【Mechanical Equipment Technology Edition】Daily Question 20171020: Refrigeration Equipment (1)
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This post was last edited by YORK Industrial Refrigeration on 2017-10-26 at 09:43. Starting today, in order to enhance communication among users, a \"Question of the Day\" activity has been launched on the Mechanical Equipment Technology forum. We hope everyone will participate actively to progress and improve together! ! ! Rewards: 3 points for active participation, 10-15 points for correct answers. This question is valid for two days; no scoring will be given after that period. There are various types of evaporators used in refrigeration, and among them, shell-and-tube evaporators are further divided into full-liquid evaporators, dry evaporators, and falling-film evaporators. Please explain separately the positions of the refrigerant and the material in these three types of evaporators. In a dry evaporator, the refrigerant flows through the heat exchange tubes, while cold water circulates outside these tubes. The heat exchange efficiency of such an evaporator is relatively low; its heat transfer coefficient is only about twice that of a tube without any special modifications. However, its advantage lies in the ease of oil return and simpler control mechanisms. Additionally, the amount of refrigerant required is approximately 1/2 to 1/3 of that needed in a flooded-type unit. The full-liquid evaporator operates in the exact opposite manner to a dry evaporator: cold water flows through the heat exchange tubes, while the refrigerant completely submerges these tubes; after absorbing heat, it evaporates outside the heat exchange tubes. The heat transfer tubes of a full-liquid evaporator are covered with numerous pin-shaped pores, and the inner surface of the tubes features spiral protrusions to enhance heat exchange on the cold water side. This efficient heat transfer tube, which simultaneously enhances external boiling and internal heat transfer, increases its heat transfer coefficient by about 5 times compared to a plain tube. Film evaporator. A film evaporator, also known as a spray evaporator, is a type of heat exchanger that is similar to a full-liquid evaporator, but it differs from it as well. In this type of evaporator, the refrigerant is sprayed from the upper part of the heat exchanger onto the heat exchange tubes; it forms only a thin layer of liquid refrigerant on these tubes. As a result, when the refrigerant boils and evaporates, the static head pressure is reduced, thereby improving the heat exchange efficiency. This efficiency is about 5% higher than that of units with fully filled tubes. ================================ High-quality promotions: Mechanical equipment——Maintenance procedures for York screw compressors http://bbs.hcbbs.com/thread-1806833-1-1.html Mechanical equipment——Upgrading of York Quinton control centers http://bbs.hcbbs.com/thread-1804837-1-1.html Mechanical equipment——Major repairs for GEA Grasso screw compressors http://bbs.hcbbs.com/thread-1800467-1-1.html Mechanical equipment——Disassembly and maintenance of British HOWDEN screw compressors http://bbs.hcbbs.com/thread-1832529-1-1.html Mechanical equipment——Disassembly and maintenance of Japanese Maekawa MYCOM screw compressors http://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1786445
In all three types of evaporators, the refrigerant flows through the tube side, while the material flows through the shell side
The refrigerant flows through the tube side, while the material flows through the shell side
In a dry-type evaporator, the refrigerant flows through the heat exchange tubes, while cold water circulates outside these tubes. The operation mode of a full-fluid evaporator is exactly the opposite of that of a dry evaporator: cold water flows through the heat exchange tubes, while the refrigerant completely submerges these tubes; after absorbing heat, it evaporates outside the heat exchange tubes. The refrigerant in the evaporator is sprayed onto the heat exchange tubes from the upper part of the heat exchanger; it forms only a thin layer of liquid refrigerant on these tubes.
In a dry-type evaporator, the refrigerant is inside the heat exchange tubes, while the material is in the shell； In a full-liquid evaporator, the material is inside the heat exchange tubes, while the refrigerant is in the shell ； In a falling-film evaporator, the refrigerant is sprayed onto the heat exchange tubes from the upper part of the heat exchanger, while the material is inside the heat exchange tubes ；
In a dry-type evaporator, the refrigerant flows through the heat exchange tubes, while the material passes outside these tubes. Such heat exchangers have a relatively low heat transfer efficiency; their heat transfer coefficient is only about twice that of tubes with no fins. However, their advantage lies in the ease of oil return and simpler control, and the amount of refrigerant required is approximately 1/2 to 1/3 of that needed in a fully liquid-filled unit. The operation mode of a full-fluid evaporator is exactly the opposite of that of a dry evaporator: cold water flows through the heat exchange tubes, while the refrigerant completely submerges these tubes; after absorbing heat, it evaporates outside the heat exchange tubes. The heat transfer tubes of a full-liquid evaporator are covered with numerous pin-shaped pores, and the inner surface of the tubes features spiral protrusions to enhance heat exchange on the cold water side. This efficient heat transfer tube, which simultaneously enhances external boiling and internal heat transfer, increases its heat transfer coefficient by about 5 times compared to a plain tube. The falling film evaporator, also known as the spray evaporator, is a type of heat exchanger that is similar to the full-liquid evaporator, but it also differs from it. In this type of evaporator, the refrigerant is sprayed from the upper part of the heat exchanger onto the heat exchange tubes; it forms only a thin layer of liquid refrigerant on these tubes. As a result, when the refrigerant boils and evaporates, the static head pressure is reduced, thereby improving the heat exchange efficiency. This efficiency is about 5% higher than that of units with fully filled tubes. Film evaporation is a type of flow boiling; due to the thin liquid film layer on the outer surface of the tube, there is no boiling point elevation caused by static pressure, resulting in a high heat transfer coefficient. In contrast, the bubbles generated by full-liquid evaporation (i.e., immersion evaporation) tend to gather on the surface of the heat exchange tubes, resulting in a decrease in heat exchange efficiency; its performance is inferior to that of falling-film evaporation. Generally speaking, falling film evaporation operates under low temperature differences, but scaling must be prevented to avoid affecting heat transfer efficiency.
In a dry-type evaporator, the refrigerant is inside the heat exchange tubes, while the material is in the shell； In a full-liquid evaporator, the material is inside the heat exchange tubes, while the refrigerant is in the shell ； In a falling-film evaporator, the refrigerant is sprayed onto the heat exchange tubes from the upper part of the heat exchanger, while the material is inside the heat exchange tubes ；
In a dry-type evaporator, the refrigerant is inside the heat exchange tubes, while the material is in the shell； In a full-liquid evaporator, the material is inside the heat exchange tubes, while the refrigerant is in the shell ； In a falling-film evaporator, the refrigerant is sprayed onto the heat exchange tubes from the upper part of the heat exchanger, while the material is inside the heat exchange tubes
In centrifugal and screw chillers, the types of evaporators are mainly full-liquid evaporators and dry evaporators. In a full-liquid evaporator, the liquid refrigerant enters the evaporator through a throttling device, and the liquid level inside the evaporator remains constant. The heat transfer tubes inside the evaporator are submerged in the refrigerant liquid. The gas-liquid mixture after endothermic evaporation still contains a large amount of liquid; therefore, the wet vapor that escapes from the evaporator is sent back to the compressor after gas-liquid separation. A dry evaporator is directly controlled by a thermal expansion valve or an electronic expansion valve to regulate the flow of liquid refrigerant into the tube side of the evaporator; the liquid refrigerant completely turns into gas inside the tubes, while the medium to be cooled flows in the shell side outside the heat transfer tubes.
In a dry-type evaporator, the refrigerant is inside the heat exchange tubes, while the material is in the shell； In a full-liquid evaporator, the material is inside the heat exchange tubes, while the refrigerant is in the shell ； In a falling-film evaporator, the refrigerant is sprayed onto the heat exchange tubes from the upper part of the heat exchanger, while the material is inside the heat exchange tubes