Chilled Water Flow Switch; Thermistor Well; Thermistor Connections To Main Base Board, J8 Connector - Carrier AquaSnap 30MPA Series Controls, Start-Up, Operation, Service, And Troubleshooting

The transducers operate on a 5 vdc supply, which is generated
by the main base board (MBB). See Fig. 31 for transducer con-
nections to the J8 connector on the MBB.
TROUBLESHOOTING — If a transducer is suspected of be-
ing faulty, first check supply voltage to the transducer. Supply
voltage should be 5 vdc ± 0.2 v. If supply voltage is correct,
compare pressure reading displayed on the scrolling marquee
display module against pressure shown on a calibrated pressure
gage. Pressure readings should be within ± 15 psig. If the two
readings are not reasonably close, replace the pressure trans-
ducer.
Chilled Water Flow Switch —
flow switch is installed in the leaving fluid piping for all units.
This is a thermal-dispersion flow switch with no field adjust-
ments. The switch is set for approximately 0.5 ft/sec of flow.
The sensor tip houses two thermistors and a heater element.
One thermistor is located in the sensor tip, closest to the flow-
ing fluid. See Fig. 32. This thermistor is used to detect changes
in the flow velocity of the liquid. The second thermistor is
bonded to the cylindrical wall and is affected only by changes
in the temperature of the liquid. The thermistors are positioned
to be in close contact with the wall of the sensor probe and, at
the same time, to be kept separated from each other within the
confines of the probe.
In order to sense flow, it is necessary to heat one of the
thermistors in the probe. When power is applied, the tip of the
probe is heated. As the fluid starts to flow, heat will be carried
away from the sensor tip. Cooling of the first thermistor is a
function of how fast heat is conducted away by the flowing
liquid.
The difference in temperature between the two thermistors
provides a measurement of fluid velocity past the sensor probe.
When fluid velocity is high, more heat will be carried away
from the heated thermistor and the temperature differential will
be small. As fluid velocity decreases, less heat will be taken
from the heated thermistor and there will be an increase in tem-
perature differential.
When unit flow rate is above the minimum flow rate, then
the output is switched on, sending 24 vac to the MBB to prove
flow has been established.
For recommended maintenance, check the flow switch op-
eration. If operation is erratic check the sensor tip for build-up
every 6 months. Clean the tip with a soft cloth. If necessary,
build-up (e.g., lime) can be removed with a common vinegar
cleansing agent.
The flow sensor cable is provided with (3) LEDs that indi-
cate if 24 vac power is present and also status of the switch
contacts. The LEDs are as follows:
• Green LED ON – 24 vac present
• One Yellow LED ON – Flow sensor switch OPEN
• Two Yellow LED ON – Flow sensor switch CLOSED
If nuisance trips of the sensor are occurring, follow the steps
below to correct the situation:
1. Check to confirm that the field-installed strainer is clean.
Use the blow-down valve provided or remove the screen
and clean it. For the case of VFD controlled pumps, en-
sure that the minimum speed setting has not been
changed.
2. Measure the pressure drop across the cooler and compare
this to the system requirements.
3. Verify that cable connections at the switch and at the ter-
minal block are secure.
A factory-installed
47
4. Check for wrong pump motor rotation. Pump must rotate
clockwise when viewed from motor end of pump.
5/8 in. HEX
6" MINIMUM
CLEARANCE FOR
THERMISTOR
REMOVAL
Fig. 30 — Thermistor Well
MAIN
BASE
BOARD
LEGEND
ACCSY — Accessory
DPT — Discharge Pressure Transducer
EFT — Entering Fluid Temperature
LFT — Leaving Fluid Temperature
LWT — Leaving Water Temperature Sensor
LVT — Low Voltage Terminal
OAT — Outdoor Air Temperature Sensor
RGT — Return Gas Temperature Sensor
SEN — Sensor Terminal Block
SPT — Space Temperature Sensor
Fig. 31 — Thermistor Connections to
Main Base Board, J8 Connector
NOTE: Dimensions are in millimeters.
Fig. 32 — Chilled Water Flow Switch
1/4-18 NPT
A30-6018
a30-499