Thermistors; Chilled Water Flow Switch; Cooler Freeze-Up Protection; Winter Shutdown - Carrier AQUASNAP 30RAP010-150 Controls, Start-Up, Operation, Service, And Troubleshooting

COOLER FREEZE-UP PROTECTION

WARNING
On medium temperature brine units, the anti-freeze solu-
tion must be properly mixed to prevent freezing at a tem-
perature of at least 15 F (8.3 C) below the leaving-fluid
temperature set point. Failure to provide the proper anti-
freeze solution mixture is considered abuse and may impair
or otherwise negatively impact the Carrier warranty.
The main base board (MBB) monitors leaving fluid temper-
ature at all times. The MBB will rapidly remove stages of ca-
pacity as necessary to prevent freezing conditions due to the
rapid loss of load or low cooler fluid flow.
When the cooler is exposed to lower ambient temperatures
(34 F [1° C] or below), freeze-up protection is required using
inhibited ethylene or propylene glycol.
HEATER CABLE — Optional factory-installed cooler and/or
hydronic package heaters are cycled based on the input from
the outside-air temperature sensor. These heaters, when
installed, are designed to protect the cooler and/or hydronic
package from freezing down to –20 F (–29 C). Power for these
heaters is supplied from the main unit power.
The input from the low pressure transducer provides a back-
up cooler freeze protection package. The MBB shuts down the
unit when a low pressure condition exists that could cause the
cooler to freeze up.
CAUTION
Do not disconnect main unit power when servicing com-
pressor(s) if ambient temperature is below 40 F (4.4 C).
The compressors have either a single circuit breaker or
multiple circuit breakers that can be used to shut off power
to the compressors. If power to the unit must be off for a
prolonged period, drain the cooler, hydronic package (if
installed) and internal piping. Add glycol according to
Winter Shutdown Step 2 below.
WINTER SHUTDOWN — Do not shut off power discon-
nect during off-season shutdown. At the end of the cooling sea-
son:
1. Drain water from system.
2. Replace drain plug(s) and add sufficient inhibited ethyl-
ene glycol (or other suitable inhibited antifreeze) to cool-
er, pump and piping to prevent freezing of residual water.
3. At the beginning of the next cooling season, refill the
cooler and add the recommended inhibitor.
Electronic control uses up to five 5 k
Thermistors —
thermistors to sense temperatures used to control operation of
the chiller. Thermistors EWT, LWT, RGTA, RGTB, and OAT
are identical in their temperature and voltage drop perfor-
mance. The SPT space temperature thermistor has a 10 k in-
put channel and it has a different set of temperature vs. resis-
tance and voltage drop performance. Resistance at various tem-
peratures are listed in Tables 27-31. For dual chiller operation,
a dual chiller sensor is required which is a 5 k thermistor.
When a digital compressor is used, a DTT (digital temperature
thermistor) is used. The DTT is an 86 k thermistor.
REPLACING THERMISTORS (EWT, LWT, RGT) — Add
a small amount of thermal conductive grease to the thermistor
well and end of probe. For all probes, tighten the retaining nut
1 / turn past finger tight. See Fig. 39 and 40. Insulate thermistor
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with cork tape or other appropriate insulating material.
THERMISTOR/TEMPERATURE SENSOR CHECK — A
high quality digital volt-ohmmeter is required to perform this
check.
1. Connect the digital voltmeter across the appropriate the-
mistor terminals at the J8 terminal strip on the Main Base
Board (see Fig. 40).
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 27-30.
3. To check thermistor accuracy, measure temperature at
probe location with an accurate thermocouple-type tem-
perature measuring instrument. Insulate thermocouple to
avoid ambient temperatures from influencing reading.
Temperature measured by thermocouple and temperature
determined from thermistor voltage reading should be
close, ± 5° F (3° C) if care was taken in applying thermo-
couple and taking readings.
If a more accurate check is required, unit must be shut down
and thermistor removed and checked at a known temperature
(freezing point or boiling point of water) using either voltage
drop measured across thermistor at the J8 terminal, by deter-
mining the resistance with chiller shut down and thermistor
disconnected from J8. Compare the values determined with the
value read by the control in the Temperatures mode using the
scrolling marquee display.
Pressure Transducers —
transducers are different part numbers and can be distinguished
by the color of the transducer body, suction (yellow) and dis-
charge (red). No pressure transducer calibration is required.
The transducers operate on a 5 vdc supply, which is generated
by the main base board (MBB). See Fig. 40 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
gauge. 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
without the factory-installed hydronic package. See Fig. 41.
Units with the optional hydronic package have the flow switch
installed in the entering fluid piping. This is a thermal-disper-
sion flow switch with no field adjustments. 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 flowing fluid. 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 af-
fected 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.
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The suction and discharge
A factory-installed