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the first to come on, unless locked out for a
malfunction or shut off on frost protection. See
for compressor cycling stages and
and
for percent cooling capacity by
stage.
R-410 compressors have belly band heaters that must
be energized 24 hours before starting compressor.
Power to the unit will energize the heaters. Heaters will
be energized during the off-cycle as long as the unit
has power. Failure to perform these pre-start
instructions could result in compressor damage.
The control system logic permits compressor operation
only after the supply fan is on. If the supply fan shuts
down, compressors will not operate. Units without
head pressure control (units with intermediate piping
packages) will lock out mechanical cooling when the
entering condenser water temperature falls below 54°F.
Mechanical cooling will resume when the entering
condenser water temperature exceeds 58°F.
When there are more than two compressors in an air
cooled unit, the first two compressors are manifolded
together. If there are four compressors, the second two
are manifolded.
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
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
,
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
for compressor staging.
Step Control
Steps of mechanical cooling are control based on
supply air or zone temperature. See
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
for the typical unit operating curve.
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
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