Minutes Left For Start; Minutes Off Time; Loading Sequence; Lead/Lag Determination - Carrier 30GTN Service And Troubleshooting Manual

MINUTES LEFT FOR START — This value is displayed
only in the network display tables (i.e., Service Tool,
Comfortworks ) and represents the amount of time to elapse
before the unit will start its initialization routine. This value
can be zero without the machine running in many situations.
This can include being unoccupied, ENABLE/OFF/REMOTE
CONTACT switch in the OFF position, CCN not allowing
unit to start, Demand Limit in effect, no call for cooling due
to no load, and alarm or alert conditions present. If the ma-
chine should be running and none of the above are true, a
minimum off time (DELY, see below) may be in effect. The
machine should start normally once the time limit has
expired.
MINUTES OFF TIME (DELY, Configuration Mode under
OPT2) — This user configurable time period is used by the
control to determine how long unit operation is delayed after
power is applied/restored to the unit. It is also used to delay
compressor restarts after the unit has shut off its lowest stage
of capacity. Typically, this time period is configured when
multiple machines are located on a single site. For example,
this gives the user the ability to prevent all the units from
restarting at once after a power failure. A value of zero for
this variable does not mean that the unit should be running.
LOADING SEQUENCE — The 30GTN,R compressor ef-
ficiency is greatest at partial load. Therefore, the following
sequence list applies to capacity control.
1. The next compressor will be started with unloaders en-
ergized on both lead compressors.
2. All valid capacity combinations using unloaders will be
used as long as the total capacity is increasing.
LEAD/LAG DETERMINATION (LLCS, Configuration Mode
under OPT2) — This is a configurable choice and is factory
set to be automatic (for sizes 080-420) or Circuit A leading
(for 040-070 sizes). For 040-070 sizes, the value can be changed
to Automatic or Circuit B only if an accessory unloader is
added to compressor B1. For 080-420 sizes, the value can be
changed to Circuit A or Circuit B leading, as desired. Set at
automatic, the control will sum the current number of logged
circuit starts and one-quarter of the current operating hours
for each circuit. The circuit with the lowest sum is started
first. 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.
CAPACITY SEQUENCE DETERMINATION (LOAD, Con-
figuration Mode under OPT2) — This is configurable as equal
circuit loading or staged circuit loading with the default set
at equal. The control determines the order in which the steps
of capacity for each circuit are changed. This control
choice does NOT have any impact on machines with only
2 compressors.
CAPACITY CONTROL OVERRIDES — The following over-
rides will modify the normal operation of the routine.
Deadband Multiplier — The user configurable Deadband Mul-
tiplier (Z.GN, Configuration Mode under SLCT) has a de-
fault value of 1.0. The range is from 1.0 to 4.0. When set to
other than 1.0, this factor is applied to the capacity Load/
Unload Factor. The larger this value is set, the longer the
control will delay between adding or removing stages of
capacity. Figure 16 shows how compressor starts can be re-
duced over time if the leaving water temperature is allowed
to drift a larger amount above and below the set point. This
value should be set in the range of 3.0 to 4.0 for systems
with small loop volumes.
First Stage Override — If the current capacity stage is zero,
the control will modify the routine with a 1.2 factor on add-
ing the first stage to reduce cycling. This factor is also ap-
plied when the control is attempting to remove the last stage
of capacity.
Slow Change Override — The control prevents the capacity
stages from being changed when the leaving fluid tempera-
ture is close to the set point (within an adjustable deadband)
and moving towards the set point.
Ramp Loading (CRMP, Configuration Mode under SLCT)
— Limits the rate of change of leaving fluid temperature. If
the unit is in a Cooling mode and configured for Ramp Load-
ing, the control makes 2 comparisons before deciding to change
stages of capacity. The control calculates a temperature dif-
ference between the control point and leaving fluid tempera-
ture. If the difference is greater than 4° F (2.2° C) and the
rate of change (°F or °C per minute) is more than the con-
figured Cooling Ramp Loading value (CRMP), the control
does not allow any changes to the current stage of capacity.
Low Entering Fluid Temperature Unloading — When the
entering fluid temperature is below the control point, the con-
trol will attempt to remove 25% of the current stages being
used. If exactly 25% cannot be removed, the control re-
moves an amount greater than 25%, but no more than nec-
essary. The lowest stage will not be removed.
Low Cooler Suction Temperature — To avoid freezing the
cooler, the control will compare the circuit Cooler Suction
temperature (T5/T6) with a predetermined freeze point. For
water circuits, the freeze point is 34 F (1.1 C). For brine
circuits, the freeze point is 8° F (4.4° C) below the cooling
set point (lower of 2 cooling set points for dual configura-
tion). If the cooler suction temperature is below the freeze
point, the unit capacity may not be allowed to increase. If
the Cooler Suction temperature falls 24 to 29° F (13.3 to
16.1° C) below the freeze point minus 2.0 F (1.1 C) for
5 minutes, Mode 7 (Circuit A) or Mode 8 (Circuit B) is ini-
tiated and no additional lag compressor stages will be added.
If the Cooler Suction temperature falls 30° F (16.7° C) or
more below the freeze point minus 2.0 F (1.1 C) for 10 min-
utes, the circuit will shut down.
Cooler Freeze Protection — The control will try to prevent
shutting the chiller down on a Cooler Freeze Protection alarm
by removing stages of capacity. The control uses the same
freeze point logic as described in the Low Cooler Suction
Temperature section above. If the cooler leaving fluid tem-
perature is less than the freeze point plus 2.0 F (1.1 C), the
control will immediately remove one stage of capacity. This
can be repeated once every 30 seconds.
MOP (Maximum Operating Pressure) Override — The
control monitors saturated condensing and suction tempera-
ture for each circuit. Based on a maximum operating set point
(saturated suction temperature), the control may lower the
EXV position when system pressures approach the set
parameters.
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