Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual

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Contents

9REFRIGERATION CYCLEThe compressor continuously draws refrigerant vapor fromthe cooler at a rate set by the amount of guide vane opening. Asthe compressor suction reduces the pressure in the cooler, theremaining refrigerant boils at a fairly low temperature (typical-ly 38 to 42 F [3 to 6 C]). The energy required for boiling is ob-tained from the water flowing through the cooler tubes. Withheat energy removed, the water becomes cold enough to use inan air conditioning circuit or for process liquid cooling.After taking heat from the water, the refrigerant vapor iscompressed. Compression adds still more heat energy, and therefrigerant is quite warm (typically 98 to 102 F [37 to 40 C])when it is discharged from the compressor into the condenser.Relatively cool (typically 65 to 90 F [18 to 32 C]) waterflowing into the condenser tubes removes heat from the refrig-erant and the vapor condenses to liquid.The liquid refrigerant passes through orifices into theFLASC (Flash Subcooler) chamber (Fig. 4 and 5). Since theFLASC chamber is at a lower pressure, part of the liquid refrig-erant flashes to vapor, thereby cooling the remaining liquid.The FLASC vapor is re-condensed on the tubes which arecooled by entering condenser water. The liquid drains into afloat valve chamber between the FLASC chamber and cooler.Here the AccuMeter™ float valve forms a liquid seal to keepFLASC chamber vapor from entering the cooler. When liquidrefrigerant passes through the valve, some of it flashes to vaporin the reduced pressure on the cooler side. In flashing, it re-moves heat from the remaining liquid. The refrigerant is now ata temperature and pressure at which the cycle began. Refriger-ant from the condenser also cools the oil and optional variablespeed drive.The refrigeration cycle for a 19XRV chiller with two-stagecompressor is similar to the one described above, with the fol-lowing exception: Liquid refrigerant from the condenserFLASC chamber linear float valve flows into an economizer atintermediate pressure (see Fig. 5). As liquid enters the cham-ber, due to the lower pressure in the economizer, some liquidflashes into a vapor and cools the remaining liquid. The sepa-rated vapor flows to the second stage of the compressor forgreater cycle efficiency. A damper valve located on the econo-mizer line to the compressor acts as a pressure regulating de-vice to stabilize low load, low condensing pressure operatingconditions. The damper will back up gas flow and thereby raisethe economizer pressure to permit proper refrigerant flowthrough the economizer valve during those conditions. Thedamper also is closed during start-up conditions to allow thesecond stage impeller to start unloaded.The subcooled liquid remaining in the economizer flowsthrough a float valve and then into the cooler.MOTOR AND LUBRICATING OILCOOLING CYCLEThe motor and the lubricating oil are cooled by liquid re-frigerant taken from the bottom of the condenser vessel(Fig. 4 and 5). Refrigerant flow is maintained by the pressuredifferential that exists due to compressor operation. After therefrigerant flows past an isolation valve, an in-line filter, and asight glass/moisture indicator, the flow is split between the mo-tor cooling and oil cooling systems.Flow to the motor cooling system passes through an orificeand into the motor. Once past the orifice, the refrigerant isdirected over the motor by a spray nozzle. The refrigerantcollects in the bottom of the motor casing and is then drainedback into the cooler through the motor refrigerant drain line.An orifice (in the motor shell) maintains a higher pressure inthe motor shell than in the cooler. The motor is protected by atemperature sensor embedded in the stator windings. Anincrease in motor winding temperature past the motor overrideset point overrides the temperature capacity control to hold,and if the motor temperature rises 10 F (5.5 C) above this setpoint, closes the inlet guide vanes. If the temperature risesabove the safety limit, the compressor shuts down.Refrigerant that flows to the oil cooling system is regulatedby thermostatic expansion valves (TXVs). The TXVs regulateflow into the oil/refrigerant plate and frame-type heat exchang-er (the oil cooler in Fig. 4 and 5). The expansion valve bulbscontrol oil temperature to the bearings. The refrigerant leavingthe oil cooler heat exchanger returns to the chiller cooler.IMPORTANT: To avoid adverse effects on chiller opera-tion, considerations must be made to condenser water tem-perature control. For steady state operation, the minimumoperating refrigerant pressure differential between coolerand condenser is approximately 20 psi (138 kPa) with amaximum evaporator refrigerant temperature of 65 F(18 C). Consult Chiller Builder for required steady stateoperational limits. Inverted start conditions are acceptablefor short durations of time, but for periods exceeding5 minutes, a special control solution strategy should beused to allow the chiller to establish a minimum refrigerantpressure differential (and thereby adequate equipmentcooling).