Oil vs Oil-Free Air Compressors | What’s the Difference?

Choosing the Right Air Compressor

With so many options available, choosing between an oil vs oil-free air compressor is one of many important decisions you’ll need to make. Portability is a key factor. Do you intend using your air compressor in a shop or on a jobsite? Output (Psi and CFM) is another important consideration and this can be affected by the lubrication method. In other words, whether the air compressor is oil lubricated or not. We’re going to be discussing the oil compressors, with an emphasis on oil vs oil-free lubrication.

I’ll be providing detailed information on air compressors. How they work and how they are lubricated will form a basis for the discussion. Before I go into all the technical details, I’ll give you a quick reference to the pros and cons of using either type of air compressor (oil or oil-free).

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Oil vs Oil-Free Air Compressors

Here are two of the most popular air compressors to illustrate the differences.

The points listed above are only intended as a guide. To fully understand oiled and oil-free air compressors, I’ll be providing the technical details for a more comprehensive understanding of these machines.

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How an Air Compressor Works?

All air compressors use a pump to force air into a tank. As more air is forced into the tank it becomes compressed, thereby increasing the pressure. The pressure is managed by a switch that controls the power supply to the pump. When the pressure drops, the pump switched on and runs until the maximum pressure is obtained, when the pump will switch off.

VIDEO | A Closer Look at How They Work

Air Compressor Components And Operation

Three types pumps are used to compress the air. Reciprocating air compressors are the most common. These use one or two pistons to force the air from the atmosphere into a tank in ever-increasing increments. Each stroke of the piston moves a small volume of air.

The rotary screw air compressor uses a helical coil or spiral lobe oil flooded screw to continuously force air into the tank. This method is generally used for high-volume, heavy duty, gas-powered air compressors. The type of machine you’d see used by road building crews. These air compressors are always oil lubricated and, therefore, don’t feature in this debate.

The final, least common, type of air compressor uses a centrifugal pump. Here a rotating impeller is used to force the air into the tank. These can be either oil or oil-free.

Since you’re most likely deciding between an oil lubricated or oil-free reciprocating air compressor, this is what I’ll be discussing.

The pistons used in a reciprocating air compressor are no different to those used in an internal combustion engine. Like the engine in your car, the pistons need lubrication. The major difference being; temperatures are lower in an air compressor because there is no combustion of gas. While air compressors run cooler than a gas engine, there is still heat generated by the friction between the piston and the sleeve in which it moves. To reduce this friction, a method of lubrication is needed.

Traditionally oil, like that used in your car’s engine, was the accepted method of lubrication for all air compressors.  Air compressors that use oil, are splash lubricated. This is a low pressure method of lubrication that uses the rotation of the pump to operate scoops that lift the oil and splash it into the piston chamber.

Since an air compressor doesn’t generate as much heat as an engine, engineers realized that a more efficient method lubrication can be used. By coating the cylinder sleeve with a friction reducing chemical (like Teflon), the need for oil lubrication is no longer necessary. This saw the introduction of oi-free air compressors.

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Which is better? Oil or Oil-Free Air Compressors.

There was a time when I would have said oil lubricated air compressors are the best. But technology changes, and opinions have to change with this. Early oil-free air compressors weren’t the best for heat dissipation and tended to produce hotter air which increases condensation of water and hot air is less dense. They were also much noisier and  tended to be less durable.

To overcome some these disadvantages, most oil-free air compressors utilize two pistons, reducing heat and noise. Using an aluminum housing with improved airflow technology, has further improved the efficiency and operating temperature of oil-free air compressors.

However, oil-free air compressors have their limitations. Despite design improvements, cooling for an oil-free air compressor is never as efficient as an oil-lubricated counterpart. Furthermore, without oil to lubricate it, the size of the piston that can be used is limited. Even when using two pistons, oil-free air compressors remain smaller, lower-volume machines.

The real advantage to using an oil-free air compressor is lower maintenance and improved portability. This makes an oil-free air compressor a good option for contractors who regularly transport their equipment form one jobsite to another. For a home shop, that doesn’t require exceptionally large volumes of compressed air, an oil-free air compressor can be a good choice.

Operating environment can also be a deciding factor. Oil-free air compressors rely heavily on air cooling. This means that in a hot climate, oil lubrication would be more beneficial. The opposite can be said for extremely cold climates. If the ambient temperature is consistently low, oil-free air compressors operate optimally. At really low temperature, oil increases in density and can form a sludge which will hinder performance. Hence, oil-free air compressors are usually better in cold conditions. While oil lubrication is preferable in hot conditions.

In professional situations, like a busy auto shop or manufacturing facility, oil-lubricated air compressors remain the preferred choice. If you’re using high-volume pneumatic tools, like impact wrenches and the like, increased CFM capabilities for the air compressor becomes more critical. This is of particular importance, when several tools are being used simultaneously. Since there is virtually no limit to the size of an oil air compressor, this is a natural choice for professional users.