The Fundamentals of Pneumatics
Part II - Humidity and Flow

This is the second in a series of articles covering the basic principles of pneumatics.  In this article, humidity and flow will be reviewed.

Humidity

Humidity is the measurement of the amount of water vapor or moisture contained in the air.  Hot air has the capacity to hold a great deal of water vapor or humidity.  Because the air molecules are hot, they move around very fast and keep the water vapor from forming into droplets.  As this hot air is cooled, the air molecules move with less velocity and are closer together.  This allows the water vapor to form into droplets.  The temperature at which the water vapor in the air begins to condense and form droplets is called dew point.  On a hot humid day, a glass of ice water will cool the air around it o below the dew point, causing the water vapor in the air to condense on the glass in  the form of droplets.  On a hot day with very little humidity in the air the outside of the glass will barely become damp.

In a pneumatic system the act of compressing the air causes it to become very hot.  This hot air has the capacity to hold a large amount of water vapor.  As compressed air is used or released from compression it becomes very cool.  Many times it will cool off below the dew point causing any water vapor to form droplets in the air line.  The act of compressing and releasing the air from compression can produce the undesirable effect of water in an air line.

Flow

Flow is the measurement of how much air is being used, measured in Cubic Feet per Minute (CFM).  The flow of air through an air line or out of an orifice is measured in CFM or Standard Cubic Feet per Minute (SCFM).  There is an important difference between pressure and flow.  Pressure is the push that causes the flow.  If we blow up a balloon and tie it off, as in Figure 1, we cause pressure.  If we untie the balloon and let the air escape we cause flow.  Pressure causes force.  Flow causes movement.

The amount of air that is discharged from an orifice or tube is dependant on basically two factors:

1. The size of the orifice, and

2. The amount of pressure in the pneumatic system pushing the air out of the orifice.

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