169
JOHNSON CONTROLS
FORM 201.21-NM4 (616)
169
JOHNSON CONTROLS
7
NUMBER OF COMPRESSORS TO START
GENERAL
The number of compressors to start control logic varies
between the standard and optional high IPLV chillers.
Standard IPLV chiller control utilizes sequential logic
that requires the micro to start 1 compressor at a time
and only add a compressor when all running compres-
sors reach maximum speed. Optional High IPLV
chillers have control algorithms that provide “smart”
anticipatory control to determine how many
compressors need to be started to satisfy the current
load. The “smart” logic is capable of reducing short
cycling, and reducing loading time on a hot water start,
and starting all compressors at the same time.
STANDARD IPLV
The standard IPLV control always starts a single
compressor under all circumstances as the first step
of loading. The Chiller Control Board does not make
decisions on the number of compressors to start based
on chilled liquid temperatures and prior compressor
operation when starting the chiller. An additional
compressor is only started when the lead compressor
has reached maximum speed and cooling requirements
are not satisfied.
OPTIONAL OPTIMIZED HIGH IPLV
On optimized IPLV chillers, the Number of
Compressors to Start Logic will be used to determine
how many compressors should be run when the unit
starts from the all compressors stopped state. This
routine will try to run all the compressors unless it is
determined that less will be needed due to light load.
The first step in the sequence is for the micro to set the
number of compressors to start equal to the number of
compressors in the chiller. The micro will look at two
prior conditions relating to the compressor operating
time the previous time it ran and how long the last
compressor has been off along with two indicators
of chilled liquid load requirements (rate of change of
chilled liquid temperature and deviation from setpoint.).
Temperature deviation is the amount of error compared
to the setpoint high limit (Se CR). Based on this
information, the micro will then determine the number of
compressors to start. The flowchart in FIG. 36 describes
the compressor starting decision process.
It is desirable to run as many compressors as possible for
increased efficiency. Optimized logic will keep as many
compressors on line and reduce speed in an effort to
optimize the use of the entire evaporator tube surface.
NUMBER OF COMPS
TO START LOGIC
NUMBER OF COMPS TO
START
REDUCTION TABLE
4 COMPS -> 3 COMPS
3 COMPS -> 2 COMPS
2 COMPS -> 1 COMP
1 COMP -> 1 COMP
SET NUM COMPS
TO START = NUM
COMPS IN UNIT
LAST RUN TIME
< 5 MIN ?
YES
REDUCE NUM COMPS
TO START PER
REDUCTION TABLE
OFF TIME
< 5 MIN ?
YES
REDUCE NUM COMPS
TO START PER
REDUCTION TABLE
NO
CONTINUE
LCHLT RATE
< 3 °F/MIN AND
LCHLT < CR+5°F?
NO
YES
REDUCE NUM COMPS
TO START PER
REDUCTION TABLE
NO
FIG. 36 - NUMBER OF COMPRESSORS TO START
LD10626
