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Maximum and minimum values of the system: Evaporator inlet temperature: 247°C; outlet temperature: 155°C; temperature difference between inlet and outlet: 82.5°C. Condenser inlet temperature: 3518°C; outlet temperature: 4523°C; temperature difference between inlet and outlet: 83.5°C. Ambient temperature: 456°C. I. Structure of the unit: It is a screw-type water-cooled chiller, which is primarily composed of a semi-hermetical twin-screw compressor, a shell-and-tube condenser, a full-liquid evaporator, an oil separator, a throttling mechanism, and an electrical control system. (1) Evaporator: During the operation of the unit, the evaporator maintains a low temperature and pressure, so that the vaporized refrigerant gas can remove the heat from the chilled water flowing through it. (2) Condenser: During the operation of the unit, the condenser maintains high temperatures and pressures so that the cooling water flowing through it can remove heat from the refrigerant. (3) Screw compressor: Continuously sends the refrigerant gas that has evaporated in the evaporator to the condenser, thereby maintaining a high pressure difference within the system. (4) Oil separator: It separates the refrigeration oil discharged along with the refrigerant gas and sends it back to the compressor, ensuring its safe and reliable operation. (5) Electronic control system: A PLC or microcontroller-based control system is used to automatically adjust the cooling capacity output by the unit to meet the actual requirements of the user ； It allows control over the usage side, as well as the heat source pump and cooling tower fans ； Display the following parameters: chilled water inlet and outlet temperatures, cooling water inlet and outlet temperatures, evaporation and condensation pressures, and other system parameters ； It allows for querying current and historical fault records. II. Introduction to the refrigeration system: The water-cooled screw chiller is a type of vapor-compression refrigeration unit. The cooling principle in all cases involves using a compressor to apply energy to the refrigerant vapor, thereby increasing its pressure and temperature. Subsequently, through processes of condensation and throttling, the refrigerant is turned into a low-pressure, low-temperature liquid. This liquid then evaporates into vapor inside the evaporator, absorbing heat from the surrounding environment (such as cold water), which results in a decrease in the temperature of that medium. This is how artificial cooling is achieved. It can be seen that the vapor compression refrigeration cycle includes four essential processes: compression, condensation, throttling, and evaporation. The principle is explained as follows: Compression process: The refrigerant vapor in the evaporator is drawn in by the screw compressor; the motor then applies energy to the compressor rotor, increasing the pressure of the refrigerant vapor so that it enters the condenser ； At the same time, the temperature of the refrigerant vapor also increases accordingly at the end of compression. Condensation process: The high-pressure, high-temperature refrigerant vapor coming from the compressor releases heat through the cooling water inside the condenser, causing its temperature to drop; at the same time, it condenses into a liquid at the saturation pressure (the pressure corresponding to the condensation temperature). At this point, the cooling water’s temperature rises as it absorbs heat from the refrigerant vapor. The temperature of the cooling water is directly related to the condensation temperature (condensation pressure). Throttling process: The high-temperature, high-pressure refrigerant liquid coming from the bottom of the condenser passes through the throttling device, where it undergoes decompression and expansion; as a result, its pressure and temperature decrease, and it turns into a low-pressure, low-temperature liquid that enters the evaporator. Evaporation process: The low-pressure, low-temperature refrigerant liquid absorbs heat from the cooling medium (such as cold water) inside the evaporator and turns into a gas, thereby lowering the temperature of the cooling medium and achieving artificial cooling. The refrigerant vapor generated in the evaporator is then drawn in by the compressor for compression, and this cycle of compression, condensation, throttling, and evaporation repeats itself. This process repeats over and over again to achieve continuous cooling. The cooling capacity is proportional to the suction flow rate of the compressor. A screw compressor is equipped with a slide valve mechanism inside it, which is used to control the suction flow rate of the compressor as well as the amount of refrigerant that evaporates, thereby enabling the cooling capacity to be adjusted steplessly within a certain range. Oil circulation system: Inside the compressor, the refrigeration oil relies on the pressure difference between high and low pressures in the system; through the built-in oil channels, it provides lubrication and cooling to both the bearings and the rotor. During the continuous exhaust process of the compressor, the refrigeration oil is discharged from the compressor along with the refrigerant gas. If this oil cannot return to the compressor, it may lead to a lack of oil inside the compressor, which can cause serious damage to it. The oil return system ensures that the refrigeration oil discharged along with the refrigerant gas returns to the compressor, thereby guaranteeing the safe and reliable operation of the unit. The oil return system is mainly divided into two parts: Oil separation and return. An efficient oil separator is installed between the compressor exhaust and the condenser; most of the refrigerant oil that is discharged along with the refrigerant gas is captured by this oil separator and sent back to the compressor through the suction port. High-pressure jet oil return: A small amount of refrigerant oil enters the condenser and eventually accumulates in the evaporator; by utilizing high-pressure liquid as a driving force, this oil can be directly returned to the compressor from the evaporator. Introduction to the control system: The control system is operated by a microcomputer. The control panel is equipped with power, operation, and fault indicator lights, allowing for easy and intuitive understanding of the current operating status of the unit. In addition, an emergency stop switch is installed on the control cabinet panel; when an urgent fault occurs in the unit and immediate shutdown is required, the emergency stop switch can be pressed to ensure the safety of the unit. Warning: Do not operate this emergency stop switch while the unit is running, as it may cause damage to the unit! Since the compressor is an important component of the unit, protections for the compressor include: ① Protection against missing phases and reverse phases in the power supply ； South Society② Overheat Protection ； ③ Current overcurrent protection ； ④ Compressor exhaust temperature protection ； ⑤ Internal overload protection of the compressor ； ⑥ High and low voltage difference protection. The main functions of a display or touchscreen are: ① Displaying system operation information. ② Displays the unit status information. ③ Displays unit fault information. ④ Set the unit operation parameters. ⑤ Set the automatic start and stop time for the unit. III. Electrical wiring installation (1) Before carrying out any electrical installation work, it is necessary to ensure that the main power supply is turned off, and a main power switch cabinet should be installed in an appropriate location. (2) Connect the main power cable and ground wire to the unit’s electrical control cabinet through the wire connection holes, and attach the wires to the corresponding terminals and ground terminals; it is essential to ensure the correct connection of each phase, L1, L2, and L3. (3) Verify that the voltage fluctuation of the main power supply is within ±10% of the value specified on the nameplate, and that the voltage imbalance is within ±2%. If these limits are exceeded, keep the unit shut down and contact the local electricity authority immediately. The formula for calculating phase voltage imbalance is: Voltage imbalance percentage = Maximum deviation from the average voltage / Average voltage × 100%. (4) Before connecting the main power cable, please check the phase sequence of each power line. (5) Connection control circuit: Properly connect the AC contactor control wires for the chilled water pump, cooling water pump, and cooling tower fan, as well as the water flow switch, to the unit’s electrical control system. The main power supply should be connected 8 hours before startup, and it must remain connected throughout the operating season, so that the compressor’s heating element can heat up when the compressor is not in use. This helps to evaporate the liquid refrigerant accumulated inside the compressor, thereby avoiding any adverse effects on the compressor resulting from starting it directly. IV. Commissioning and trial operation: After completing procedures such as installation, water connection, wiring, and power supply, the unit can enter the commissioning phase. Personnel designated by the manufacturer must be present to supervise and guide the startup testing as well as subsequent operation. Warning: The crew can control the chilled water pumps, cooling water pumps, and cooling tower fans. However, before debugging, it is strictly prohibited to operate these pumps via the main unit; temporary wiring can be used to test these devices. (1) Before starting up, the unit must be powered first (the unit cannot start) so that the compressor’s heating element can operate, with a heating time of no less than 8 hours. (2) Carefully adjust the flow control valve on the freezing/cooling water system or the inlet stop valve of the unit to ensure that the water flow rate in the system meets the required levels. (3) By operating the display, check the unit’s fault records; if any faults are present, they must be checked and resolved to confirm that the unit is free of faults. (4) And only after verifying that the controller parameters are set correctly can the start button be pressed to activate the unit. (5) After the unit is started, check the rotation direction of the compressor; if it is rotating in the wrong direction, adjust two of the phase wires in the power connection ； Check the compressor lubricating oil; the oil level in the oil sight glass should be kept at a visible level. (6) Run the system for 30 minutes, and set the inlet water temperature based on the user’s load and the actual conditions of the water system to ensure normal operation of the unit. After shutting down, restart the unit after an interval of 10 minutes to prevent frequent startups. Finally, carefully check the parameter settings on the operation display to conclude the trial run. Appendix 1: Pre-debugging Inspection and Record Form. Pre-debugging Inspection and Record Form. V. Maintenance: Water-cooled screw chillers are generally used for around 3,000 hours per year (depending on the climatic and geographical conditions in China). To ensure the long-term safe and reliable operation of the unit, extend its service life for continued use, and reduce operating costs, regular scientific maintenance of the unit is extremely important. 1. Daily start-up and shutdown: The daily start-up and shutdown of the unit can be carried out in either manual or automatic mode. For settings regarding automatic start-up and shutdown of the unit, please refer to the relevant sections on automatic start-up and shutdown in the \"Controller Operation Manual\" provided by each manufacturer. The on/off button on the controller is used for manual startup and shutdown, while the emergency stop switch on the control box door is used to shut down the unit during maintenance, debugging, or in emergency situations; it is generally not used under normal conditions. 2. Maintenance of main components: (1) During operation, close attention should be paid to the system’s exhaust and intake pressures; if any abnormalities are detected, the cause should be identified promptly to resolve the issue. (2) Do not adjust the setpoints of control and protection components arbitrarily. (3) Regularly check the electrical connections for any looseness; if there is looseness, tighten it promptly. (4) Regularly check the reliability of electrical components, and replace any defective or unreliable components promptly. 3. After long-term operation for descaling, calcium oxide or other minerals will accumulate on the water-side heat transfer surfaces of the unit’s shell-and-tube heat exchangers. When an excessive amount of these minerals forms scale on the heat transfer surfaces, it affects the heat transfer efficiency, leading to increased electricity consumption and higher exhaust pressure. Organic acids such as formic acid, citric acid, and acetic acid can be used for cleaning. 4. Shutdown in winter: When shutting down in winter, the inner and outer surfaces of the unit should be cleaned and dried ； The drain valve must be opened to empty the water remaining in the shell-and-tube heat exchanger, in order to prevent freezing accidents. 5. When starting up the unit in spring after a long period of shutdown, the following preparatory steps should be taken: (1) Thoroughly inspect and clean the unit. (2) Clean the water pipeline system. (3) Check the water pump. (4) Tighten all wire connections. (5) Preheat the unit compressor as required. 6. Part replacement: When replacing parts, it is preferable to use accessories supplied by the brand manufacturer; similar alternatives should not be used casually. 7. Complete refrigerant leakage: In such a case, it is necessary to use high-pressure nitrogen (15–20 kg of pressure) or refrigerant to detect leaks in the system. If welding is required, the gas inside the system must be completely removed before welding can take place. Before charging the refrigerant, the entire refrigeration system must be dry and evacuated. 8. Refilling refrigerant: 1) Connect the refrigerant filling nozzle on the compressor suction pipe to the refrigerant filling bottle. 2) Circulate the chilled/cooling water and start the unit. 3) Slowly charge the system with refrigerant, and check the suction and discharge pressures. 4) Note: When performing leak detection and airtightness tests, it is absolutely forbidden to fill the refrigeration system with oxygen, acetylene, or similar gases. 9. System anti-freezing: If severe freezing occurs in the flow channels of a shell-and-tube heat exchanger, this may lead to the rupture and leakage of the heat exchanger; damage caused by freezing is not covered under the warranty. Therefore, special attention must be paid to preventing freezing in the unit. When the unit is shut down for standby at lower ambient temperatures, if it is placed in an environment with an outdoor temperature below 0°C, the water in the evaporator and condenser must be drained. The specific steps for draining water are indicated on the drainage label on the unit. During operation, if the chilled water flow switch fails, it may cause the water pipes to freeze; therefore, the flow switch must be interlocked with the unit. During maintenance, when charging the unit with refrigerant or releasing it for repair, it is possible for water inside the evaporator to freeze. It is necessary to keep the water in the evaporator flowing or to drain it completely. Unit system maintenance schedule: Check the compressor lubricant level weekly; check the water flow rate in the circulating water system monthly; check the voltage and power supply monthly; check the amount of refrigerant (using the sight gauge) monthly; check the tightness of wire connections and electrical insulation quarterly; check and adjust the temperature settings quarterly; check the dry filter quarterly; replace the compressor oil filter after 40,000 hours; replace the compressor bearings after 40,000 hours. Fault analysis and troubleshooting methods: Detect possible causes of faults and apply corresponding troubleshooting methods. Excessively high exhaust pressure – there may be air or other non-condensable gases in the system; remove these gases through the fluorine injection port, and evacuate the system again if necessary. Repair of cooling tower fan faults – restore the operation of the cooling tower fan. Excessively high inhalation pressure: see \"Excessively high inhalation pressure\". Excessively high ambient temperature, or too low flow rate of cooling water: check the cooling water system and increase the water flow rate. If the compressor oil level is too low, check the level through the sight glass and add more refrigerant oil. If the exhaust pressure or suction pressure is too low, refer to the section on \"Low Suction Pressure.\" In the event of a refrigerant leak or insufficient charging, conduct a leak detection and top up with fluorine gas. If the temperature of the cooling water is too low, check whether the capacity of the cooling tower is too large or if the ambient temperature is too low. Excessively high suction pressure and excessively high discharge pressure – see “Excessively high discharge pressure”. Excessive amount of refrigerant charged: drain some of the refrigerant. Liquid refrigerant flows from the evaporator to the compressor. The inlet temperature of the chilled water is higher than the allowed maximum value; check and adjust the expansion valve to ensure that its temperature sensor is in close contact with the suction pipe and is completely insulated from the outside environment. The suction pressure is too low; the dry filter is clogged, so it needs to be replaced. The expansion valve is not set properly or is malfunctioning – adjust it to the appropriate superheat temperature, or check whether there are any leaks in the expansion valve’s temperature sensor. The system has insufficient refrigerant; leak detection and refilling with fluorine are required. The inlet temperature of the chilled water is significantly lower than the specified value; check whether the flow rate of the chilled water is insufficient, and verify if the pressures in the inlet and outlet pipes of the evaporator are too low, then adjust the chilled water flow rate. The compressor shuts down due to high-pressure protection; the temperature of the cooling water is too high; there is a fault with the cooling tower fan – it needs to be repaired. The setting value for high-pressure shutdown is incorrect; check the high-pressure switch. The compressor shuts down due to motor overload; the voltage is either too high or too low – ensure that the voltage does not exceed or fall below the rated voltage by more than ±10%. Excessively high exhaust pressure: see \"Excessively high exhaust pressure\". If the cooling water temperature is too high, check whether the cooling tower has insufficient capacity. In the case of a faulty component, check the compressor current and compare it with the rated full-load current specified for the compressor. Check for short circuits in the motor or wiring terminals by measuring the corresponding resistances of the motor and terminals. The compressor stops operating due to the activation of its built-in temperature protection switch; check the voltage to ensure it remains within the specified range. Excessively high exhaust pressure: See “Excessively high exhaust pressure”. Excessively high inlet temperature of the chilled water; faulty temperature protection switch inside the compressor – replace the component. Insufficient refrigerant in the system: Check for leaks. The compressor stops operating due to low-pressure protection; clogged dryer filter – replace the dryer filter element. Faulty expansion valve: Adjust or replace the expansion valve. Incorrect setting for low-pressure shutdown: Check the low-pressure switch. Insufficient refrigerant: Fill in more refrigerant. Excessive noise from the compressor: Insufficient refrigeration oil in the compressor – check the oil level via the sight glass and add more refrigeration oil. The compressor will not start: Overcurrent relay has tripped, or the fuse is blown. Replace the damaged components. Check that the control circuit is connected; examine the wiring of the control system. If there is no current, check the power supply. For high-voltage or low-voltage protection issues, refer to the section on suction and exhaust pressure faults earlier. If the contactor coil is damaged, replace it with a new one. If the phase sequence of the power supply is incorrect, reconnect it and adjust two of the wiring connections. Water system failure; the water flow switch is open. Check the water system; if there are alarm signals from the controller, identify the type of alarm and take appropriate action. The start/stop time setting is incorrect; check and reset it. The temperature detected by the temperature sensor exceeds the set value; check and reset it