Judging from the lubrication system the Air Compressor is divided into:

  1. Oil-Free Compressor
  2. Oil / Oil Flooded Injection Compressor

Oil Free Compressor is a compressor where the main part is not lubricated. For example in a screw compressor, the thread is not lubricated by oil. Because the threads are not lubricated, the air delivered to the system is oil free.

An oil injection compressor is a compressor where the main part is lubricated by oil. For example, in a screw compressor, the threads are lubricated by oil. So there is a possibility of oil carried by compressed air. So that in this type of compressor there is an oil separator. Its function is to separate the mixed oil and air. It is through the oil separator that the oil and air are separated. The oil will flow to the bottom to the lubrication line and the air will flow to the top which will continue to the compressor discharge. Even if there is an oil separator, there is no guarantee that the air sent to the system will be oil-free. The oil carried over to the system (oil carryover) in this compressor is usually in the range of 2-5 ppm.

In an oil-free compressor, the discharge pressure is limited. Usually a maximum of 10 barg. It’s different with the oil inject compressor which can be up to 16 barg. Of course, from this point of view, oil injection will benefit more. So why the resulting discharge pressure can be greater on the oil inject compressor? The answer lies in the ideal gas formula. I will not discuss in detail here.

When it comes to the quality of the air produced, we will definitely choose an oil free compressor. Because the air produced is clean (oil free) compared to oil injection compressors which still carry 2-5 ppm oil. However, this then became the reason for choosing an oil free compressor in a project. Again, it depends on the usage and application. If the use is still allowed to contain oil, I prefer to use injet oil. Of course this is the reason for the price (expensive). However, if its use requires oil-free air, such as in food & beverage applications, an oil-free compressor should be chosen.

In terms of life time, in my opinion, oil inject compressors have a better lifespan than oil free ones. the size of the engine components that are regularly lubricated will increase the life of these components. Especially if the component is classified as a rotating part.

So, the selection of a compressor will always look at various aspects and perspectives. Hopefully this brief explanation can provide an overview and additional knowledge for engineering friends.