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Figure 31. Typical pulldown curve for unit operating
properly within control band
Figure 32. Typical pulldown curve for unit operating
improperly outside control band
Compressor Safety Devices
If a compressor low pressure cutout opens during
compressor startup, the UCM will not shut the compressor
off during the first two to three minutes after startup. This
prevents possible nuisance trips during low ambient start
conditions. See
Table 28, p.
Each compressor's discharge line contains a high pressure
cutout. Under abnormal operating conditions, the cutout
will open to stop compressor operation.
Table 28. Pressure cutouts (open/close)
High Pressure
Unit Model
SXWG
Low Ambient Compressor Lockout
This function will lock out the compressor if the outdoor air
temperature sensor reads an outdoor temperature below
the low ambient compressor lockout temperature setpoint.
This setpoint is adjustable at the human interface panel.
Compressors will lock out when outdoor air temperature
falls below that selected temperature and will start again
when the temperature rises 5°F above the setpoint.
Evaporator Coil Frost Protection
FROSTAT
The FROSTAT™ system eliminates the need for hot gas
bypass. It utilizes an evaporator temperature sensor
SCXF-SVX01T-EN
55.
Low Pressure
Cutout
Cutout
553/424
49/74
mounted on the suction line near the TXV bulb of each
circuit to protect the evaporator from freezing.
If the evaporator temperature approaches the specified
setpoint (adjustable between 25 and 35°F at the HI) the
compressor(s) will cycle off. The supply fan remains on to
help de-ice the coil. The compressors will restart when the
evaporator temperature has risen 10°F above the specified
cutout temperature and when the compressor(s) have been
off a minimum of three minutes. This prevents rapid cycling
of the compressors.
Service Valve Option
If ordered, service valves are factory installed on each
circuit before and after the compressor to allow compressor
isolation for servicing.
Waterside Components
Waterside components consist of water piping, water
valves, water flow switch option, water cooled condensers
(SXWF only), and the economizer option.
Water Purge
NOTICE
Proper Water Treatment Required!
The use of untreated or improperly treated water
could result in scaling, erosion, corrosion, algae or
slime.
Use the services of a qualified water treatment
specialist 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.
This user-definable feature allows the user to select a
purge schedule to automatically circulate water through the
economizer and condensers periodically during non-
operational times. This allows fresh chemicals to circulate
in waterside heat exchangers. This feature is on all units
and is defined at the HI.
Water Piping Options
Water piping is factory-installed with left-hand connections
on units without a waterside economizer. Units can be
ordered with either basic piping or intermediate piping.
Also, units with waterside economizers can be set for either
variable or constant water flow at the HI. See
57
and
Figure 34, p. 57
for detailed piping configuration
information.
With compatible piping configurations, the unit can be
configured to provide:
•
Constant water flow with basic or intermediate piping
or
•
Variable water flow (head pressure control) with
intermediate piping only.
Operating Principles
Figure 33, p.
55
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