Classification of evaporators and condensers

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This post was last edited by yinkuilin6868 on 2016-6-4 09:25. I. Types and characteristics of condensers Condensers can be divided into three categories based on their cooling medium: water-cooled, air-cooled, and evaporative types. II. Classification of evaporators: Depending on the type of medium to be cooled, evaporators can be divided into two main categories: (1) Evaporators using liquid cooling agents. Used to cool liquid refrigerants such as water, brine, or ethylene glycol water solutions. Common types of such evaporators include horizontal evaporators, vertical tube evaporators, and spiral tube evaporators. (2) Evaporator for cooling air. These types of evaporators have cooling coils and air coolers. The following mainly introduces the evaporators for cooling liquid refrigerants commonly used in air conditioning systems. I. Horizontal evaporator A horizontal evaporator is also known as a horizontal shell-and-tube evaporator. Its structure is basically similar to that of a horizontal shell-and-tube condenser. Based on the liquid supply method, they can be divided into shell-and-tube evaporators and dry evaporators. 1. Horizontal shell-and-tube evaporator: The horizontal shell-and-tube evaporator is a full-liquid evaporator. That is, the coolant flows inside the tubes at a speed of 1–2 m/s, while the space between the tubes outside is mostly filled with the refrigerant; heat exchange takes place efficiently between the two through the tube walls. The heat-absorbing evaporated refrigerant vapor passes through the liquid separator at the top of the evaporator before entering the compressor. To ensure the proper operation of the refrigeration system, the fill level of the refrigerant in such an evaporator should be moderate. An excessively high liquid level may cause liquid to be entrained in the return gas, leading to liquid slugging in the compressor ； Conversely, if the liquid level is too low, some of the evaporation tubes will protrude above the liquid surface and fail to perform heat exchange, thereby reducing the heat transfer capacity of the evaporator. Therefore, for ammonia evaporators, the filling height is generally 70–80% of the cylinder diameter, while for Freon evaporators, it is generally 55–65% of the cylinder diameter. Horizontal shell-and-tube evaporators are widely used in closed brine circulation systems. Its main features are: a compact structure, good contact between the liquid and the heat transfer surface, and a high heat transfer coefficient. However, it requires a large amount of refrigerant to be charged, and the liquid column will have a certain impact on the evaporation temperature. Furthermore, when the saltwater concentration decreases or the saltwater pump stops operating for some reason, the saltwater inside the pipes may freeze. If the refrigerant is Freon, the lubricating oil dissolved in the Freon finds it difficult to return to the compressor. Furthermore, work must be stopped during cleaning. 2. Dry fluorine evaporator: The appearance and structure of this type of evaporator are essentially the same as those of a horizontal shell-and-tube evaporator. The main difference between them is that the refrigerant flows inside the tubes, while the heat transfer fluid flows outside the tubes. The throttled Freon liquid enters the evaporator from the bottom of one of the end caps, and after passing through several paths, it exits from the top of the end cap. As the refrigerant flows within the tubes, it continues to evaporate; as a result, a portion of the tube walls is occupied by vapor. Therefore, its heat transfer efficiency is lower than that of a full-liquid system. However, it has no effect of the liquid column on the evaporation temperature, and since the flow rate of Freon is high (≥4 m/s), oil return is good. Furthermore, the large amount of refrigerant filled outside the tube reduces the risk of freezing. The amount of refrigerant required in such an evaporator is only 1/2 to 1/3, or even less, of that needed in a full-liquid type, which is why it is called a \"dry evaporator\". To increase the flow rate of the coolant and enable it to scour the tube bundle laterally, multiple baffle plates are installed inside the shell to enhance heat transfer efficiency. Dry Freon evaporators are commonly used for cooling fresh water, with a water flow rate of generally 0.5–1.5 m/s; for copper tubes, this value is usually 1.0 m/s. II. Vertical-tube and coiled-tube evaporators: The common feature of vertical-tube and coiled-tube evaporators is that the refrigerant evaporates inside the tubes, with the entire set of evaporator tubes submerged in a tank (or basin, pool) filled with the coolant. To ensure that the coolant circulates at a constant speed within the tank, longitudinal partitions are welded inside it, along with spiral agitators. The coolant flow rate is generally 0.3 to 0.7 m/s to enhance heat transfer. These two types of evaporators can only be used in open-loop systems; therefore, the refrigerant must be a non-volatile substance, with brine and water being common choices. If salt water is used, the evaporator tubes are prone to oxidation, and the salt water tends to absorb moisture, resulting in a decrease in concentration. These two types of evaporators allow for direct observation of the flow of the refrigerant, and they are widely used in brine cooling systems that utilize ammonia as a refrigerant. 1. Vertical tube evaporator: The vertical tube evaporator is entirely fabricated by welding seamless steel tubes. The tubes of the evaporator are organized in groups; depending on different capacity requirements, the evaporator can be composed of multiple such groups of tubes. Each group of tubes consists of two horizontally arranged headers with larger diameters; the one above is called the steam header, while the one below is called the liquid header. It consists of thin risers and thick risers with both ends bent, welded to the header. The diameter of the upper and lower horizontal manifolds is generally D108×4 or D121×4, the diameter of the thin risers is generally D57×3.5 or D38×3, and the diameter of the thick risers is generally D76×4. One end of the upper header is connected to the vapor-liquid separator, which separates the liquid droplets carried in the returned vapor. One end of the lower pipe is connected to the oil collector. The high-pressure, room-temperature liquid refrigerant from the reservoir enters the evaporator after throttling, via the inlet pipe located in the middle of the upper header; this inlet pipe is situated within the large vertical tube and extends downward to the lower header. This ensures that the liquid is distributed more evenly among the individual risers. Since the relative heat exchange area of thin risers is larger than that of thick risers, the liquid inside the thin risers evaporates first, generating a large amount of vapor that drives the liquid upward. After the vapor-liquid mixture enters the upper header, most of the liquid returns to the lower header through the rough vertical tubes, thus forming an internal circulation. The steam containing some of the liquid droplets enters the vapor-liquid separator; the separated liquid returns to the lower collector, while the steam is drawn away by the compressor. The lubricating oil from the evaporator accumulates in the oil collector for regular removal. 2. Coiled tube evaporator: The coiled tube evaporator is a variant that has been developed as an improvement on the counterflow tube evaporator. Its overall structure is similar to that of a vertical pipe type. The difference between the two is that in the spiral tube evaporator, two rows of spiral tubes replace the vertical tubes in the weld between the upper and lower manifolds. When the distances between the manifolds are equal, the heat transfer area increases; as a result, the structure is compact, there are fewer weld joints, and it is easier to manufacture. On this basis, double-spiral tube evaporators have been developed and produced in China, whose spiral tubes are composed of an inner and an outer coil with different diameters. This makes the structure of the evaporator more compact.