The machine to produce compressed air is called compressing air machine, or air compressor machine, air compressor in short.

1. Working principle of air compressor

1.1 Classification of air compressor

The air compressor can be divided into the following categories: screw air compressor(divided into single screw air compressor and double screw air compressor) , centrifugal air compressor, piston air compressor, sliding vane compressor, scroll compressor, rotary vane compressor.

1.2 Working principle of air compressor

Double screw single grade compressor: A pair of parallel female and male rotors (that is screw) rotates in the cylinder and that makes the air volume in the rotor tooth space change periodically . The air flows from the intake side to the exhaust side along rotor axis to realize the whole process of air inlet, compression, and air outlet. The air inlet and air outlet ports are located at the two ends of the housing respectively. The rotation of the rotors are driven by a electric motor.

Piston air compressor: The electric motor drives compressor directly and this makes crankshaft rotate, driving the connecting rods to make pistons move up and down in the cylinders. The air volume and air pressure in the cylinder will change at the same time. In the intake process, the free air will enter into cylinders through air filters and intake valves. Then in the compress process, because of the volume decreases, the air pressure increases and the compressed air enters into the air tank through exhaust valves, pipe, and one-way valve. When the pressure in the tank gets to the rated pressure, the pressure switch will cut off the power to stop the compressor automatically. When the pressure drops to the specified pressure, the pressure switch will connect the power to start the compressor automatically. Please pay attention to related terms about others.

2. Lubricating oil of compressor

The requirements of each type compressor for lubricating oil are different.

2.1 Rotary vane compressor

The function of lubricating oil for the rotary vane compressor is to lubricate the vanes which slide in and out during compressing process. The lubricating oil can also be used as sealant between the vane and , to make gas compression possible. The products ISO 60-150 can meet the viscosity requirements of rotary vane compressor.

2.2 Reciprocating compressor

Reciprocating compressor provides a wide exhaust pressure range: from 1 bar to 1000 bar. The lubricating oil lubricates cylinders, crankcase components, pistons, air valves and connecting-rob. The crankcase components include: cross-head bearing, cross joints, cross-head guiding mechanism and crank pins. Recent applications on refrigeration shows that ISO15 lubricating oil with operating viscosity less than 10cst can provide suitable lubrication. However, relying on gas molecular weight and fluid pressure, the classical uses of reciprocating compressors are ISO 68-680 products.

In most reciprocating compressors, a kind of fluid used as lubricant is used in all parts. The splash lubrication method is used in smaller compressors. An oil pump system is used to lubricate upper crankcase parts in larger compressors. Two different kinds of lubricating oils are used in large compressors, one is used in cylinders and the other is used in others needed lubricated.

2.3 Screw compressor

The pressure range of screw compressors is from 1 to 25 bar. They have many advantages, including high efficiency, low discharged temperature, high reliability and less maintenance owing to the simple mechanical construction. The lubricating oil of screw compressors must have several functions: lubricate bearings, provide seal between the screw and housing, remove the heat produced in the compressing process, wash away any particles and protect system to avoid corrosion. The lubricating oil viscosity of the upper part depends on the ability to provide sufficient lubricating oil for bearings. The classical upper part viscosity limitation is 30-100cst. The lubricating oil ISO 68-220 usually can meet the viscosity requirements of the screw compressor, the accurate viscosity degree relies on operation condition and the ingredient of airflow.

Because of the closed-loop design, synthetic products are very suitable for screw compressors. The lubricating oil and compressed gas enter into the separator. The oil separated from the separator flows back to compressor through an oil cooler. If the oil is degradation in this process, it will lead to some problems, such as: the bearings will breakdown, the seals will not be tight and there will be corrosion, etc.. In many applications, the usage of synthetic lubricating oil makes compressor can compress hydrocarbon and produce gas.