Michael Mathe
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Know the Basics and Rely on AMSOIL Technical Services for Success in the Field

In the April 2015 issue of AMSOIL Magazine I touched on the oil requirements and basic operation of reciprocating compressors. In this month's inssue I'd like to cover another very important type of compressor: the oil-injected rotary screw.

Rotary screws have been around since the early 1960s and are currently the dominant compressor in industrial facilities. Outside of the factory, rotary screws have gained in popularity with many commercial operations, such as auto maintenance and body shops. They are commonly seen at construction sites mounted on trailers and it wouldn't be unusual to find an affordable 3 horsepower unit in a well-equipped home shop.

To compress air, an oil-injected rotary screw compressor relies on two intermeshed screws mounted in a heavy steel case. This assembly is called an air end. An electric motor turns one of the screws, which in turn spins the other. When compressed air is required, an intake valve above the screw opens, allowing them to grab air and squeeze it as it moves down their length. At this point, the compressor is producing compressed air. When air isn't required, the compressor goes into an unloaded state by closing the intake, allowing the screws to keep spinning without compressing air.

The electronics and sensors governing this process may be relatively complex, but the parts that we're concerned about from a lubrication standpoint are very simple. To lubricate the compressor, oil moves from the reservoir through a filter to the bearings and onto the screws to reduce friction between them and create an airtight seal. Since the compressed air is in direct contact with the oil, it travels through an air/oil separator filter to recover the oil so it doesn't leave the compressor with the air. Last but not least, the oil acts as a coolant, carrying the heat generated by compressing air to an oil cooler, which is much like a car's radiator. This is the reason you may hear some refer to rotary screw compressor oil as coolant.

Rotary screws have a gauge that displays the temperature of the compressed air as it leaves the air end. This is referred to as the discharge temperature, and the lifespan of any rotary-screw compressor oil is heavily influenced by it. There is no "normal" discharge temperature, but 180-195 degrees fahrenheit is common. Being substantially below this range can result in high amounts of water remaining in the machine, causing sludge and plugged filters. Substantially higher temperatures can shorten oil life, and poor-quality oil in those high temperatures can produce varnish to the point the machine won't run. As a safety precaution, many machines will automatically shut off at approximately 230 degrees fahrenheit to avoid damage and prevent fires.

Drain intervals for screw-compressor oils vary widely. Mineral oils may last from 2,000 to 4,000 hours, and some very expensive synthetics are rated for 12,000 hours under ideal conditions. The most widely accepted expectation for oil life is 8,000 hours. A vast majority of screw compressors use ISO 46 non-detergent oil, such as AMSOIL PC Series ISO 46 Synthetic Compressor Oil (PCI), to replace hydrocarbon-based oils, or AMSOIL SIROCCO ISO 32/46 to replace PAG (Polyalkylene Glycol)-based oil.

Polyalkylene Glycol is used by several compressor manufacturers. There is nothing mysterious about PAG; it is simply just another type of oil. The important thing to remember is PAG oils aren't compatible with any hydrocarbon-based conventional or synthetic oil, including our PC Series ISO 46 product. Mixing them in a compressor will at the very least require a thorough flushing with several filter changes. Lastly, understand that there may be a requirement for a higher or lower viscosity on occasion.

Our knowledgeable Technical Services staff helps our customers with compressor oil recommendations every day. If you're ever in doubt on which oil to choose for a rotary-screw compressor, just give us a call at (715) 399-TECH (8324). Published, AMSOIL Magazine 07/15.

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