Maintenance And Repair - Gardner Denver RDS Series Operation And Maintenance

RDS Series —50-1000 SCFM

Maintenance and Repair

Thermistors
Several functions on RDS Series refrigerated air dryers
are controlled by the main control board. Variable
resistance values from thermistors provide input signals
to the control board, which then adjusts several functions
of the dryer, based on these signals. Resistance values
can be found in the thermistor resistance value tables.
Thermistor location and function.
Inlet air thermistor - located at the inlet air port to the
dryer. Provides signal for timing of automatic drain. As air
warms up, water vapor holding capacity increases. At this
point the dryer must be able to drain more water. The inlet
thermistor will sense a rise in inlet air temperature,
causing the drain to open more frequently to allow more
drainage from the separator. Drain timing is set by jumper
position on the control board. (see drain valve timer
options table)
Dewpoint thermistor - Located on small machines (50-250
scfm) at the outlet of the evaporator. In these machines
the thermistor is not accessible because of the
construction of the evaporator assembly. The wires are
accessible at the control board terminals. In larger dryers
(300 to 1000 scfm) the thermistor is located in a well on
the control panel side of the evaporator. The dew point
thermistor senses the air temperature at the out let of the
heat exchanger. At this point the air is the coldest in the
system. At 100% RH (saturation) temperature and dew
point are the same. The signal from the thermistor is
transmitted to the control board and lights the dew point
monitor on the front of the dryer.
Condenser out thermistor - located on the liquid
refrigerant line as it leaves the condenser in the proximity
of the liquid line dryer. This thermistor senses the
temperature of the refrigerant as it leaves the condenser.
A signal is sent to the control board, which cycles the
condenser fans. In case of fan failure or poor condensing
conditions (dirty condenser, high ambient, etc.) the
temperature will continue to rise at the thermistor. At
approximately 150 Deg F the dryer will go into alarm, the
alarm light will light, and the dryer will shut down.
Compressor discharge thermistor - located on the
refrigerant discharge line of the refrigerant compressor.
This thermistor senses the temperature of the refrigerant
gas as it leaves the compressor. When the refrigerant
temperature rises indicating overload conditions to a
temperature of 245 Deg F, the liquid injection solenoid will
open. This causes liquid refrigerant to flow through a
capillary tube into the suction of the refrigerant
compressor, reducing return gas temperature to the
compressor, providing additional cooling to the
compressor. If this is not sufficient, the temperature will
continue to rise and will shut the dryer down when it
reaches 265 Deg F.
Procedure for verifying thermistor operation
1. Remove the thermistor in question from the well or
refrigerant line that it is installed on.
2. Remove the plug that contains the control wires from
CN3 on the control board.
3. Verify the thermistor terminal location by checking
the wiring diagram on the lid of the dryer.
4. With a resistance meter, measure the resistance.
Compare the reading to the value at on the table at
ambient conditions.
5. Place the probe in a bottle of ice water. Read the
resistance at 32 Deg F on the table. The values
should be close. If the resistance changes in value,
the thermistor is responding. If there is no resistance
reading, replace the thermistor.
- 16 -
Section 6