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moisture to the atmospheric 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 power is applied to one solenoid and power removed from the other. The deenergized solenoid allows air from the compressor to pass through its 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 valve remains inactive until the drying cylinder pressure builds up to a value greater than 60 psi (414 kPa) which is required for proper puring of the desiccant. When the 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
dehyrdrator 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. If it is on, check for loose or broken wiring, or a defective pressure sensitive switch. Bypass the switch by appling ac power directly to the compressor motor windings; if the motor runs, the switch is defective. If the motor does not run, disconnect the ac power and rotate the fan blades; they should move freely. If not, an obstruction has caused motor overheating and activation of the thermal overload switch, disconnecting the motor: replace the motor-compressor unit. |
