46
The valve(s) is factory-set to maintain between 8° and 10° F
(4.4° and 5.6° C) of superheat leaving the cooler. Check the
superheat during operation after conditions have stabilized. If
necessary, adjust the superheat to prevent refrigerant floodback
to the compressor.
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 14.
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 the circuit and refrigeration system from
unsafe high pressure conditions. For 30MP050-071, two
different high pressure switches are used, depending on unit
configuration. See Table 44 for high-pressure switch settings.
The high-pressure switch is mounted in the discharge line of
the circuit. If an unsafe, high-pressure condition should exist,
the switch opens and shuts off the unit. The MBB senses the
HPS feedback signal and generates an appropriate alarm. The
MBB prevents the circuit from restarting until the alert condi-
tion is reset. The switch should open at the pressure corre-
sponding to the appropriate switch setting as shown in
Table 44.
Table 44 — Factory Settings, High-Pressure
Switch (Fixed)
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
also to 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.
COOLER FREEZE-UP PROTECTION
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.
Thermistors — Electronic control uses up to five 5,000-
ohmthermistors to sense temperatures used to control opera-
tion of the chiller. Thermistors EWT, LWT, RGT.A, CNDE,
CNDL, and OAT are identical in their temperature and voltage
drop performance. The SPT space temperature thermistor has a
10,000-ohm input channel and it has a different set of tempera-
ture vs. resistance and voltage drop performance. Resistance
values at various temperatures are listed in Tables 45-49. For
dual chiller operation, a dual chiller sensor is required which is
a 5,000-ohm 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. 30.
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. 31).
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 45-49.
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 — The suction and discharge
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.
UNIT
CUTOUT CUT-IN
PART
NUMBER
Psig kPa Psig kPa
All 30MP015-045;
30MPA050-071,
30MPW050-071
High Condensing
650 4482 500 3447 HK02ZZ001
30MPW050-071 558 3848 435 3000 HK02ZZ003
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.
To Next Page
Loading...
