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82 SCXG-SVX01B-EN
Input Devices and System
Functions
Following are basic input device and
system function descriptions used within
the UCM network on IntelliPak self-
contained units. Refer to the unit wiring
diagrams for specific connections.
Water Purge
NOTICE
Proper Water Treatment!
The use of untreated or improperly
treated water in coils may result in
scaling, erosion, corrosion, algae or
slime. It is recommended that the
services of a qualified water treatment
specialist be engaged to determine what
water treatment, if any, is required. Trane
assumes no responsibility for equipment
failures which result from untreated or
improperly treated water or saline or
brackish water.
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 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.
Compressor Motor Winding Thermostats
A thermostat is embedded in the motor
windings of each compressor. Each
thermostat opens if the motor windings
exceed approximately 221°F. The
thermostat resets automatically when the
winding temperature decreases to
approximately 181°F. Rapid cycling, loss
of charge, abnormally high suction
temperatures, or the compressor running
backwards could cause the thermostat to
open. During a request for compressor
operation, if the compressor module
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.
LP cutouts are mounted on the suction
lines near the compressors. The LP
control contacts close when the suction
pressure exceeds 27 ± 4
psig. If the LP
control is open when a compressor
starts, none of the compressors on that
circuit will operate. They are locked out
and a manual reset diagnostic initiates.
The LP cutouts open if the suction
pressure approaches 7 ± 4 psig. If the LP
cutout opens after a compressor starts,
all compressors operating on that circuit
will turn off immediately and will remain
off for a minimum of three minutes.
If the LP cutout trips four consecutive
times during the first three minutes of
operation, the compressors on that circuit
will lock out and a manual reset diagnos-
tic initiates.
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.
Air-cooled units: During a request for
compressor operation when the con-
densing 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 condens-
ing temperature to within the control
band, more fans turn on. As the satu-
rated condensing temperature ap-
proaches 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 tempera-
ture 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.
general
information
Operation