What are the working principles of the full-liquid evaporator and the dry evaporator on the unit?

Full-liquid shell-and-tube evaporator: This type of evaporator is commonly used in air conditioning systems with cooling functions, to cool water or brine. Its structure is similar to that of a horizontal shell-and-tube condenser, as shown in the figure. It is widely used due to its advantages such as good heat transfer performance, compact structure, small footprint, and ease of installation; it is particularly suitable for chillers used in air conditioning systems. The shell-and-tube evaporator used for Freon is shown in the figure. All shell-and-tube evaporators are horizontal. In the traditionally used process, the refrigerant liquid outside the tubes absorbs heat as it evaporates, while the cooling medium flowing inside the tubes releases heat. The low-temperature, low-pressure liquid, after being depressurized by the expansion valve, enters from the lower part of the cylinder and fills the space outside the tubes. Due to the large amount of liquid stored, it is a full-liquid evaporator. The vapor formed by the vaporization of the refrigerant rises continuously toward the liquid surface, where the separation vessel at the top removes any liquid droplets that may be present in the vapor; the dry vapor is then sent back to the compressor. The water flow path is also designed as multi-pass, just like in shell-and-tube condensers; that is, it flows back and forth multiple times within the array of heat transfer tubes through the end caps to exchange heat with the refrigerant. The water inlets and outlets are generally also located on the same side of the end cover, with water entering from below and exiting from above. The housing is equipped with several pipe fittings for connecting to other devices in the refrigeration system. The heat transfer tubes in ammonia evaporators are usually seamless steel tubes with dimensions of 25×25 or 32×25. In Freon evaporators, copper or brass tubes are commonly used; for those with a diameter of less than 20, low-ribbed tubes are often employed to enhance heat transfer efficiency. Generally speaking, the heat transfer coefficient of a shell-and-tube evaporator is slightly lower than that of a shell-and-tube condenser, due to the smaller heat transfer temperature difference and lower heat flux density. It should be noted that in such a full-liquid evaporator, bubbles are generated especially when the refrigerant vaporizes, causing the liquid level to rise compared to when the unit is not in operation. To prevent the compressor from drawing in unvaporized liquid, space must be left at the upper part of the tank. For ammonia refrigerants, the liquid filling level should be kept at no more than 70%–80% of the cylinder diameter; the foaming problem is more severe with Freon, so the liquid filling level for it should be around 55%–65%. The heat transfer tubes exposed above the liquid surface are wetted by the refrigerant foam after the evaporator is put into operation, and they can also perform an excellent heat exchange function. Furthermore, when used to cool fresh water, its evaporation temperature should not be below 0, in order to avoid the risk of freezing that could cause the heat transfer tubes to crack.

Dry fluorine refrigerant shell-and-tube evaporator: A dry fluorine refrigerant shell-and-tube evaporator used for cooling fresh water. In this type of evaporator, the refrigerant liquid evaporates inside the tubes, and as a result, the amount of liquid refrigerant required can be reduced by 80%–85% compared to conventional systems. It is structurally similar to a full-liquid evaporator with external evaporation, but its operating principle is completely different. Here, the Freon liquid enters the heat transfer tube array through two paths at the lower part of the front end cover, undergoing four round trips; the vapor generated by evaporation is drawn out from the upper part of the same end cover. Obviously, the end dance plays a guiding role in the flow of the refrigerant. The cooled water flows outside the tubes, entering from one end at the top of the housing and exiting from the other end. To increase the flow velocity and enhance heat transfer, equal-length sleeves are installed on the tie rods between several crescent-shaped baffle plates inside the evaporator shell, in order to maintain the spacing between the baffle plates.

Reply to 3# zhaoqian1326: Hello, senior. I am a graduate student currently working on projects related to evaporator design. I have very little information available, and I really don’t know where to start. Could you please recommend some resources or share any information you have on this topic? I would be extremely grateful! Thank you~~ Email: mz_lnfs@163.com QQ: 123979245 Thank you~

Reply 1# sdhklh Dear senior! I am currently learning about the design of evaporators. Recently, I posted many questions related to evaporators on HaiChuan; I am really eager to find a starting point for both my studies and my career. Therefore, I earnestly request those with experience in such design work to offer some guidance; I’m in urgent need of information on the classification of evaporators ； 2 Standard and design drawings for full-liquid evaporators and dry evaporators, along with actual calculation examples? I am extremely grateful~~ Email: mz_lnfs@163.com QQ: 123979245 Thank you~