Oil Free Air Compressors

When we compare oil-free and oil-injected, we shall also make a clear understanding of technically oil-free. Understanding the differences of these three types of compressor solutions could be of vital importance in making your operation and manufacturing activities free of oil contamination.

Technically oil-free compressors: Technically oil-free solutions are usually achieved through oil-injected compressors with downstream filtration systems. Technically oil-free compressors are usually providing compressed air ranking as Class 1. No matter how many stages of filters are used after the compressor, there surely exists certain oil in the compressed air.

Oil-injected compressors: This type is the most common one and the easiest to understand. There exists oil as lubrication in the compressed chamber.

Oil-free compressors: Class 0 oil free compressors guarantee totally contamination free compressed air. In the compressing chamber, no matter if it is a piston oil free compressor, screw oil free compressor, centrifugal compressor or diaphragm compressor, there is no oil at all. So it could ensure totally oil free compressed air.

Whether we shall consider oil-free compressors or oil-injected compressors, it depends on the applications and requirements. The main principle is whether oil will contaminate final products, pollute the environment, affect health and safety.

The following industries and applications will require oil-free compressed air.

Food & beverage: in any applications or operations where the air will get into contact with food ingredients or beverage fluids, the air shall be totally oil free. However, if the air is used for factory or plant cleaning, the air doesn’t need to be totally oil free.

Medical & healthcare: there are numerous compressed air applications in the medical & healthcare industry. Air shall be strictly oil free no matter if it is used for dental practice, ICU care or others.

Electronics: oil will contaminate highly sensitive parts, which will cause short circuits and final products failure.

Pharmaceuticals: for any pharmaceutical manufacturing, the medicines to be produced will be eaten by humans or animals. So they shall not be polluted or contaminated at all.

Oil-injected compressors are more frequently seen in applications where oil existence will have no influence on the operation and final products, including powering jack hammers, water borehole drilling, sand blasting etc.

To answer this question, we shall firstly divide oil free air compressors by their compressing methods. There are screw oil free compressor, water lubricated oil free compressor, oil free piston compressor, scroll oil free compressor, centrifugal compressor, and diaphragm compressor. Their basic working principles are explained below:

Screw oil free compressor: After the air filter, the air will be sucked into the air end through the air inlet valve. The two rotors in the air end are driven by external gears. Thus the two rotors don’t come into contact with each other without any friction. So there is no need for lubrication oil inside the air end and the discharged air could be totally oil free.

Water lubricated oil free compressor: Instead of lubrication oil, water is used inside the air end as lubrication and cooling substance. This type of oil free compressor could possibly be the most highly efficient, because water has the highest specific heat capacity.

Oil free piston compressor: It is similar to screw oil free compressor. In the compressing chamber, there is no oil mixing with compressed air.

Scroll oil free compressor: This type of oil free compressor is the quietest one and discharges quite a small amount of compressed air, restricted by its working principle. Inside the air end, there is a turbine rotor and a turbine stator. The air is sucked into the space between the turbine rotor and the turbine stator. As the turbine rotor moves, the compression chamber gradually decreases and finally compressed air is discharged through the air outlet of the central portion.

Centrifugal compressor: After the air is sucked inside, it gets a very high speed powered by an impeller. So it has a very large kinetic energy. Then it will be thrown into the diffuser, where its high kinetic energy will turn into pressure energy. During all this compressing process, no lubrication oil is involved so its compressed air could be totally oil free.

Diaphragm compressor: This is an ideal compressor for compressing noble gases, toxic gases, or other flammable and explosive gases, because it has the lowest leakage rate. Because the flexible diaphragm membrane totally isolates the air from the oil, it could reach oil free compressing.

Compared with an oil-lubricated compressor, oil free piston compressor and oil free screw compressor may also have some disadvantages. They are listed below:

Higher purchasing cost: oil free piston compressor and oil free screw compressor normally have a much higher initial cost, because their air end and materials have to be specially treated and made to be oil free and suitable for sensitive applications.

Higher running noise: without lubricating oil to weaken noise and minimise frictions, their running will cause higher noise.

Higher maintenance cost: this normally happens to oil free piston compressors, because the piston ring will wear much faster without lubrication oil.

Higher compressed air temperature: lubrication oil will take out some heat in the compressing chamber. Without lubrication oil, the compressed air temperature from oil free piston compressor and oil free screw air compressor will be much higher and we will need a more complicated and efficient cooler to lower its pressure dew point.

Centrifugal compressor, diaphragm compressor and scroll compressor are all designed to be oil free and there is no oil lubricated type for these three. So we can just check the main parts for oil free screw compressor, water lubricated oil free compressor and oil free piston compressor.

Oil free piston compressor: this type compressor has almost the same parts as oil lubricated piston compressor.

Oil free screw compressor: it has an air end with two rotors inside. The two rotors are driven by their own gears separately. Unlike the oil injected screw air compressor, it doesn’t have an oil gas tank, oil gas separator etc. The air after the air end is cooled by the cooler and discharged directly.

Water lubricated oil free compressor: inside the air end, there is a screw and two star wheels. After the air is compressed, the air will flow inside a water gas tank, which is similar to an oil gas tank for the oil injected screw air compressor. The air is separated from water inside the water gas tank and then discharged.