Honeywell AUTOMATIC CONTROL Engineering Manual page 502

GENERAL ENGINEERING DATA
Use a psychrometric chart to determine that the ambient air
at 80F and 80 percent rh contains 0.0177 pounds of moisture
per pound of dry air. See Psychrometric Chart Fundamentals
section. On Figure 7 locate intersection of 90F and 30 psi lines.
Follow to right of chart and determine that the compressed air
is saturated at 0.0099 pounds of moisture per pound of dry air.
The difference between 0.0177 and 0.0099 is 0.0078 pounds
of moisture per pound of dry air which condenses and collects
on the bottom of the air compressor storage tank.
When the air pressure is reduced to 20 psi and distributed to
the control system, the temperature will drop to approximately
the 80F ambient. The air can now contain only 0.0091 pounds
of moisture per pound of dry air. This means that 0.0008 pounds
of moisture per pound of dry air condenses and collects in low
places in the tubing.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
As the compressed air temperature is reduced further as it
passes through air at a lower ambient temperature, additional
moisture will condense. This may plug filters and restrictors in
controllers or collect in valve or damper actuators.
The problem is controlled by use of an air dryer. The
suggested alternative of a much higher pressure can have the
same condensation problem.
EXAMPLE:
Using the design summer conditions for Phoenix, Arizona
(Table 2), the air contains 0.0142 pounds of moisture per
pound of dry air. If the air is compressed to 90 psi and the
tank temperature is maintained at 115F, then the air leaving
the tank will contain 0.0088 pounds of moisture per pound
of dry air. At 20 psi the air is saturated at 78F. If the
compressed air temperature drops below 78F, moisture will
condense out of the air.
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