Figure 21-9. Evacuation Hookup
EVACUATING THE SYSTEM.
If the system has been operated in a discharged condition or anytime the system has been open to
atmospheric pressure, the receiver-dehydrator must be replaced and the system evacuated to remove any
trapped air and moisture which has entered it. A vacuum pump capable of pulling 29 inches of mercury or
better should be used. As we lower the pressure in the air conditioning system, we lower the boiling
temperature of the water (moisture) that may be present. Then we are able to pull this water, in the form of
vapor, out of the system. The following table demonstrates the effectiveness of moisture removal under a given
vacuum.
CHART 2105. SYSTEM VACUUM
System Vacuum Temperature °F.
27.99 100
COMPOUND GAUGE 28.89 80
READING IN INCHES 29.40 60
OF MERCURY VACUUM 29.71 40
29.82 20
29.88 0
—NOTE—
For each 1,000 feet of elevation above sea level, the compound
gauge reading will be about one inch lower, numerically.
21-50-11
Page 21-22
Revised: December 21, 1981
1F21
PIPER AIRCRAFT
PA-28RT-201 / 201T
MAINTENANCE MANUAL
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