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5.8 - Demand limit

Generally, demand limit is used by an energy management

system in order to restrict the unit electricity consumption.

The PRO-DIALOG Plus control system for 30GX & 30HX

provides two methods of demand limit:

• 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 para-

meters are configurable in the User1 menu (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 4.3.3) 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

0-10 V dc demand limit

No demand limit

Total demand limit

Maximum permitted capacity

5.9 - Limiting the unit running current

This feature is used to avoid tripping the circuit breaker by:

• prohibiting an increase in compressor capacity when the

current reaches a first threshold.

• unloading one or more capacity stages when a second

protection threshold is reached.

See chapter: Description of the user 1 configuration sub-menu.

5.10 - 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 tempera-

ture 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.

5.11 - 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.

5.12 - 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 4.5.7.3).

• 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 compres-

sors. The screw compressor efficiency is better at full load

than at part load. By default the 'close control'* configuration

is not validated, and the control will always try to optimise

unit efficiency.

temperature precision has priority over compressor capacity. This parameter

is only accessible to Carrier Service.

5.13 - Compressor start-up sequence in one circuit

The first compressor to start is the one with the least number of

start-ups and operating hours. If both compressors are operat-

ing and the load decreases, the compressor that started first

shuts down. This avoids cycling of ther same compressor.

5.14 - EXV control

The electronic expansion valves (EXVs) control charging and the

refrigerant flow in the evaporator, maintaining the optimum

evaporator throttle and a correct discharge superheat. Opening the

valve permits reduction of the throttle, and thus improves the

heat exchange in the evaporator. This opening can be limited to

maintain the condenser subcooling and correct superheat,

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