Unit installation

Operating
SCXF-SVX01K-EN 55
Compressor Cycling
Compressors cycle to maintain the operating state
required by the temperature controls. In the event of a
compressor failure, the next available compressor turns
on. Refer to Table 35, p. 55 for compressor cycling by unit
model and tons.
During normal conditions, compressors will not shut off
until they have been on for at least three minutes and will
not turn on until they have been off for at least three
minutes. Normal operating conditions are established on
an individual compressor basis. When a compressor
starts, its timer also starts.The compressor evaporator
circuit frost protection can override the “minimum” timer
and reduce the five minute minimum required time
period.
When the unit is powered up, or manually reset there will
be a three to eight minute delay before the first
compressor may be turned on as requested by the unit
temperature control algorithm.
Compressor Lead/Lag Operation
Compressor lead/lag is a user-selectable feature at the HI
panel and is available on all units. After each request for
compressor operation, the lead refrigeration circuit or
compressor switches, thereby causing a more equitable or
balanced run time among compressors.
When lead/lag is enabled, each time the system cycles, it
will alternate between the standard compressor staging
and the lead/lag staging. Using Table 35, p. 55, a SXWF 29-
ton unit will first stage compressor B then A, then AB for
first cycle and A, then AB for the second cycle. Appropriate
condenser valves (water-cooled and condenser fans (air-
cooled) will stage with appropriate compressors to
maintain saturated condensing temperature. Enabling
lead/lag may drop a cooling stage when compared to
standard staging. See Table 35, p. 55 for compressor
staging.
Step Control
Steps of mechanical cooling are control based on supply
air or zone temperature. See Table 35, p. 55 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 temperature 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 temperature 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 approaches setpoint, the time duration
between changing levels of cooling capacity increases.
See Figure 35, p. 56 for the typical unit operating curve.
Figure 36, p. 56 shows typical unit performance when
supply air temperature swings exceed the control band
limits.
Adjust the supply air temperature control band according
to the desired unit performance. Increasing the control
band reduces the equipment cycle rate and increases the
maximum potential supply air temperature deviation from
setpoint. Conversely, decreasing the control band reduces
the maximum potential temperature deviation, but
increases the compressor cycle rate.
Follow these recommendations concerning the supply air
temperature control band settings based on expected unit
sizing:
2 Cooling stage unit: 9°F
3 Cooling stage unit: 7°F
4 Cooling stage unit: 6°F
Table 35. Compressor stage
Unit
size
Refrigerant
circuit type
Compressor by stage
Compressor
staging
Compressor
stagingABCDEF
SXWF 20, 22, 25
SXRF 20
Independent 10 10 B/AB A/AB
SXWF 29, 32
SXRF 25, 29
Independent 15 10 B/A/AB A/AB
SXWF 35, 38 Independent 10 10 10 B/BC/ABC A/AC/ABC
SXRF 30, 35 Manifolded 10 10 10 B/BC/ABC A/AC/ABC
SXWF 42, 46 Independent 15 10 10 B/A/AC/ABC C/AC/ABC
SXRF 40 Manifolded 15 10 10 B/A/AC/ABC C/AC/ABC
SXWF 52, 58 Independent 15 15 15 B/BC/ABC A/AC/ABC
SXRF 50 Manifolded 15 15 15 B/BC/ABC A/AC/ABC