Because the oil-flooded rotary screw is a popular type of air compressor used by manufacturing plants, their upkeep is essential -- especially during the cold winter months. Dan Wise, webmaster at, Dover, Del., shares some common cold weather myths.

Myth No. 1. If a compressor is inside a heated building, it is not affected by cold weather. The truth is that many air-cooled compressors are installed so that their oil coolers and aftercoolers are vented to outside air. The ductwork often is insulated, and it may not feel cold to the touch. However, compressor oil trapped in a cooler that is exposed to cold outside air can become thick as tar. Remember, the oil will be returning to lubricate bearings and seals. Overly thick oil will not circulate fast enough to prevent air-end seizure.

Myth No. 2. There is no need to worry about winterizing because the compressor has a thermostatic valve. Some rotary screw compressors are furnished with a thermal or bypass valve in the coolant (oil) piping system. This valve is controlled by a thermostat and will bypass the oil cooler when necessary to achieve and maintain a minimum oil temperature. This process helps protect the compressor during cold weather startups. It is critical to make sure the thermal valve is operating properly during startup as well as when the compressor reaches its normal operating temperature. A simple test is to remember that warm oil should be returning through the thermal valve and oil filter to the air end within a few minutes of startup. Myth No. 3. My compressor is safe from winter temperatures because I use synthetic oil. A quality synthetic oil will flow better at low temperatures than a petroleum-based oil with the same viscosity. However, cold temperatures still can have a negative impact on a compressor with synthetic oil. The real threat to the oil is the accumulation of water in the oil sump. The source of the liquid is condensation, which happens when a rotary screw compressor operates under 140oF (60oC). The water contamination will settle in the oil sump during cold weather conditions. A small amount of water in the oil will accelerate bearing wear and corrosion regardless of the type of oil. This is why compressor manufacturers recommend draining a small amount of lubricant on a regular basis during cold weather to check for water before starting the compressor.

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