Capacity Control; Determining The Lead Circuit; Circuit Loading Sequence; Compressor Start-Up Sequence In One Circuit - Carrier 30GX Installation, Operation And Maintenance Instructions

• By reference to a limiting signal from a user-controlled volt-
free contact: the capacity of the unit cannot exceed the
demand limit setpoint (which can be modified in the
Setpoints menu) when the limit contact is closed.
• By reference to an external 0-10 V d.c. signal: the capacity of
the unit cannot exceed the demand limit imposed by this
external signal. It is a linear function and its parameters are
configurable by Carrier Service (voltages at 0% limitation
and 100% of limitation). This function is not available if
Reset by reference to an external 0-10 V d.c. signal has
already been selected.
Whatever the method used, demand limit is active in all
operating types: Local, Remote or CCN. However, in Local
operating type, demand limit can be disabled with keypad
commands (see section 4.2.4) and in CCN operating type,
demand limit can be controlled directly with the aid of CCN
commands.
Note: A limitation value of 100% means that the unit may call
upon the full array of its capacity stages.
Here is an example of demand limit by an external 0-10 V d.c.
signal. This example assumes that the limitation parameters are
such that at 0 volt the authorised capacity shall be maximum
capacity, and at 10 volts the authorised capacity shall be zero
(this is the default configuration).
Demand limit by 0-10 V d.c. signal
NO DEMAND LIMIT
0-10 V DC DEMAND LIMIT

5.7 - Capacity control

This function adjusts the number of active compressors and
loaders to keep the leaving water temperature at its setpoint.
The precision with which this is achieved depends on the
capacity of the water loop, the flow rate, the load, and the
number of stages available on the unit.
The control system continuously takes account of the
temperature error with respect to the setpoint, as well as the
rate of change in this error and the difference between entering
and leaving water temperatures, in order to determine the
optimum moment at which to add or withdraw a capacity stage.
NOTE: If the same compressor undergoes too many starts (per
hour) this automatically brings about reduction of compressor
starts, which makes leaving water temperature control less
precise.

5.8 - Determining the lead circuit

This function commands the start/stop sequence of the two
refrigerant circuits called A and B. The circuit authorised to
start first is the lead circuit. Three methods can be configured
by the user in the Configuration menu:
• Auto mode: the control system determines the lead circuit so
as to equalise the number of starts on each circuit (value
weighted by the operating times of each circuit). Thus, the
circuit with the least number of starts is always given
precedence to start. The lead circuit is stopped last.
• Circuit A as leader: Circuit A is always the lead circuit. It is
the first to start and the last to stop.
• Circuit B as leader: Circuit B is always the lead circuit. It is
the first to start and the last to stop.

5.9 - Circuit loading sequence

Two circuit loading sequences are available. The choice of
sequence can be configured by the user in the Configuration
menu (see section 4.2.10).
• Balanced circuit loading: If this sequence is selected, the
control system tries to keep the capacity of circuits A and B
equal as the total load on the unit increases or decreases.
• Loading with priority given to one circuit: If this sequence
is selected, the control system loads the lead circuit
completely before the second circuit starts up. When there is
a demand limit, the second circuit is unloaded first.
NOTE: 30HX and 30GX units use 06N twin screw compressors.
The screw compressor efficiency is better at full load than at
part load. By default the close control configuration (tempera-
ture precision has priority over compressor efficiency - this
parameter is only accessible by Carrier Service) is not vali-
dated, and the control will always try to optimise unit efficiency.

5.10 - Compressor start-up sequence in one circuit

TOTAL DEMAND LIMIT
The first compressor to start is the one with the least number of
start-ups and operating hours. If both compressors are
operating and the load decreases, the compressor that started
first shuts down. This avoids cycling of ther same compressor.

5.11 - Controlling the EXV

EXVs control the refrigerant flow in the evaporator. Each
circuit has a liquid level detector that is mounted vertically
high in the evaporator shell. The level detector consists of a
small electric resistance heater and of three thermistors wired
in series and mounted at different levels inside the well body.
The heater is used so that the thermistors reach approximately
93°C dry air. As the refrigerant level increases in the
evaporator, the resistance of the nearest thermistor(s) changes
considerably. This difference in resistance permits precise
control of the specified level, modulating the EXVs.
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