Unit installation

Controls
SCXF-SVX01K-EN 71
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 compressors are protected by circuit breakers that
interrupt the compressor power supply if the current
exceeds the breakers “must trip” value. During a request
for compressor operation, if the compressor module
(MCM or SCM) detects a problem outside of it’s 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 theWSM.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), andWSM 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.
Air-cooled units. 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.
Water-cooled. 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 temperature. If not economizing,
the condenser valve will modulate to maintain condensing
saturated temperature. Water-cooled units without head
pressure control will lock out mechanical cooling at
entering condenser water temperatures below 54°F.
Mechanical cooling will resume when the entering
condenser water temperature exceeds 58°F.
Low Ambient Control (Air-Cooled Units Only)
The low ambient modulating output on the compressor
module is functional on all units with or without the low
ambient option. When the compressor module stages up
to it's highest stages 2 or 3 depending on unit size), the
modulating output is 100% (10 VDC). When the control is
at stage 1, the modulating output (0-10 VDC) controls the
saturated condensing temperature to within the
programmable condensing temperature low ambient
control point.
Low Ambient Compressor Lockout (Air-Cooled
Units Only)
The low ambient compressor lockout utilizes an analog
input device. When the system is configured for low
ambient compressor lockout, the compressors will not