Dry evaporators and full liquid evaporators are widely used in the refrigeration and air conditioning industries, and their main differences are reflected in the following aspects:
1、 Working principle and structure
Dry evaporator
Working principle: The dry evaporator uses gas as the heat transfer medium and evaporates liquid into gas through indirect evaporation. In a dry evaporator, liquid and gas do not come into direct contact, and their heat transfer methods are mainly convective heat transfer and radiative heat transfer.
Structural features: The dry evaporator consists of a shell with multiple heat exchange tubes arranged in parallel or series inside. The outer side of the heat exchange tube may be equipped with baffles or flow guides to guide the direction of water flow, ensuring that the cooling water can evenly cover all surfaces of the heat exchange tube and improve heat transfer efficiency. There is often a gas separation space at the top for separating gaseous refrigerant from liquid refrigerant, ensuring that only gaseous refrigerant is fed into the compressor.
Full liquid evaporator
Working principle: The full liquid evaporator uses liquid as the heat transfer medium and directly evaporates the liquid into gas. In a full liquid evaporator, liquid and gas come into direct contact, and the heat transfer method is phase change heat transfer.
Structural features: The full liquid evaporator consists of a closed shell containing a large number of parallel arranged high-efficiency heat exchange tubes. These heat exchange tubes can be smooth or have special surface treatments or internal structures (such as spiral protrusions) to enhance heat transfer efficiency. The bottom of the shell is equipped with a liquid supply port, and the top is equipped with a gas-liquid separation device to ensure that only gaseous refrigerant is fed into the compressor. In order to improve the heat transfer efficiency on the cooling water side, baffles are sometimes installed between the heat exchange tubes to make the water flow path more tortuous, thereby increasing the contact area and disturbance.
2、 Heat transfer efficiency and refrigerant usage
Heat transfer efficiency: The heat transfer efficiency of a full liquid evaporator is usually higher than that of a dry evaporator. Because the refrigerant in the full liquid evaporator is completely immersed in the heat exchange tubes, boiling and evaporating outside the tubes, more efficient heat exchange can be achieved. Dry evaporators, on the other hand, use natural convection for heat transfer, resulting in relatively low overall heat transfer efficiency.
Refrigerant usage: The refrigerant charge required for a dry evaporator is relatively small, only about one-third of that for a full liquid evaporator. And a full liquid evaporator requires injecting a large amount of refrigerant to ensure that the heat exchange tubes are completely submerged.
3、 Applicable scenarios and advantages and disadvantages
Applicable scenarios:
Dry evaporators are suitable for situations with high evaporation capacity (>1000kg/h) and high concentration (>20%), such as treating wastewater containing organic substances, concentrating chemical products, etc.
Full liquid evaporator is suitable for situations with low evaporation capacity (<1000kg/h) and low concentration (<5%), such as processing solutions in food and medicine, seawater desalination, etc.
Advantages and disadvantages:
Dry evaporator: Its advantages include good oil return performance, lubricating oil can be directly returned to the compressor with the refrigerant, and the refrigerant charging amount is relatively small. The main disadvantage is that the heat transfer efficiency is relatively low, and when multiple processes are used, the separation of gas and liquid two-phase refrigerants will occur when they turn inside the end cap, resulting in uneven distribution of refrigerant flow in each tube in the next process.
Full liquid evaporator: Its advantages include high heat transfer coefficient, simple control, and convenient operation and management. The main disadvantage is that the refrigerant charge is relatively large, about 55% to 65% of the effective volume of the cylinder, and there is a risk of water freezing in the pipe when the evaporation temperature is below 0 ℃. In addition, the lower part of the cylinder is prone to oil accumulation, and effective oil return measures are needed to ensure safe operation.
There are significant differences between dry evaporators and full liquid evaporators in terms of working principles, structural characteristics, heat transfer efficiency, refrigerant usage, applicable scenarios, and advantages and disadvantages. When choosing the type of evaporator, comprehensive consideration should be given based on specific application scenarios and requirements.
