The mention of oil-free air is a vexatious question. Just how free from oil does compressed air need to be to be considered "Oil Free"? The international standard for measuring the quality of compressed air is ISO 8753. The oil content is measured by weight and 5 classes of air are defined according to the oil content. The highest standard (Class 1) is defined as having an oil content of not more than 0.01 mg/Nm3 whilst the lowest standard (Class 5) has an oil content of 25 mg/Nm3 It should be noted however that using an oil-free compressor will not necessanly produce air of this quality, especially on board an FPSO vessel or plafform.
The atmosphere on board such vessels contains a significant amount of hydrocarbons in vapour form. These are sucked into the compressor and their concentration (by weight) is increased when the air is compressed. As such it is necessary to use filters to remove the oil. Tanabe offer complete compressed air systems (including the filters, air dryers, receiver tanks etc).
7. Screw Compressors - Oil Free or Oil Injected.
Tanabe produce lubricated and oil-free compressors of both the piston and rotary screw types. For this application however we recommend the use of oil-injected compressors. The basic reason is that, as you must use virtually the same filtration system with either type then why go to the additional expense and trouble of installing oil-free screw compressors? Both systems will produce air of the same Class 1 standard.
Oil free screw compressors require two stages of compression and the maximum pressure is 10.5 bar. An additional intercooler is required between the two air screw ends, which are linked by a complex gear chain.
The initial buying cost is higher and maintenance costs are significantly higher.
Tanabe oil-injected screw compressors provide outputs up to 14 bar from a single compression stage. They have an extremely efficient oil separation system which provides air with only about 10% of the oil content of a standard industrial screw compressor. Both the prime cost and maintenance costs are considerably lower than oil-free screw compressors.
