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The compressor draws air through a filter in both air
intake assemblies, compresses it, and feeds it to the manifold
for distribution on to solenoid valves. The air is then passed
through both dryer cylinders and to the back pressure regulator
valve. When the drying cylinder reaches a predetermined pressure,
the regulator valve opens and connects the dehydrator output to
the transmission line. Each dryer cylinder contains a molecular sieve type desiccant that absorbs (sucks in) moisture from the air passing through it. The moisture clings to the desiccant until the pressure is reduced to a valve that causes the desiccant to expel the moisture. A stream of dry air is diverted through the cylinder and carries the expelled moisture to the atmostphere through the moisture outlet of the solenoid valve. The valve has two ports, a compressed air inlet and a moisture outlet. When power is applied to the solenoid, the air inlet closes and the moisture outlet opens as shown in Figure 5. The solenoid valves are controlled by a timing motor that drives a cam-operated solenoid switch. Every 30 seconds (36 seconds for 1921E) power is applied to one solenoid and power removed from the other. The deenergized solenoid allows air from the compressor to pass through the cylinder. The dry air output of the |
drying cylinder is diverted into two streams. One stream
passes through a fixed orifice into the other (purging) cylinder
to purge it of moisture. The orifice regulates the amount of
purging air and provides enough back pressure for the other (main)
stream which is passed to the back pressure regulator valve. The air at the back pressure regulator valve remains inactive until the drying cylinder pressure builds up to a value greater than 60 psi (414 kPa) which is required for proper purging of the desiccant. When this pressure exceeds the preset value for the back pressure regulator valve, the valve opens and passes dry air through the humidity indicator to the transmission line. The humidity indicator should remain a dark blue color, indicating a relative humidity of less than 10% in the air flow. The normal operating maximum dew point of -40°F/C is equivalent to about 0.5% relative humidity at room temperature. A change in color to pale blue or pink indicates some part of the dehydrator has failed. When the transmission line pressure reaches 8 psi (55 kPa), the pressure sensitive switch opens and removes power from the motors and the purging-cylinder solenoid valve. The back pressure regulator valve then closes to prevent the line pressure from "bleeding" back into the dehydrator where the pressure has fallen to zero. |
