Carrier 30GTN Service And Troubleshooting Manual

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Carrier 30GTN Service And Troubleshooting Manual Manual pdf 16 page image

Electronic Expansion Valve (EXV) (See Fig. 14)— Standard units are equipped with a bottom seal EXV. Thisdevice eliminates the use of the liquid line solenoid pump-down at unit shutdown. An O-ring has been added to bottomof orifice assembly to complete a seal in the valve on shut-down. This is not a mechanical shut-off. When service is re-quired, use the liquid line service valve to pump down thesystem.High pressure refrigerant enters bottom of valve where itpasses through a group of machined slots in side of orificeassembly. As refrigerant passes through the orifice, it dropsin pressure. To control flow of refrigerant, the sleeve slidesup and down along orifice assembly, modulating the size oforifice. The sleeve is moved by a linear stepper motor thatmoves in increments controlled directly by the processor. Asstepper motor rotates, the motion is translated into linear move-ment of lead screw. There are 1500 discrete steps with thiscombination. The valve orifice begins to be exposed at320 steps. Since there is not a tight seal with the orifice andthe sleeve, the minimum position for operation is 120 steps.Two thermistors are used to determine suction superheat.One thermistor is located in the cooler and the other is lo-cated in the cylinder end of the compressor after refrigeranthas passed over the motor. The difference between the2 thermistors is the suction superheat. These machines areset up to provide approximately 5 to 7 F (2.8 to 3.9 C) super-heat leaving the cooler. Motor cooling accounts for approxi-mately 22 F (12.2 C), resulting in a superheat entering com-pressor cylinders of approximately 30 F (16.7 C). This increasesperformance of cooler by reducing the amount of superheatneeded.Because the valves are controlled by the EXV module, itis possible to track the position of the valve. Valve positioncan be used to control head pressure and system refrigerantcharge.During initial start-up, the EXV module will drive eachvalve fully closed. After initialization period, valve positionis controlled by the EXV module and the MBB.The EXV is used to limit the maximum cooler saturatedsuction temperature to 55 F (12.8 C). This makes it possiblefor the chiller to start at high cooler fluid temperatures with-out overloading the compressor.Energy Management Module (Fig. 15) — Thisfactory-installed option or field-installed accessory is usedfor the following types of temperature reset, demand limit,and/or ice features:• 4 to 20 mA leaving fluid temperature reset (requires field-supplied 4 to 20 mA generator)• 4 to 20 mA cooling set point reset (requires field-supplied4 to 20 mA generator)• Discrete inputs for 2-step demand limit (requires field-supplied dry contacts capable of handling a 5 vdc, 1 to20 mA load)• 4 to 20 mA demand limit (requires field-supplied 4 to 20mA generator)• Discrete input for Ice Done switch (requires field-supplieddry contacts capable of handling a 5 vdc, 1 to 20 mA load)See Demand Limit and Temperature Reset sections onpages 43 and 45 for further details.Capacity Control — The control system cycles com-pressors, unloaders, and hot gas bypass solenoids to main-tain the user-configured leaving chilled fluid temperature setpoint. Entering fluid temperature is used by the Main BaseBoard (MBB) to determine the temperature drop across thecooler and is used in determining the optimum time to addor subtract capacity stages. The chilled fluid temperature setpoint can be automatically reset by the return temperaturereset or space and outdoor-air temperature reset features. Itcan also be reset from an external 4 to 20 mA signal (re-quires Energy Management Module FIOP/accessory).With the automatic lead-lag feature in the unit, the controldetermines randomly which circuit will start first, A or B. Atthe first call for cooling, the lead compressor crankcase heaterwill be deenergized, a condenser fan will start, and the com-pressor will start unloaded.NOTE: The automatic lead-lag feature is only operative whenan even number of unloaders is present. The 040-070 unitsrequire an accessory unloader for the lead-lag feature to bein effect.If the circuit has been off for 15 minutes, and the unit isa TXV unit, liquid line solenoid will remain closed duringstart-up of each circuit for 15 seconds while the cooler andsuction lines are purged of any liquid refrigerant. For unitswith EXVs, the lead compressor will be signaled to start.The EXV will remain at minimum position for 10 secondsbefore it is allowed to modulate.After the purge period, the EXV will begin to meter therefrigerant, or the liquid line solenoid will open allowing theTXV to meter the refrigerant to the cooler. If the off-time isless than 15 minutes, the EXV will be opened as soon as thecompressor starts.The EXVs will open gradually to provide a controlled start-upto prevent liquid flood-back to the compressor. During start-up, the oil pressure switch is bypassed for 2 minutes to al-low for the transient changes during start-up. As additionalstages of compression are required, the processor control willadd them. See Tables 5A and 5B.If a circuit is to be stopped, the control will first start toclose the EXV or close the liquid line solenoid valve.For units with TXVs, the lag compressor(s) will be shut downand the lead compressor will continue to operate for 10 sec-onds to purge the cooler of any refrigerant.For units with EXVs, the lag compressor(s) will be shut downand the lead compressor will continue to run. After the lagcompressor(s) has shut down, the EXV is signaled to close.The lead compressor will remain on for 10 seconds after theEXV is closed.During both algorithms (TXV and EXV), all diagnosticconditions will be honored. If a safety trip or alarm condi-tion is detected before pumpdown is complete, the circuitwill be shut down.Fig. 14 — Electronic Expansion Valve (EXV)16