SCXF-SVX01B-EN 79
Owner
Sequence of
Operation
Compressor Safety Devices
The compressors have motor
temperature cutout switches in the motor
windings. These switches are provided
to take the compressors off line during
high motor winding temperature
conditions.
If a compressor low pressure cutout
opens during compressor start-up, the
UCM will not shut the compressor off
during the first two to three minutes after
start-up. This prevents possible nuisance
trips during low ambient start conditions.
See Table O-SO-2.
Each compressor’s discharge line
contains a high pressure cutout. Under
abnormal operating conditions, the cutout
will open to stop compressor operation.
Table O-SO-2. Pressure Cutouts.
Unit High Pressure Low Pressure
Model Cutout Cutout
SXWF 360/270 20/35
SXRF 405/350 12/27
Step Control
Steps of mechanical cooling are control
based on supply air or zone temperature.
See Table O-SO-1 on page 78 for
compressor staging.
Capacity is based on an integrating
control concept. The unit capacity
matches the existing load and maintains
an average supply air temperature within
the supply air setpoint temperature
control band region.
The supply air temperature control band
is centered around supply air tempera-
ture setpoint and is adjustable from 2 to
12 F. In a steady state, the unit will either
maintain a constant level of cooling
capacity with the supply air temperature
within the control band, or the highest
active cooling level will cycle to provide
an average supply air temperature equal
to the setpoint.
If the supply air temperature swings
outside the limits of the control band, the
mechanical cooling capacity will increase
or decrease by one level accordingly. The
change occurs by integrating the tem-
perature offset from the control band
limit.
A minimum time delay of five minutes
follows each change in cooling level. This
time delay promotes stability by allowing
the system to respond to the change
before any further control action occurs.
As the supply air temperature ap-
proaches setpoint, the time duration
between changing levels of cooling
capacity increases.
See Figure O-SO-2 on page 80 for the
typical unit operating curve. Figure O-SO-
3 on page 80 shows typical unit perfor-
mance when supply air temperature
swings exceed the control band limits.
Table O-SO-3. Cooling Capacity Percent.
Cooling Stage % Utilized
Unit Tons Cool 1 Cool 2 Cool 3 Cool 4
SXWF 20, 22 53 100 - -
SXRF 20, 25 49 100 - -
SXWF 25
SXRF 25 39 92 100 -
SXWF 29
SXRF 29 39 59 100 -
SXWF 32
SXRF 30 31 65 100 -
SXWF 35
SXRF 35 30 65 100 -
SXWF 38
SXWF 42 26 43 71 100
SXRF 40 30 41 70 100
SXWF 46
SXWF 52 32 65 100 -
SXRF 50 30 65 100 -
SXWF 58
SXWF 65 24 44 71 100
SXWF 72 23 43 71 100
SXRF 60 25 50 74 100
SXWF 80
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