I. Types and Characteristics of Condensers

Condensers can be divided into three major categories according to different cooling media: water-cooled type, air-cooled type, and evaporative type.

II. Classification of Evaporators:

According to different types of cooled media, evaporators can be divided into two major categories:

(1) Evaporators for cooling liquid secondary refrigerants. They are used to cool liquid secondary refrigerants—such as water, brine, or ethylene glycol aqueous solution. Common types of such evaporators include horizontal evaporators, vertical tube evaporators, and spiral tube evaporators.

(2) Evaporators for cooling air. Such evaporators include cooling coils and air coolers. The following mainly introduces the evaporators for cooling liquid secondary refrigerants commonly used in air conditioning systems.

I. Horizontal Evaporators

Horizontal evaporators are also known as horizontal shell-and-tube evaporators. Their structure is basically similar to that of horizontal shell-and-tube condensers. According to the liquid supply method, they can be divided into shell-and-tube evaporators and dry-type evaporators.

1. Horizontal Shell-and-Tube Evaporators

Horizontal shell-and-tube evaporators are flooded evaporators. That is, the secondary refrigerant flows in the tubes at a speed of 1～2m/s, and most of the space between the tube bundles outside the tubes is filled with refrigerant liquid. The two conduct sufficient heat exchange through the tube walls. The refrigerant vapor generated by heat absorption and evaporation enters the compressor through the liquid separator at the upper part of the evaporator.

To ensure the normal operation of the refrigeration system, the filling height of refrigerant in this type of evaporator should be moderate. If the liquid level is too high, the return gas may carry liquid, causing liquid slugging in the compressor; on the contrary, if the liquid level is too low, part of the evaporation tubes will be exposed above the liquid level and fail to perform heat exchange, thereby reducing the heat transfer capacity of the evaporator. Therefore, the filling height for ammonia evaporators is generally 70～80% of the cylinder diameter, and 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. Their main characteristics are: compact structure, good contact between liquid and heat transfer surface, and high heat transfer coefficient. However, they need to be filled with a large amount of refrigerant, and the liquid column will have a certain impact on the evaporation temperature. In addition, when the brine concentration decreases or the brine pump stops working due to failure, the brine in the tubes may freeze. If the refrigerant is Freon, the lubricating oil dissolved in Freon is difficult to return to the compressor. Besides, the operation must be stopped during cleaning.

2. Dry-Type Freon Evaporators

The appearance and structure of this type of evaporator are basically the same as those of horizontal shell-and-tube evaporators. The main difference between them is that the refrigerant flows inside the tubes, while the secondary refrigerant flows outside the tubes. The throttled Freon liquid enters the evaporator from the lower part of the end cover on one side, and is led out from the upper part of the end cover after passing through several passes. The refrigerant continuously evaporates as it flows inside the tubes, so part of the wall surface is occupied by vapor. Therefore, its heat transfer effect is not as good as that of the flooded type. However, it is free from the influence of liquid column on evaporation temperature, and due to the high flow rate of Freon (≥4m/s), the oil return is good. In addition, since a large amount of secondary refrigerant is filled outside the tubes, the risk of freezing is reduced.

The refrigerant charge in this type of evaporator only needs 1/2～1/3 or less of that in the flooded type, so it is called a "dry-type evaporator". To increase the flow rate of the secondary refrigerant and make it scour the tube bundle transversely, multiple baffle plates are installed in the shell to improve the heat transfer effect. Dry-type Freon evaporators are often used to cool fresh water, with the water flow rate generally being 0.5～1.5m/s, and 1.0m/s for copper tubes. II. Vertical Tube and Spiral Tube Evaporators

The common point of vertical tube and spiral tube evaporators is that the refrigerant evaporates inside the tubes, and the entire evaporator tube bundle is immersed in a tank (or pool, trough) filled with secondary refrigerant. To ensure the secondary refrigerant circulates in the tank at a certain speed, longitudinal baffles and spiral agitators are welded in the tank. The flow rate of the secondary refrigerant is generally 0.3～0.7m/s to enhance heat transfer.

These two types of evaporators can only be used in open circulation systems, so the secondary refrigerant must be non-volatile substances, commonly brine and water. If brine is used, the evaporator tubes are prone to oxidation, and the brine is easy to absorb moisture and reduce its concentration. These two types of evaporators allow direct observation of the flow of secondary refrigerant, and are widely used in brine refrigeration systems using ammonia as refrigerant.

1. Vertical Tube Evaporators Vertical tube evaporators are all welded with seamless steel pipes. The tubes of the evaporator are grouped into units, and the evaporator can be composed of several groups of tubes according to different capacity requirements. Each group of tubes consists of two horizontal headers with larger diameters—the upper one is called the vapor header, and the lower one is called the liquid header—and thin vertical tubes and thick vertical tubes with bent ends welded to the headers.

The diameter of the upper and lower horizontal headers is generally D108×4 or D121×4, the diameter of thin vertical tubes is generally D57×3.5 or D38×3, and the diameter of thick vertical tubes is generally D76×4. One end of the upper header is connected to the vapor-liquid separator to separate liquid droplets carried in the return vapor. One end of the lower header is connected to the oil collector. The high-pressure, normal-temperature liquid refrigerant from the receiver enters the evaporator through the liquid inlet pipe in the middle of the upper header after throttling. The liquid inlet pipe is located in the thick vertical tube and extends down to the lower header. This ensures that the liquid can be evenly distributed to each vertical tube. Since the relative heat exchange area of thin vertical tubes is larger than that of thick vertical tubes, the liquid in thin vertical tubes evaporates first to generate a large amount of vapor and drive the liquid upward. After the vapor-liquid mixture enters the upper header, most of the liquid in it returns to the lower header through the thick vertical tubes, forming an internal circulation. The vapor carrying some liquid droplets enters the vapor-liquid separator, the separated liquid returns to the lower header again, and the vapor is sucked away by the compressor. The lubricating oil of the evaporator deposits in the oil collector for regular discharge.

2. Spiral Tube Evaporators Spiral tube evaporators are a modified version of vertical tube evaporators. Their overall structure is similar to that of vertical tube evaporators. The difference is that the spiral tube evaporator uses two rows of spiral tubes instead of vertical tubes welded between the upper and lower headers. When the header spacing is the same, the heat transfer area is increased, so the structural size is compact, and there are fewer welding joints, making processing and manufacturing easier.

On this basis, domestic manufacturers have developed and produced double-head spiral tube evaporators, whose spiral tubes are composed of inner and outer rings with different spiral diameters. This makes the structure of the evaporator more compact.