Compressor Staging; Lead/Lag Determination (38Apd Units); Loading Sequence Select; Circuit Compressor Staging - Carrier 38APS025 Manual

COMPRESSOR STAGING — Several factors determine
which circuit and compressor to start:
Lead/Lag Determination (38APD Units) — Lead/Lag deter-
mination is a configurable choice and is factory set to be auto-
matic, Configuration OPT2 LLCS=1 for all units, and
applies specifically to the 38APD units.
When the 38APD unit is equipped with a digital scroll com-
pressor and enabled (Configuration UNIT A1.TY=YES),
or minimum load is installed and enabled (Configuration
OPT1 MLV.S=YES), then circuit A is always the lead circuit
regardless of the LLCS value. For 38APD units without digital
scroll compressor (A1.TY=NO), or minimum load is not in-
stalled or enabled (MLV.S=NO), the value can be changed to
Circuit A leads (LLCS=2) or Circuit B leads (LLCS=3) as de-
sired. Set at automatic LLCS=1, the control will sum the cur-
rent number of logged circuit starts and one-quarter of the cur-
rent operating hours for each circuit. The circuit with the low-
est sum is started first.
Circuit Wear Factor = Circuit Starts + (Circuit Run Hours/4)
Changes to which circuit is the lead circuit and which is the lag
are also made when total machine capacity is at 100% or when
there is a change in the direction of capacity (increase or
decrease) and each circuit's capacity is equal.
Lead/Lag determination will obey the Time Guard function.
If a compressor is unavailable due to Time Guard, it will be
skipped in the selection process. For additional information on
Time Guard, see MDTG - Time Guard Active on page 135.
Loading Sequence Select — This feature is configurable as
equal circuit loading or staged circuit loading with the default
set at equal (Configuration OPT2 LOAD=1). The control
determines the order in which the steps of capacity for each
circuit are changed. Set to equal, the unit will alternate starting
compressors in each circuit as the requirement increases. Set to
staged (Configuration OPT2 LOAD=2), one circuit will
load completely before the second circuit is started. This con-
trol choice does NOT have any impact on machines with only
one circuit or units with 2 compressors, one in each circuit.
Circuit Compressor Staging — The control has an automatic
lead-lag feature built in which determines the wear factor for
each compressor. Wear factor is calculated for each compres-
sor as 6 times the number of starts plus the number of run
hours.
Compressor Wear Factor = 6 x (Compressor Starts +
Compressor Run Hours)
If all compressors are off and less than 30 minutes have
elapsed since the last compressor was turned off, the wear fac-
tor is used to determine which compressor to start next. As ad-
ditional stages of compression are required, the unit control
will add them after the Time Guard timer has expired, if appli-
cable. Time Guard may change the loading sequence. If a cir-
cuit is to be stopped, the compressor with the lowest wear fac-
tor will be shut off first after its Time Guard timer has expired,
if applicable. For additional information on Time Guard,
MDTG - Time Guard Active on page 135.
Tables 39 and 40 are examples that depict the capacity stag-
ing for equal circuit loading and staged circuit loading, without
digital compressor. These are two of many possible staging se-
quences and are only meant to provide examples of staging
sequences.
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