54
SCXF-SVX01T-EN
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 27, p. 54, 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 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 27, p. 54
for compressor staging.
Step Control
Steps of mechanical cooling are control based on supply
air or zone temperature. See Table 27, p. 54 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 31, p. 55 for the typical unit operating curve.
Figure 32, p. 55 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 27. Compressor stages
Unit Size
Refrigerant
Circuit Type
Compressor by Stage
Compressor Staging Compressor Staging
A B C D E F
SXWF 20, 22, 25
Independent
10 10 B/AB A/AB
SXWF 29, 32
Independent
15 10 B/A/AB A/AB
SXWF 35, 38
Independent
10 10 10 B/BC/ABC A/AC/ABC
SXWF 42, 46
Independent
15 10 10 B/A/AC/ABC C/AC/ABC
SXWF 52, 58
Independent
15 15 15 B/BC/ABC A/AC/ABC
SXWF 65, 72
Independent
15 15 15 10 B/BD/ABD/ABCD A/AD/ACD/ABCD
SXWF 80
Independent
15 15 15 15 B/BD/ABD/ABCD A/AC/ABC/ABCD
SCWF 90
Independent
15 15 15 15 15 A/AB/ABC/ABCDE C/CDE/ACDE/BCDEF
SCWF 100
Independent
15 15 15 15 10 10 A/AB/ABCD/ABCDEF CD/CDEF/ACDEF/ABCDEF
SCWF C1
Independent
15 15 15 15 15 15 A/AB/ABCD/ABCDEF CD/CDEF/ACDEF/ABCDEF
Operating Principles
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