Brief introduction to the basic characteristics of screw air compressors

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Introduction: Replacing piston air compressors, which have many vulnerable components and poor reliability, with highly reliable screw air compressors has become an inevitable trend. According to statistics, screw compressors accounted for only 27% in Japan in 1976, rising to 85% by 1985. In the developed Western countries, the market share of screw air compressors is 80%, and this figure continues to rise. Screw air compressors have advantages such as simple structure, small size, no vulnerable parts, reliable operation, long service life, and easy maintenance. A single-screw air compressor, also known as a worm air compressor, has a meshing pair consisting of a 6-threaded screw and two 11-toothed star wheels. The worm engages with two star wheels simultaneously. This not only ensures a balanced load on the worm, but also doubles the displacement volume. As a result, the compressor is compact in size, with a displacement of just 9 cubic meters per minute (9 m³/min). The weight of this worm-type compressor is merely 1/6 that of a piston-type compressor. The force balance in the worm gear gives screw air compressors a series of advantages: high performance and efficiency. These compressor units – screw-type air compressors – utilize high-capacity compression components; the low speed of the rotor’s outer surface facilitates effective oiling, resulting in high efficiency and reliability. By 2012, manufacturers’ designs were able to ensure extremely low system temperatures and compressed air temperatures. Ensure that all components achieve better cooling performance and a longer service life. Foldable drive concept: Air compressor equipment – screw air compressors use efficient transmission systems to drive the compression components at an optimal speed suitable for the intended application. No maintenance is required at all during normal operation. It boasts the advantages of maintenance-free operation, high reliability, and high efficiency. Built-in intelligent control: To reduce operating costs, precise operational control is essential. All screw compressors are equipped with an intelligent control system, whose control menu is easy to use. Detailed analysis of the working process of a screw air compressor – the intake process: As the rotor rotates, the grooves between the male and female rotors reach the opening at the intake side wall, at which point the space within these grooves becomes larger. At this stage, the grooves are connected to the intake port. During exhaust, the gas in these grooves is completely expelled, leaving the grooves in a vacuum state. When the rotor reaches the intake port, external air is drawn in and enters the grooves of the male and female rotors along the axial direction. When the gas fills the entire tooth groove, the end face on the intake side of the rotor moves away from the intake port of the casing, thereby sealing off the gas in the tooth groove. Detailed analysis of the working process of screw air compressors – the exhaust process: When the mating surfaces of the rotors come into alignment with the exhaust port of the casing, the compressed gas begins to be discharged. This continues until the mating surfaces between the teeth move to the exhaust end face; at this point, the space between the mating surfaces of the male and female rotors and the tooth grooves at the exhaust port is zero, marking the completion of the exhaust process. Meanwhile, the length of the tooth grooves between the rotating surfaces and the intake port of the casing increases again, allowing the intake process to commence once more. Working pressure: There are many ways to express pressure units, and here we mainly introduce the units commonly used for indicating pressure in screw air compressors. ① Working pressure, which is often referred to as exhaust pressure by users in China. Working pressure refers to the higher pressure of the gas discharged from the air compressor; ② The commonly used units for working pressure are: bar or MPa; 1 bar = 0.1 MPa; ③ Generally, users refer to the pressure unit as “Kg” (kilograms). In reality, “kg” is simply an abbreviation for kgf/cm²; roughly speaking, 1 bar equals 1 kgf/cm². Power ① Generally speaking, the power of an air compressor refers to the rated power of the drive motor or diesel engine associated with it; ② The units of power are KW (kilowatts) or HP (horsepower), with 1 KW ≈ 1.333 HP. Maintenance: Intake air filter element. The air filter is a component that removes dust and impurities from the air; the cleaned air then enters the compression chamber of the screw rotor for compression. The internal clearance within the screw machine allows only particles under 15µ to be filtered out. If the air filter element is clogged or damaged, a large amount of particles larger than 15u enter the screw compressor’s internal circulation. This not only **reduces the service life of the oil filter and the oil-gas separation element**, but it also allows a large number of particles to reach the bearing chamber, accelerating bearing wear and increasing the rotor clearance. As a result, the compression efficiency declines, and in severe cases, the rotors may even stick together. Oil filter: The oil filter element should be replaced after 500 hours of operation of the new machine. Use a special wrench to unscrew the filter element in a counter-clockwise direction, install a new element, and then fill it with screw-on oil. Tighten the seal of the element by turning it back into the filter holder with both hands, applying firm pressure. It is recommended to replace the filter element every 1,500–2,000 hours; it is better to replace the oil filter element at the same time as the engine oil. In harsh environments, the replacement interval should be shortened. It is strictly prohibited to use the oil filter element beyond its specified lifespan; otherwise, due to severe clogging of the filter element, the pressure difference will exceed the threshold that the bypass valve can handle, causing the bypass valve to open automatically. As a result, large amounts of dirt and particles will enter the screw compressor along with the oil, leading to serious consequences. The replacement of the diesel engine oil filter and diesel fuel filter in diesel-driven screw compressors shall be carried out in accordance with the maintenance requirements for diesel engines; the replacement procedure is similar to that for the screw compressor oil filter. Oil-gas separator: An oil-gas separator is a component that separates screw lubricant from compressed air. Under normal operating conditions, its service life is around 3000 hours; however, the quality of the lubricant and the precision of air filtration have a significant impact on its lifespan. It is evident that in harsh operating conditions, the maintenance and replacement cycle of the air filter element must be shortened, and even the installation of a pre-filter should be considered. The oil-gas separator must be replaced when it reaches its expiration date or when the pressure difference before and after it exceeds 0.12 Mpa. Otherwise, it may cause the motor to become overloaded, and the oil-gas separator to be damaged, resulting in oil leakage. Replacement method: Remove the various control pipe connectors installed on the fuel tank cover. Remove the oil return pipe that extends into the oil tank from the lid of the oil tank, and unscrew the fastening bolts on the lid of the oil tank. Remove the top cover of the oil/gas cylinder and take out the oil filter. Remove the asbestos gasket and dirt stuck to the upper cover. Install a new oil-gas separator, making sure to staple the asbestos gaskets at the top and bottom; when pressing them in place, the gaskets must be arranged neatly, otherwise it may cause damage to the gaskets. Reinstall the upper cover, oil return pipe, and various control pipes as they were, and check for any leaks. Oil replacement: The quality of the oil used in screw compressors has a decisive impact on their performance. Good-quality oil features excellent antioxidant stability, rapid separation of impurities, good defoaming properties, high viscosity, and strong anti-corrosion capabilities; therefore, users must use genuine screw compressor oil supplied by our company. For new machines, the first oil change should be performed after 500 hours of operation; thereafter, new oil should be changed every 2,000 operating hours. It is better to replace the oil filter at the same time as changing the oil. Use in harsh environments shortens the replacement cycle. Replacement method: Start the air compressor and run it for 5 minutes to raise the oil temperature above 50°C, thereby reducing the viscosity of the oil. Stop operation; when the pressure in the oil and gas tank is 0.1 Mpa, open the drain valve at the bottom of the tank and connect it to the oil storage tank. The oil drain valve should be opened slowly to prevent lubricant, which is under pressure and at high temperature, from splashing and causing injury or contamination. Close the drain valve once the lubricant becomes droplet-shaped. Unscrew the oil filter element, drain all the lubricating oil from the various pipelines, and replace it with a new one. Open the filler cap, pour in new oil to keep the oil level within the range indicated by the dipstick, tighten the filler cap, and check for any leaks. During use, lubricating oil must be checked regularly; when the oil level is found to be too low, fresh oil should be added promptly. Condensate water also needs to be drained frequently while using the lubricating oil – generally once a week, and in hot climates, every 2-3 days. If the machine has been shut down for more than 4 hours and there is no pressure in the oil and gas tank, open the drain valve to release the condensed water; once organic oil begins to flow out, close the valve immediately. It is strictly prohibited to mix lubricants of different brands. Never use lubricants beyond their expiration date, as this will lead to a decline in their quality, poor lubricating properties, and a lower flash point; this can easily result in shutdowns due to high temperatures as well as spontaneous combustion of the oil. Fault detection: 1. Fault symptoms of screw air compressors: High fuel consumption by the unit or high oil content in the compressed air. This fault occurs due to an excessive amount of coolant; the correct level should be checked when the unit is under load, and it should not exceed half of the tank’s capacity. A clogged return oil pipe can also cause problems with screw air compressors; improper installation of the return oil pipe can lead to high fuel consumption. If the exhaust pressure is too low while the unit is running, it can cause faults. A damaged oil separation element can also result in problems with screw air compressors. The partition inside the separation cylinder may be damaged as well. The coolant may have deteriorated or been used for too long. 2. Fault symptoms of screw compressors: Low unit pressure. The cause of this fault is that the actual air consumption exceeds the air output capacity of the unit; issues with the unloader or intake valve of the screw compressor (the valve fails to close during loading); abnormalities in the drive system, excessively high ambient temperature, or a clogged air filter; malfunction of the load solenoid valve (1SV); sticking of the pressure valve; leaks in the user’s piping network; faults in components such as pressure sensors, pressure gauges, and pressure switches can also lead to low unit pressure; air leakage from the input hoses of the pressure sensor or pressure gauge.
3. Fault symptoms of screw compressors: Overload of the fan motor; deformation of the fan; malfunction of the fan motor; failure of the thermal relay in the fan motor (due to aging); loose wiring; clogged cooler; high exhaust resistance. 4. Fault symptoms of screw air compressors: High current in the unit and low voltage; loose wiring; pressure in the unit exceeding the rated value; clogged oil separation element; contactor failures; main unit failures; main motor failures. 5. Fault symptoms of screw air compressors: High exhaust temperature (above 100 °C); too low level of coolant in the unit; dirty oil cooler. How to determine whether the cooler is dirty? It can be determined by checking the temperature difference between the inlet and outlet of the oil – a normal temperature difference is between 20-30 degrees. If external dust has blocked the cooling fins, it can be cleaned simply by blowing compressed air on them; if this doesn’t work and the inside of the radiator is dirty, a specialized cleaning agent must be used. In cases where the blockage inside the radiator is severe, a water pump may be used for circulation cleaning, with the cleaning time depending on the situation. For water-cooled radiators that are blocked, it’s better to remove the front and rear covers and use iron bars to clean the inside of the copper tubes. A clogged oil filter element can also cause faults in screw air compressors. The oil cut-off solenoid valve may not be receiving power, its coil may be damaged, or its diaphragm may be ruptured or aged. Oil cut-off solenoid valves are commonly found in older models. There can also be failures in the cooling fan motor. The exhaust ducts may not be unobstructed, or there may be high exhaust resistance (back pressure). The ambient temperature may exceed the specified range (38°C or 46°C). 6. Fault symptoms of screw air compressors: Excessively high exhaust pressure of the unit; faults in the intake valve; faults in the hydraulic cylinder; faults in the load solenoid valve (1SV); excessively high pressure setting; faults in the pressure sensor (for Intellisys-controlled units); faults in the pressure gauge (for relay-controlled units); faults in the pressure switch (for relay-controlled units). 7. Fault symptoms of screw air compressors: The unit cannot start – blown fuse; faulty temperature switch; loose wiring; thermal relay of the main motor activating; thermal relay of the fan motor activating; faulty transformer; no power supply to Intellisys (Intellisys-controlled unit); fault in the Intellisys controller. 8. Fault symptoms of screw air compressors: High current or tripping when the unit starts – issue with the user’s air switch; too low input voltage; too short star-delta conversion time (it should be 10–12 seconds); faults in the hydraulic cylinder (failure to return to its normal position); faults in the intake valve (excessive opening degree or sticking); loose wiring; faults in the main motor or the main unit, which can lead to failures in the entire screw air compressor; damaged 1TR time relay (the relay is used to control the unit). Knowledge for selecting screw air compressors 1. Discharge pressure: Under normal circumstances, it is advisable to choose a larger diameter for the gas delivery pipeline in order to reduce resistance losses and thus lower energy consumption over the long term. The higher the exhaust pressure of the compressor, the more energy it consumes. Therefore, do not choose compressors with too high an exhaust pressure. 2. Displacement: The total actual gas consumption is multiplied by a coefficient of 1.1–1.2 to determine the appropriate displacement for the compressor model to be selected. 3. Air quality: Based on the requirements of the gas-using equipment in the industry, it is necessary to choose a screw air compressor and related equipment that meet those requirements. Operating Procedures
I. Pre-start preparations:
1. Check whether all parts of the air compressor are in good condition, and whether any protective devices, instruments, valves, pipelines, and joints are damaged or loose. 2. Slightly open the drain valve at the bottom of the oil and gas tank to release the condensed water and debris accumulated at the bottom of the lubricating oil; close it as soon as oil begins to flow out, in order to prevent the lubricating oil from emulsifying and deteriorating prematurely. 3. Check whether the oil level in the oil and gas tank is between the two marks on the gauge; if it is low, top it up. Make sure there is no pressure in the system before refueling (the oil level should be checked 10 minutes after the machine has stopped; it is slightly lower while the machine is running than when it is stopped). 4. When turning on a new machine for the first time, or after it has been idle for an extended period, first remove the air filter cover and add approximately 0.5 liters of lubricating oil through the air intake to prevent oil starvation and damage to the internal components during startup. 5. Confirm that there is no pressure in the system. 6. Open the exhaust valve. 7. Check the cable connected to the air compressor to ensure that the voltage meets the manufacturer’s specifications (between 360V and 410V). It is necessary to conduct this check both when the equipment is not in operation and after it has been turned on. In some factories, the static voltage is normal, but the voltage drops under load, which can lead to motor overload. II. Starting Steps 1. Close the switch handle. 2. Operate in jog mode to check whether the motor rotates in the correct direction, except in cases where a commutation protection device is installed. 3. Confirm that the manual valve is in the “unloaded” position; pressing the “start” button will initiate operation. A few seconds later, move the manual valve to the “loaded” position, and the pressure will gradually rise to the rated value, while the lubricating oil pressure will be around 0.25 Mpa lower than the exhaust pressure. 4. Observe whether the operation is smooth, the sound is normal, air convection is unobstructed, the gauge readings are normal, and there are no leaks. III. Precautions during operation 1. Regularly check whether all instruments are functioning properly. 2. Frequently listen to determine whether the operating sounds from various parts of the air compressor are normal. 3. Regularly check for any leaks. 4. If the oil level cannot be seen on the oil level gauge during operation, the machine should be stopped immediately. Check the oil level again after 10 minutes; if it is insufficient, add more oil once there is no pressure in the system. 5. Always keep the exterior of the air compressor and the surrounding area clean; it is strictly prohibited to place any objects on the compressor, such as tools, rags, clothing, gloves, etc. 6. In case of special circumstances, follow the “emergency stop” procedure. IV. Shutdown: First, move the manual valve to the “unload” position to unload the air compressor; after about ten seconds, press the “stop” button, the motor will cease operating, and the switch handle should be moved to the zero position. V. Emergency Shutdown 1. An emergency shutdown should be carried out in any of the following situations: (1) When abnormal noises or vibrations occur; (2) When the exhaust pressure exceeds the set pressure of the safety valve yet the safety valve does not open; (3) When the exhaust temperature exceeds 100 degrees Celsius and the machine does not shut down automatically; (4) In case of an emergency in the surrounding area. 2. During an emergency shutdown, there is no need to unload the machine first; simply press the “Stop” button.