Check Unit Safeties; Thermistors; Pressure Transducers - Carrier AQUASNAP MPW015-045 Operation And Service Manual

MINIMUM LOAD VALVE — On units equipped with the
factory-installed hot gas bypass option, a solenoid valve and
discharge bypass valve (minimum load valve) are located
between the discharge line and the cooler entering-refrigerant
line. The MBB cycles the solenoid to perform minimum load
valve function and the discharge bypass valve modulates to the
suction pressure set point of the valve. The bypass valve has an
adjustable opening setting between 95 to 115 psig (655 to
793 kPa). The factory setting is 105 psig (724 kPa).
The amount of capacity reduction achieved by the mini-
mum load valve is not adjustable. The total unit capacity with
the minimum load valve is shown in Table 19.
PRESSURE RELIEF DEVICES — All units have one pres-
sure relief device per circuit located in the liquid line which re-
lieves at 210 F (100 C).
The 30MPW unit does not have a condenser pressure relief
valve, because the brazed-plate condenser is not considered a
pressure vessel, as defined in ANSI/ASHRAE 15 (American
National Standards Institute/American Society of Heating,
Refrigerating, and Air Conditioning Engineers) safety code
requirements.
For 30MPA condenserless units, pressure relief devices de-
signed to relieve at the pressure determined in local codes,
must be field-supplied and installed in the discharge line piping
in accordance with ANSI/ASHRAE 15 safety code require-
ments. Additional pressure relief valves, properly selected,
must be field-supplied and installed to protect high side equip-
ment and may be required by applicable codes.
Most codes require that a relief valve be vented directly to
the outdoors. The vent line must not be smaller than the relief
valve outlet. Consult ANSI/ASHRAE 15 for detailed informa-
tion concerning layout and sizing of relief vent lines.

Check Unit Safeties

HIGH-PRESSURE SWITCH — A high-pressure switch is
provided to protect each compressor and refrigeration system
from unsafe high pressure conditions. See Table 31 for high-
pressure switch settings.
The high-pressure switch is mounted in the discharge line of
each circuit. If an unsafe, high-pressure condition should exist,
the switch opens and shuts off the affected circuit. The CSB
(current sensing board) senses the compressor feedback signal
and generates an appropriate alarm. The MBB prevents the cir-
cuit from restarting until the alert condition is reset. The switch
should open at the pressure corresponding to the appropriate
switch setting as shown in Table 31.
Table 31 — Factory Settings, High-Pressure
Switch (Fixed)
CUTOUT
UNIT
Psig
30MP015-045 650
Clear the alarm using the scrolling marquee display. The
unit should restart after the compressor anti-short-cycle delay,
built into the unit control module, expires.
PRESSURE TRANSDUCERS — Each unit is equipped with
a suction and discharge pressure transducer. These inputs to the
MBB are not only used to monitor the status of the unit, but to
also maintain operation of the chiller within the compressor
manufacturer's specified limits. The input to the MBB from the
suction pressure transducer is also used to protect the compres-
sor from operating at low pressure conditions. If suction return
gas thermistors are installed, then additional low superheat
conditions are detected. In some cases, the unit may not be able
to run at full capacity. The control module will automatically
reduce the capacity of a circuit as needed to maintain specified
maximum/minimum operating pressures.
CUT-IN
kPa Psig kPa
4482 500 3447
COOLER FREEZE-UP PROTECTION
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 cooler leaving fluid
temperature at all times. The MBB will rapidly remove stages
of capacity 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 temperatures (40 F
[4.4 C] or below), freeze-up protection is required using inhib-
ited ethylene or propylene glycol.
Electronic control uses up to five 5 k
Thermistors —
thermistors to sense temperatures used to control operation of
the chiller. Thermistors EWT, LWT, RGTA, CNDE, CNDL,
and OAT are identical in their temperature and voltage drop
performance. The SPT space temperature thermistor has a
10 k input channel and it has a different set of temperature vs.
resistance and voltage drop performance. Resistance at various
temperatures are listed in Tables 32-35. For dual chiller opera-
tion, a dual chiller sensor is required which is a 5 k
thermistor.
REPLACING THERMISTORS (EWT, LWT, RGT, CNDE,
CNDL) — Add a small amount of thermal conductive grease
to the thermistor well and end of probe. For all probes, tighten
the retaining nut ¼ turn past finger tight. See Fig. 22.
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. 23).
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 32-35.
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. 23 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
38
WARNING
The suction and discharge