Table of Contents
Advertisement
UNITS WITH ELECTRONIC EXPANSION VALVE (EXV)
— The EXV remains closed for 10 seconds to purge cooler
and suction line of any liquid refrigerant that may have mi-
grated to these areas during off period. After 10 seconds, EXV
starts to open. As more cooling is required, control brings on
additional stages of capacity. Loading sequence for com-
pressors is shown in Table 13. Automatic lead-lag control is
provided on all units.
Lag compressor will shut down, and lead compressor will
continue to run. After lag compressor has shut down, the EXV
is signaled to close. Lead compressor remains on until EXV
is less than 300 steps open, and the saturated suction tem-
perature is less than 25 F (−4 C) as sensed by cooler ther-
mistor T5 or T6, or until one minute has elapsed.
30GT080-110 UNITS WITH OPTIONAL THERMOSTATIC
EXPANSION VALVE (TXV)
NOTE: The TXV option is available for 080-110 units only
and does not apply to associated modular units.
Liquid line solenoid valve is not energized for first 10 sec-
onds of compressor operation. This is called pre-pumpout
cycle.
Microprocessor determines how rapidly capacity stages are
added or subtracted based on deviation from leaving chilled
fluid temperature set point and rate of change of leaving fluid
temperature. If fluid temperature is very warm and pulldown
option is being used, microprocessor limits rate of tempera-
ture drop of leaving fluid to 1° F (0.56° C) per minute to
avoid high peak kW charges. If the capacity is being
limited by pulldown, the control display shows
the display button is pressed. Once capacity has been sat-
isfied, the unit starts to shut down.
Lag compressor will be shut down and lead compressor
continues to run for 10 seconds to purge cooler of any
refrigerant.
LOAD SHED — If load shed option is being used, control
limits maximum capacity to load shed input value. Refer to
Controls and Troubleshooting literature for details. If
capacity is limited by a load shed signal, display shows
when display button is pressed.
TEMPERATURE RESET — If temperature reset is being
used, microprocessor adjusts leaving fluid temperature to ob-
tain greater part-load efficiency. Refer to Controls and Trouble-
shooting literature for details. If leaving fluid temperature is
being reset, display shows
pressed.
when
when the display button is
HEAD PRESSURE CONTROL
Units with EXV — Microprocessor controls EXV to
maintain a superheat of 30° F (16.7° C) entering compressor
cylinders.
Microprocessor control also cycles condenser fans on and
off to maintain an adequate pressure differential across EXVs.
Fans are controlled by position of EXV and saturated con-
densing temperature thermistors. When EXV is fully open
and superheat is greater than 40° F (22° C), fan stages are
removed; when the valve is approximately half open, fan stages
are added. This allows unit to run at very low condensing
temperatures at part load. Thus chiller has very high part-
load EERs (energy efficiency ratios). Fan sequence is shown
in Fig. 20. Refer to Controls and Troubleshooting literature
for more head pressure control information.
Units with TXV (30GT080-110 Only) — There is one TXV
for each refrigerant circuit, and each TXV is factory set to
maintain 8 to 10° F (5 to 6° C) superheat of vapor leaving
cooler to control flow of liquid refrigerant into cooler. Super-
heat can be reset but should be reset only if necessary.
Logic to cycle microprocessor-controlled fans is based
on saturated condensing temperature only. This temperature
is sensed by thermistors T3 and T4 (Fig. 21 and 22). The
microprocessor turns on an additional stage of fans when
either coil thermistor (T3 or T4) is greater than 113 F (45 C)
and turns off a fan stage when T3 and T4 are both below
73 F (23 C). Between each change in fan stage, control waits
one minute to allow head pressure to stabilize unless either
T3 or T4 is greater than 125 F (52 C), in which case all
microprocessor-controlled fans come on immediately.
Condenser fan sequence is shown in Fig. 20.
REMOTE ON-OFF — In applications where controlling the
starting and stopping of the chiller from a remote location is
desired, such as a remote timeclock, the remote on-off fea-
ture is used. See Fig. 16 and 17 for wiring information. If
the chiller is being held off by the remote on-off switch,
appears in the display when the display button is pressed.
NOTE: DO NOT USE FLUID PUMP to cycle chiller on and
off except as a safety feature. Cycling of chiller must be ac-
complished through the remote on-off switch, since the fluid
pump must continue to run for 1 minute after initialization
of pumpdown.
REMOTE ALARM — See Fig. 16 and 17 for remote alarm
field wiring.
52
Hide quick links:
Advertisement
Table of Contents
