What is the difference between piston air compressor and screw air compressor?

The difference between piston air compressor and screw air compressor

structure and working principle

1. Piston-type oil-free lubrication air compressor Piston-type oil-free lubrication air compressor is composed of compressor host, cooling system, adjustment system, lubrication system, safety valve, motor and control equipment. The compressor and motor are fastened on the base with bolts, and the base is fixed on the foundation with anchor bolts. When working, the motor directly drives the crankshaft through the coupling, drives the connecting rod, crosshead and piston rod, so that the piston reciprocates in the cylinder of the compressor to complete the processes of suction, compression, and discharge. The machine is a double-acting compressor, that is, the piston up and down movement of air suction, compression and discharge.

2. Screw air compressor

screw air compressor is composed of screw head, motor, oil and gas separation barrel, cooling system, air conditioning system, lubrication system, safety valve and control system. The whole machine is installed in a box, self-contained, and can be directly placed on a flat cement floor without being fixed on the foundation with anchor bolts. The screw head is a double-shaft positive displacement rotary type 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, while the auxiliary (female) rotor has 6 teeth. The main rotor diameter is large, and the auxiliary rotor diameter is small. The teeth form a spiral and the two mesh with each other. The two ends of the main and auxiliary rotors are respectively supported and positioned by bearings. When working, the motor directly takes the main rotor through the coupling (or belt). Because the two rotors mesh with each other, the main rotor directly drives the auxiliary rotor to rotate together. The coolant is directly sprayed into the meshing part of the rotor from the nozzle at the lower part of the compressor casing, and mixed with air to take away the heat generated by compression to achieve the cooling effect. At the same time, a liquid film is formed to prevent direct contact between metal and metal between rotors and to close the gap between rotors and housings. The injected coolant can also reduce the noise generated by high-speed compression.

The main components of screw air compressor are screw head and oil and gas separation barrel. The screw head is sucked through the suction filter and the intake control valve, and the oil is injected into the air compression chamber at the same time to cool and seal the head and lubricate the screw and bearings. The compression chamber produces compressed air. The oil-gas mixture generated after compression is discharged into the oil-gas separation barrel, and most of the oil is separated from the oil-gas mixture due to mechanical centrifugal force and gravity. The air passes through the oil and gas separation cylinder core made of borosilicate glass fiber, and almost all the oil mist is separated. The oil separated from the oil and gas separation cylinder core is returned to the screw head through the oil return pipe. An oil filter is installed on the oil return pipe. After the oil return is filtered by the oil filter, the clean oil flows back to the screw head. After the oil is separated, the compressed air leaves the oil cylinder through the minimum pressure control valve and enters the aftercooler. The after-cooler cools the compressed air and then transfers it to the air storage tank for use by each air consuming unit. The condensed water is concentrated in the air storage tank and discharged through the automatic drain or manually.

Features

1. Piston type oil-free air compressor

the piston ring in the cylinder of the oil-free air compressor and the packing in the packing device are filled with PTFE with self-lubricating characteristics as the sealing element. Therefore, the cylinder and the packing device do not need to inject lubricating oil lubrication, under normal circumstances after the compressed gas is basically pure and free of oil, no need to increase the oil removal device. The shortcomings of the 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 large, the maintenance workload is large, and the maintenance cost is high.

2. Screw air compressor

the precise coordination between the female and male rotors of the screw air compressor and the rotor and the body shell reduces the gas return leakage and improves the efficiency; only the mutual meshing of the rotors and no reciprocating motion of the cylinder reduces the source of vibration and noise. The unique lubrication method has the following advantages. Relying on the pressure difference generated by itself, the coolant is continuously injected into the compression chamber and the bearing, which simplifies 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 drives without the use of high-precision synchronous gears; the injected coolant can increase the airtightness and reduce the noise generated by high-frequency compression, it can also absorb a large amount of compression heat. Therefore, even if the single-stage compression ratio is as high as 16, the exhaust temperature will not be 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 low vibration, no need to fix it on the foundation with anchor bolts, low motor power, low noise, high efficiency, stable exhaust pressure, and no vulnerable parts. The disadvantage of the machine is that the compressed air contains oil, and its oil content is 1~3 × 10-6. The oil removal device needs to be added to the process with strict requirements on the oil content of the compressed gas. The plant's compressed air system has added a two-stage oil removal device. Since the compressed air in the ADC process is directly in contact with the product ADC foaming agent, the quality requirements for the air are higher, and a three-stage oil removal device is added to the gas used in the ADC process. The comparison of compressor performance parameters is shown in Table 1.

Main fault

1. Piston type oil-free air compressor

the piston ring and packing device of the machine do not need oil lubrication. Under normal circumstances, the compressed gas is basically pure and free of oil stains. However, due to the incomplete oil scraping and poor sealing of the oil scraping ring, oil often runs to the packing device or even the piston ring, resulting in oil in the compressed gas. In addition, the exhaust temperature is high, sometimes as high as 200 ℃; the cooler is blocked, so that the cooling effect is not good; the piston ring is stained with oil and is particularly easy to wear; the valve is leaking; the cylinder liner is worn.

2. Screw air compressor

screw air compressor failure is very few, as long as the regular maintenance of oil and gas separator, air and oil filter, etc., can ensure its normal operation. The maintenance outside the maintenance of the 2 10m3 screw machines used is the blockage of the sewage pipe and the failure of the control panel. For 2 years, the main engine system has been running normally.