In the refrigeration and air conditioning industry, the evaporator is one of the most important components, responsible for converting the low-pressure liquid refrigerant into a gaseous state, thereby absorbing heat and achieving a cooling effect. According to the relative position and flow pattern between the refrigerant and the heat exchange tube, the evaporator is mainly divided into two categories: dry evaporator and flooded evaporator.
1. Dry evaporator
The dry evaporator consists of a shell with multiple heat exchange tubes arranged in parallel or in series. Baffles or guide plates may be provided on the outside of the heat exchange tube to guide the direction of water flow, ensure that the cooling water can evenly cover all the heat exchange tube surfaces, and improve the heat transfer efficiency. A gas separation space is often provided at the top to separate the gaseous refrigerant from the liquid refrigerant to ensure that only the gaseous refrigerant is sent to the compressor.
Working process: The refrigerant liquid enters the heat exchange tube from the bottom of the evaporator and begins to absorb heat from the cooling water outside the tube. As the heat is absorbed, the refrigerant gradually evaporates into a gaseous state, and the pressure and temperature of the refrigerant remain relatively constant during this process. The evaporated refrigerant gas rises to the top of the evaporator, where it is further removed by the gas separation device and then sucked into the compressor.
At the same time, the cooling water in the external circulation continuously takes away heat to maintain the temperature difference conditions required for the evaporation process. Since the dry evaporator uses natural convection to transfer heat, its overall heat exchange efficiency is lower than that of the flooded evaporator; but it also means that less refrigerant filling is required to meet the working requirements, reducing costs and potential risks.
Advantages: Good oil return performance, lubricating oil can be directly returned to the compressor with the refrigerant, and the refrigerant filling amount is small, about only about one-third of the flooded evaporator.
Disadvantages: Compared with the flooded evaporator, the heat transfer efficiency is low, about twice the heat transfer coefficient of the bare tube.
2. Flooded evaporator
The flooded evaporator is a high-efficiency heat exchange equipment, widely used in refrigeration and air-conditioning systems. Its main feature is that cold water flows inside the heat exchange tubes, while the refrigerant completely immerses these heat exchange tubes and boils and evaporates outside the tubes.
Structural features: The flooded evaporator consists of a closed shell containing a large number of high-efficiency heat exchange tubes arranged in parallel. These heat exchange tubes can be smooth or have special surface treatments or internal structures (such as spiral protrusions) to enhance heat transfer efficiency. In some designs, the heat exchange tubes may also have pinholes or other forms of surface features to promote the boiling process of the refrigerant.
There is a liquid supply port at the bottom of the shell and a gas-liquid separation device at the top to ensure that only gaseous refrigerant is sent to the compressor. In order to improve the heat exchange effect on the cooling water side, baffles are sometimes set between the heat exchange tubes to make the water flow path more tortuous, thereby increasing the contact area and disturbance.
Working process: Liquid refrigerant enters the shell from the bottom of the evaporator and completely immerses the heat exchange tubes. Refrigerant heating and evaporation: When the low-temperature and low-pressure liquid refrigerant contacts the higher-temperature heat exchange tubes, it begins to absorb heat from the cold water in the tubes and gradually evaporates into a gaseous state. In this process, since the refrigerant is in direct contact with the heat exchange surface, a very high heat transfer efficiency can be achieved.
As more refrigerant evaporates, the mixture rises to the top of the evaporator. Here, the unevaporated liquid droplets are removed by a specially designed gas-liquid separator to ensure that only gaseous refrigerant is sent to the compressor for the next cycle. At the same time, the cold water passing through the heat exchange tube cools down due to the release of heat, and then flows out of the evaporator to continue participating in the cooling cycle of the system. Since a certain amount of lubricating oil will accumulate during the entire heat exchange process, effective oil return measures are required, such as using an ejector pump or gravity to regularly discharge the lubricating oil accumulated at the bottom to ensure the normal operation of the system.
Advantages: high heat transfer coefficient, simple control, and convenient operation and management.
Disadvantages: When the evaporation temperature is lower than 0℃, there is a risk of water freezing in the tube. The refrigerant filling volume is large, about 55%~65% of the effective volume of the cylinder. Affected by the height of the refrigerant liquid column, the evaporation temperature at the bottom of the cylinder is high, which reduces the overall heat transfer temperature difference. The lower part of the cylinder is prone to oil accumulation, and effective oil return measures are required to ensure safe operation.
