Controls - Carrier AQUASNAP 30RA010-055 Product Data

Controls

Microprocessor — The ComfortLink™ microprocessor
controls overall unit operation. Its central executive routine
controls a number of processes simultaneously. These
include internal timers, reading inputs, analog to digital
conversions, fan control, display control, diagnostic con-
trol, output relay control, demand limit, capacity control,
head pressure control, and temperature reset. Some pro-
cesses are updated almost continuously, others every 2 to
3 seconds, and some every 30 seconds. The microproces-
sor routine is started by switching the Emergency ON-OFF
switch to ON position. Pump control of external pumps
(where so configured) or optional internal pump, will
energize the cooler pump to the internal (or CCN) time
schedule (or input occupied signal from external system).
Where dual pumps are utilized, control will also stagger
start of pumps based on last started. 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
cycling compressors at a rate to satisfy actual dynamic load
conditions. The control maintains leaving-fluid temperature
set point shown on the scrolling marquee display board
through intelligent cycling of compressors. Accuracy
depends on loop volume, loop flow rate, load, outdoor-air
temperature, number of stages, and particular stage being
cycled off. No adjustment for cooling range or cooler flow
rate is required, because the control automatically compen-
sates for cooling range by measuring both return-fluid tem-
perature and leaving-fluid temperature. This is referred to
as leaving-fluid temperature control with return-fluid tem-
perature compensation.
The basic logic for determining when to add or remove a
stage 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 set point and slowly
moving closer, logic prevents addition of another stage. If
leaving-fluid temperature is less than 34 F (1.1 C) for
water, or 6° F (3.3° C) below the set point for brine units,
the unit is shut off until the fluid temperature goes to 34 F
(1.1 C) or to 6° F (3.3° C) above the set point to protect
against freezing.
If 1° F per minute (0.6° C per minute) pulldown control
has been selected (adjustable setting), no additional steps of
capacity are added as long as difference between leaving-
fluid temperature and set point is greater than 4° F (2.2° C)
and rate of change in leaving-fluid temperature is less than
1° F per minute (0.6° C per minute). 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
on periods.
Sensors — Three thermistors are used for temperature-
sensing inputs to microprocessor. Additional thermistor
sensors may be used as remote temperature sensors for
optional LCWT reset.
• Cooler leaving chilled fluid temperature (T1)
• Cooler entering fluid (return) temperature (T2)
• Outside air temperature (T9)
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
The microprocessor uses these temperatures and pres-
sures to control capacity and fan cycling.
Control sequence
Off cycle — If ambient temperature is below 36 F (2 C),
cooler heaters (if equipped) 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. The microprocessor limits
supply-fluid temperature decrease (start-up only) to 1° F
(0.6° C) per minute.
Capacity control — On first call for cooling, micropro-
cessor starts initial compressor and fan stage on lead
circuit.
As additional cooling is required, additional compressors
are energized.
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 temperature of
supply chilled water to cycle the compressor(s) and to con-
trol compressor unloading and loading to match cooling
load requirements.
Minimum load control valve is energized by the MBB.
Valve allows hot gas to pass directly into the cooler circuit
on the final step of unloading, maintaining constant suc-
tion pressure and permitting the unit to operate at lower
loads with less compressor cycling.
CAPACITY CONTROL STEPS
UNIT
30RA
010
015
018
022
025
030
035
040
045
050
055
STANDARD
CAPACITY STEPS (%)
0,100
0,100
0, 50, 100
0, 41, 100
0, 50, 100
0, 50, 100
0, 25, 60, 100
0, 32, 63, 100
0, 22, 44, 72, 100
0, 25, 50, 75, 100
0, 25, 50, 75, 100
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