Demand Limit - Carrier AquaEdge 19XR Controls Operation And Troubleshooting

DEMAND LIMIT SOURCE exceeds the ACTIVE DEMAND
LIMIT by 5% or less, capacity will be inhibited. If the DE-
MAND LIMIT SOURCE exceeds the ACTIVE DEMAND
LIMIT by more than 5%, capacity will be decreased.
Alternatively, the limit may be applied by AUTO DE-
MAND LIMIT INPUT, an optional 4 to 20 mA input. This de-
mand limit control option (4 to 20 mA DEMAND LIMIT
TYPE) is externally controlled by a 4 to 20 mA signal. The op-
tion is set up on the Configuration Menu  GENERAL CON-
FIGURATION screen. When enabled, 4 mA will set ACTIVE
DEMAND LIMIT to 100% of the DEMAND LIMIT
SOURCE (regardless of the value of BASE DEMAND LIM-
IT), and 20 mA will set ACTIVE DEMAND LIMIT to the val-
ue configured for DEMAND LIMIT AT 20 mA in the Config-
uration Menu  SERVICE PARAMETERS screen.
OVERRIDE CONTROL — Capacity overrides can prevent
some safety shutdowns caused by exceeding the motor am-
perage limit, evaporator refrigerant low temperature safety
limit, motor high temperature safety limit, and condenser
high pressure limit. In these cases there are two stages of ca-
pacity control:
1. When the value of interest crosses the first stage set point
into the override region, the capacity is prevented from
increasing further, and the status line on the PIC 5 con-
troller indicates the reason for the override. Normal ca-
pacity control operation is restored when the value cross-
es back over the first stage set point, leaving the override
region.
2. When the value of interest is in the override region and
further crosses the second stage set point, the capacity is
decreased until the value meets the override termination
condition. The PIC 5 controls resume normal capacity
control operation after the override termination condition
has been satisfied. (In the case of high discharge super-
heat, there is an intermediate stage.)
Table 15 summarizes these override parameters.
Other types of override events do not override control guide
vane or VFD operation, but are reported:
• High compressor discharge temperature override — For
compressor/frame 6 and 7, if the COMP DISCHARGE
TEMP is greater than the COMP DISCHARGE ALERT
threshold, then high discharge temperature override will
be displayed in the main screen until the COMP DIS-
CHARGE TEMP is less than the COMP DISCHARGE
ALERT threshold – 2° F (1.1° C).
• High compressor bearing temperature override — For
compressor/frame 6 and 7, if one of the compressor bear-
ing temperatures is greater than the compressor bearing
temperature Alert (Configuration Menu  Protective
Limit Config) threshold, then High Bearing Temp Over-
ride shall be active until all of the compressor bearing
temperatures are less than Comp Bearing Temp Alert
minus 2° F (1.1° C). For compressors 2-6, D, and E, if
the calculated compressor bearing temperature is greater
than the bearing temperature alert threshold, then high
bearing temperature override will be active until the cal-
culated bearing temperature is less than the threshold
minus 2° F (1.1° C).
• Low Discharge Superheat Temperature Override — This
override is ignored during the first 5 minutes after chiller
start-up. There are additional requirements for normal
override function after start-up. For compressor E, con-
denser pressure must be 10 psi greater than cooler pres-
sure; for compressor/frame 6 and 7, the damper must
have been open for 20 consecutive seconds.
RECYCLE CONTROL — The chiller may cycle off and
wait until the load increases to restart when the compressor
is running in a lightly loaded condition. This normal cycling
is known as "recycle."
In cooling mode, a recycle shutdown is initiated when either
of the following conditions is true:
• Leaving chilled water temperature (or entering chilled
water temperature, if the EWT CONTROL OPTION is
enabled) is more than 5° F (2.8° C) below the CON-
TROL POINT.
• Leaving chilled water temperature (or entering chilled
water temperature, if the EWT CONTROL OPTION is
enabled) is below the CONTROL POINT, and the chilled
water temperature difference is less than the RECYCLE
SHUTDOWN DELTA T.
In heating mode, a recycle cycle shutdown occurs when ei-
ther of the following conditions is true:
• Leaving condenser water temperature (or entering con-
denser water temperature, if the EWT CONTROL
OPTION is enabled) is more than 5° F (2.8° C) below the
CONTROL POINT.
• Leaving condenser water temperature (or entering con-
denser water temperature, if the EWT CONTROL
OPTION is enabled) is above the CONTROL POINT,
and the condenser water temperature difference is less
than the RECYCLE SHUTDOWN DELTA T.
NOTE: Recycle shutdown will not occur if the CONTROL
POINT has been changed by more than 1° F (0.56° C) within
the previous 5 minutes of operation.
When the chiller is in RECYCLE mode, the chilled water
pump relay remains energized so the chilled water temperature
can be monitored for increasing load. The recycle control uses
RECYCLE RESTART DELTA T to check when the compres-
sor should be restarted. In cooling mode, the compressor will
restart when the leaving chilled water temperature (or entering
chilled water temperature, if the EWT CONTROL OPTION is
enabled) is greater than the CONTROL POINT plus the RE-
CYCLE RESTART DELTA T for 5 consecutive seconds. In
heating mode, the compressor will restart when the leaving
condenser water temperature (or entering condenser water tem-
perature, if the EWT CONTROL OPTION is enabled) is less
than the CONTROL POINT minus the RECYCLE RESTART
DELTA T for 5 consecutive seconds.
RUNNING TIMERS AND COUNTERS — The PIC 5 con-
trol maintains two run-time clocks: COMPRESSOR ONTIME
and SERVICE ONTIME. COMPRESSOR ONTIME indicates
the total lifetime compressor run hours. SERVICE ONTIME is
a resettable timer that can be used to indicate the hours since
the last service visit or any other event. A separate counter tal-
lies compressor starts as TOTAL COMPRESSOR STARTS.
All of these can be viewed on the RUN TIMES screen. Both
Ontime counters roll over to 0 at 500,000 hours. Manual
changes to SERVICE ONTIME from the screen are permitted
at any time. If the controller is replaced, one opportunity before
the first start-up with the new controller is provided to set
COMPRESSOR ONTIME and TOTAL COMPRESSOR
STARTS to the last readings retained with the prior controller.
The chiller also maintains a start-to-start timer and a stop-
to-start timer. These timers limit how soon the chiller can be
started and are displayed on the system overview (home) and
RUN TIMES screens. They can be configured in the Config-
uration Menu  GENERAL CONFIGURATION screen.
They must expire before the chiller starts. If the timers have
not expired, the RUN STATUS parameter on the System
Overview (Home) and GENERAL PARAMETERS screen
reads TIMEOUT.
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