Controls - Carrier Aquasnap 30RAP010 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, only one pump will be
used at a time. The control will start the pump with the
least number of operating hours. 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 microproces-
sor controls the capacity of the chiller by cycling compres-
sors 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 compensates
for cooling range by measuring both return-fluid tempera-
ture and leaving-fluid temperature. This is referred to as
leaving-fluid temperature control with return-fluid tempera-
ture 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 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 greater
than the selected pulldown control rate. If it has been less
than 90 seconds since the last capacity change, compres-
sors will continue to run unless a safety device trips. This
prevents rapid cycling and also helps return oil during short
on periods.
Sensors — Thermistors are used for temperature-sensing
inputs to microprocessor. Additional thermistor sensors
may be used as remote temperature sensors for optional
LCWT (leaving chilled fluid temperature) reset.
• Cooler leaving chilled fluid temperature
• Cooler entering fluid (return) temperature
• Outside air temperature
• Compressor suction temperature
Two refrigerant pressure transducers are used in each
circuit for sensing suction and discharge pressure. The
microprocessor uses these inputs to control capacity, the
electronic expansion valve, and fan cycling.
• Saturated condensing temperature
• Cooler saturation temperature
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.
Hot gas bypass 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 suction pres-
sure and permitting the unit to operate at lower loads with
less compressor cycling.
On units equipped with the digital compressor option,
the control will intigrate the modulation of the digital com-
pressor into the capacity routine to match cooling load
requirements. The digital compressor will modulate in 13
steps for sizes 010 and 015, 22 steps (11 per compressor)
for sizes 020-030, and 44 steps (11 per compressor) for
sizes 035-060.
The digital scroll option provides better capacity control by
incrementally modulating capacity effectively, increasing
the number of compression stages compared to chillers
that are not equipped with this option. The digital scroll
compressor is not a variable speed device, it modulates the
capacity output by allowing the scroll sets to separate dur-
ing operation, alternating between full capacity and zero
capacity. Utilizing a fixed timeframe ratio, the percentage
of time that the scroll set is engaged is the percentage
capacity of that compressor.
There are 2 major advantages of this type of capacity con-
trol. First, there is closer capacity control operation with all
the available capacity steps compared to the on/off cycling
control of conventional scrolls. Second, there is much less
wear factor on digital scrolls compared to standard scroll
compressors because the digital scrolls are not subject to as
many of the shutdown/restart cycles as conventional
scrolls. Digital scrolls, rather than shutting off, tend to
remain on as they vary to deliver the correct capacity step.
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