Carrier AQUASNAP AIR COOLED CHILLERS WITH COMFORTLINK CONTROLS 30RAP010-060 Operation And Service Manual

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Carrier AQUASNAP AIR COOLED CHILLERS WITH COMFORTLINK CONTROLS 30RAP010-060 Operation And Service Manual Manual pdf 52 page image

52MINIMUM LOAD VALVE — On units equipped with thefactory-installed capacity reduction option, a solenoid valveand discharge bypass valve (minimum load valve) are locatedbetween the discharge line and the cooler entering-refrigerantline. The MBB cycles the solenoid to perform minimum loadvalve function and the discharge bypass valve modulates to thesuction pressure set point and the valve.The amount of capacity reduction achieved by the mini-mum load valve is not adjustable. The total unit capacity withthe minimum load valve is shown in Table 19.PRESSURE RELIEF DEVICES — All units have one pres-sure relief device per circuit located in the liquid line which re-lieves at 210 F (100 C).Check Unit SafetiesHIGH-PRESSURE SWITCH — A high-pressure switch isprovided to protect each compressor and refrigeration systemfrom unsafe high pressure conditions. See Table 32 for high-pressure switch settings.The high-pressure switch is mounted in the discharge line ofeach circuit. If an unsafe, high-pressure condition should exist,the switch opens and shuts off the affected circuit. The CSBsenses the compressor feedback signal and generates an appro-priate alarm. The MBB prevents the circuit from restarting un-til the alert condition is reset. The switch should open at thepressure corresponding to the appropriate switch setting asshown in Table 32.Table 32 — Factory Settings, High-PressureSwitch (Fixed)Clear the alarm using the scrolling marquee display as de-scribed on page 63. The unit should restart after the compressoranti-short-cycle delay, built into the unit control module,expires.PRESSURE TRANSDUCERS — Each refrigerant circuit isequipped with a suction and discharge pressure transducer.These inputs to the MBB are not only used to monitor the sta-tus of the unit, but to also maintain operation of the chillerwithin the compressor manufacturer's specified limits. The in-put to the MBB from the suction pressure transducer is alsoused to protect the compressor from operating at low pressureconditions and low superheat conditions. In some cases, theunit may not be able to run at full capacity. The control modulewill automatically reduce the capacity of a circuit as needed tomaintain specified maximum/minimum operating pressures.COOLER FREEZE-UP PROTECTIONThe main base board (MBB) monitors leaving fluid temper-ature at all times. The MBB will rapidly remove stages of ca-pacity as necessary to prevent freezing conditions due to therapid loss of load or low cooler fluid flow.When the cooler is exposed to lower ambient temperatures(34 F [1° C] or below), freeze-up protection is required usinginhibited ethylene or propylene glycol.HEATER CABLE — Optional factory-installed cooler and/orhydronic package heaters are cycled based on the input fromthe outside-air temperature sensor. These heaters, when in-stalled, are designed to protect the cooler and/or hydronic pack-age from freezing down to –20 F (–29 C). Power for these heat-ers is supplied from the main unit power.The input from the low pressure transducer provides a back-up cooler freeze protection package. The MBB shuts down theunit when a low pressure condition exists that could cause thecooler to freeze up.WINTER SHUTDOWN — At the end of the cooling season:1. Drain the fluid from the cooler, hydronic package (if in-stalled) and internal piping.2. Fill the cooler and hydronic package with at least 2 gal-lons (7.6 L) of inhibited propylene glycol or other suit-able inhibited antifreeze solution to prevent any residualwater in the cooler and hydronic package/piping fromfreezing.3. At the beginning of the next cooling season, refill thecooler and add the recommended inhibitor.Thermistors — Electronic control uses up to five 5 kthermistors to sense temperatures used to control operation ofthe chiller. Thermistors EWT, LWT, RGTA, RGTB, and OATare identical in their temperature and voltage drop perfor-mance. The SPT space temperature thermistor has a 10 k in-put channel and it has a different set of temperature vs. resis-tance and voltage drop performance. Resistance at various tem-peratures are listed in Tables 33-37. For dual chiller operation,a dual chiller sensor is required which is a 5 k thermistor.When a digital compressor is used, a DTT (digital temperaturethermistor) is used. The DTT is an 86 k thermistor.REPLACING THERMISTORS (EWT, LWT, RGT) — Adda small amount of thermal conductive grease to the thermistorwell and end of probe. For all probes, tighten the retaining nut¼ turn past finger tight. See Fig. 28.THERMISTOR/TEMPERATURE SENSOR CHECK — Ahigh quality digital volt-ohmmeter is required to perform thischeck.1. Connect the digital voltmeter across the appropriate the-mistor terminals at the J8 terminal strip on the Main BaseBoard (see Fig. 29).2. Using the voltage reading obtained, read the sensor tem-perature from Tables 33-37.3. To check thermistor accuracy, measure temperature atprobe location with an accurate thermocouple-type tem-perature measuring instrument. Insulate thermocouple toavoid ambient temperatures from influencing reading.Temperature measured by thermocouple and temperaturedetermined from thermistor voltage reading should beclose, ± 5° F (3° C) if care was taken in applying thermo-couple and taking readings.If a more accurate check is required, unit must be shut downand thermistor removed and checked at a known temperature(freezing point or boiling point of water) using either voltagedrop measured across thermistor at the J8 terminal, by deter-mining the resistance with chiller shut down and thermistorUNIT CUTOUT CUT-INPsig kPa Psig kPa30RA 650 4482 500 3447WARNINGOn medium temperature brine units, the anti-freeze solu-tion must be properly mixed to prevent freezing at a tem-perature of at least 15 F (8.3 C) below the leaving-fluidtemperature set point. Failure to provide the proper anti-freeze solution mixture is considered abuse and may impairor otherwise negatively impact the Carrier warranty.CAUTIONDo not disconnect main unit power when servicing com-pressor(s) if ambient temperature is below 40 F (4.4 C).The compressors have either a single circuit breaker ormultiple circuit breakers which can be used to shut offpower to the compressors. If power to the unit must be offfor a prolonged period, drain the cooler, hydronic package(if installed) and internal piping. Add glycol according toWinter Shutdown Step 2 below.