Rotary Screw Compressor Winterizing Tips
Author : Dan Wise
Webmaster - CompressorWise.com
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
The company featured on our website at
http://www.CompressorWise.com/act 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
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
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
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
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
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
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
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
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 firstname.lastname@example.org 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