There is water on your car’s windshield in the morning and it did not rain the night before. Where did the water come from? Psychrometry or hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. Although the principles of psychrometry apply to any physical system consisting of gas-vapor mixtures, the most common example is the mixture of water vapor and air, more commonly known as dew point.

Dew point is the temperature at which water vapor begins to condense out of the air. Alternatively, it can be defined as the temperature at which air becomes completely saturated. In dehumidification by cooling and condensation, it is the temperature to which the moist air must be cooled to allow water removal. The lower the absolute amount of moisture in air, the lower the dew point of that air sample will be.

Dew points can be defined and specified for ambient air or for compressed air. The higher the pressure of the air, the higher the “pressure dew point” will be. Pressure dew point can be a proxy for allowable moisture content. Pressure dew points are specified for air drying and handling equipment to avoid condensation in compressed air distribution lines exposed to low temperatures.

So you ask, “What does this have to do with my compressor maintenance?” First, understand that your compressor’s health is dependent on the rotor bearings’ capability to maintain zero axial motion. When this capability is diminished, the rotors will begin to contact the compressor housing and failure is imminent. Second, consider the components that make up your unloading circuit that provide compressed air, or not, as the operator requires.

Several key points in your compressor circuit require regular maintenance:

  1. Start by draining the moisture that has condensed in your air/oil tanks at the end of every shift. Allow at least 15 minutes after shutting down for the water to separate from the oil. Then open the drain valve, observing the flow that typically starts as oil, and transitions to clear water then back to oil. Close the valve.
  2. Use suitable compressor oil that meets the ASTM D665 rust inhibitor requirement. That should prevent water in your oil from oxidizing on the axial control bearings.
  3. Drain the coalescing filter in the unloading controls circuit as environmental conditions require. This could be as often as once an hour when there is high relative humidity. Failure to drain this filter will cause water carry over that will lead to component failure in the pneumatic unloading controls, pilot valves, running blow down valve, and other critical areas.
  4. Change the coalescing filter element (Schramm part number 5000-7205) at 1,000 hour intervals or once a year.

Following these simple steps can lead to a long and healthy compressor life and proper operation of the pneumatic unloading circuit components.

Recognizing that we are headed into the winter months, we need to remember the physical properties of water once the temperature goes below 32 degrees Fahrenheit. Anyone who has started to operate a compressor on a day where the overnight temperatures were below freezing is familiar with the sound of a safety valve blowing off when they expect the compressor to unload. Schramm has introduced the air-CONTROL unload system as an alternative to eliminate some of the maintenance issues associated with pneumatic unloading control circuits. Speak with your local authorized Schramm dealer to learn more about what air-CONTROL can do to improve your operations and cut your operating costs.

For additional service questions or to speak with a Service Technician today, call 610-696-2500 or email

Steve Hanley is a veteran service technician with Schramm. For more Tech Topics columns, visit