How oil-free rotary screw air compressors work
For absolutely 100% oil-free compressed air, you need an oil-free compressor. The basic principle of the oil-free screw compressor is same as for the oil-injected compressor. 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
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.
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 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.
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.
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.
While oil-injected compressors, with their single element are normally direct-coupled to the electro motor, or through a (relatively cheap) pulley system, we need a gear-box to drive two compressor elements from one electro 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).
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.
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.