Carrier 30GX Installation, Operation And Maintenance Instructions

Also see for 30GX Series: Installation, operation and maintenance instructions Installation, operation and maintenance instructions

29• By reference to a limiting signal from a user-controlled volt-free contact: the capacity of the unit cannot exceed thedemand limit setpoint (which can be modified in theSetpoints menu) when the limit contact is closed.• By reference to an external 0-10 V d.c. signal: the capacity ofthe unit cannot exceed the demand limit imposed by thisexternal signal. It is a linear function and its parameters areconfigurable by Carrier Service (voltages at 0% limitationand 100% of limitation). This function is not available ifReset by reference to an external 0-10 V d.c. signal hasalready been selected.Whatever the method used, demand limit is active in alloperating types: Local, Remote or CCN. However, in Localoperating type, demand limit can be disabled with keypadcommands (see section 4.2.4) and in CCN operating type,demand limit can be controlled directly with the aid of CCNcommands.Note: A limitation value of 100% means that the unit may callupon 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 aresuch that at 0 volt the authorised capacity shall be maximumcapacity, and at 10 volts the authorised capacity shall be zero(this is the default configuration).Demand limit by 0-10 V d.c. signal0-10 V DC DEMAND LIMITNO DEMAND LIMITTOTAL DEMAND LIMITMAXIMUM PERMITTED CAPACITY5.7 - Capacity controlThis function adjusts the number of active compressors andloaders to keep the leaving water temperature at its setpoint.The precision with which this is achieved depends on thecapacity of the water loop, the flow rate, the load, and thenumber of stages available on the unit.The control system continuously takes account of thetemperature error with respect to the setpoint, as well as therate of change in this error and the difference between enteringand leaving water temperatures, in order to determine theoptimum moment at which to add or withdraw a capacity stage.NOTE: If the same compressor undergoes too many starts (perhour) this automatically brings about reduction of compressorstarts, which makes leaving water temperature control lessprecise.5.8 - Determining the lead circuitThis function commands the start/stop sequence of the tworefrigerant circuits called A and B. The circuit authorised tostart first is the lead circuit. Three methods can be configuredby the user in the Configuration menu:• Auto mode: the control system determines the lead circuit soas to equalise the number of starts on each circuit (valueweighted by the operating times of each circuit). Thus, thecircuit with the least number of starts is always givenprecedence to start. The lead circuit is stopped last.• Circuit A as leader: Circuit A is always the lead circuit. It isthe first to start and the last to stop.• Circuit B as leader: Circuit B is always the lead circuit. It isthe first to start and the last to stop.5.9 - Circuit loading sequenceTwo circuit loading sequences are available. The choice ofsequence can be configured by the user in the Configurationmenu (see section 4.2.10).• Balanced circuit loading: If this sequence is selected, thecontrol system tries to keep the capacity of circuits A and Bequal as the total load on the unit increases or decreases.• Loading with priority given to one circuit: If this sequenceis selected, the control system loads the lead circuitcompletely before the second circuit starts up. When there isa 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 atpart load. By default the close control configuration (tempera-ture precision has priority over compressor efficiency - thisparameter 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 circuitThe first compressor to start is the one with the least number ofstart-ups and operating hours. If both compressors areoperating and the load decreases, the compressor that startedfirst shuts down. This avoids cycling of ther same compressor.5.11 - Controlling the EXVEXVs control the refrigerant flow in the evaporator. Eachcircuit has a liquid level detector that is mounted verticallyhigh in the evaporator shell. The level detector consists of asmall electric resistance heater and of three thermistors wiredin series and mounted at different levels inside the well body.The heater is used so that the thermistors reach approximately93°C dry air. As the refrigerant level increases in theevaporator, the resistance of the nearest thermistor(s) changesconsiderably. This difference in resistance permits precisecontrol of the specified level, modulating the EXVs.