Carrier 30GB060 - Manual (Page 13)

Control sequence
Off cycle — During unit off cycle, crankcase and, if
ambient temperature is below 36 F (2 C), cooler and
control box heaters are energized. Electronic expansion
valves are closed.
Start-up — After control circuit ON/OFF circuit breaker
switched ON, prestart process takes place for 2 minutes,
when microprocessor checks itself and waits for tem­
perature to stabilize. First circuit to start may be no. 1 or 2,
(automatic lead/lag feature). The controlled pull down
feature limits compressor loading on start-up to reduce
demand on start-up and unnecessary compressor usage.
The microprocessor limits supply water temperature
decrease (start-up only) to 1°F (0.6 C) per minute.
Capacity control — On first call for cooling, micro­
processor starts initial compressor and fan stage on lead
circuit. The electronic expansion valve remains closed
for 10 seconds, permitting a pumpout on start-up. After
pumpout, the valves open and, if necessary, additional
outdoor fans are energized. Crankcase heaters are de­
energized when a compressor is started. As additional
cooling is required, lag circuit starts. If further cooling is
needed, compressors are added, alternating between
lead and lag circuits. Speed at which capacity is added or
decreased is controlled by temperature deviation from
set point and rate of temperature change of chilled water.
As less cooling is required, circuits shut down (or
unload) in an order that evens out each circuit’s com­
pressor run time. When no further cooling is called for
On each compressor circuit), expansion valve closes and
compressor and fans continue to run while pumping
down cooler.
Low-temperature override feature prevents LCWT
from overshooting the set point and possibly causing a
nuisance trip-out by the freeze protection.
High-temperature override feature allows chiller to
add capacity quickly during rapid load variations.
Demand limit — If applied, unit step controls limit
total power draw of unit to selected point by controlling
number of operational compressors during periods of
peak electrical demand or time clock shutdown. Consult
Accessory Demand Limit Control Module Installation
Instructions for further details.
Reset accessory — If applied, microprocessor compares
either return water, space or outdoor temperature with
the accessory board settings, and adjusts leaving chilled
water temperature appropriately.
Electronic expansion valve and outdoor fan control —
Expansion valve opens and closes on signal from micro­
processor to maintain an approximate 20 F (11C)
refrigerant superheat entering the compressor cylinders.
(The compressor motor increases the refrigerant super­
heat from the approximate 5 F [3 C] leaving the cooler to
that entering the cylinders.) Outdoor fans (operated by
microprocessor) run to as low an ambient as possible to
maintain a minimum expansion valve pressure differential.
Abnormal conditions (alarm light) — All control safeties
in chiller operate through compressor protection board
or control relay and microprocessor. High-pressure switch
and discharge gas thermostat directly shuts down
compressor(s) through compressor protection board or
control relay. For other safeties, microprocessor (1) makes
appropriate decision to shut down a compressor due to
a safety trip or bad sensor reading (2) energizes alarm light
and (3) displays appropriate failure code on the display.
Chiller holds in safety mode until reset. It then reverts to
normal control when unit is reset.
Safeties include: Oil-pressure switch cuts out if pressure
differential is below minimum. Switch is bypassed on
start-up for 45 seconds. Loss-of-charge switch cuts out if
system pressure drops below minimum. High-pressure
switch cuts out compressors if compressor discharge
pressure increases to 395 psig (2724 kPa). Discharge gas
thermostat opens when discharge gas temperature
exceeds maximum 295 F (146 C). Ground current safety
opens on sensing a current-to-ground in compressor
windings in excess of 2.5 amps. Loss of flow protection
provided by temperature differences between entering
and leaving water temperature sensors. Flow switch is
not required. Freeze-up protection provided by leaving
water temperature sensor if cooler temperature drops
to 35 F (1.8 C). Sensor failures are detected by the
microprocessor.
Diagnostics — Microprocessor may be put through
Quick Test (see Controls and Troubleshooting book)
without additional equipment or tools. Quick Test
confirms microprocessor is functional, informs observer
through LED display the condition of each sensor and
switch in chiller, and allows observer to check for proper
operation of fans and compressor(s).
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