In certain situations, a rotary screw compressor is a much better option than a piston compressor. This is often the case in instances where you need a very large amount of highly pressurised air. Jackhammers, nail guns, spray paint guns, and compressed air dryers all often use a rotary screw compressor, but the type used depends on a number of different criteria. The two types of compressors are oil-injected and oil-free.
An oil-injected compressor requires oil to function correctly. The oil must be injected into the actual compressor element while the device is active. This element is where the air is actually compressed. Once the oil has lubricated the device, it is pulled out of the air by a separator. While this filter removes a very large amount of the oil from the compressed air, a miniscule amount remains in the air when it leaves the device. This oil, which is about 0.1% of the total amount of oil used, is called the oil carryover.
Even though it is a very tiny amount, this oil carryover makes oil-injected compressors unusable in certain applications. If the air must be oil-free, oil-injected rotary screw compressors cannot be used. In most factory and workshop tasks, the oil carryover is not an issue.
In cases where the air must be oil-free, an oil-free rotary screw compressor is required. These compressors are very different in design from oil-injected compressors, even though both function on the same basic principle and produce the same results. The main difference is that oil-free compressors do not use any oil. This means that the mechanism must not require any lubrication to function. The compression occurs in two stages rather than in a single stage as it does in oil-injected compressors.
When an oil-free compressor is turned on, the air is compressed anywhere from two to five bars, which is the measure of air compression. During this first stage, the air becomes very hot. To cool down the air, an intercooler is placed in between the first compression chamber and the second. In this second stage, the air is compressed another two to five bars. The final air compression level is usually seven bars in total.
Often, a single motor will drive both of the compression phases. This allows for a continuous flow of compressed air because air is being compressed in both stages at once.
The biggest design difference between these two options is that the oil-free compressor’s rotors must be designed to never touch. If they do touch without any lubrication, they will quickly wear down. Instead, they’re built to rotate very closely to each other without ever making contact.
While oil-free compressors are required in some industries, oil-injected compressors are used much more often. They are found in many different factories and machining plants where oil carryover does not affect the final product.