Oil-Free (Oil-Less) Air Compressors

Air purity is critical for many applications where even the tiniest drop of oil, or air contaminated with oil can cause product spoilage, product recall or damage production equipment. Various industries have developed stringent quality standards to ensure safety of both manufacturing processes and end customers. Oil Free compressors are guaranteed to meet ISO 8573-1 Class Zero.
Oil-free air compressors can be the perfect solution for the compressed air applications where meeting the highest air purity standards is key. A 100% dry oil-free air supply can be achieved with the use of a number of different oil-less compressor technologies such as scroll, water injected screw , 2 stage dry screw, depending on your requirements.

Oil-Free Rotary Screw Compressors

Rotary screw compressors are positive displacement compressors. The principle of compression in oil-less rotary screw compressors is similar to that of oil-injected models, but without oil being introduced into the compression chamber. Two distinct types are available - the dry type and the water-injected type.

In the dry type oil-free rotary screw compressors, the intermeshing rotors do not touch and their relative positions are maintained through lubricated timing gears outside the compression chamber. In the water injected type, similar timing gear construction is used, but water is injected into the compression chamber to act as a seal in internal clearances, and to remove the heat of compression.


oil-free screw air end
Oil-Less Screw Air End

Lubrication

Oil-free rotary screw compressors utilize oil for lubrication of bearings and gears, which are isolated from the compression chamber. The lubricating oil, therefore, does not cause compressed air contamination.

Cooling

The cooling system in dry type oil-free rotary screw compressors normally consists of an air cooler after each stage and an oil cooler. These may be water-cooled or air-cooled radiator type. Water-injected oil-free rotary screw compressors use the injected water to remove the heat of compression. The injected water is then removed from the discharged compressed air by a conventional moisture separation device.

Oil-Free Rotary Lobe Compressor

Oil-Free Rotary Lobe Compressors

Rotary lobe air compressors are positive displacement, non-contact or clearance type compressors. Two intermeshing rotors have lobe profiles that intermesh during rotation. Air flows into the compression chamber from the two inlet ports while the discharge ports are sealed by the rotor lobes. Rotation continues until the two discharge ports are exposed to the compression chamber, when the air is discharged. The dual inlet/outlet ports eliminate any axial thrust loads.

Lubrication
With no mechanical contact within the compression chamber of a rotary lobe compressor, lubrication within this chamber is eliminated. Lubrication of bearings, timing gears and speed increasing gears is all external to the compression chamber, and shaft seals prevent any migration of oil to the compression chamber. This ensures oil-free compression and air delivery.

Cooling

As for most industrial air compressors operating in the 80 - 125 psig range, two stages of compression are required to handle the heat of compression in oil-less rotary lobe compressors. Air leaving the first stage is passed through an air intercooler, where its temperature is reduced as close as possible to atmospheric temperature and resulting condensate drained off, before it enters the second stage compression chamber where it is compressed to the desired system pressure. A typical air end consists of a cast iron stator housing that may have passage for air or water cooling.


Oil-Free Rotary Scroll Compressors

Oil-free by design, the rotary scroll compressor has become a popular compressor as a domestic air conditioning refrigerant compressor. More recently, it has been introduced to the standard air compressor market in the lower end of the horsepower range of rotary air compressors.
The operating compression principle involves two intermeshing spirals or scrolls, one scroll being stationary and the other orbiting in relation to the stationary scroll. Air entering through the suction port in the stationary scroll fills the suction chamber, then the portion of the compression chamber at an intermediate pressure is sealed by adjacent portions of the two scrolls. As orbiting continues, the space occupied by the air becomes progressively reduced and moves progressively toward the discharge port in the center of the stationary scroll.

Lubrication
There is no metal to metal contact between the scrolls, eliminating the need for lubrication in the compression chamber and ensuring oil-free air delivery from the scroll compressor.


Cooling
Current models are air cooled, and noise levels with a sound attenuating canopy are extremely low, in the range 52 - 59 dBA at 1 meter, in accordance with the CAGI/Pneurop test code.

 

 
rotary-scroll-principle
Operating Principle for a Scroll Compressor

Oil-Free Reciprocating Compressors (Piston Compressors)

The heart of a reciprocating compressor is the piston, which acts to produce pressure. Single-acting reciprocating compressors have working fluid enter in one side and push the piston, while double-acting reciprocating compressors have fluid on both ends with a return line. There are oil-free and oil-lubricated piston compressors.

Lubrication
The oil-free designs do not allow oil in the compression chamber and use pistons of self-lubricating materials or use heat resistant, non-metallic guides and piston rings that are self-lubricating. Piston rings and rod packing usually are of PTFE-based materials, carbon, or other synthetic materials, which can operate without added lubrication. 

Cooling
Single-acting air compressors have different arrangements for removing the heat of compression. Air cooled versions have external fanning for heat dissipation on the cylinder, cylinder head and, in some cases, the external heat exchanger. Air is drawn or blown across the fans and the compressor crankcase by a fan that may be the spokes of the drive pulley/flywheel.

 
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