Precautions before and after the refrigeration startup of screw chiller units

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I. Preparatory work before starting the screw-type refrigeration unit. Proper commissioning of the refrigeration unit is a crucial step in ensuring the normal operation of the cooling system, conserving energy, and prolonging its service life. For large and medium-sized refrigeration systems installed on-site, before commissioning, it is necessary first to familiarize oneself with the layout and connections of the entire system as specified in the design drawings, to understand the physical structure of each device and the performance of its components, as well as the electrical control system and water supply system. To this end, engineers specializing in refrigeration, water, and electricity should be present during debugging. Before debugging, the user should carefully read the product operation manual provided by the manufacturer and proceed step by step in accordance with the instructions. Operators must receive specialized training from the manufacturer and obtain an operation certificate for the unit before they can operate it, in order to prevent fatal damage to the unit resulting from incorrect operations. 1 Preparations before commissioning (1) Since screw-type chiller units are medium to large-sized refrigeration machines, close coordination is required in terms of design, installation, and operation during commissioning. To ensure that the commissioning process proceeds in an orderly manner, it is necessary to form a temporary trial-operation team composed of personnel from relevant parties to comprehensively oversee the commissioning activities. (2) The personnel responsible for commissioning must have a thorough understanding of the structure and performance of the unit’s equipment, as well as of the safety techniques related to refrigeration machines. They need to be aware of the methods, steps involved in commissioning, and the technical requirements that need to be met. A detailed and specific commissioning plan should be developed, so that the personnel working in various positions know exactly what their tasks and responsibilities are. (3) Check whether the installation of the unit meets the technical requirements, whether the foundation of the unit is suitable, and whether the dimensions, specifications, and material of the connecting pipes comply with the design requirements. (4) The power supply system of the unit shall be fully installed and passed testing. (5) Conduct water flow tests separately on the cold water and cooling water systems to flush out debris from the piping systems; the pumps should operate properly, and the volume of circulating water should meet the requirements of the operating conditions. (6) Clean and prepare the debugging environment to ensure it is clean, well-lit, and unobstructed. (7) Prepare all the general and specialized tools required for debugging. (8) Prepare all the measuring instruments and devices required for debugging, such as those for pressure, temperature, flow rate, mass, time, etc. (9) Prepare the safety protection equipment necessary for commissioning and operation. Unit 2 commissioning: After refrigerant charging is completed (if refrigerant charging is required), load commissioning can be carried out. In recent years, screw chillers have made significant improvements in terms of performance and electrical control. Many of these chillers allow for simulated testing of their main electrical control systems before they are put into operation; that is, the main unit of the chiller is not powered on, while the control system is powered on. Then, through settings within the chiller, the electrical control system is tested to determine whether all components are functioning properly. If there are any problems with the electronic control system, they can be resolved promptly. Finally, reconnect the main unit’s power supply and perform debugging. During the debugging process, special attention should be paid to the following points: a) Check whether all valves in the refrigeration system are in their normal open position, especially the exhaust stop valve, which must not be closed. b. Open the cooling water valve of the condenser and the chilled water valve of the evaporator; the flow rates of the chilled water and cooling water should meet the requirements specified by the manufacturer. c. Before starting, it is necessary to check whether the power supply voltage of the unit is normal. d. Start the unit in accordance with the startup manual provided by the manufacturer. e. After the unit is started, refer to the startup manual provided by the manufacturer to check whether all parameters of the unit are normal. f. The various data of the unit can be recorded based on the unit operation data record sheet provided by the manufacturer; in particular, some key parameters must be recorded clearly. g. During the operation of the unit, it is necessary to check whether the up/down mechanisms of the compressor are functioning properly. h. The safety protection devices installed in the refrigeration system, such as high/low pressure protectors, flow switches for cold water and cooling water interruptions, safety valves, etc., should be used properly. Any damaged components must be replaced promptly. i. If any abnormality occurs in the unit, it should be stopped immediately for inspection. Before commissioning the refrigeration system, it is essential to thoroughly clean and dry the interior of the air conditioning system. If the preliminary work is not carried out carefully, it will increase the amount of work during debugging, and it will also create many potential problems for the subsequent operation of the refrigeration system. II. Startup procedures for screw refrigeration units: After the screw refrigeration compressor has undergone trial operation and any issues identified have been resolved, it can proceed to its normal operating mode. The operating procedure is as follows: 1. Verify whether the states of all relevant valves in the unit meet the requirements for startup. 2. Supply power to the unit’s electrical control device and turn on the power switch so that the power control indicator light comes on. 3. When starting the cooling water pump, cooling tower fan, and refrigerant water pump, the operation indicator lights of all three should be seen to be on. 4. Check whether the temperature of the lubricating oil has reached 30 °C. If the temperature is below 30 °C, the electric heater should be turned on to provide heat; at the same time, the oil pump can be started to ensure a uniform increase in the temperature of the lubricating oil. 5. After the oil pump starts operating, set the energy regulation control valve in the unloaded position and ensure that the spool is in the zero position. 6. Adjust the oil pressure control valve to bring the oil pressure to 0.5–0.6 MPa. 7. Turn off the power to the compressor, activate the control switch, and open the compressor’s suction valve. After a delay, the compressor starts running; once it is operating, adjust the lubricating oil pressure so that it is 0.15–0.3 MPa higher than the exhaust pressure. 8. Turn off the solenoid valve control circuit in the supply line, activate the solenoid valve to supply liquid refrigerant to the evaporator, place the energy regulation device in the loaded position, and gradually increase the load over time. At the same time, monitor the suction pressure; by adjusting the expansion valve, keep the suction pressure stable between 0.36 and 0.56 MPa. 9. After the compressor starts operating, when the lubricating oil temperature reaches 45 °C, cut off the power supply to the electric heater. At the same time, open the inlet and outlet valves of the cooling water for the oil cooler, so that during the compressor’s operation, the oil temperature is maintained within the range of 40–55 °C. 10. If the cooling water temperature is low, the fan of the cooling tower can be turned off temporarily. 11. Turn the fuel injection valve 1/2 to 1 full turn, while ensuring that the intake valve and the liquid outlet valve of the unit are in the fully open position. 12. Set the energy control device to 100%, and adjust the expansion valve so that the suction superheat remains above 6°C. 13. Inspections during the unit’s startup and operation. After the unit is started and put into operation, attention should be paid to checking the following items to ensure its safe operation. 1) The sound and vibration levels of the refrigerant water pump, cooling water pump, and cooling tower fans during operation; whether various parameters such as the outlet pressure of the pumps and water temperature are within the normal operating range. 2) Is the oil temperature of the lubricating oil below 60 ℃, is the oil pressure 0.15–0.3 MPa higher than the exhaust pressure, and is the oil level normal? 3) When the compressor is operating at full load, is the suction pressure within the range of 0.36–0.56 MPa? 4) Is the discharge pressure of the compressor below 1.55 MPa, and is the discharge temperature below 100 °C? 5) During the operation of the compressor, is the motor’s operating current within the specified range? If the current is too high, it should be adjusted to operate under reduced load to prevent the motor from burning out due to excessive operating current. 6) Are the sound and vibration levels normal when the compressor is operating? In any of the aforementioned situations, if any abnormalities are detected, the machine should be shut down immediately. After identifying the cause and rectifying the fault, the unit can be restarted. Under no circumstances should the unit be operated with issues present, to avoid serious accidents. III. Indicators of normal operation of screw refrigeration compressors The indicators of normal operation of screw refrigeration compressors are as follows: 1. The compressor’s discharge pressure is 1.8–1.47 MPa (gauge pressure) ； 2. The compressor discharge temperature is 45–90 °C; the maximum value shall not exceed 1.05 ℃ ； 3. The oil temperature of the compressor is around 40–55 °C ； 4. The oil pressure of the compressor should be 0.2–0.3 MPa (gauge pressure). 5. During operation, the compressor should produce a uniform and steady sound, without any abnormal noises ； 6. The condensation temperature of the unit should be 3–5 degrees higher than the cooling water temperature ℃ ； The cold spot temperature should generally be maintained at around 40 °C; the inlet water temperature of the condenser should be below 32 °C ； 7. The evaporation temperature of the unit should be 3–4 °C lower than the outlet temperature of the refrigerant water, which is generally around 5–7 °C. IV. Shutdown procedures for screw refrigeration compressors The shutdown of screw refrigeration compressors can be carried out in various ways, including normal shutdown, emergency shutdown, automatic shutdown, and long-term shutdown. 1 Operating procedure for normal shutdown: (1) Place the manual unloading control device in the unloaded position. (2) Close the solenoid valve and the outlet valve on the liquid supply line between the condenser and the evaporator. (3) Stop the compressor from running and close its suction valve at the same time. (4) Once the energy is reduced to zero, stop the operation of the oil pump. (5) Set the energy regulating device to the “Stop” position. (6) Close the cooling water inlet valve of the oil cooler. (7) Stop the operation of the cooling water pump and the cooling tower fan. (8) Stop the operation of the refrigerant water pump. (9) Turn off the main power supply. Emergency shutdown of 2 units: In the normal operation of screw-type refrigeration compressors, if any abnormal phenomena occur ； To protect the safety of the crew, an emergency shutdown should be initiated. The operation method is: (1) Stop the compressor from running. (2) Close the suction valve of the compressor. (3) Close the solenoid valve on the liquid supply pipe of the unit and the liquid outlet valve of the condenser. (4) Stop the oil pump from operating. (5) Close the cooling water inlet valve of the oil cooler. (6) Stop the refrigerant water pump, cooling water pump, and cooling tower fan. (7) Cut off the main power supply. The methods for shutting down the unit in case of failures such as power outages or water supply interruptions during operation can be handled by referring to the relevant procedures outlined for emergency shutdown of centrifugal compressors. After the unit is shut down urgently, the cause of the fault should be identified promptly. Once the fault is resolved, the unit can be restarted using the normal startup procedure. Automatic shutdown of the unit: During operation of a screw-type refrigeration compressor, if the pressure and temperature values of the unit exceed the specified ranges, the protective devices in the unit’s control system come into action to automatically stop the compressor from working. This phenomenon is known as the automatic shutdown of the unit. When the unit shuts down automatically, the corresponding fault indicator light on its electrical control panel will light up to indicate the location of the fault. When this situation occurs, after the main unit shuts down, the shutdown of other components can be handled using the emergency shutdown method. After completing the shutdown procedures, the unit should be inspected; only after any faults have been rectified can it be restarted following the normal startup procedure. Long-term shutdown of the 4 units: Since the screw refrigeration compressors used as the cooling source for central air conditioning operate on a seasonal basis, the units are shut down for extended periods of time. To ensure the safety of the unit, during seasonal shutdowns, the shutdown procedure can be followed as described below. (1) When the unit is operating normally, close its liquid outlet valve to enable it to operate under reduced load, and pump all the refrigerant in the unit into the condenser. To prevent the unit from shutting down due to too low suction pressure, the setting value of the low-pressure pressure relay can be adjusted to 0.15 MPa. When the suction pressure drops to around 0.15 MPa, the compressor stops operating; after the compressor stops, the low-pressure value can be adjusted back again. (2) Drain the water from the oil coolers, condensers, and evaporators that have stopped operating, and remove any remaining water to prevent the heat transfer tubes inside them from being damaged by freezing in winter. (3) Close the relevant valves in the unit and check for any leaks. (4) The lubricating oil pump should be started and operated for 10–20 minutes per week, so that the lubricating oil can be distributed evenly throughout all the working surfaces inside the compressor over time. This prevents a lack of oil on the surfaces of the components due to prolonged shutdowns, which could cause difficulties when restarting the unit.