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Rotary Screw Compressor Winterizing Tips
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Rotary Screw Compressor Winterizing Tips

Author : Dan Wise
Webmaster -

The oil flooded rotary screw is the most popular type of air compressor used by industrial plants. However, there is still a mystique about some aspects of this type of compressor.

The company featured on our website at has repaired thousands of rotary screw airends. We wanted to gather ideas from their experience to share with you.

In our research, we found that cold weather causes unique problems that can lead to expensive repairs. We were able to gather information that can help you prevent airend failures due to cold weather.

7 Cold Weather Myths

The following deals with some common misconceptions surrounding the topic of winterizing oil flooded rotary screws.

1. Our compressor is inside a heated building and not affected by cold weather.

A large number of air cooled compressors are installed so that their oil coolers and aftercoolers are vented to outside air. It may be directly adjacent to an outside wall or connected by extensive ductwork.

The ductwork is often 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 airend seizure.

2. We don't worry 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 airend within a few minutes of startup. The oil will not be diverted to the oil cooler until the oil temperature reaches the minimum operating temperature.

3. We use synthetic oil so our compressor is safe from winter temperatures.

A quality synthetic oil will flow better at low temperatures than a petroleum based oil with the same viscosity. However, cold temperatures can still 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 140 degrees F.

The water from condensation is boiled off and moved out of the compressor during warm weather. This contamination is passed out with the compressed air flow and will be removed at the aftercooler and other locations.

The water contamination will settle in the oil sump during cold weather conditions. This is a threat because a small amount of water in the oil will accelerate bearing wear and corrosion regardless of the type of oil.

Water will separate from the oil if the compressor is shut down at the end of the work day. 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.

4. We were assured that our new control system can safely handle temperatures up to 50 degrees F below zero.

It is common for the computer board in the new control systems to be capable of withstanding low temperatures. The problem is that this protection is not always there for the devices that that send vital information to the computer board.

Cold weather can cause condensation to collect in the control air lines. This will foul the pressure transducers and other solid state sensors that send signals to the control system.

There are many problems that can develop if the control system is not getting the right information. One example would be for the compressor to run fully loaded, when more air is not needed, until the lifting of the pressure relief valves alerts someone to the problem.

5. Our compressor, aftercooler and air dryer are all in a heated space. So, the cold temperatures can not affect our condensation.

The condensation in a compressed air system is removed through drains that are strategically located in the air system. It is very common for this water mixture to be taken from the drains to a discharge point outside.

These discharge lines are typically located in an inconspicuous place and can easily become clogged with ice during freezing temperatures. This will force the condensation to flow back towards the air system.

This liquid contamination in the air piping can find its way back into expensive compressor components if the compressor is shut down for the night.

6. The High Air Temp switch shuts down the compressor once or twice on cold mornings. Everything still feels cold so, the switch must be malfunctioning.

The high air temp switch is located at the airend discharge and is set to shut the compressor down when the air reaches 220 degrees F. It is typically the first safety device to react to problems in the compressor.

One function of the lubricant is to keep the airend cool during operation. Cold weather can thicken the oil which will slow the flow of the lubricant through the airend.

The heat of compression will quickly drive the air discharge temperatures to the shut down point. Hitting the reset several times or jumping out the shutdown switch can lead to a compressor failure or even a fire.

7. We only use the rotary screw compressor during the Summer.

It is important for compressors that are idle for the winter to be given fresh oil and a filter change before storage. Lubricants that have been used will form acids that can etch the bearings if left to sit for an extended time. Fresh oil eliminates this problem.

The airend and other heavy iron parts will sweat during the winter months if they are placed in unheated storage. The moist morning air condenses on the inside and outside of the castings and can create problems.

One way to protect the compressor is to manually rotate the airend a few times each month to keep a coating of oil on the rotors. In addition, high voltage motors should be thoroughly dried and inspected before power is applied after a long cold storage to minimize the possibility of moisture induced electrical shorts.

A Final Comment

A knowledgeable maintenance staff will allow you to eliminate the expense of an outside service company and take control of your compressors. Also, your people are more likely to prevent costly downtime and reduce the operating and maintenance costs.

Send an email to if you have any questions or want additional information on the subject of rotary screw compressors. If you want to speak to someone with experience preparing compressors for winter, give us a phone number and the best time to reach you.

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Revised: Thursday, 08-Oct-2015 11:54:37 AEDT
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