ROTARY SCREW COMPRESSORS ARE AVAILABLE AS OIL-INJECTED AND OIL-FREE MODELS.
You can use the former when the air quality is of secondary importance. That is because they introduce oil, which is a contaminant in compressed air, into the compression process. This oil can be filtered out, but not to guaranteed zero. However, that does not make a difference if the air is used for applications like powering pneumatic tools, for gas refrigeration or hydrocarbon processing.
But there are also many cases in which air quality matters a great deal and you do not want to add any new contaminants to the process. This applies to applications in the pharmaceutical industry, in medical research, the food & beverage sector or in semiconductor manufacturing. Here, an oil-free rotary screw compressors is usually the right choice.
Let’s take a closer look at what distinguishes the two technologies:
In oil-injected screw compressors (although their technical name is “oil-lubricated compressors”), the male rotor drives the female rotor. Oil is injected into the compression chamber to cool and lubricate the compressor element. This oil also helps to remove the heat from the compression process and minimizes any leakage in the compression chamber. Oil-injected screw compressor elements can be manufactured for high pressure ratios, with one compression stage usually being sufficient for a pressure up to 14 and even 17 bar, albeit at the expense of reduced energy efficiency and bearing life. In an oil-free compressor, on the other hand, timing gears drive both rotors for harmonic running. In this technology, the clearance between both elements is calculated to be minimal.
Modern high-speed, oil-free screw compressors have asymmetric screw profiles. Due to the reduced internal leakage, this results in a significantly improved energy efficiency.
External gears are most often used to synchronize the position of the counter-rotating rotors. Because the rotors don’t come into contact with each other, nor with the compressor housing, no lubrication is required inside the compression chamber. Therefore, the compressed air is completely oil-free.
In this technology, the built-in pressure ratio is restricted by the limiting temperature difference between the inlet and the discharge. This is why oil-free screw compressors frequently work in several stages and with interstage cooling to reach higher pressures.
It is important to note, however, that these compressors will still use lubrication for other processes, such as motor cooling or removing heat from the rotors. In some cases, this might be oil. However, Atlas Copco also offers models that use water as an alternative lubricant. Other variants rely on a special coating applied to the rotor elements, which eliminates the need for lubricants.
Rotary screw compressors are available as fixed-speed models or as those with a variable speed drive (VSD).
First, the basic difference can be found right in the name: Fixed-speed compressors run at a continuous, fixed motor speed while a variable speed drive will adapt the motor speed to the amount of air that is required at any given time.
Fixed-speed models are an extremely efficient solution whenever a compressor has to continuously operate at 100% capacity. However, it should be noted that the vast majority of compressed air applications do not require this type of consistently high output.
In that case, fixed-speed compressors quickly become less efficient. The reason is that they waste energy (and therefore money) when the motor comes to a stop but first continues to run without any air being produced.
This type of inefficiency and waste is especially apparent in facilities that operate multiple shifts or have flow demand fluctuations.
Variable speed drive compressors, on the other hand, increase or decrease their motor speed based on the real-time air demand.
On slower production days or when the compressor is used for applications with a varying air demand, this can greatly reduce the amount of energy it consumes. In turn, this can considerably lower operating costs.
Considering that the cost of energy accounts for up to 80% of the total cost of ownership of a compressor, and that VSD technology can reduce energy consumption up to 60%, these savings very quickly add up.
