Brief Introduction of Compressor Working Principle
Screw type single stage compressor is a pair of parallel teeth Yinyang rotors (or screw) rotating in the cylinder, so that the air between the slots of the rotor continuously produces periodic volume changes. Air is transported from the suction side to the output side along the axis of the rotor to realize the whole process of suction, compression and exhaust of the screw type compressor. The air inlet and outlet of the air compressor are located at both ends of the shell. The slot of the cathode rotor and the teeth of the cathode rotor are rotated by the main motor.

The motor directly drives the compressor, which makes the crankshaft rotate, drives the connecting rod and makes the piston reciprocate, which causes the volume change of the cylinder. Because of the change of the pressure in the cylinder, the air is brought into the cylinder through the air filter (muffler) through the intake valve. During the compression stroke, due to the reduction of the volume of the cylinder, the compressed air passes through the exhaust valve and enters the gas storage tank through the exhaust pipe and the one-way valve (check valve). When the exhaust pressure reaches the rated pressure of 0.7MPa, the compressed air is automatically shut down by the pressure switch. When the pressure of the gas storage tank is reduced to 0.5-0.6 MPa, the pressure switch is automatically connected and started.

2. Compressor lubricants
2.1 Rotary Vane Compressor
Each type of compressor has different requirements for lubricating oil. The lubricating oil function of rotary vane compressor is to lubricate the blades sliding in and out during compression. Lubricating oil is also used as a sealant between blade and rack, making gas compression possible. Usually ISO68-150 products meet the viscosity requirements of rotary vane compressor.
2.2 Reciprocating Compressor
Reciprocating compressors provide a large range of outflow pressure capacity from 1 bar g to 1000 bar G. Oil-lubricated cylinders, crankcase components, coils, pistons, valves and filling rods of reciprocating compressors. Crankcase components include cross head bearings, cross joints, cross head bearings and crank pins. Recent refrigeration applications have shown that ISO15 lubricants with operating viscosity less than 10 cSt can provide suitable lubrication. However, depending on the molecular weight of the gas and pressure operation, the classical use of processing and hydrocarbon gas reciprocating compressors is ISO68-680 products.

In most reciprocating compressors, a fluid is used as a lubricant for all components. Smaller reciprocating compressors use splash lubricants. Larger devices usually use an oil pump system to lubricate the crankcase components above. Some large equipment uses two different lubricants, one for cylinders and the other for other parts that need lubrication. Since cylinder lubricants must coexist with gases, they must be compatible with downflow processes. Cylinder lubricants can be designed as special gases or operating conditions to provide lubrication.

2.3 Spiral Compressor
Compressed hydrocarbons and gases are commonly used in screw compressors with a flow pressure range of 1-25 bar g (5). They have many advantages, including improved compression efficiency, low outflow temperature, high reliability and less maintenance due to simple mechanical construction. Spiral gas compressor must have several functions. They lubricate bearings, provide adequate sealing between the screw and the rack, remove heat during compression, flush any particles in the compressor and protect the system from corrosion. The lower viscous limit is 10-20cSt oil supply temperature to the bearing and 5cST under the condition of outflow to ensure proper sealing. The viscosity of the upper lubricant depends on the ability to provide sufficient lubricant for the bearing. The typical upper viscosity limit is 30-100 cSt. Usually ISO68-220 lubricant satisfies the viscosity requirement of screw compressor. Accurate viscosity levels depend on operating conditions and airflow composition.
Because of the closed-loop design of the system, the synthetic products are especially suitable for screw compressors (Fig. 1). Lubricating oil and compressed gas enter the separator. The separated oil passes through an oil cooler and then flows back into the compressor. In this process, the degradation of lubricating oil can lead to compressor problems such as bearing failure, insufficient sealing or corrosion. In many applications, the use of synthetic compressor lubricants can result in effective hydrocarbon compression and gas production.