[Mechanical Equipment Technology Edition] Daily Question 201804028: Common Faults of Screw Refrigeration Compressors (4)

Inspection and maintenance: Strictly follow the procedures for inspecting and maintaining screw compressors, improve the technical skills of maintenance personnel, and ensure the quality of equipment inspection and maintenance； Process: Strictly adhere to the operational parameters for screw compressors, conducting frequent inspections and thorough analyses. The final point is that process engineers and maintenance personnel should communicate more with each other, learn from one another, and share their experiences, in order to ensure the safe, stable, and long-term operation of the equipment.

1. Establish rules and regulations for the cleaning and maintenance of screw-type chillers. It is essential to clean the evaporator, condenser, pipes, filters, cooling towers, as well as fans of the unit on an annual basis, in order to prevent impurities in the water from causing scaling and affecting the proper operation of the chiller. 2. Operators should always pay attention to the operation of the unit; if the chiller emits abnormal noises or shows any faults, it is necessary to shut it down promptly to prevent further damage to the unit. 3. Operators should regularly record all the data related to the operation of the unit (this applies equally to maintenance tasks as well), to facilitate future reference. After long-term use, it is necessary to inspect various components of the unit to ensure they are functioning properly.

First, we need to establish rules and procedures for the cleaning and maintenance of screw chillers, and keep records of each maintenance task performed. When cleaning the unit, it is not just a matter of cleaning its exterior; what’s more important is to clean the unit’s evaporator, condenser, pipes, filters, cooling towers, and fans on an annual basis, so as to prevent impurities in the water from causing scaling and affecting the proper operation of the chiller. Secondly, it is necessary to replace the compressor lubricant based on the usage time of the screw chiller. The main functions of lubricant in a compressor are lubrication, cooling, and capacity control. It is recommended to check the cleanliness of the lubricant every 10,000 hours of operation of the chiller, and to replace the lubricant every 20,000 hours or every 4 years. When changing the lubricant, it is necessary to use oil that has passed quality certification; oils from different brands cannot be mixed together! When using a screw-type chiller, operators should always pay close attention to its operation. If the chiller emits unusual noises or shows any signs of malfunction, it is essential to shut it down promptly to prevent further damage to the unit. Furthermore, troubleshoot the issue based on the error messages displayed on the chiller control panel; remember not to start the chiller until the fault has been resolved. If you encounter a fault that cannot be resolved on your own, be sure to contact the maintenance personnel from the screw chiller manufacturer promptly for after-sales support. Furthermore, operators of screw chillers should regularly keep records of various data related to the unit’s operation (the same applies to maintenance tasks), to facilitate future reference. After long-term use, it is necessary to inspect all components of the unit to ensure they are functioning properly. When a screw chiller is not needed to operate for an extended period of time, the water inside the unit and in the pipes should be drained to prevent damage to the unit’s components.

1. Proper commissioning of the refrigeration unit is an important aspect in ensuring the proper operation of the refrigeration system, saving energy consumption, and extending its service life. 2. For screw refrigeration compressors, there should be operation records documenting the performance of the units, and maintenance technical files must also be established. Complete technical documentation helps identify potential faults, allowing timely measures to be taken to prevent their occurrence. 3. Regular maintenance of the screw compressor should be paid attention to. 4. The condenser and evaporator should be cleaned at least once a year to remove scale and other contaminants from the tubes. 5. After long-term use, the quality of the lubricating oil deteriorates, with an increase in impurities and moisture within it; therefore, it is necessary to regularly monitor and check the quality of the oil. Once a problem is detected, it should be replaced promptly, and the grade of the lubricating oil used for replacement must comply with the technical specifications. 6. The filter element inside the dry filter must be replaced regularly. 7. Regular calibration of the safety valve is extremely important for the safety of the entire unit. 8. Refilling of the refrigerant. Unless there are other special reasons, normal units do not experience significant leaks. If a certain amount of refrigerant leaks due to improper use or after maintenance, it is necessary to add more refrigerant. When charging the refrigerant, attention must be paid to the type of refrigerant specified for use in the unit.

I. Carry out proper preparations before starting the screw-type refrigeration unit. Proper commissioning of the refrigeration unit is an important step in ensuring its normal 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. 1. Preparations before debugging: 1) 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. 2) Check whether the power supply system of the unit is functioning properly. 3) Conduct water flow tests separately on the cold water and cooling water systems to flush out debris from the water pathways; the pumps should operate properly, and the circulation volume should meet the requirements of the operating conditions. 4) Check whether the pressure gauges, thermometers, and sensors on the pipes are functioning properly. 5) Check if the water pressure in the pipes is normal. 6) Prepare the safety devices necessary for operation. 2. Unit commissioning 1) Check that all valves in the refrigeration system are in their normal open position, especially the exhaust stop valve, which must not be closed. 2) 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. 3) Before starting, it is necessary to check whether the power supply voltage of the unit is normal. 4) Start the unit in accordance with the startup manual provided by the manufacturer. 5) After the unit is started, check whether all its parameters are normal according to the startup manual provided by the manufacturer. 6) 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. 7) During the operation of the unit, it is necessary to check whether the upper and lower loading mechanisms of the compressor are functioning properly. 8) 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. 9) If any abnormality occurs in the unit, it should be stopped 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. Follow the procedures specified in the operation manual strictly when starting up screw refrigeration units. After the screw refrigeration compressors have undergone trial operations and any issues identified have been resolved, they can proceed to the normal operating procedure. 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 to illuminate the power control indicator light. (3) When starting the cooling water pump, the cooling tower fan, and the refrigerant water pump, their operation indicator lights should be visible. (4) Check whether the lubricating oil temperature 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 load-reduction 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 operating; once it is running, adjust the lubricating oil pressure so that it is 0.15–0.3 MPa higher than the exhaust pressure. (8) Turn on the control circuit of the solenoid valve in the closed liquid supply line to activate the solenoid valve; 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 temporarily turned off. (11) Turn the fuel injection valve 1/2 to 1 turn, while ensuring that the suction valve and the liquid discharge 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. (14) The sound and vibration levels of the chilled water pumps, cooling water pumps, 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. (15) 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? (16) When the compressor is operating at full load, is the suction pressure within the range of 0.36–0.56 MPa? (17) Is the exhaust pressure of the compressor below 1.55 MPa, and is the exhaust temperature below 100 °C? (18) During the operation of the compressor, is the motor 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. (19) Are the sound and vibration levels of the compressor normal during operation? Among the items mentioned above, if any abnormality is detected, the machine should be stopped immediately; after identifying the cause and fixing the fault, the unit can be restarted. Under no circumstances should the unit be operated with issues present, to avoid serious accidents. III. Perform the shutdown procedure for screw refrigeration compressors in accordance with the specified procedures. 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: (l) Set the manual unloading control device to 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 adjustment 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. 2. Emergency shutdown of the unit: During 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 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 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. 3. 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 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 using the normal startup procedure only after any faults have been resolved. 4. Long-term shutdown of the unit: Since the screw refrigeration compressors used as the cooling source for central air conditioning operate on a seasonal basis, the unit is shut down for extended periods of time. To ensure the safety of the unit, during seasonal shutdowns, the shutdown procedure can be carried out as follows. (l) During normal operation of the unit, 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. (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 machinery due to prolonged shutdowns, which could cause difficulties when restarting the unit. IV. Thorough maintenance of screw chiller units The main aspects of maintaining screw chiller units include routine maintenance and periodic inspections. Regular maintenance ensures the long-term proper operation of the unit, extends its service life, and also helps to save energy consumption for cooling. For screw chillers, there should be operation records to document the performance of the units, and maintenance technical files must also be established. Complete technical documentation helps identify potential faults, allowing timely measures to be taken to prevent their occurrence. 1. Screw compressor: The screw compressor is a very critical component in the unit; the quality of the compressor directly affects the stability of the unit. If the compressor fails, due to the high precision requirements for its installation, it is generally necessary to have the manufacturer carry out the repairs. 2. Cleaning of the condenser and evaporator: In water-cooled condensers, the cooling water uses an open-loop circulation system, with tap water being circulated through a cooling tower. When the levels of calcium and magnesium salts in water are high, they tend to decompose and deposit on the cooling water pipes, forming scale that impairs heat transfer. Excessive scaling also reduces the flow cross-section of the cooling water, decreases the water volume, and raises the condensation pressure. Therefore, when the quality of the cooling water used is poor, the cooling water pipes should be cleaned at least once a year to remove scale and other contaminants from within them. There are generally two methods for cleaning the water pipes in a condenser: (1) Using a specialized pipe-cleaning gun to clean the pipes. (2) Use a specialized cleaning agent for cyclic flushing, or add it to the cooling water; wait for 24 hours before changing the solution until it is thoroughly cleaned. 3. Replace the lubricating oil: After long-term use, the quality of the lubricating oil deteriorates, with an increase in impurities and moisture within it; therefore, it is necessary to regularly monitor and check the quality of the oil. Once a problem is detected, it should be replaced promptly, and the grade of the lubricating oil used for replacement must comply with the technical specifications. 4. Replacement of the dry filter – The dry filter is an important component that ensures proper circulation of the refrigerant. Since water and refrigerant are immiscible, the presence of water in the system will **affect the operating efficiency of the unit; therefore, it is very important to keep the system dry. The filter elements inside the dryer-filter must be replaced regularly. 5. Calibration of safety valves: The condenser and evaporator in screw-type chillers are both considered pressure vessels. According to regulations, a safety valve must be installed at the high-pressure end of the unit, that is, on the condenser itself. In case the unit operates under abnormal conditions, the safety valve can automatically release pressure, thereby preventing potential harm to people caused by high pressures. Therefore, the regular calibration of the safety valve is extremely important for the safety of the entire unit. 6. Refrigerant charging: Unless there are other special reasons, normal units do not experience significant leaks. If a certain amount of refrigerant leaks due to improper use or after maintenance, it is necessary to add more refrigerant. When charging the refrigerant, attention must be paid to the type of refrigerant specified for use in the unit. V. Operation Management and Precautions during Shutdown 1. Precautions for the operation management of screw-type chiller units (1) The normal start-up and shutdown of the unit must be carried out strictly in accordance with the steps outlined in the operation manual provided by the manufacturer. (2) During operation, the unit must record parameters in a timely and accurate manner. (3) If an alarm triggers a shutdown during the operation of the unit, relevant personnel should be notified promptly to inspect the unit. It is also possible to contact the factory directly. (4) It is strictly prohibited to short-circuit the water flow switch during the operation of the unit, to prevent the water pipes from freezing. (5) The machine room should be supervised by dedicated staff; unauthorized personnel are strictly prohibited from entering the machine room to operate the equipment. (6) The machine room should be equipped with appropriate safety protection devices and maintenance inspection tools, such as pressure gauges and thermometers; these tools should be stored in designated locations. 2. Precautions for shutting down screw-type chiller units: (1) The main power switch should be turned off after the unit is shut down. (2) If the unit is out of service for an extended period, all water accumulated within the chilled water and cooling water systems should be drained to prevent rust formation. The water chamber end cover should be sealed. (3) During long-term shutdowns, the unit should undergo proper maintenance. (4) During shutdown, the entire unit should be covered to prevent dust accumulation. (5) During shutdown, personnel unrelated to the unit shall not touch the machine.

Inspection and maintenance: Strictly follow the procedures for inspecting and maintaining screw compressors, improve the technical skills of maintenance personnel, and ensure the quality of equipment inspection and maintenance； Process: Strictly adhere to the operational parameters for screw compressors, conducting frequent inspections and thorough analyses. The final point is that process engineers and maintenance personnel should communicate more with each other, learn from one another, and share their experiences, in order to ensure the safe, stable, and long-term operation of the equipment.

1) Select the switch to enable manual startup； 2) Open the compressor exhaust stop valve ； 3) Set the compressor load to “0”, which corresponds to a 10% load level ； 4) Start the cooling water pump and the coolant water pump to supply water to the condenser, oil cooler, and evaporator ； 5) Start the oil pump ； 6) 30 seconds after the oil pump starts, when the pressure difference between the oil pressure and the exhaust pressure reaches 0.4~0.6 MPa, press the compressor start button; the compressor will start up, and at the same time, bypass solenoid valve A will also open automatically. Once the motor is operating normally, valve A closes automatically ； 7) Monitor the suction pressure gauge, gradually open the suction shut-off valve and increase the load manually, making sure that the suction pressure does not drop too low. After the compressor starts operating normally, adjust the oil pressure control valve so that the oil pressure difference is 0.15~0.3 MPa. 8) Check whether the pressure and temperature of various parts of the equipment, especially the temperature of moving components, are normal. If anything abnormal occurs, the machine should be stopped for inspection. 9) The initial operation time should not be too long; it is possible to shut down after about half an hour. The shutdown sequence is to unload, shut down the main machine, close the suction stop valve, stop the oil pump, and stop the water pump, thereby completing the first startup process. When the main unit’s stop button is pressed, bypass solenoid valve B opens automatically, and it closes automatically after shutdown.

Inspection and maintenance: Strictly follow the procedures for inspecting and maintaining screw compressors, improve the technical skills of maintenance personnel, and ensure the quality of equipment inspection and maintenance； Process: Strictly adhere to the operational parameters for screw compressors, conducting frequent inspections and thorough analyses. The final point is that process engineers and maintenance personnel should communicate more with each other, learn from one another, and share their experiences, in order to ensure the safe, stable, and long-term operation of the equipment.