The main advantage is the reliability and significantly reduced maintenance requirement with this type of machine. There are no valves, piston rings or cylinder heads to de-coke. Service intervals are longer and running costs are low.
The operating principle is of two helical screws (one male, one female) in which the interstices gets progressively smaller as they turn, thus increasing the pressure. Oil is simultaneously injected into the air screw-end. This acts as a lubricant, a sealant and a coolant. The output from the screw-end is thus a mixture of air and oil mixed together, which must be separated before the compressed air is delivered to the system.
Efficient separation is essential, since equipment generally requires clean compressed air. Also, any oil that is blown away with the compressed air, (rather than being recirculated), has to be replaced, adding to operating costs. Tanabe screw compressors boast one of the most efficient separation systems avaiiable, consuming only 0.0035 mg oil/m3 compressed air, further contributing to the minimal operating costs.
Separation takes place in a tank immediately below the air screw-end. This tank contains the compressed air/oil mixture at a pressure slightly higher than the final air delivery pressure. As such, this is a pressure vessel and in terms of class regulations it must be class certified. Similar regulations may also apply to the heat exchangers for the oil cooler and the air after-cooler, depending on the volume. Furthermore, the electrical control panel must also comply with class regulations and although this panel does not need certification all electrical equipment used on board, must satisfy the class requirements. Most industrial screw compressors do not comply.
Not all screw compressors use a separation tank, as smaller sizes may employ an alternative system of filters to remove the oil from the air. This eliminates the need for a separation tank but can present problems in isolating the screw end during hydrostatic testing of the piping system during the acceptance tests. Furthermore, oil seals used within this patented system may not be compatible with all types of synthetic oil. Problems have been experienced with swollen and perished seals.
