Advantages of screw air compressors compared with piston air compressors
Structure and working principle
1. Piston oil-free lubrication air compressor
Piston oil-free lubricated air compressor consists of compressor main engine, cooling system, regulating system, lubrication system, safety valve, electric motor and control equipment. The compressor and motor are fastened to the machine base with bolts, and the machine base is fixed to the foundation with anchor bolts. During operation, the electric motor directly drives the crankshaft through the coupling, driving the connecting rod, crosshead and piston rod to make the piston reciprocate in the cylinder of the compressor to complete the suction, compression, discharge and other processes. This machine is a double-acting compressor, that is, air is sucked in, compressed and discharged when the piston moves up and down.
2. Screw air compressor
Screw air compressor consists of screw head, electric motor, oil and gas separation barrel, cooling system, air conditioning system, lubrication system, safety valve and Control system and other components. The whole machine is installed in a box and is self-contained. It can be placed directly on the flat cement floor without the need to fix it to the foundation with anchor bolts. The screw head is a dual-shaft positive displacement rotary compressor head. A pair of high-precision main (male) and auxiliary (female) rotors are installed horizontally and parallel inside the casing. The main (male) rotor has 5 teeth, and the auxiliary (female) rotor has 6 teeth. The main rotor has a large diameter and the auxiliary rotor has a small diameter. The teeth form a spiral shape and the two mesh with each other. Both ends of the main and auxiliary rotors are supported and positioned by bearings respectively. When working, the motor directly drives the main rotor through the coupling (or belt). Since the two rotors mesh with each other, the main rotor directly drives the auxiliary rotor to rotate together. The coolant is sprayed directly into the rotor meshing part from the nozzle at the bottom of the compressor casing, and mixes with the air to take away the heat generated by compression to achieve a cooling effect. At the same time, a liquid film is formed to prevent direct metal-to-metal contact between the rotors and close the gap between the rotors and the casing. The injected coolant also reduces the noise produced by high-speed compression.
The main components of a screw air compressor are the screw head and the oil and gas separation barrel. The screw head inhales air through the suction filter and air inlet control valve. At the same time, oil is injected into the air compression chamber to cool, seal and lubricate the screw and bearings. The compression chamber generates compressed air. The oil-gas mixture generated after compression is discharged into the oil-gas separation barrel. Due to the action of mechanical centrifugal force and gravity, most of the oil is separated from the oil-gas mixture. The air passes through the oil and gas separation core made of boron silicate glass fiber, and almost all the oil mist is separated. The oil separated from the oil and gas separation cylinder core returns to the screw head through the oil return pipe. An oil filter is installed on the oil return pipe. After the return oil is filtered by the oil filter, the clean oil flows back into the screw head. After the oil is separated, the compressed air leaves the oil and air cylinder through the minimum pressure control valve and enters the aftercooler. The aftercooler cools the compressed air and then discharges it to the air storage tank for use by various gas consuming units. The condensed water is concentrated in the air storage tank and drained through an automatic drainer or manually.
Features
1. Piston oil-free lubrication air compressor
The piston ring in the oil-free lubrication air compressor cylinder and the packing in the packing device are all Filled polytetrafluoroethylene with self-lubricating properties is used as the sealing element. Therefore, the cylinder and packing device do not need to be lubricated with lubricating oil. Under normal circumstances, the compressed gas is basically pure and contains no oil, and there is no need to add an oil removal device. The disadvantages of this machine are that the motor power is too large, the exhaust pressure is not stable enough, the exhaust temperature is high, the noise is too loud, the maintenance workload is large, and the maintenance cost is high.
2. Screw air compressor
The precise cooperation between the male and female rotors of the screw air compressor and between the rotor and the body shell reduces gas backflow leakage and improves efficiency; Only the rotors mesh with each other and there is no reciprocating motion of the cylinder, which reduces the source of vibration and noise.
The unique lubrication method has the following advantages. By virtue of the pressure difference generated by itself, coolant is continuously injected into the compression chamber and bearings, simplifying the complex mechanical structure; the injection of coolant can form a liquid film between the rotors, and the auxiliary rotor can be directly Driven by the main rotor, there is no need to rely on high-precision synchronous gears; the injected coolant can increase the airtight effect, reduce the noise caused by high-frequency compression, and absorb a large amount of compression heat. Therefore, even if the single-stage compression ratio Up to 16 can also prevent the exhaust temperature from being too high, and there will be no friction between the rotor and the casing due to different thermal expansion coefficients. Therefore, the screw air compressor has the advantages of small vibration, no need to fix it on the foundation with anchor bolts, low motor power, low noise, high efficiency, stable exhaust pressure, and no wearing parts. The disadvantage of this machine is that the compressed air contains oil, and its oil content is 1 to 3 × 10-6. Processes that require strict oil content in compressed air need to add an oil removal device. The plant's compressed air system has added a two-stage oil removal device. Since the compressed air in the ADC process is in direct contact with the product ADC foaming agent, the air quality requirements are higher. A three-stage oil removal device is added to the air used in the ADC process. The comparison of compressor performance parameters is shown in Table 1.
Main faults
1. Piston oil-free lubrication air compressor
The piston ring and packing device of this machine do not require oil lubrication. Under normal circumstances, the compressed gas is basically pure and contains no oil. However, because the oil scraper ring often does not scrape the oil completely and the seal is not good, oil often runs onto the packing device and even the piston ring, causing the compressed gas to contain oil. In addition, the exhaust temperature is high, sometimes as high as 200°C; the cooler is clogged, resulting in poor cooling effect; the piston ring is stained with oil and is particularly prone to wear; the valve flap is leaking; the cylinder liner is worn, etc.
2. Screw air compressor
Screw air compressors have very few faults. As long as the oil and gas separator, air and oil filters, etc. are regularly maintained, they can be guaranteed to be normal. run. The two 10m3 screw machines used had maintenance problems other than maintenance, including blocked sewage pipes and faulty control panels. Over the past two years, the host system has been operating normally.
Conclusion
From the perspective of use effect, the screw air compressor has the unparalleled advantages of the piston air compressor. It not only reduces the labor intensity of the operator, but also does not need to be equipped for maintenance. work, greatly reducing maintenance costs. On the other hand, when using a piston machine, the exhaust pressure will occasionally be too low, causing the ion membrane control system to alarm. After switching to a screw machine, the exhaust pressure was set at 0.58MPa. The pressure remained stable and no exhaust has occurred. If the pressure is too low, the ion membrane control system will alarm, thus ensuring the safe production of the ion membrane system.