Carrier 30GK Demand limit by 0-10 V d.c. signal, Capacity control, Circuit loading sequence

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

		No demand limit
Maximum permitted	capacity	Total demand limit
		0-10 V dc demand limit

5.9 - Capacity control

This function activates the compressors and capacity unloaders to keep the entering or 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.

5.10 - 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. This function is only available in two-circuit units.

NOTE: If one circuit has more unloaders than the other, this circuit is always the lead circuit, independent of the configuration and the operating hours of the two circuits.

•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. This choice is only available in two-circuit units.

5.11 - 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). This function is only available in two-circuit units.

•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: If the unit is air-cooled and if the saturated condensing temperature of one of the two circuits is lower than 0°C at the start-up of one circuit, the priority capacity loading sequence of the circuits is used by the control, independent of the configuration.

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