Precautions for commissioning, starting up, and shutting down screw refrigeration units

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I. Preparatory work before starting a screw-type refrigeration unit. Proper commissioning of the refrigeration unit is an important step in ensuring its proper operation, saving energy consumption, and extending 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 involved during commissioning. Before debugging, users should carefully read the product operating instructions provided by the manufacturer and proceed step by step in accordance with the operating requirements. 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 chillers are medium to large-sized refrigeration units, close coordination is required in terms of design, installation, and operation during commissioning. To ensure that the commissioning work is carried out in an orderly manner, it is necessary to form a temporary commissioning team composed of relevant personnel to oversee the entire process. (2) The personnel responsible for commissioning should have a thorough understanding of the structure and performance of the unit equipment, be familiar with the safety techniques related to refrigerators, clearly understand the methods, procedures, and technical requirements for commissioning. They should develop a detailed and specific commissioning plan, and ensure that all commissioning personnel at each post are aware of their tasks and requirements. (3) Check whether the installation of the unit meets the technical requirements, whether the foundation of the unit is satisfactory, and whether the dimensions, specifications, and material of the connecting pipelines comply with the design requirements. (4) The power supply system of the unit shall be fully installed and passed testing. (5) Conduct a water flow test on the cold water and cooling water systems separately to flush out any contaminants from the water circuits. The water pumps should operate normally, and the volume of circulating water must 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 various general and special tools required for debugging. (8) Prepare various measuring instruments and gauges required for commissioning, such as those for pressure, temperature, flow rate, mass, time, etc. (9) Prepare the safety protection equipment necessary for commissioning and operation. For the commissioning of 2 units, after the refrigerant charging is completed (if refrigerant charging is required), the load commissioning can be carried out. In recent years, screw chillers have made significant advancements in terms of performance and electrical control. Many of these chillers allow for simulated tests 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 any problems arise with the electronic control system, they can be resolved promptly. Finally, connect the power to the host computer to carry out debugging. During the commissioning process, special attention should be paid to the following points: a. Check whether all valves in the refrigeration system are in their normal open state; in particular, the discharge stop valve must not be closed. b. Open the cooling water valve of the condenser and the cold water valve of the evaporator; the flow rates of cold 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, check whether all its parameters are normal according to the startup manual provided by the manufacturer. 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 upper and lower loading mechanisms of the compressor are functioning properly. h. The safety protection devices installed in the refrigeration system, such as high and low pressure protection devices, flow switches for cold water and cooling water supply interruption, safety valves, etc., should be used properly; any damaged devices must be replaced promptly. i. If any abnormality occurs in the unit, it should be shut down immediately for inspection. Before debugging the refrigeration system, it is essential to clean and dry the interior of the air conditioning system thoroughly. If the preliminary work is not carried out carefully, it will increase the workload during commissioning and pose many potential problems for the subsequent operation of the refrigeration system. II. Startup procedures for screw-type refrigeration units: After the screw-type refrigeration compressor has undergone trial operation and any issues identified have been resolved, it can proceed to its normal operating mode. The operation method is as follows: 1. Verify whether the status of all relevant valves in the unit meets the requirements for starting up. 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. Start the cooling water pump, the cooling tower fan, and the refrigerant water pump; the operation indicator lights for all three should be visible. 4. Check whether the lubricating oil temperature has reached 30 ℃. 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 running, set the energy regulation control valve to the unloading position, and ensure that the slide valve is in the zero position. 6. Adjust the oil pressure regulating valve so that the oil pressure reaches 0.5–0.6 MPa. 7. Turn off the compressor power supply, activate the control switch, and open the compressor suction valve. After a delay, the compressor starts running. Once the compressor is in operation, adjust the lubricating oil pressure so that it is 0.15–0.3 MPa higher than the discharge 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 temperature of the lubricating oil reaches 45 °C, the power supply to the electric heater is disconnected. At the same time, the inlet and outlet valves for the cooling water in the oil cooler are opened, so that the oil temperature remains within the range of 40–55 °C during operation of the compressor. 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 discharge valve of the unit are in the fully open position. 12. Set the energy regulating device to the 100% position, and at the same time 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 fan 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 exhaust pressure of the compressor below 1.55 MPa, and is the exhaust temperature below 100 °C? 5) During the compressor’s operation, is the operating current of the motor 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 of the compressor normal during operation? 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 exhaust pressure should be between 1.8 and 1.47 MPa (gauge pressure); 2. The compressor’s exhaust temperature should range from 45 to 90 °C, with a maximum of 1.05 °C; 3. The oil temperature within the compressor should be around 40 to 55 °C; 4. The oil pressure in the compressor should be between 0.2 and 0.3 MPa (gauge pressure); 5. During operation, the compressor should produce a uniform and steady sound, without any abnormal noises; 6. The condensing temperature of the unit should be 3 to 5 °C higher than the temperature of the cooling water. The temperature of the cold medium should generally be around 40 °C, while the inlet temperature of the coolant should be below 32 °C; 7. The evaporation temperature of the unit should be 3 to 4 °C lower than the outlet temperature of the coolant, with the outlet temperature of the coolant usually being around 5 to 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. Procedure for normal shutdown: (1) Place the manual unloading control device in the unloaded position. (2) Close the solenoid valve and the liquid 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: During normal operation, if any abnormal conditions occur in screw-type refrigeration compressors, an emergency shutdown should be initiated to protect the safety of the units. 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 unit’s supply pipe 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 faults 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 protection 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 stops, the shutdown of other components can be handled using the emergency shutdown method. After completing the shutdown procedures, the unit should be inspected; it can be restarted according to the normal startup procedure only after the faults have been resolved. 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 outlined 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 excessively low suction pressure, the set value of the low-pressure relay can be adjusted to 0.15 MPa. When the suction pressure drops to around 0.15 MPa, the compressor shuts down. After the compressor has stopped, the low-pressure value can be readjusted. (2) Drain the water from the oil coolers, condensers, and evaporators that have been taken out of service, 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 operated for 10–20 minutes each week, so that the lubricating oil can be evenly distributed over all working surfaces inside the compressor over a long period of time. This prevents oil deficiency on the surfaces of components due to prolonged downtime, thereby avoiding difficulties when restarting the unit.