How Do Oil-Free Air Compressors Work?

You can find more information on our web, so please take a look.

For industries where product contamination can be a costly problem, compressed air purity is a top-of-mind concern. For other manufacturers where sensitive production equipment must remain free of contamination to avoid expensive downtime, compressed air cleanliness is also a high priority. Unfortunately, even with the best filtration systems, oils used to lubricate traditional air compressors may still find a way into the compressed air and spoil finished product or damage delicate equipment. In industries such as food and beverage, pharmaceuticals, electronics, medical devices and others where compressed air contamination is a major worry, oil-free air compressors may be the solution.

What is an Oil-Free Air Compressor?

Oil-free air compressors do not use any oil within the compression chamber. Instead, some versions of this specialized air compressor type use alternative materials such as water or Teflon to protect components while allowing them to move smoothly without the use of oil-based or synthetic lubricants. Other oil-free compressor designs have completely removed metal-to-metal contact from the compression chamber, eliminating the need for any lubrication at all. It should be noted that in some oil-free models, the bearings and gears within the gearbox will still require lubrication; however, this is external to the compression chamber and proper sealing of the gearbox ensures that oil will not contaminate the delivered compressed air.

It should also be explained that there is a difference between oil-free and oil-less air compressors. Oil-less air compressors have no oil present in the compression package. While oil-less compressors do reduce contamination risks, they work by first expelling air, filtering the air to eliminate contamination and then pressurizing and cooling the air for use.

Compressed air purity is ranked according to ISO Standards from 0 to 5, with Class 0 providing the purest, cleanest, highest quality air. Oil-free air compressors provide 100% oil-free air and usually have an ISO Class 0 designation. For this reason, oil-free air compressors are likely the best solution for compressed air applications where meeting the highest air purity standards is essential. Oil-less air compressors may be considered ISO Class 1. For this reason, it&#;s best to consult with an expert when deciding whether an oil-free or oil-less air compressor meets the compressed air purity level needed for your specific application.

Image curtsey of Gardner Denver. Featured Product: Ultima

How Do Oil-Free Air Compressors Work?

Simplistically, all air compressors function in the same manner: As air is drawn into the compression chamber, its volume is decreased and its pressure is increased as the air is forced into a pressurized storage tank. While oil-free air compressors operate via the same basic principle, different oil-free compressor designs will have slightly different operating principles. There are three common technologies:

Oil-free rotary screw compressors: This type of oil-free air compressor uses external gears to synchronize the position of the counter-rotating screw elements. Because the rotors do not come in contact with each other, there is no friction and, therefore, no lubrication is needed in the compression chamber. The gearbox that drives the rotors will likely contain oil, but is contained in a separate, sealed chamber so no oil can enter the compression chamber during normal operation. An internal air cooler or a water-cooled radiator may be used to cool the compression chamber.

Oil-free piston compressors: In this type of oil-free compressor, mechanical components, such as the piston and valves, are coated in Teflon or another material that reduces friction and allows the mechanism to move freely without the use of oil lubricants. The compression chamber is usually cooled by an external fan.

Oil-free scroll compressors: These oil-free air compressors offer two spiral-shaped metal pieces that fit one inside the other and reside inside the compression chamber. As air is brought into the compression chamber, the inner spiral oscillates inside the outer spiral. The two work together to compress the air by decreasing its volume. Air is then sent to the center of the compressor to be cooled. The spiral scrolls in this design do not make contact, so this type of oil-free compressor does not require lubrication in the compression chamber. Movement of the spirals is generated from a sealed gearbox, which does require oil to lubricate gearbox components; however, the oil within the gearbox is prevented from entering the compression chamber with proper sealing.

What are the Benefits of Oil-Free Air Compressors?

Aside from the obvious benefits of delivering the purest compressed air to applications that must avoid product spoilage, product recalls or damage to sensitive production equipment, there are other worthwhile benefits associated with oil-free air compressors.

Oil-free air compressors do not require downstream filter replacements because they are not filtering oil out of the compressed air system. Additionally, because there is no oil, there is no need to separate oil from the condensate, which reduces both equipment and disposal costs. Oil and maintenance costs are also reduced, as there is no need to continually refill the compressor with oil or change oil-catching filters. Oil-free air compressors also tend to be kinder on the environment as there is no oil leakage or disposal associated with their use. Further, energy requirements are reduced as pressure drops caused by filtration are eliminated.

What is the Lifespan of an Oil-Free Air Compressor?

Typically, oil-free air compressors do have a shorter lifespan in terms of service hours than traditional oil-lubricated units because the Teflon (or other coating) used to lubricate the internal cylinder will eventually wear off or degrade over time. Service life may be further reduced in applications where the compressor sees high temperatures or runs for longer-than-normal periods.

Oil-free compressors also tend to generate more heat than their oiled counterparts, which means if they are subjected to high pressures or stress, the internal components are more likely to burn out or warp. The compressor design will also have an impact on the service life. For example, oil-free piston air compressors typically have a shorter lifespan than oil-free rotary screw compressors.

However, keep in mind that oil-free compressors tend not to run continuously, so the reduction of service hours may work out to be the same in terms of years of service when compared to traditional oil-lubricated air compressors that run 24/7. In addition, a solid preventive maintenance program that routinely examines moving parts, the coating on the cylinders and the seals will help elongate the service life of an oil-free air compressor.

For more information on whether your application may benefit from an oil-free air compressor, please contact an expert at JHFOSTER.

For absolutely 100% oil-free compressed air, you need an oil-free compressor.

The basic principle of the oil-free screw compressor is same the same as for oil-injected compressors. But as the name suggests, there is no oil injected during compression.

The oil-free screw compressor element

No oil means that there is no oil for sealing the rotors and for cooling the compressed air, elements and rotors.

Because there is no oil for sealing, the rotors need to be very precise and have very small tolerances. The rotors don't touch each other, but the air-gap between the two is very small (for optimal performance).

The element is cooled by cooling water that flows through special pockets in the element casing. Of course this is less efficient as injecting relatively cold oil, and only the casing is cooled, not the rotors or the air itself.

For this reason, the pressure ratio of the oil-free screw element is much lower compared to the oil-injected element. Remember, the pressure ratio is the outlet pressure divided by the inlet pressure (around 13 for oil-injected compressor, about 3.5 for oil-free elements).

If we would use the oil-free element to compress air directly to 7 bar, the element will get too hot and grind to a stop (literally). So how do we get to 7 bar, the typical system pressure for compressed air systems? Easy&#; just install two elements in series.

The first element (stage 1) compresses the air to about 3.5 bar. The air is cooled down by the intercooler. The second element (stage 2) compresses the air further to the end pressure of 7 bar.

Now we see why the oil-free screw compressors are more expensive: they have two compression elements, compared to only one in oil-injected compressors. Also, they require a gear box to drive two elements from one compressor. On top of that, the compressor elements used in oil-free types are more expensive than oil-injected types, since they are manufactured with much smaller clearances compared to oil-injected compressor elements.

The two compressor elements, stage 1 and stage 2 work together to produce the required output pressure. The first stage pumps air to the intercooler. The second takes the air from the intercooler and compresses it to the final pressure. The two stages are designed so that they work in a perfect balance.

If there's a problem with one of the stages, it will usually result it less capacity (less liters per second, or m3 per minute) for that stage. This means that the balance between stage 1 and stage 2 will be disturbed.

This can easily be seen by keeping an eye on the temperatures (stage 1 and stage 2) and the intercooler pressure.

How it works

Outside air

The air is sucked in through the unloader valve and inlet air filter. The filter protects the compressor elements from damage, by keeping all dust and dirt outside of the compressor.

The unloader valve is opened and closed by the control system. When the valve is open, the compressor is in loaded condition (it is actually pumping air). When the valve is closed, the compressor is in unloaded condition; the compressor is running, but since it cannot suck in any air, it is not delivering any compressed air to the system.

When the compressor is in loaded condition and the unloader (inlet-) valve is open, the air is sucked into the first (low pressure) compressor element.

With competitive price and timely delivery, Comair sincerely hope to be your supplier and partner.

The low-pressure compressor element

In the low pressure element, the air is compressed to about 2 &#; 2.5 bar. Because of the compression, the air becomes really hot.

Normal temperatures for low pressure element outlet temperature are between 160 and 180 degrees Celsius.

The compression is done without oil, only air (as opposed to oil-injected rotary screw compressors). Because of this, the compressed air becomes very hot.

Where oil-injected screw elements have an outlet temperature of about 80 degrees Celsius, oil-free elements outlet temperature s are twice as high! And the (low-pressure) oil-free element only compresses it to about 2.5 bar, compared to 7 &#; 13 bar for oil-injected screw elements.

The intercooler

The air is cooled by the intercooler. It will cool down the air to about 25 &#; 30 degrees Celsius. There's a moisture trap installed after the intercooler to remove and water from the air.

The high-pressure compressor element

The air is further compressed by the high-pressure element to the final pressure. This pressure depends on the compressor specifications and are normally anywhere between 7 and 13 bar.

The aftercooler

Because of the compression, the air is (again) very hot. This time somewhere between 140 &#; 175 degrees Celsius. So, it is cooled again, by the aftercooler. But before it enters the aftercooler, it normally passed a pulsation damper and a check-valve. The check-valve makes sure that compressed air doesn't flow back into the compressor when it is stopped.

After the after-cooler, the air reaches its outlet temperature of about 25 degrees Celsius. There's another moisture trap installed to remove any water that may have formed inside the after cooler.

Compressor built-up

As we see, the air system is quite simple, in terms of number of components: low-pressure element, intercooler, high-pressure element, after-cooler.

But we need a lot of extra stuff to keep the compressor running, and the physics are a lot more complicated.

The low pressure and high pressure element operate in a perfect balanced situation. All the air that is compressed by the low-pressure elements needs to be sucked in by the high-pressure element. If there is no balance, the pressure in the intercooler will rise or fall.

The elements are designed for a certain pressure ration. That's the outlet pressure divided by the inlet pressure. If the pressure ratio over a compressor element becomes too big, it will eventually break down.

If one of the elements wears down or breaks down, it disturbs the balance and can take the other element down with it.

Go to our screw air compressor element page for more information about air compressor screw elements in general.

The gearbox

While oil-injected compressors, with their single element are normally direct-coupled to the electric motor, or through a (relatively cheap) pulley system, we need a gear-box to drive two compressor elements from one electric motor on the oil-free kind of air compressor.

Gear-boxes are expensive, they require lubrication, make noise, and lower to overall efficiency of a machine (any machine).

Gearbox Oil

We need oil to lubricate the gears and bearings. Yes, there is oil in an oil-free compressor. But it's completely separated from the compressed air side.

Oil is used to lubricate the gears, the bearings inside the gearbox, and the bearings and the timing gear inside the compressor elements. On bigger and air-cooled compressor, the oil is also used to cool the compressor elements.

The oil is pumped up from the oil sump inside the gearbox, through the oil cooler and oil filter, to the gears and the bearings. The oil filter removes any dirt from the oil, to protect the bearings and the gears.

Compressor cooling

On smaller and air-cooled machines, the oil flows through the cooling jackets of the compressor elements, to cool them, before it flows to the oil filter.

On air-cooled oil-free rotary screw compressors, outside air is used to cool the compressed air and the oil, and the oil is in turn used to cool the compressor elements.

On water-cooled oil-free rotary screw air compressors, water is used to cool the oil, the compressed air and the compressor elements.

When the machine is water cooled, the cooling system is often divided in two circuits: one for the oil cooler, the low-pressure element and the intercooler, and one for the high-pressure element and the after cooler.

If you want to learn more, please visit our website High-Pressure Oil Free Screw Air Compressor.