Controls; Microprocessor; Thermistors; Electronic Expansion Valve (Exv) - Carrier 30GTN 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 control, output relay control,
demand limit, capacity control, head pressure control, and
temperature 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 circuit breaker switch (switch 2) to ON
position.
When the unit receives a call for cooling (either from the
internal control or CCN network command), the unit stages
up in capacity to maintain the cooler fluid set point. The first
compressor starts 11/2 to 3 minutes after the call for cooling.
The lead circuit can be specifically designated or randomly
selected by the controls, depending on how the unit is field
configured. A field configuration is also available to
determine if the unit should stage up both circuits equally or
load one circuit completely before bringing on the other.
The ComfortLink microprocessor controls the capacity of
the chiller by cycling compressors on and off at a rate to
satisfy actual dynamic load conditions. The control maintains
leaving-fluid temperature set point shown on 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 temperature and leaving-fluid temperature.
This is referred to as leaving-fluid temperature control with
return-fluid temperature 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 (factory 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.
Lead/lag operation can be configured to balance
compressor operating hours when set to automatic. When
lead/lag operation is configured to automatic, a compressor
wear factor is used to determine which circuit to start first by
utilizing a combination of actual run hours and number
starts. Lag compressors in a circuit would also be started to
maintain even wear factors. Either circuit can be set to
always lead, if desired.
The control also performs other special functions when
turning on or off. When a circuit is to be turned off, EXV
is closed first, and compressor is run until conditions are
met to terminate pumpout to remove refrigerant that was in
the cooler. At start-up, if a circuit has not run in the last 15
minutes, circuit is run to remove any refrigerant that has
migrated to the cooler. The oil pressure switch is bypassed
for 2 minutes during start-up and for 1 minute during
normal operation.

Thermistors

Eight thermistors are used for temperature-sensing inputs
to microprocessor. (A ninth [T9] and/or tenth [T10] may
be used as a remote temperature sensor for optional LCWT
reset.)
T1 Cooler leaving chilled fluid temperature
T2 Cooler entering fluid (return) temperature
T3 Saturated condensing temperature — Circuit A
T4 Saturated condensing temperature — Circuit B
T5 Cooler saturation temperature — Circuit A
T6 Cooler saturation temperature — Circuit B
T7 Return gas temperature entering compressor
cylinder — Circuit A
T8 Return gas temperature entering compressor
cylinder — Circuit B
T9 Outdoor air temperature sensor (accessory)
T10 Remote space temperature sensor (accessory)
The microprocessor uses these temperatures to control
capacity, fan cycling, and EXV operation.

Electronic expansion valve (EXV)

To control flow of refrigerant for different operating
conditions, EXV piston moves up and down over slot
orifices through which refrigerant flows to modulate size
of opening. Piston is moved by a stepper motor through
1500 discrete steps. The piston is repositioned by the
microprocessor every 3 seconds as required.
The EXV is used to control superheat in compressor.
The difference between 2 thermistors (compressor return
gas temperature minus cooler saturation temperature) is
used to determine superheat. The EXV is controlled to
maintain superheat entering pistons at approximately 29 F
(16.1 C), which results in slightly superheated refrigerant
leaving cooler.
The electronic control provides for a prepurge and
pumpout cycle each time the lead compressor in a circuit is
started or stopped. These pumpout cycles minimize
amount of excess refrigerant that can go to compressor on
start-up and cause oil dilution (which would result in
eventual bearing wear).
The microprocessor software is programmed so that
EXV functions as an MOP (maximum operating pressure)
valve, limiting the suction temperatures to 55 F (12.8 C).
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