Answer: The compressed air from the precooler of the cold dryer (which has been partially dehydrated in advance, but the water content is still quite large) enters the evaporator and moves in the shell side. During the zigzag process, it conducts convection heat exchange with the low-temperature refrigerant vapor in the evaporator tube side. The refrigerant liquid in the tube absorbs heat and boils (commonly known as evaporation) into refrigerant vapor, which is a phase change process. Before the refrigerant liquid completely changes into gas, the evaporation pressure remains unchanged. The evaporation temperature also remains unchanged. The temperature of the compressed air will get closer and closer to the evaporation temperature of the refrigerant liquid during the heat exchange process. However, due to the structural limitations of the cold dryer, the heat exchange area of the evaporator cannot be increased indefinitely, and the heat transfer temperature difference between the compressed air and the refrigerant vapor always exists. Therefore, the cooling temperature that the compressed air can reach cannot be equal to or lower than the evaporation temperature at any time.
The evaporator is the lowest temperature place in the cold dryer. And the evaporation temperature corresponds to the evaporation pressure. The lower the evaporation pressure, the lower the evaporation temperature. The compressed air in the evaporator conducts convection heat exchange with the evaporation temperature of the refrigerant in the tube. Since the low-pressure refrigerant liquid in the tube absorbs heat isothermally during the evaporation process, the temperature of the compressed air outside the tube gradually decreases during the flow process; the temperature to which the air is finally cooled depends on many factors, such as: the evaporation temperature of the refrigerant liquid, the heat exchange area of the evaporator, the streamline shape of the compressed air (advection or plain flow), the air flow rate, etc. These are all determined one by one in the design. Under certain conditions of evaporation temperature, the heat exchange area of the evaporator has the greatest impact on the final temperature of the compressed air. The larger the heat exchange area, the smaller the temperature difference between the final temperature of the air and the evaporation temperature. Theoretically, as long as the heat exchange area of the evaporator is large enough, the final cooling temperature of the compressed air can be infinitely close to the evaporation temperature of the refrigerant liquid in the tube, but in fact it is impossible to do so. Under the actual conditions of the dryer, it is common for the final cooling temperature of the compressed air (i.e. the theoretical "pressure dew point") to be 3~5℃ higher than the evaporation temperature.
In the dryer, the evaporator is the main heat exchange component that absorbs the heat of the compressed air (to cool it down). At the same time, it uses the cooled compressed air as a carrier to provide part of the cooling capacity to the precooler to cool the upstream compressed air with a higher temperature, which in turn reduces its own heat load. The ultimate effect of this series connection of cooling is to reduce the system's energy demand for the outside world. Due to the existence of working fluid cold loss (mainly the discharge of condensed water), the evaporator cannot completely provide the absorbed cooling capacity to the precooler, and due to the existence of the saturated hot air condensation phase change in the precooler, it is also impossible to convert all the absorbed cooling capacity into a reduction in the apparent temperature of the hot air. Compared with other refrigeration equipment, this complex relationship between the supply and demand of cooling capacity is unique to the cold dryer. This also shows that in the cold dryer, refrigeration is not the purpose of the work, but an intermediate means to reduce the water content of the air.
