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Today, Gonglenghui will share a case study of a client from Jinhua, Zhejiang, for everyone’s reference. In this case, the display screen of the York RWB II856E screw compressor unit went black; the unit produced obvious abnormal noises during operation, and the cooling temperature did not meet the required standards, resulting in low operational efficiency of the unit. Since this customer operates in the fluorine chemicals industry, their process requirements call for refrigeration units to cool calcium chloride solutions. It is known that this model of refrigeration unit has a cooling capacity of 1627 KW; it uses R22 as a refrigerant, and its drive motor has a power of 1700 HP at 10 KV. It is capable of cooling calcium chloride solutions with a mass concentration of 30% at a rate of 329 m3/h, from -30°C to -35°C. The problems currently encountered by the customer are: 1. The unit’s display screen is black. 2. The noise level during unit operation is significant. 3. The cooling temperature does not meet the requirements, resulting in low operating efficiency of the unit. I. Possible reasons for abnormal noises and failures in the refrigeration unit: 1. Wear of the thrust bearings. 2. There is a foreign object in the rotor slot. 3. Shaft seal wear. 4. Lack of lubricating oil. 5. Moving parts such as the rotor are loose. Fault detection and cause identification: 1. A rattling noise from the bearing may be caused by rough surfaces on the shaft surface, balls, or rollers, as well as abnormal loads, improper installation, insufficient lubricant, excessive play, the intrusion of foreign objects, or contact with other parts. 2. There are foreign objects in the rotor slots or the rotor is loose; operational errors may lead to insufficient oil supply, causing the compressor to operate in a dry-running condition. Or, impurities in the oil and the inhaled gases can cause scratches on the male and female rotors as well as the casing, leading to internal leakage. 3. Shaft seal wear – the shaft seal may have been corroded and worn by gases, resulting in a failure of the seal. Or water entering the gas accelerates the wear of the rotor and sealing components; in addition, leaks in the heat exchange tubes can also lead to wear of the shaft seals. Solution: A final inspection revealed that the shaft seal was worn, resulting in a failure of the seal and causing abnormal noises in the unit. The shaft seal was replaced again, and the abnormal noise was eliminated. II. The main reasons for failures due to the refrigeration unit not reaching the required temperature are: 1. Evaporator scaling. 2. Lack of refrigerant. 3. The condensing capacity of the condenser has decreased. Fault detection and cause identification: 1. The circulating cooling water in the evaporator contains large amounts of salts, corrosion products, and various microorganisms. If the water quality is not treated, impurities such as calcium and magnesium carbonate scales will accumulate over time as the evaporator operates, leading to poor heat transfer and a reduction in the unit’s cooling efficiency. 2. A lack of refrigerant is the main reason for the cooling temperature not meeting the required standards. 3. A decrease in the condensing capacity of the condenser may be due to scaling on it. During the water circulation process in the refrigeration unit, scaling can easily occur in the condenser pipes. If not addressed promptly, this scaling will affect the heat exchange efficiency, leading to an increase in the condensing temperature and consequently a reduction in the cooling capacity. Solution: A final inspection revealed that both the evaporator and the condenser were covered with scale to varying degrees; after descaling them and optimizing the water quality, the cooling temperature reached the required level. Note: The main reasons for the refrigeration temperature not meeting the specified standards are the three mentioned above; in addition, issues such as excessive resistance in the suction pipeline, leaks in the high and low pressure systems, and poor thermal insulation and sealing can also lead to problems with the refrigeration temperature. Since these three types of failures do not occur during regular maintenance of the unit, no further explanation is needed. III. Fault of black screen on the display screen of the refrigeration unit 1. Fault in the control board of the refrigeration unit – The control board serves as the control center of the refrigeration unit, regulating its operation. When the control panel fails, issues such as distorted displays, an inability to accurately control the unit, and difficulties in interpreting the unit’s data may occur; in severe cases, power protection will be activated. Fault detection and root cause analysis: Test the control board, focusing on circuit connections and protective switches. 2. Oil shortage in the refrigeration unit: A lack of oil in the refrigeration unit can cause the unit to activate its power-off protection mechanism, and the display screen will go black as a result of an error message. We all know that the compressor is the core of a refrigeration unit; its operation requires sufficient lubricating oil to facilitate the compression of the refrigerant. A lack of oil in the compressor can lead to wear inside it, and over time this will result in damage to the compressor. Therefore, the protection system of the refrigeration unit will cut off power as a form of protection when it detects a lack of oil in the compressor, causing the display screen to go black. Fault detection and cause identification: The compressor lacks oil, mainly due to poor oil return, lubricant leakage, degraded lubricant, or blocked oil return lines. During operation, poor oil return in the compressor is related to blockages inside the refrigeration system’s pipelines as well as abnormalities in the oil return lines; therefore, it is necessary to check and address the airtightness of the compressor’s connection pipes, the patency of these pipes, and their cleanliness.