Controls - Carrier AQUAFORCE 30XA080-500 Product Data

Controls

Microprocessor
The ComfortLink™ microprocessor controls overall unit
operation and controls a number of processes simulta-
neously. These processes include internal timers, reading
inputs, analog to digital conversions, fan control, display
control, diagnostic control, output relay control, demand
limit, capacity control, head pressure control, and tempera-
ture reset. Some processes are updated almost continuously,
others every 2 to 3 seconds, and some every 30 seconds.
The microprocessor routine is started by switching the
Emergency ON-OFF switch to ON position. Pump control
of external pumps (where configured) will energize the
cooler pump to the internal (or CCN) time schedule (or input
occupied signal from external system).
When the unit receives a call for cooling (based on a
deviation from chilled water set point), the unit stages up in
capacity to maintain the cooler fluid set point. The first
compressor starts 1 to 3 minutes after the call for cooling.
The ComfortLink microprocessor controls the capacity of
the chiller by varying the number of compressors on and
each loading capacity to satisfy actual dynamic load condi-
tions. The control maintains leaving-fluid temperature set
point shown on the Navigator™ device through intelligent
positioning of the slide valve and compressor cycling.
Accuracy depends on loop volume, loop flow rate, load,
and outdoor-air temperature. No adjustment for cooling
range or cooler flow rate is required, because the control
automatically compensates for cooling range by measuring
both return-fluid temperature and leaving-fluid tempera-
ture. This is referred to as leaving-fluid temperature control
with return-fluid temperature compensation.
The basic logic for determining when to add or remove
capacity is a time band integration of deviation from set
point plus rate of change of leaving-fluid temperature.
When leaving-fluid temperature is close to the set point
and slowly moving closer, logic prevents additional capac-
ity. If leaving-fluid temperature is less than 34 F (1.1 C) for
water, or 6° F (3.3° C) below the brine freeze set point for
brine units, the unit is shut off until the water temperature
for brine reaches 34 F (1.1 C) or to 6° F (3.3° C) above the
set point for brine to protect against freezing.
If pulldown control has been selected (adjustable setting),
no additional capacity is added as long as the difference
between leaving-fluid temperature and the set point is
greater than 4° F (2.2° C) and rate of change in leaving-
water temperature is greater than the adjustable setting. If
it has been less than 90 seconds since the last capacity
change, compressors will continue to run unless a safety
device trips. This prevents rapid cycling and also helps
return oil during short operating periods.
Control sequence
Off cycle — If ambient temperature is below 36 F (2.2 C),
cooler heaters are also energized.
Start-up — After control circuit switches on, the prestart
process takes place, then microprocessor checks itself,
starts pump (if configured) and waits for temperature to
stabilize. The controlled pulldown feature limits compres-
sor loading on start-up to reduce demand on start-up and
unnecessary compressor usage.
Capacity control — On the first call for cooling, the
microprocessor starts initial compressor and fan stage on
lead circuit.
As additional cooling is required, the capacity of the
compressor is increased by changing the position of the
slide valve. As the load increases above the compressor's
capacity, another compressor is started and both staged
together.
The speed at which capacity is added or reduced is con-
trolled by temperature deviation from set point and rate of
temperature change of chilled fluid.
The Main Base Board (MBB) responds to the supply
chilled water temperature to cycle the compressors to
match cooling load requirements.
The Minimum Load control valve is energized by the
MBB. The valve allows hot gas to pass directly into the
cooler circuit on the initial step of unloading, permitting
the unit to operate at lower loads with less compressor
cycling.
Sensors
Thermistors are used to control temperature-sensing inputs
to the microprocessor. No additional thermistor sensors are
required for optional leaving chilled water temperature,
return water, or outdoor air reset.
The following sensors can be used on 30XA units:
• Cooler leaving chilled fluid temperature (T1)
• Cooler entering fluid (return) temperature (T2)
• Outside-air temperature (T9)
• Space Temperature (T10)
Two refrigerant pressure transducers are used in each
circuit for sensing suction and discharge pressure. The
microprocessor uses these inputs to control capacity and
fan cycling.
• Saturated condensing temperature
• Cooler saturation temperature
• Oil
• Economizer
Additional information
Detailed information on controls and operation is available
in the Controls, Start-Up, Operation, Service, and Trouble-
shooting guide included with each unit. Packaged Service
Training programs are also available. Contact a local
Carrier representative for more information.
Dynamic ComfortLink controls keep the chiller on
line during periods of extreme operating conditions. If the
entering fluid temperature is 95 F (35 C) and the saturated
suction temperature is 50 F (10 C) or higher the maximum
operating pressure (MOP) feature limits the suction to keep
the chiller online. The control automatically starts the
chiller in the unloaded state to eliminate the potential of
compressor overload due to high head pressure or low suc-
tion pressure. The controller will equalize run time on each
circuit through the lead/lag feature. If a circuit becomes
disabled, the control will automatically set the active circuit
to lead, keeping the chiller online at a reduced capacity.
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