Carrier 30GK Slave compressor start-up sequence, Controlling the EXV, Active setpoint selection

5.12 - Slave compressor start-up sequence

Lag compressors are started and stopped in a sequence designed to equalise their number of start-ups (value weighted by their operating time).

5.13 - Controlling the EXV

EXVs, if used, control the flow of refrigerant in the evaporator. They are controlled in order to maintain a constant level of superheat at the thermistor for the lead compressor inlet gas (located between the compressor motor and the cylinders).

A thermistor and a pressure sensor, in the lead compressor of each circuit, are used to measure this superheat. The thermistor measures the temperature of the superheated gas entering the cylinders. The pressure sensor measures the suction gas pressure. The controller converts this value into a saturated temperature. The difference between the superheated gas temperature and the saturated temperature is the superheat. The control system sets the position of the EXV to hold this superheat at the setpoint configured for the unit.

Since the EXVs are driven by the controller their positions are always known. During start-up of a circuit its EXV is fully closed to ensure pump down. After pump down, the system continuously controls and monitors the valve position. Similarly, on shutdown of a circuit the EXV is closed again to ensure pumping down.

EXVs are also used to restrict suction temperature, making it possible to start the unit at higher water and suction tempera- tures without overloading the compressors. This procedure controls what is known as Maximum Operating Pressure (MOP).

5.14 - Head pressure control on air-cooled units

There are two methods, configurable only by Carrier Service, for controlling the condenser fans:

•Controlling condensation by reference to a setpoint (default): The saturated head pressure is controlled by reference to a fixed setpoint (user-definable in the Setpoints menu). This temperature is maintained by cycling fans on and off, as well as by varying the speed of a fan in appropriate cases.

•Controlling condensation by reference to the position of the EXV: The saturated head pressure is controlled by reference to the position of the EXV and the superheat, by cycling fans on and off as well as by varying the speed of a fan in appropriate cases. The control system seeks to keep the EXVs as wide open as possible whilst maintaining the correct level of superheat with the fans. When the second condensing setpoint is selected (reclaim setting), the control system will automatically revert to controlling by reference to a setpoint even if control by reference to the EXV has been selected. The original configuration is reinitialised when the first condensing setpoint is re-selected.

NOTE

Certain units can have up to 8 fan stages, of which one per circuit is a variable-speed fan, depending on their configura- tion and wiring.

5.15 - Head pressure control on water-cooled units

There are two configurable methods for controlling the conden- ser water valves (optional):

•Controlling head pressure by reference to a setpoint: Saturated head pressure is controlled by reference to a user- definable fixed setpoint. This temperature is maintained by using the valves to control the flow of water in each conden- ser circuit.

•Controlling head pressure by reference to the position of the EXV: Saturated head pressure is controlled by reference to the position of the EXV and the superheat, by adjusting the flow of water in each condenser circuit. The valve control system seeks to keep the EXVs as wide open as possible whilst maintaining the correct level of superheat. When the second head pressure setpoint is selected (reclaim setting), the control system will automatically revert to controlling by reference to a setpoint even if control by reference to the EXV has been selected. The original configuration is reinitialised when the first head pressure setpoint is re- selected.

5.16 - Active setpoint selection

Two setpoints can be selected as active in cooling mode, and one setpoint in heating mode. Usually, the second cooling setpoint is used for unoccupied periods or for ice storage (brine unit). Depending on the current operating mode, the active setpoint can be selected either by choosing the item in the Information menu. or with the user volt-free contacts, or with network commands, or by the setpoint time schedule program (Schedule 2).

The following tables show a summary of the possible selctions as a function of the control typpes (local, remote or network), and the parameters below:

•Local setpoint selection: item No. 11 of the Information Menu permits selection of the active setpoint, if the unit is in the local operating type.

•Heating/cooling operating mode.

•Control contacts: status of the remote heating and cooling control contact. This contact is only active, if the unit is under remote operating control. See section 3.6.6.

•Schedule 2 program status: schedule program for setpoint selection. See section 4.3.11.6.

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