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GENERAL ENGINEERING DATA
Use a psychrometric chart to determine that the ambient air
at 25 C and 80 percent rh contains 16 grams of moisture per
kilogram of dry air. See Psychrometric Chart Fundamentals
section. On Figure 7 locate intersection of 30 C and 310 kPa
lines. Follow to right of chart and determine that the compressed
air is saturated at 10 grams of moisture per kilogram of dry air.
The difference between 16 and 10 is 6 grams of moisture per
kilogram of dry air which condenses and collects on the bottom
of the air compressor storage tank.
When the air pressure is reduced to 240 kPa and distributed to
the control system, the temperature will drop to approximately
the 25 C ambient. The air can now contain only 9.2 grams of
moisture per kilogram of dry air. This means that 0.8 grams of
moisture per kilogram of dry air condenses and collects in low
places in the tubing.
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 of 40 C dry bulb and 25 C wet bulb, the air
contains 13.8 grams of moisture per kilogram of dry air.
If the air is compressed to 725 kPa and the tank
temperature is maintained at 45 C, then the air leaving
the tank will contain 9.6 grams of moisture per kilogram
of dry air. At 240 kPa the air is saturated at 26 C. If the
compressed air temperature drops below 26 C, moisture
will condense out of the air.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
480
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