangers are more complex, but the basic equations remain the same.