76
SCXF-SVX01Q-EN
Water Purge
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During the unoccupied mode, water-cooled units will
periodically circulate water through the condensers
and waterside economizer if the user has enabled the
purge function at the HI. The water purge function
circulates water to introduce fresh water-treatment
chemicals and help prevent water stagnation. The
number of hours between each periodic purge, or
purge duration, is user-defined at the HI between 1-999
hours. If the periodic purge timer expires while the unit
is in occupied mode, it will wait for the next available
unoccupied time before initiating water purge.
Contrary, if a request for cooling occurs during a purge
sequence, purge will terminate and cooling will
commence.
Compressor Circuit Breakers
The Scroll Compressors are protected by circuit
breakers which interrupt the power supply to the
compressors if the current exceeds the breakers “must
trip” value. During a request for compressor operation,
if the Compressor Module detects a problem outside
normal parameters, it turns any operating compressor
(s) on that circuit “Off”, locks out all compressor
operation for that circuit, and initiates a manual reset
diagnostic.
Low Pressure Control
Low pressure (LP) control is accomplished using a
binary input device mounted on the suction line, near
the compressor. If suction pressure drops to 49 (water-
cooled), 36 (air-cooled) ± 6 psig, or below, the switch
opens.
If the switch is open at start, no compressors on that
circuit will operate. They are locked out and a manual
reset diagnostic initiates. If the LP switch opens after a
compressor start, all compressors on that circuit will
stop and remain off a minimum 3 minutes before
restarting. If the LP cutout trips four times in the first
three minutes of operation, all compressors on that
circuit lockout and a manual reset diagnostic initiates.
LP switches close at 74 (water-cooled) and 61 (air-
cooled)± 6 psig.
Evaporator Temperature Sensor Frostat
The evaporator temperature sensor is an analog input
device used to monitor refrigerant temperature inside
the evaporator coil to prevent coil freezing. It is
attached to the suction line near the evaporator coil
with circuits 1 and 2 connected to the SCM/MCM and
circuits 3 and 4 connected to the WSM. The coil frost
cutout temperature is factory set at 30°F. It is adjustable
at the HI from 25-35°F. The compressors stage off as
necessary to prevent icing. After the last compressor
stages off, the compressors will restart when the
evaporator temperature rises 10°F above the coil frost
cutout temperature and the minimum three minute
“off” time elapses.
Saturated Condenser Temperature Sensors
The saturated condenser temperature sensors are
analog input devices. They are mounted inside a
temperature well located on a condenser tube bend on
air-cooled units, and in the condenser shell on water-
cooled units. The sensors monitor the saturated
refrigerant temperature inside the condenser coil and
are connected to the SCM/MCM for circuits 1 and 2 (air
or water cooled), and WSM for circuits 3 and 4 (only
water-cooled).
Head Pressure Control
Head pressure control is accomplished using two
saturated refrigerant temperature sensors on air-
cooled units and up to four sensors on water-cooled
units.
AAiirr--ccoooolleedd uunniittss:: During a request for compressor
operation when the condensing temperature rises
above the lower limit of the control band, the
compressor module (SCM/MCM) sequences condenser
fans on. If the operating fans cannot bring the
condensing temperature to within the control band,
more fans turn on. As the saturated condensing
temperature approaches the lower limit of the control
band, fans sequence off. The minimum on/off time for
condenser fan staging is 5.2 seconds. If the system is
operating at a given fan stage below 100% for 30
minutes the saturated condensing temperature is
above the efficiency check point setting, a fan stage will
be added. If the saturated condensing temperature falls
below the efficiency check point setting, fan control
remains at the present operating stage. If the fan stage
cycles four times within a 10 minute period, the lower
limit temperature is redefined as being equal to the
lower limit minus the temporary low limit suppression
setting. The unit will utilize this new low limit
temperature for one hour to reduce condenser fan
short cycling.
WWaatteerr--ccoooolleedd:: Units without WSE, the condenser
valve modulates to maintain an average saturated
condenser temperature. Units with WSE, if
economizing and mechanical cooling is necessary the
economize valve will sacrifice free cooling and
modulate to maintain condensing saturated
CCoonnttrroollss
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