Ramp Loading; Rampdown; Capacity Override; High Discharge Temperature Control - Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual

To determine what caused the alarm, the operator should read
both the primary and secondary default screen messages, as well
as the alarm history. The primary message indicates the most
recent alarm condition. The secondary message gives more
detail on the alarm condition. Since there may be more than one
alarm condition, another alarm message may appear after the
first condition is cleared. Check the ALARM HISTORY screen
for additional help in determining the reasons for the alarms.
Once all existing alarms are cleared (by pressing the
softkey), the default ICVC display returns to normal operation.
The ramp loading control slows down
Ramp Loading —
the rate at which the compressor loads up. This control can pre-
vent the compressor from loading up during the short period of
time when the chiller is started and the chilled water loop has to
be brought down to CONTROL POINT. This helps reduce
electrical demand charges by slowly bringing the chilled water
to CONTROL POINT. The total power draw during this period
remains almost unchanged.
There are several methods of ramp loading with the PIC III.
Ramp loading can be based on LEAVING CHILLED WATER,
ENTERING CHILLED WATER, PERCENT LINE CURENT,
or PERCENT MOTOR KILOWATTS. PULLDOWN RAMP
TYPE is selected from the RAMP__DEM screen.
1. Temperature ramp loading (TEMP PULLDOWN DEG/
MIN) limits the degrees per minute rate at which either
LEAVING CHILLED WATER or ENTERING CHILLED
WATER temperature decreases. This rate is configured by
the operator on the TEMP_CTL screen.
NOTE: If chiller control power has been off for 3 hours or
more, the next start-up (only) will follow temperature ramp
loading using the minimum rate regardless of the ramp loading
method and rate which are configured in the screens. This is
used to maximize oil reclaim during start-up.
2. Motor load ramp loading (AMPS OR KW RAMP %/
MIN) limits the rate at which the compressor motor cur-
rent or compressor motor load increases. The AMPS OR
KW RAMP %/MIN rate is configured by the operator
on the RAMP_DEM screen in line current or motor
kilowatts.
If kilowatts is selected for the DEMAND LIMIT SOURCE,
the MOTOR RATED LOAD KILOWATTS must be entered in
the VFD_CONF screen.
The TEMP PULLDOWN DEG/MIN may be viewed or
modified on the TEMP_CTL screen which is accessed from
the EQUIPMENT SERVICE screen. PULLDOWN RAMP
TYPE, DEMAND LIMIT SOURCE, and AMPS OR KW RAMP
%/MIN may be viewed or modified on RAMP_DEM screen.
The Rampdown control applies to VFD
Rampdown —
equipped chillers. At machine start-up the VFD speed is set to
"Start Speed" which is the lower of VFD MAXIMUM SPEED
or the configured value of VFD START SPEED. VFD START
SPEED is configured on the SETUP2 screen. After Ramp
Loading is complete the control begins to reduce the VFD
speed. Speed reduction continues until the leaving water tem-
perature is within the control point deadband and any condition
is reached that would require the VFD speed to hold or in-
crease. The chiller is then out of Rampdown and running in
normal capacity control.
Capacity overrides can prevent
Capacity Override —
some safety shutdowns caused by exceeding the motor amper-
age limit, low evaporator temperature safety limit, high motor
temperature safety limit, and high condenser pressure limit. In
all cases, there are two stages of compressor capacity control
applied by guide vane operation:
1. When the value of interest crosses the First Stage Set
Point into the Override Region, the guide vanes are
prevented from opening further, and the status line on the
ICVC indicates the reason for the override. Normal
capacity control operation is restored when the value
crosses back over the First Stage Set point, leaving the
Override Region. See Table 7.
2. When the value of interest is in the Override Region and
further crosses the Second Stage Set Point, the guide
vanes are closed until the value meets the Override
Termination Condition. The PIC III controls resume
RESET
normal capacity control operation after the override
termination condition has been satisfied. (In the case of
high discharge superheat, there is an intermediate stage.)
Whenever the motor current demand limit set point
(ACTIVE DEMAND LIMIT) is reached, it activates a capacity
override, again, with a 2-step process. Exceeding 110% of the
rated load amps for more than 30 seconds will initiate a safety
shutdown.
The high compressor lift (surge prevention) set point will
cause a capacity override as well. When the surge prevention
set point is reached, the controller normally will only prevent
the guide vanes from opening. If so equipped, the hot gas
bypass valve will open instead of holding the vanes. The hot
gas bypass will only open if the compressor is at 100% speed.
See the Surge Prevention Algorithm section, page 54.
High Discharge Temperature Control —
COMP DISCHARGE TEMP increases above 160 F (71.1 C),
the guide vanes are proportionally opened to increase gas flow
through the compressor. If the LEAVING CHILLED WATER
temperature decreases 5 F (2.8 C) below the control set point
temperature, as a result of opening the guide vanes, the PIC III
will bring the chiller into the recycle mode.
Compressor Bearing Temperature —
bearing temperature (MTRB) is a calculated value which is the
temperature of the oil leaving the bearing plus an internal incre-
ment calculated with the THRUST BRG RESET FACTOR. The
Comp Thrust Bearing Reset is calculated using the rate of
change of the oil leaving the bearing. If the oil temperature is
stable the COMP THRUST BRG TEMP equals the COMP
THRUST LVG OIL TEMP. As the oil temperature rises, the
COMP THRUST BRG RESET is calculated to account for any
lag between actual bearing temperature and the temperature of
the leaving oil.
In the SETUP1 screen an adjustment called the THRUST
BRG RESET FACTOR is an adjustment to the COMP THRUST
BRG TEMP calculation. The default value is 1.4 and will nor-
mally be left at that value unless other wise advised by Carrier
Service Engineering.
Oil Sump Temperature and Pump Con-
The oil sump temperature is regulated by the PIC III,
trol —
with the oil heater relay when the chiller is shut down.
As part of the pre-start checks executed by the controls, the
oil sump temperature (OIL SUMP TEMP) is compared to
the cooler refrigerant temperature (EVAPORATOR REFRIG
TEMP) if the OIL SUMP TEMP is less than 150 F (65.6). If the
difference between these 2 temperatures is 50 F (27.8 C) or
less, the start-up will be delayed until either of these conditions
is no longer true. Once this temperature criteria is satisfied, the
start-up continues.
The oil heater relay is energized whenever the chiller com-
pressor is off and the oil sump temperature is less than 140 F
(60.0 C) or the OIL SUMP TEMP is less than the EVAP
REFRIG TEMP plus 53 F (29.4 C). The oil heater is turned off
when the OIL SUMP TEMP is either:
• more than 152 F (66.7 C), or
• more than 142 F (61.1 C) and more than the EVAP REFRIG
TEMP plus 55 F (30.6 C).
The oil heater is always off during start-up or when the
compressor is running.
50
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