27
Low Cooler Suction Temperature
— To avoid freezing the
cooler, the control will compare the circuit Cooler Suction tem-
perature (T5/T6) with a predetermined freeze point. If the cool-
er fluid selected is Water, the freeze point is 34 F (1.1 C). If the
cooler fluid selected is Brine, the freeze point is 8° F (4.4 ° C)
below the cooling set point (or lower of two cooling set points
in dual set point configurations). If the cooler suction tempera-
ture is 24° to 29° F (13.3° to 16.1° C) below the cooler leaving
water temperature and is also 2° F (1.1° C) less than the freeze
point for 5 minutes, Mode 7 (Circuit A) or Mode 8 (Circuit B)
is initiated and no additional capacity increase is allowed. The
circuit will be allowed to run in this condition. If the cooler suc-
tion temperature is more than 30° F (16.7° C) below the cooler
leaving water temperature and is also 2° F (1.1° C) below the
freeze point for 10 minutes, the circuit will be stopped without
going through pumpdown.
Cooler Freeze Protection
— The control will try to prevent
shutting the chiller down on a Cooler Freeze Protection alarm
by removing stages of capacity. The control uses the same
freeze point logic as described in the Low Cooler Suction Tem-
perature section above. If the cooler leaving fluid temperature
is less than the freeze point plus 2.0° F (1.1° C), the control will
immediately remove one stage of capacity. This can be repeat-
ed once every 30 seconds.
MOP (Maximum Operating Pressure) Override
— The con-
trol monitors saturated condensing and suction temperature for
each circuit. Based on a maximum operating set point (saturat-
ed suction temperature), the control may lower the EXV posi-
tion when system pressures approach the set parameters.
Head Pressure Control
COMFORTLINK™ UNITS (With EXV) — The Main Base
Board (MBB) controls the condenser fans to maintain the low-
est condensing temperature possible, and thus the highest unit
efficiency. The fans are controlled by the saturated condensing
temperature set from the factory. The fans can also be con-
trolled by a combination of the saturated condensing tempera-
ture, EXV position and compressor superheat. Fan control is a
configurable decision and is determined by the Head Pressure
Control Method (HPCM) setting in the Configuration Mode
under the OPT1 sub-mode. For EXV control (HPCM = 1),
when the position of the EXV is fully open, T3 and T4 are less
than 78 F (25.6 C), and superheat is greater than 40 F (22.2 C),
fan stages will be removed. When the valve is less than 40%
open, or T3 and T4 are greater than 113 F (45 C), fan stages
will be added. At each change of the fan stage, the system will
wait one minute to allow the head pressure to stabilize unless
either T3 or T4 is greater than 125 F (51.6 C), in which case all
MBB-controlled fans will start immediately. This method
allows the unit to run at very low condensing temperatures at
part load.
During the first 10 minutes after circuit start-up, MBB-
controlled fans are not turned on until T3 and T4 are greater
than the head pressure set point plus 10 F (5.6 C). If T3 and T4
are greater than 95 F (35 C) just prior to circuit start-up, all
MBB-controlled fan stages are turned on to prevent excessive
discharge pressure during pull-down. Fan sequences are shown
in Fig. 17.
UNITS WITH TXV — The logic to cycle MBB-controlled
fans is based on saturated condensing temperature only, as
sensed by thermistors T3 and T4 (see Fig. 8 and 10). When
either T3 or T4 exceeds the head pressure set point, the MBB
will turn on an additional stage of fans. For the first 10 minutes
of each circuit operation, the head pressure set point is raised
by 10° F (5.6° C). It will turn off a fan stage when T3 and T4
are both below the head pressure set point by 35° F (19.4° C).
At each change of a fan stage the control will wait for one
minute for head pressure to stabilize unless T3 and T4 is great-
er than 125 F (51.6 C), in which case all MBB-controlled fans
start immediately. If T3 and T4 are greater than 95 F (35.0 C)
just prior to circuit start-up, all MBB-controlled fan stages are
turned on to prevent excessive discharge pressure during pull-
down. Fan sequences are shown in Fig. 18.
Motormaster® Option
— For low-ambient operation, the lead
fan(s) in each circuit can be equipped with the Motormaster III
head pressure controller option or accessory. Wind baffles and
brackets must be field-fabricated for all units using accessory
Motormaster III controls to ensure proper cooling cycle opera-
tion at low-ambient temperatures. The fans controlled are those
that are energized with the lead compressor in each circuit. All
sizes use one controller per circuit. Refer to Fig. 18 for con-
denser fan staging information.
Pumpout
EXV UNITS — When the lead compressor in each circuit is
started or stopped, that circuit goes through a pumpout cycle to
purge the cooler and refrigerant suction lines of refrigerant. If a
circuit is starting within 15 minutes of the last shutdown, the
pumpout cycle will be skipped.
The pumpout cycle starts immediately upon starting the
lead compressor and keeps the EXV at minimum position for
10 seconds. The EXV is then opened an additional percentage
and compressor superheat control begins. At this point, the
EXV opens gradually to provide a controlled start-up to pre-
vent liquid flood-back to the compressor.
At shutdown, the pumpout cycle continuously closes the
EXV until all lag compressors are off and the EXV is at 0%.
The lead compressor continues to run for an additional 10 sec-
onds and is then shut off.
TXV UNITS — Pumpout is based on timed pumpout. On a
command for start-up, the lead compressor starts. After 15 sec-
onds, the liquid line solenoid opens. At shutdown, the liquid
line solenoid closes when the lead compressor has stopped.
47
46
45
44
43
42
41
0 200 400 600 800 1000
TIME (SECONDS)
2 STARTS
3 STARTS
DEADBAND EXAMPLE
L
WT (F)
MODIFIED
DEADBAND
STANDARD
DEADBAND
8
7
6
5
L
WT (C)
Fig. 17 — Deadband Multiplier
LEGEND
LWT —
Leaving Water Temperature
