It’s September and winter is coming fast to our part of the country. Each year we see dozens of preventable rotary screw and vane airend failures blamed on the cold weather. Because we warranty our remanufactured equipment, we usually try to get some information about the more dramatic failures. Often the culprit responsible for the demise of your last airend is waiting to pounce on its replacement. Let’s look at some of these gremlins and the misconceptions that accompany them.

#1. My compressor is located inside a heated building and not affected by cold weather.

Most air cooled compressors are located 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 or air handling equipment. As this ductwork is often insulated, it may not even feel cold to the touch. However, compressor oil trapped in a cooler exposed to this cold outside air can be thick as tar. This is the oil that will be returning to lubricate bearings and seals.  Overly thick oil will not circulate fast enough to prevent airend seizure.

#2. I don’t have to worry. My compressor has a thermostatic valve.

To aid in cold weather startups and stabilize running temps, some compressor packages are fitted with a thermal mixing or bypass valve. This valve bypasses the oil cooler as needed to achieve and maintain a minimum oil temperature of 140 degrees F or more. Does yours work? It is important to prove that your thermal valve operates properly on startup as well as when up to temperature. Warm oil should be returning through the thermal valve and oil filter to the airend within a very few minutes of startup. Only when this oil temperature reaches 140 F or so, will some of the oil be diverted through the oil cooler.

#3. I run synthetic oil. My machine is immune to winter temperatures.

While most synthetic lubricants flow better at low temperatures than conventional petroleum oils of the same viscosity rating, they are not a cure all. And they are by no means all equally adept at handling plunging temperatures. More of a concern, when rotary compressors operate at less than 140 degrees F for any length of time they can accumulate water in their oil sump. In warm temperatures, this condensate water is boiled off and sent downstream in vapor form to be removed in aftercoolers and condensate separators. However at the lower temperatures it collects where it can do the most damage… in your compressor oil. Even the slightest amount of water in the compressor oil will lead to accelerated bearing wear in running machines and corrosion in those at rest, synthetic or not.  Because water settles out from the compressor oil if shut off overnight, many manufacturers recommend draining a small amount of lubricant on a regular basis during cold weather to check for water before starting the compressor.

#4. The salesman said this new computer control system is good to 50 below.

Though the PC board at the heart of you new state of the art compressor may be frost proof, the interfaces between this control and your air system are not. Condensate which collects in your control air lines can foul the pressure tranducers, thermistors and other solid state sensors that feed vital information to the controller. Frozen condensate in the air control lines can easily cause most any compressor to run fully loaded until (hopefully) lifting pressure relief valves alert someone to the problem.

#5. My compressor, aftercooler, air dryer etc. are all in a heated space. How can this condensate water be backing up in my compressor?

Unless your condensate traps dump into a condensate oil separator or an open inside drain, they are most likely piped outside. In freezing temperatures these drains, usually located in an inconspicuous place, can get clogged with ice, and condensate flow backs up. This water contaminates your air pipe work and is free to find its way back into expensive compressor components after the machine is shut down for the night. Do you know where your condensate drains dump?

#6. My compressor shuts down on overtemp once or twice each cold morning, but everything still feels cold. What’s wrong with my overtemp sensor?

The temperature sensor that usually reacts first is the hot air temperature switch located in the airend’s discharge port or pipe work. If the compressor oil is too thick to flow freely through the airend, heat of compression will very quickly drive air discharge temperatures beyond the 220 degree F set point of the switch. Hitting the reset several times or jumping out the shutdown switch can lead to a failure or even fire. 

#7. We only use this machine during the summer season.

Two things come to mind here. For those compressors idle for the winter, such as some construction or drilling equipment, it is important for the compressor to be given fresh oil and a filter change before storage. Acids which commonly form in compressor lubricants will etch your bearings if left to sit for extended periods of time. Fresh oil will not. So it is far better to change oil before winter than after a long shutdown. This applies to engines as well. Secondly, heavy iron parts, including motors and airends sweat in unheated storage during the winter months. Moist morning air condenses on the inside and outside of these castings and can create a problem. Airends stored for the winter should be manually rotated a few times at least once per month to keep a coating of oil on the rotors. High voltage motors should be thoroughly dried and meggered before power is applied after a long cold storage to minimize any possibility of moisture induced electrical shorts.