Air Supply UK: Specialist Compressed Air Engineers

Published: 24/09/2021

How do oil free air compressors work?

Air compressors are crucial for the operation of factories, workshops and other industrial applications. They’re used to power equipment and provide a more compact, lightweight source of power than centralised power sources. 

However, compressed air is only as good as its purity. When the process is exposed to oil, it becomes more difficult to keep the air clean and can increase costs. As a result, oil free air compressors are becoming more common. This is especially true for those applications where oil contaminated air can lead to product spoilage, product recall or damage to equipment. 

This guide will look at what oil free air compressors are and how they operate: 

  • What are oil free air compressors? 
  • How do oil free air compressors work? 
  • How long do oil free air compressors last?

What are oil free air compressors?

For most applications, lubricated air compressors are acceptable for use. The oil within them reduces friction between the moving parts which can improve efficiency, reduce maintenance costs and provide safety for explosive environments where friction can raise heat levels to a dangerous extent. 

However, these air compressors introduce some oil contamination into the air they output which can contaminate the air they provide. Oil free air compressors serve the same function but can provide additional savings without the risk of contamination. 

Unlike regular air compressors, oil free air compressors do not contain any lubrication in the compression chamber. They use materials such as water or a Teflon coating which protects the pump and allows the mechanism inside to move smoothly without the need for oil-based or synthetic lubrication. 

Oil free air compressors are the perfect solution for applications where a certain level of air quality is required by the ISO (International Organisation for Standardisation). Applications such as food and beverage production, medical uses or applications using chemicals require a higher quality of air than other applications. Whilst oil free air compressors contain oil, a chamber keeps it separate from the compressor itself to prevent contamination. 

How do oil free air compressors work? 

Air compressors function on a very simple principle: when the air is compressed, its volume increases and the pressure decreases. Oil free and oil injected air compressors work in exactly the same way but oil free air compressors do not introduce oil into the compression chamber.

With oil free air compressors, the meshing rotors never touch, which eliminates the need for oil. However, the timing gears inside are still lubricated to allow for maintained positioning but oil is applied outside the compression chamber to avoid air contamination. 

Understanding how air free air compressors work and why they are so long lasting can be demonstrated by looking at how they function step-by-step. 

Step 1: Draw in air 

Oil free air compressors draw in outside air through an unloader valve and then pass it through an inlet filter to make sure the air is clean. This filter helps to limit damage to the compressor and its internal components. 

The unloader valve opens to allow the compressor to pump air into its chamber, which places it in the loaded position. Once the valve closes, the compressor enters the unloaded condition and starts running. Once the compressor is running and actively delivering compressed air, it usually won’t be able to draw in any additional air. 

When the compressor is turned on and is drawing air through the open unloader valve, the air will enter the low-pressure compressor element. 

Step 2: The first compressor element 

Air compressors create heat and this usually involves the low pressure compressor element because it’s working without any oil. 

The average oil free compressor element operates at around 2.5 bar and compressing air can make the unit operate at a temperature of up to 180 degrees. This can be almost twice as high as the temperature of oil-lubricated compressors, due to a lack of a flowing medium that takes the heat away. 

Oil free elements start by compressing the air and then move it through the compressor to cool it down so the air can be used by the relevant applications. 

Step 3: Intercooler access 

After it has been compressed initially, the air is pushed through an intercooler where it is cooled so it can be compressed further. Depending on the nature of your compressor, the air will then be moved into a second stage of compression or the final stage. Cooling systems can either be water cooled or air cooled, depending on the type of air compressor. 

Compressing air can generate heat which limits the oxygen content of the air, reducing its density. A cooling system allows for denser and more oxygen rich air to be used. This provides more fuel and improves the power output. 

Intercoolers are important for two reasons:

  1. They cool the air to a proper temperature to minimise heat damage 
  2. They allow air to be compressed at much higher PSIs in two-stage pumps and means second stage compression faces less wear

Cooling air can lead to condensation so intercoolers are usually fitted with a filter, or moisture trap, which is designed to remove moisture and water from the air. 

Once the air has been through the cooling system, it is returned to the compressor for additional compressing. 

Step 4: Second, higher pressure compression

The air moves back into the main chamber of the air compressor (or this could be the second chamber, depending on how the compressor is designed) and is further compressed by a high pressure element. 

Again, the air becomes hot due to a lack of lubrication in the surrounding elements so it needs to be cooled again. 

Step 5: Air prep and aftercooler 

During the second stage of compression, the air can reach up to 150 degrees in temperature which means it needs to be cooled again before it can be used in other equipment. In its final stages, the air enters the aftercooler which then allows it to be properly stored. 

When the air flows into the aftercooler, it passes through a check valve which will prevent backflow and ensure the air continues to compress and fill the tank. Any backflow can damage the equipment and cause a major failure of the air compressor. 

After this, the air is stored or sent to your equipment for use. 

Step 6: Pressure switches 

The air compressor tank contains detection equipment which monitors the level of air. Once it falls below the specified level, the air compressor turns back on an works to rebuild the amount of air in the tank. These pressure switches are set by the factory and arrive at a predefined level.

Types of oil free air compressor and how they work 

Understanding the type of air compressor available and how they work can help to choose the right air compressor for your application. 

There are a number of different types of oil free air compressor available:

Oil free rotary screw air compressor 

Within an oil free rotary screw compressor, external gears synchronise the position of the counter-rotating screw elements. The rotors do not come into contact with each other or create friction so no lubrication is needed within the compression chamber. This provides oil free compressed air. 

Oil free piston compressor 

A regular piston compressor contains a crankshaft with a connecting rod and piston, cylinder and valve heat. These components work together to produce pressure. Piston compressors can be either oil free or lubricated. An oil free piston compressor does not inject oil into the compression chamber. It uses Teflon coated rings for protection. 

Oil free piston compressors prevent oil from entering the compression chamber and use self-lubricating materials or heat resistant, non-metallic guides and piston rings that are self-lubricating. 

Oil free scroll compressors

These compressors work as a single spiral-shaped rotor oscillates against a similar fixed spiral and, as these spirals move against each other, the cavity trapping air between them becomes progressively smaller. This decrease in volume forces the fixed volume of intake air to increase in pressure. 

There is no metal to metal contact between the scrolls which removes the need for lubrication in the compression chamber and allows for oil free air delivery. 

Oil free rotary lobe compressors 

These compressors are positive displacement, non-contact or clearance type compressors. They consist of two intermeshing rotors with lobe profiles that intermesh during rotation. Rotation continues until the two discharge ports are exposed to the compression chamber, where the air is discharged. 

How long do oil free air compressors last? 

Oil free air compressors do not usually last as long as an oil lubricated compressor because the pre-lubricated parts can wear away or degrade over time. 

Oil free compressors can last for a number of years if you keep up with the maintenance but it can require more time and cost than an oil lubricated compressor. 

Air Supply UK: Specialist compressed air engineers

Look no further than Air Supply UK for highly-skilled air compressor engineers throughout Yorkshire and the North East of England. Whether you require scheduled maintenance services or it’s time to invest in new air compression equipment for your UK business, we have a solution for you. 

Get in touch with our team to learn more about new and used air compressor equipment. We look forward to hearing from you.  

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