41 Questions and Answers on Refrigeration Systems

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1. What are the three necessary conditions for the safe operation of a refrigeration system? 　 Answer: (1) The refrigerant pressure within the system must not reach abnormally high levels to prevent the equipment from breaking. 　　 (2) Misoperations such as wet strokes, liquid explosions, or liquid slugs must not occur (as they may lead to such conditions), in order to prevent damage to the equipment. 　　 (3) Moving parts must be free of defects and fasteners must not be loose, to prevent damage to the machinery. 2. What is the evaporation temperature? 　　Answer: The temperature at which the refrigerant in the evaporator boils and vaporizes under a certain pressure is called the evaporation temperature. 3. What is the condensation temperature? 　　Answer: The temperature at which the gaseous refrigerant in the condenser condenses into a liquid under certain pressure is called the condensation temperature. 4. What is the subcooling (or supercooling) temperature? 　　Answer: The temperature to which the condensed liquid refrigerant is cooled, under the condensation pressure, to a level below the condensation temperature is called the subcooling temperature (or supercooling temperature). 5. What is intermediate temperature? 　　Answer: In a two-stage compression system, the saturation temperature of the refrigerant at the intermediate pressure in the intercooler is referred to as the intermediate temperature. 6. (How to detect, how to control) the suction temperature of the compressor? 　　Answer: The suction temperature of the compressor can be measured with a thermometer placed in front of the compressor’s suction valve. The suction temperature is generally higher than the evaporation temperature, and the extent of this difference depends on the length of the return pipe and the insulation of the pipes; it is usually 5 to 10 degrees higher than the evaporation temperature. The superheat can be adjusted by changing the liquid supply volume. 7. (How to measure) Compressor discharge temperature; (What factors affect the discharge temperature)? 　　Answer: The exhaust temperature of the compressor can be measured using a thermometer on the exhaust pipe. The exhaust temperature is directly proportional to the pressure ratio (of intake and exhaust) and the intake temperature. The higher the intake superheat and the greater the pressure ratio, the higher the exhaust temperature ； Otherwise, the opposite is true. Generally, the exhaust pressure is slightly higher than the condensation pressure. 8. What is surge (liquid slugging)? 　　Answer: When the refrigerant fails to absorb heat and evaporate properly, the liquid or wet vapor form of the refrigerant is drawn into the compressor, a condition known as wet compression. 9. What causes water in the ears? 　　Answer: (1) The level control of the gas-liquid separator or low-pressure surge tank malfunctions, resulting in an excessively high liquid level. 　　 (2) Excessive liquid supply volume and too rapid supply. Internal leakage or excessive opening of the throttle valve. 　　 (3) Excessive liquid accumulation in the evaporator or gas-liquid separator (low-pressure circulation tank), low heat load, and too rapid loading during startup. 　　 (4) Sudden increase in thermal load ； Or the suction valve was not adjusted in time after defrosting. 10. What consequences can occur after a tide car incident? 　　Answer: For piston machines: (1) The refrigerant enters the compressor, causing numerous bubbles to form in the lubricating oil, which destroys the oil film on the lubricated surfaces and also leads to unstable oil pressure. 　　 (2) Operating moving parts without proper lubrication, resulting in scuffing ； In severe cases, the bearing shaft and the Babbitt alloy of the main shaft bearings melt. 　　 (3) The refrigerant enters the compressor, causing the cylinder liner to cool down and contract rapidly, thereby gripping the piston ； In severe cases, it can damage the cylinder liner, piston, connecting rod, and piston pin. 　　 (4) Since liquids are incompressible, the forces acting on the connecting rods and pistons under surge conditions far exceed the design values, making damage very likely ； Since the liquid is incompressible, under flooding conditions, the exhaust valve assembly along with the dummy cover will be lifted by the force of the liquid ； In severe cases, it can cause the safety spring to deform, leading to catastrophic accidents such as the destruction of the engine block and cylinder head, as well as the breakdown of gaskets, which can injure people. 　　For screw compressors: moisture ingress can cause vibration and increased noise, as well as damage to the rotor and bearings due to excessive stress ； Severe water intrusion can also damage equipment and cause accidents. 11. How should one handle a tide-affected vehicle when discovered? 　　Answer: (1) When a piston compressor experiences slugging, the suction shut-off valve of the compressor should be immediately reduced, and the throttle valve should be closed to stop liquid supply. If the suction temperature continues to drop, the suction valve must be further closed or even shut off, while also reducing the load until it reaches zero. The frictional heat generated between the crankshaft and the bearing shells during rotation is used to evaporate the refrigerant inside the crankcase. Once the pressure within the crankcase rises, a set of cylinders is activated; after the pressure decreases, they are deactivated. Repeat this multiple times until the refrigerant inside the crankcase is completely evaporated. After that, slightly open the intake shut-off valve and gradually increase the load. If there is still refrigerant liquid in the suction line, repeat the previous process. Until the liquid is completely drained, slowly fully open the suction stop valve, and the compressor will resume normal operation. When surging occurs, it is necessary to observe carefully and adjust the oil pressure. If there is no oil pressure or the pressure is too low, the machine should be stopped immediately; the lubricating oil and coolant in the crankcase should be drained, new oil should be added, and then the machine can be restarted. (2) When a wet rot occurs in the screw compressor, the compressor’s suction shut-off valve should be immediately reduced, and the throttle valve should be closed to stop liquid supply. If the suction temperature continues to drop, it is necessary to further reduce the opening of the suction valve, but not close it entirely, in order to prevent abnormal noises and vibrations caused by excessively low suction pressure; meanwhile, reduce the load until it reaches zero. Taking advantage of the screw compressor’s insensitivity to wet strokes, the liquid in the return gas line is slowly drained into the oil. After that, open the suction shut-off valve and gradually increase the load until the compressor starts operating normally. When surge pressure occurs, it is necessary to pay attention and adjust the oil pressure. To prevent the oil temperature from dropping too low, the oil heating device can be turned on or the oil cooling water valve can be closed. 12. What causes excessive exhaust pressure, and how can it be resolved? 　　Answer: (1) A large amount of mixed gas in the system and high-pressure sections can cause excessively high exhaust pressure. Air should be released. In ammonia systems, to reduce atmospheric pollution caused by ammonia, air separators are generally used to discharge non-condensable gases from the system. 　　In small fluorine systems, air can be discharged directly through the air release valve on the condenser. Slightly open the vent valve to release air; when the gas emitted appears as white smoke, it indicates that a large amount of Freon is being released, and the valve should be closed to stop the venting process. 　　 (2) Scaling or accumulation of debris on the water side of the condenser heat exchange tubes. The water covers on both sides of the condenser should be opened for inspection and cleaning (flush with a high-pressure water jet, scrub back and forth with a brush or cloth, or have it cleaned by a professional company). 　 　 (3) Excessive water accumulation and oil accumulation in the condenser. Check whether the condenser outlet valve and the balance pipe valve are fully open (they should be fully open); if necessary, check whether the valve seats have come loose. Excess refrigerant is released, along with accumulated refrigeration oil. 　　 (4) The split gasket of the condenser end cover is damaged, resulting in a short-circuited cooling water flow. The water covers on both sides of the condenser should be opened to remove rust from the area of the thrust washer, and the gaskets should be replaced. 　　 (5) The inlet and outlet temperatures of the cooling water exceed the design requirements. Clean the dirt from the cooling water tower, check whether the distributor is loose or tilted, and ensure that there are no foreign objects blocking the inlet. 　　 (6) Insufficient cooling water flow. It is manifested as a temperature difference between the inlet and outlet water of the cooling water exceeding the specified value. Check: Whether there is excessive mechanical wear on the water pump ； Is there any foreign object blocking inside the water pump? ； Are the water valve, check valve, and filter screen abnormal? ； Does the water pump’s head meet the requirements? ； Are the layout and specifications of the water pipes reasonable? 13. What are the reasons why a compressor will not start, and what are the solutions? 　　Answer: (1) Electrical fault ； Inspect and repair. 　　 (2) Failure of the pressure relay or oil pressure relay ； Check and adjust the interlocking contacts of the pressure relay and the oil pressure relay. 　　 (3) Excessively high crankcase pressure or intermediate pressure ； Inspect or replace the exhaust valve plates, or reduce the crankcase and intermediate pressure. 　　 (4) (Piston machine) Unloading mechanism failure ； Inspect and repair. 14. What are the causes of knocking sounds in the cylinder of a piston engine and how to deal with them? 　　Answer: (1) During operation, the piston hits the exhaust valve ； Open the cylinder head where there is noise, and increase the clearance between the piston and the inner valve seat. (2) The valve bolts are loose ； Tighten the air valve bolt. 　　 (3) The valve disc is damaged and breaks off, falling into the cylinder; the clearance between the piston pin and the piston pin boss is too large, and the clearance between the piston and the cylinder is too large ； After removing the cylinder, conduct inspections, make adjustments, and carry out repairs. 　　 (4) The fake cover spring is deformed, resulting in insufficient elasticity ； Add a pad to increase the spring force or replace it. 　　 (5) Refrigerant liquid entering the cylinder causes liquid slugging ； Close down or temporarily shut off the suction shut-off valve and the liquid supply throttle valve to drain the liquid. 15. What are the causes of knocking sounds in the crankcase of a piston engine, and what are the solutions? 　　Answer: (1) The clearance between the big-end bearing of the connecting rod and the crank pin is too large ； Check and adjust its clearance or replace it. 　　 (2) Excessive clearance between the main journal and the main bearings ； Check and adjust the clearance. 　　 (3) Loose fit between the flywheel and the shaft or key ； Check and adjust the clearance, then repair it. 　　 (4) Breakage of the split pin in the connecting rod bolt, loosening of the connecting rod nut ； Tighten the linkage nut and secure it with a split pin. 16. What are the reasons for no oil pressure after a piston compressor starts up, and what are the solutions? 　　Answer: (1) Failure of the oil pump’s transmission components ； Disassemble for maintenance. 　　 (2) Blockage at the oil pump inlet ； Check and remove dirt. 　　 (3) Oil pressure gauge malfunction ； Replace the oil pressure gauge. 　　 (4) The fine oil filter and shaft seal are oil-free ； Before starting the machine, oil should be added to the fine oil filter and shaft seal to prevent vacuum conditions from occurring during operation. 17. What are the causes of low oil pressure in piston compressors and how to address them? 　Answer: (1) The oil suction filter is clogged ； Remove and clean. 　　 (2) Failure of the oil pressure control valve ； Repair or replace. 　　 (3) Excessive clearance and wear between the oil pump gear and the pump cover ； Repair or replace. 　　 (4) Low crankcase oil level ； Refuel or return oil from the oil fraction. 　　 (5) Severe wear of bearings in various parts leads to excessive clearance or oil leakage in some oil circuits ； Inspect and repair. 18. What are the reasons for increased oil consumption in piston compressors and how to address them? 　　Answer: (1) The refrigerant liquid enters the crankcase ； Reduce the flow of the suction shut-off valve and the liquid supply throttle valve, or close them temporarily (refer to the methods for handling flooded vehicles). 　　 (2) The sealing ring, oil scrapper ring, or cylinder is severely worn, or the piston ring landings are aligned on one line ； Inspect and adjust, and replace severely worn parts if necessary. 　　 (3) Excessively high crankcase oil level or excessive high exhaust temperature ； Release some lubricating oil or take measures to reduce the exhaust temperature. 19. What are the causes of oil or air leakage from the shaft seal, and what are the solutions? 　　Answer: (1) Poor shaft seal assembly or roughness on the shaft seal sealing surface ； Inspect and adjust, replace, or grind the sealing ring. 　　 (2) The “O”-rings of the moving and stationary rings are aged and deformed, or their tightness is not appropriate ； Replace the sealing rubber ring. 　　 (3) High content of liquid refrigerant in the oil ； Increase the oil temperature or release refrigerant. 　　 (4) Excessively high crankcase pressure in piston compressors ； Reduce crankcase pressure. 20. What are the causes of failure in the unloading mechanism of piston compressors, and what are the solutions? 　　Answer: (1) Insufficient oil pressure ； Adjust the oil pressure so that it is 0.12~0.2 MPa higher than the suction pressure. 　　 (2) Clogged oil pipe ； Take it apart and clean it. 　　 (3) Debris inside the cylinder has caused it to get stuck ； Take it apart and clean it. 　　 (4) Improper assembly of the oil distribution valve, incorrect installation of the pull rod or rotating ring, or the rotating ring getting stuck ； Disassemble for maintenance. 21. What are the causes of excessive compressor suction superheat (suction temperature being higher than the evaporation temperature) and what are the solutions? 　　Answer: (1) There is insufficient refrigerant in the refrigeration system ； Top up the refrigerant. 　　 (2) Insufficient refrigerant in the evaporator ； Open the throttle valve wider to increase the liquid supply. 　　 (3) Poor insulation of the suction pipeline in the refrigeration system ； Inspect and repair. 　　 (4) Excessive water content in the refrigerant ； Check the moisture content of the refrigerant. 　　 (5) The throttle valve is open to a small degree, resulting in a low liquid supply volume ； Open the throttle valve wider to increase the liquid supply volume. 22. What are the reasons for high exhaust temperature in piston compressors and how to address them? 　　Answer: (1) The temperature of the inhaled gas is too high ； Adjust the suction superheat (refer to question 22). 　　 (2) Exhaust valve disc cracked ； Open the cylinder head, inspect and replace the exhaust valve plates. 　　 (3) Safety valve leaking ； Inspect the safety valve and adjust/repair it. 　　 (4) Piston ring leakage ； Inspect the piston rings and adjust/repair them. 　　 (5) Cracked cylinder liner gasket, air leakage ； Inspect and replace. 　　 (6) Excessive clearance at the top dead center of the piston ； Check and adjust the top dead center clearance. 　　 (7) Insufficient cooling capacity of the cylinder head ； Check the water volume and temperature, and make adjustments. 　　 (8) The compression ratio of the compressor is too high ； Measure the evaporation pressure and condensation pressure. 23. What are the reasons for too low suction pressure in a compressor and how to address it? 　　Answer: (1) The supply throttle valve or suction filter is blocked (dirty or frozen) ； Disassemble for inspection and cleaning. 　　 (2) Insufficient refrigerant in the system ； Top up the refrigerant. 　　 (3) Insufficient refrigerant in the evaporator ； Open the throttle valve wider to increase the liquid supply. 　　 (4) There is too much refrigerant oil in the system and within the evaporator ； Identify the areas in the system where oil has accumulated and drain the accumulated oil. 　　 (5) Low heat load ； Adjust the compressor stage and perform proper unloading. 24. What are the causes of abnormal vibration in screw air compressors and how to deal with it? 　　Answer: (1) The foundation bolts of the unit are not tightened or are loose ； Tighten the foot bolts. 　　 (2) The compressor shaft and motor shaft are misaligned or out of alignment ； Realignment. 　　 (3) Pipeline vibration exacerbates the vibration of the unit ； Add support points or change the support points. 　　 (4) The compressor draws in too much oil or refrigerant liquid ； Shut down the machine and rotate the crank to discharge the liquid from the compressor. 　　 (5) The spool cannot stay in the desired position, but vibrates there ； Check for leaks in the oil piston, four-way valve, or load/unload solenoid valve, and repair them if necessary. 　　 (6) Excessively high vacuum level in the intake chamber ； Open the suction shut-off valve and check whether the suction filter is clogged. 25. What are the reasons for insufficient cooling capacity in screw chiller units and how to address them? 　　Answer: (1) The position of the spool is incorrect or there are other faults (the spool cannot rest against the fixed end) ； Check the position of the indicator or angular displacement sensor, and inspect the spool valve. 　　 (2) The intake filter is clogged, resulting in excessive intake pressure loss, a drop in intake pressure, and reduced volumetric efficiency ； Remove the intake air filter and clean it. 　　 (3) Abnormal wear of the machine leads to excessive gaps ； Inspect, adjust, or replace parts. 　　 (4) Excessive resistance loss in the intake pipeline; the intake pressure is much too low compared to the evaporation pressure ； Check the suction stop valve and suction check valve; repair any issues found. 　　 (5) Leakage between high-voltage and low-voltage systems ； Inspect the bypass valves for starting and stopping as well as the return valve; repair any issues found. 　　 (6) Insufficient fuel injection, preventing proper sealing ； Inspect the oil circuit, oil pump, and oil filter to increase the fuel injection volume. 　　 (7) The exhaust pressure is much higher than the condensation pressure, resulting in a decrease in volumetric efficiency ； Inspect the exhaust system pipes and valves to remove any obstructions in the exhaust system. If air has entered the system, it should be released. 26. What are the causes and solutions for abnormal noises occurring during the operation of screw compressor units? 　　Answer: (1) There are foreign objects in the rotor slots ； Inspect the rotor and intake filter. 　　 (2) Thrust bearing damage ； Replace the thrust bearing. 　　 (3) Main bearing wear; friction between the rotor and the housing ； Inspect and replace the main bearings. 　 　 (4) Spool skew ； Inspect the slide valve guide block and guide posts. 　 　 (5) Loose connections at moving parts ； Disassemble the machine for maintenance and strengthen relaxation measures. 27. What are the causes and solutions for excessively high exhaust temperature or oil temperature? 　　Answer: (1) The compression ratio is too high ； Measure the intake and exhaust pressures to reduce the pressure ratio. 　　 (2) Decreased cooling efficiency of the water-cooled oil cooler ； Clean the oil cooler, lower the water temperature, or increase the water flow. 　　 (3) Insufficient liquid supply to the liquid ammonia oil cooler ； Analyze the reasons and increase the liquid supply volume. 　　 (4) Inhaling severely superheated steam ； Increase the liquid supply volume, improve the insulation of the suction pipeline, and check for leaks in the bypass valve. 　　 (5) Insufficient fuel injection volume ； Inspect and analyze the cause, then increase the fuel injection volume. 　　 (6) Air infiltration into the system ； It should be released, and the cause of air infiltration should be investigated for repair. 28. What are the reasons for a drop in the exhaust temperature or oil temperature of a screw compressor, and what are the solutions? 　　Answer: (1) Inhaling moist vapor or liquid refrigerant ； Reduce the amount of liquid supplied to the evaporation system. 　　 (2) Continuous no-load operation ； Check the spool valve. 　　 (3) Exhaust pressure is abnormally low ； Reduce the water supply volume or decrease the number of condensers in use. 29. What are the reasons for inflexible operation or no operation of the slide valve in screw compressors, and what are the solutions? 　　Answer: (1) The four-way directional valve or solenoid valve does not operate smoothly ； Check the four-way directional valve or solenoid coil and wiring. 　　 (2) The oil pipeline system is blocked ； Maintenance. 　　 (3) Oil piston stuck or oil leakage ； Inspect the oil piston or replace the seal ring. 　　 (4) Low oil pressure ； Inspect and adjust the oil pressure. 　　 (5) The slide valve or guide key is stuck ； Maintenance. 30. What are the causes of excessive temperature in the screw compressor casing and how to address them? 　　Answer: (1) The moving parts are abnormally worn ； Inspect the compressor and replace damaged parts. 　　 (2) Severe overheating during inhalation ； Reduce the suction superheat. 　　 (3) Bypass pipeline leakage ； Check whether the bypass valves for starting and stopping the machine are leaking. 　　 (4) Excessively high compression ratio ； Measure the intake and exhaust pressures to reduce the pressure ratio. 31. What are the causes of leakage at the shaft seals of compressors and oil pumps, and what are the solutions? 　　Answer: (1) Insufficient oil supply to the shaft seal causes damage to the sealing ring ； Overhaul, inspect the oil circuit, and adjust the oil pressure. 　　 (2) Deformation or damage to the “O” ring ； Replace. 　　 (3) Poor assembly ； Disassemble and repair. 　　 (4) Inadequate contact between the stationary and rotating rings ； Remove and re-grind. 　　 (5) Impurities in the oil cause wear to the sealing surfaces; there is too much refrigerant liquid in the oil ； Check the essential oil filter and ensure the oil supply temperature. 32. What are the causes of low oil pressure and how to deal with it? 　　Answer: (1) The oil pressure control valve is not adjusted properly ； Readjust the oil pressure control valve. 　　 (2) High oil leakage inside the compressor ； Inspect and repair. 　　 (3) Oil temperature too high ； Inspect the oil cooler to eliminate factors that affect its heat transfer capacity. 　　 (4) Poor oil quality and insufficient oil quantity ； Change oil, add fuel. 　　 (5) Oil pump wear or failure ； Maintenance. 　　 (6) The coarse and fine oil filters are dirty and clogged ； Clean the filter element. 　　 (7) The oil contains a large amount of refrigerant ； Stop the machine and heat the oil. 33. What are the reasons for increased oil consumption in compressors and how to address them? 　　Answer: (1) The oil separation efficiency of the oil separator has decreased ； Service the oil separator. 　　 (2) Excessive oil accumulation in the oil separator, with too high an oil level ； Oil drainage and oil level control. 　　 (3) Excessively high exhaust temperature reduces the efficiency of the oil separator ； Enhance oil cooling to reduce exhaust temperature. 　　 (4) Excess oil pressure, excessive fuel injection, liquid return to the compressor ； Adjust the oil pressure or service the compressor, and deal with the liquid returning to the compressor. 　　 (5) Clogged return oil line ； Maintenance. 34. What are the causes of rising oil level in the oil separator and how to deal with it? 　　Answer: (1) The oil in the system returns to the compressor ； Too much oil is being released. 　　 (2) Excessive refrigerant entering the compressor oil ； Increase the oil temperature to accelerate the evaporation of the refrigerant dissolved in the oil. 　　 (3) Blockage in the oil return pipeline of the oil separator ； Maintenance. 　　 (4) The liquid level gauge of the vertical oil separator contains condensed refrigerant liquid ； At this time, the liquid level height may be inaccurate; the actual oil level height should be estimated. 35. What are the reasons for the compressor to run in reverse when a screw compressor is shut down, and what are the solutions? 　　Answer: (1) The intake and exhaust check valves are not sealing properly ； Inspect and repair to eliminate valve plate jamming. 　　 (2) The bypass valve to prevent reversal was not opened in time ； Inspect and repair. 36. What are the causes of too low inhalation temperature and how to address them? 　　Answer: (1) There is too much refrigerant in the evaporator, gas-liquid separator, or low-pressure circulation tank ； Adjust the liquid supply valve to stop or reduce the amount of liquid supplied, and it is even possible to drain excess refrigerant into a drain tank. 　　 (2) Decreased heat exchange efficiency of the evaporator ； Clean the evaporator or drain the oil. 37. How are the safety protection values for refrigeration equipment specified? Answer: The safety protection values for refrigeration equipment are specified in the product’s user manual. The safety protection values for LG series screw refrigeration compressors are as follows (for reference): (1) High oil injection temperature protection: 65°C (shutdown) ； (2) Low suction pressure protection: -0.03 Mpa (machine shuts down); this value can be modified ； (3) High exhaust pressure protection: 1.57 Mpa (shutdown) ； (4) Essential oil filter high pressure difference protection: 0.1 Mpa (shutdown) ； (5) Main motor overload protection (protection value according to the requirements of the installed motor) ； (6) Low oil pressure to exhaust pressure difference protection: 0.1 Mpa (shutdown) ； (7) Oil pump overload protection (protection value based on the requirements of the associated motor) ； (8) Protection against low outlet water temperature for chiller units, brine-based cooling units, and ethylene glycol-based cooling units; protection against water supply interruption to the evaporator and condenser. (9) Opening pressure of safety valves for condensers, liquid storage tanks, oil separators, and oil collectors: 1.85 Mpa ； Opening pressure of the safety valve for the full-liquid evaporator, gas-liquid separator, low-pressure circulation liquid tank, intercooler, and economizer: 1.25 Mpa. 38. How are the vacuum tests for the system specified? 　　Answer: The purpose of conducting a vacuum test on the system is to check its sealing performance under vacuum conditions, while also preparing it for the filling of refrigerant and refrigeration oil. The system should be evacuated to 5.33 kPa (40 mmHg) and held at this pressure for 24 hours; the pressure increase during this period should not exceed 0.67 kPa (5 mmHg). 39. How to schedule major, medium, and minor equipment repairs? 　　Answer: The cycles for major, medium, and minor repairs of the equipment should be determined by the user, who must take into account the specifications in the equipment’s user manual as well as factors such as the operating environment, working conditions, annual operating hours, and production pace in order to plan these repairs. Perform maintenance at the appropriate time. The contents of major, medium, and minor repairs for the equipment are determined in accordance with the equipment’s operating instructions and taking into account the actual usage conditions of the equipment. 40. How should major, medium, and minor repairs of piston refrigeration compressors be scheduled? (Reference) (1) What is the cycle for major repairs? 　　Answer: A major overhaul is carried out every 8,000 hours of operation or so. (2) What are the contents of the major repair? 　　Answer: Inspect and clean the components, and measure the degree of wear on them: such as cylinders, pistons, piston rings, crankshafts, bearings, connecting rods, intake and exhaust valves, oil pumps, etc. Those with minor wear can be repaired and used, while those with more severe wear should be replaced. Check the safety valves and various instruments (to be carried out by a qualified department). Clean filters and other components in the refrigeration oil system, refrigerant system, and water system. (3) What is the cycle for mid-term repairs? 　　Answer: A mid-term maintenance is carried out every 3,000–4,000 hours of operation. (4) What is the content of the mid-term repair? 　　Answer: Apart from minor repair tasks, it is necessary to check and adjust the clearance between the cylinder and piston, the clearance at the piston ring seats, the clearance between the big and small end bearings of the connecting rod and the crank pin, the clearance between the main bearings and the shaft diameter, as well as the clearance between the valve and the piston. Check the degree of wear on components such as piston pins, cylinders, and crankshaft. Check the lubrication system. Check whether the couplings and anchor bolts are loose. (5) What is the cycle for minor repairs? 　　Answer: After the mid-overhaul, a minor overhaul is carried out every 1,000–1,200 hours of operation. (6) What does minor maintenance include? 　　Answer: Clean the cooling water pump ； Inspect the piston, gas rings, oil rings, and intake and exhaust valves; replace any damaged valve plates and valve springs, etc ； Inspect the bearing shells of the rod ends, and clean the crankcase, oil filter, air intake filter, etc ； Replace the refrigeration oil ； Check the coaxiality of the motor and the crankshaft. 　　41. How to schedule major, medium, and minor repairs for screw refrigeration compressors? (Reference) Answer: The maintenance schedule for screw compressor units is related to many factors; the information below is provided for reference. (1) Motor of screw compressor: Disassembly, maintenance, and part replacement, as well as bearing lubrication; valid for 2 years. Refer to the motor operation manual. (2) Coupling: Check the coaxiality of the compressor and the motor (check whether the elastic coupling elements are damaged or the rubber pins are worn). The inspection interval is 3–6 months. (3) Oil separator: Clean the interior, every 2 years. (4) Oil cooler: Remove scale (for water cooling) and oil sludge; maintenance interval is half a year ； It depends on the water quality and level of dirt. (5) Oil pump: Leak testing and maintenance, valid for 1 year. (6) Oil filter (including coarse oil filter), intake air filter: cleaning, every six months. It should be cleaned after 100–150 hours of driving for the first time. (7) Oil pressure control valve: Check of regulation capacity, validity period of 1 year. (8) Spool valve: Functional check, deadline 3–6 months. (9) Safety valves, pressure gauges, thermometers: calibration, validity period of 1 year. (10) Check valves, air intake and exhaust stop valves, pressure gauge valves: maintenance, period of 2 years. (11) Pressure relays, temperature relays: Calibration, with a validity period of about half a year. Refer to the instructions. (12) Electrical equipment: Functional check, with a deadline of about 3 months. Refer to the instructions. (13) Automatic protection and automatic control systems: Duration of about 3 months.