The evaporator is a cooling capacity output device in a refrigeration unit. The refrigerant evaporates in the evaporator and absorbs heat from the low-temperature heat source medium (water or air) to achieve the purpose of refrigeration.

Evaporators are classified according to the different media they cool into: evaporators for cooling air, and evaporators for cooling liquids (water or other liquid secondary refrigerants).

Evaporators for Cooling Air:

A bare coil structure is mostly adopted when air is in natural convection

A finned tube structure is adopted when air is in forced convection

Evaporators for Cooling Liquids (Water or Other Liquid Secondary Refrigerants):

Shell-and-Tube Type

Submerged Type

According to Different Refrigerant Supply Modes:

Flooded Evaporator

Dry-Type Evaporator

Circulating Evaporator

Spray-Type Evaporator

Flooded Evaporator    

According to their structure, they are divided into several structural types such as horizontal shell-and-tube type, water tank straight tube type, and water tank type. Their common feature is that the evaporator is filled with liquid refrigerant, and the refrigerant vapor generated by heat absorption and evaporation during operation is continuously separated from the liquid. Due to the full contact between the refrigerant and the heat transfer surface, the boiling heat transfer coefficient is higher. However, the disadvantages are that the refrigerant charge is large, the hydrostatic pressure of the liquid column will have an adverse effect on the evaporation temperature; if a refrigerant soluble in lubricating oil is used, it is difficult for the lubricating oil to return to the compressor.

Shell-and-Tube Flooded Evaporator:

It is generally of horizontal structure, see the figure. The refrigerant evaporates outside the tubes inside the shell; the secondary refrigerant flows inside the tubes, usually in a multi-pass form. The inlet and outlet of the secondary refrigerant are arranged on the end cover, with the flow direction of bottom-in and top-out. The refrigerant liquid enters the shell from the bottom or side of the shell, and the vapor is led out from the upper part and returned to the compressor. The refrigerant in the shell always maintains a static liquid level height of about 70%~80% of the shell diameter.

Attention Points for Shell-and-Tube Flooded Evaporators:

① When water is used as the secondary refrigerant, if the evaporation temperature drops below 0℃, the inside of the tubes may freeze, and in severe cases, the heat transfer tubes may burst. At the same time, the evaporator has a small water capacity and poor thermal stability during operation;

② At low evaporation pressure, the hydrostatic liquid column in the shell will increase the temperature at the bottom and reduce the heat transfer temperature difference;

③ For refrigerants miscible with lubricating oil, it is difficult to return oil when using flooded evaporators;

④ The refrigerant charge is relatively large. Meanwhile, it is not suitable for the machine to work under moving conditions, and the shaking of the liquid level may lead to compressor cylinder washing accidents; 

⑤ In flooded evaporators, a large number of bubbles are generated due to refrigerant vaporization, which raises the liquid level. Therefore, the refrigerant charge should not submerge all the heat transfer surfaces.

Water Tank Evaporator:

Water tank evaporators can be composed of parallel straight tubes or spiral tubes (also known as vertical evaporators). They all work immersed in liquid secondary refrigerant. Due to the action of the agitator, the liquid secondary refrigerant circulates in the water tank to enhance the heat transfer effect. The refrigerant liquid evaporates and absorbs heat inside the tubes, reducing the temperature of the secondary refrigerant outside the tubes.

Vertical Tube Water Tank Evaporator

1—Oil Collector; 2—Equalizing Tube; 3—Vapor-Liquid Separator; 4—Upper Gas Header; 5—Water Tank; 6—Drain Outlet; 7—Agitator; 8—Overflow Pipe; 9—Baffle; 10—Vertical Tube Bundle; 11—Lower Liquid Header

It can overcome the possible freezing problem of shell-and-tube flooded evaporators and the disadvantage of poor thermal stability during operation. At the same time, the refrigerant enters from the bottom and exits from the top, which conforms to the movement law of liquid during boiling, with good circulation and high boiling heat transfer coefficient.

Non-Flooded Evaporator

Non-Flooded Evaporator (Dry-Type Evaporator)

A dry-type evaporator is an evaporator in which the refrigerant liquid can be completely vaporized inside the heat transfer tubes. The cooled medium outside the heat transfer tubes is secondary refrigerant (water) or air, while the refrigerant absorbs heat and evaporates inside the tubes, with its hourly flow rate about 20%-30% of the internal volume of the heat transfer tubes. Increasing the mass flow rate of the refrigerant can increase the wetting area of the refrigerant liquid inside the tubes. At the same time, the pressure difference at its inlet and outlet increases with the increase of flow resistance, resulting in a decrease in the coefficient of performance (COP) of the refrigeration system.

Classified by Cooling Medium:

Dry-Type Evaporator for Cooling Liquid

Dry-Type Evaporator for Cooling Air (Direct Expansion Air Cooler)

Dry-Type Evaporator for Cooling Liquid:

Dry-Type Shell-and-Tube Welded Plate

Type Evaporator

1. Dry-Type Shell-and-Tube Evaporator

Dry-type shell-and-tube evaporators are divided into straight tube type and U-tube type structural forms. Their common feature is that multiple segmental baffles are installed in the shell, aiming to increase the flow rate of the secondary refrigerant outside the tubes and enhance the heat transfer effect

Characteristics of Dry-Type Shell-and-Tube Evaporators:

① It can ensure that the lubricating oil entering the refrigeration system returns to the compressor smoothly;

② The required refrigerant charge is small, only 1/3 of that of a flooded evaporator with the same capacity;

③ When used for cooling water, even if the evaporation temperature reaches 0℃, freezing accidents will not occur;

④ It can adopt thermal expansion valve for liquid supply, which is more reliable than the float valve liquid supply of flooded type.

Direct Expansion Air Cooler

Direct Expansion Air Cooler

According to the movement state of air, such evaporators are divided into two types: evaporators for cooling free-moving air and evaporators for cooling forced-flow air.

 1. Evaporator for Cooling Free-Moving Air: Since the cooled air is in a free-moving state, its heat transfer coefficient is low. Therefore, this type of evaporator is made into a bare tube coil assembly, usually called a cooling coil. It is generally used in cold storage and low-temperature test devices.

Cooling coils have the advantages of small liquid holding capacity (the liquid charge is about 40% of the internal volume of the coils) and convenient operation and maintenance. However, they have large refrigerant flow resistance inside the tubes, and the vapor after evaporation is not easy to discharge. At the same time, since the air outside the tubes is in free movement, the heat transfer coefficient is low, generally in the range of 6.3-8.1 W/(m²·K). 

 2. Evaporator for Cooling Forced-Flow Air (also known as Air Cooler): Since the heat transfer coefficient K of bare tube air coolers is very low, to strengthen the heat exchange on the air side, fins are often installed outside the tubes to increase the heat transfer coefficient. However, under normal circumstances, the installation of finned tubes will cause large flow resistance due to the small fin spacing, so measures must be taken to force air to pass through the finned tube bundle at a certain flow rate to obtain a good heat exchange effect. 

This type of evaporator has the advantages of compact structure, good heat transfer effect, ability to change the moisture content of air, and wide application range. However, from the analysis of manufacturing process requirements, the close contact between fins and heat transfer tubes is the key to improving its heat transfer effect.

Circulating Evaporator                                             

In this type of evaporator, the refrigerant repeatedly circulates, absorbs heat and evaporates inside the tubes until it is completely vaporized, so it is called a circulating evaporator. Circulating evaporators are mostly used in large-scale liquid pump liquid supply and gravity liquid supply cold storage systems or low-temperature environmental test devices.

The advantage of circulating evaporators is that the inner surface of the evaporator pipeline can always be completely wetted, and the surface heat transfer coefficient is very high. But it has a large volume and requires a large amount of refrigerant charge.