Troubleshooting Compressor Faults (Screw compressor equipped chillers.)

Transcription

1 Troubleshooting Compressor Faults (Screw compressor equipped chillers.) A problem with the compressors operation is displayed with a Compressor Failure Event (alarm) on the FX16 or Carel controller. The compressor failure event can be caused by one of the following problems. Refer to the individual compressor menu selection on the FX16 or Carel controller display to determine which problem caused the compressor Event. Circuit Failure The Circuit Failure for a compressor can be caused by a failure of the High Pressure Safety (HP), the Freeze Safety (FZ), the compressor Overloads (OVL) or the compressor Power Module (PM). The Circuit Failure for a compressor can be reset by resetting the device that caused the failure Event, the High Pressure Safety, the Freeze Safety, compressor Overload or the compressor Power Module. These safeties are wired in series with the compressor contactor. When all of the safeties are in their normal position the compressor fail relay is energized which opens the contacts that supply the digital input to the controller. When any of the safeties trip the relay, compressor contactor, and all other components downstream are de-energized in order to shut down the circuit. The compressor safety contacts close indicating a Circuit Failure. Note that whenever a unit phase failure monitor trips it will cut control power to all components downstream of its contacts. This causes the compressor failure contacts to de-energize and their contacts to close which will manifest as all circuits tripping on Circuit Failure. Generally the event summary and event history will also log the phase monitor failure which will make it easier to differentiate what caused the faults. Any other items that will affect control power to the compressor fail relays will also result in multiple compressors failing on Circuit Failure at the same time. When troubleshooting a circuit failure it is recommended that a meter is placed from the neutral to the load side of each safety contact working from the innermost safety back out to determine which safety has tripped. Normally the power module is the innermost safety. A failure here can indicate that the compressor is running too hot or there is a problem with phasing. Next out are the compressor overloads. If these are tripped the compressor may have shorted out, the unit is having general power issues, or there are loose wires on the compressor wiring. Next will be the freeze switch. A trip here indicates that the suction pressure is running too low. If the suction pressure is below the setting for over 120 seconds the freeze will trip. Suction limiting, superheat, and hot gas should be checked.

2 Next is the high pressure switch. A trip here indicates that the discharge pressure is too high. Fan cycling, condenser cleanliness, and discharge transducer readings should be checked. Make sure to check the transducer against calibrated gauges for the appropriate refrigerant. If the safeties are not tripping or the compressors are tripping on startup the following items will need to be checked. The wiring to the compressor status relays will need to be checked for tightness. Check or replace any crimp on connections to the relay coils to ensure that the connections are tight and do not have insulation pinched in the crimp. Check the relay to make sure that it is operating correctly. If it is suspected that the relay may be going bad or sticking, replace it. Check the wiring between the relay contact for the compressor status input to the controller. Check the terminals for tightness and check or replace any crimp on connections. If the wiring itself is suspected of having an open or intermittent open, replace with the same type of cable. If the compressor status inputs are on an expansion module the expansion module may be losing communications. This will cause the controller to not register compressor status and cause a trip. See Johnson Controls expansion module troubleshooting guide. Mechanical Low Pressure The Mechanical Low Pressure failure for a compressor is caused when the Low Pressure Safety (LO) opens when the compressor has been commanded to run. The Mechanical Low Pressure failure for a compressor can be reset by depressing the compressor logic reset button. If this trip is displayed and the only thing required to get the circuit operating again is pushing the compressor logic reset button then the LO switch is opening while the compressor is commanded to run. This means that the suction pressure has fallen below the LO switch cutout setpoint. Check to make sure that the LO switch is set according to the pressure table on the middle left of the electrical prints. Put gauges on the suction line and slowly throttle the suction valve closed to determine if the LO is cutting out at the proper pressure. Caution! Do not throttle the valve completely closed while the compressor is running! Check to make sure hot gas bypass is operating correctly if equipped. Other items that may cause the LO switch to open is improper glycol concentration, setpoints set lower than design conditions, improper superheat, or low refrigerant charge. This failure depends upon the compressor status digital input for correct operation. The controller looks at the status of this input and whether or not it has commanded the circuit to run in order to evaluate for this failure. When the compressor has been commanded and the compressor status input does not close the controller assumes that the LO switch has not closed and trips on Mechanical Low Pressure.

3 Due to the controller relying on the compressor status input, anything that affects the compressor interlock relay can also manifest as this failure. If the relay is malfunctioning, the expansion module that the contact is tied into is malfunctioning, or the interconnecting wiring is damaged or loose, this trip may occur even if the LO switch is made. Under certain circumstances a freeze trip or other safety string trip may occur that will also cause this fault on the controller. This is due to where the trip is caught in the controllers processing loop and which fault posts first on the alarm summary. Troubleshoot by determining if the compressor status input is being read properly at the controller when the compressor is on and off. This can be seen on the compressor page on the controller. If it is being read correctly check actual pressure on the suction line with calibrated gauges to see if the suction pressure is too low to keep the LO switch made when the compressor is running. If it is not reading correctly determine if the expansion modules are working. If they are check the digital input by pulling the wire off to see if it registers open and jumpering it see if it registers closed. If it reads correctly check the compressor interlock relay and interconnecting wiring for proper operation. Check the safety string as described in Compressor Failure above. Check the superheat boards to make sure that they are energized and working properly. Check to make sure that the superheat has been set correctly. If the superheat board loses power the valve will not open which will result in a low pressure failure. If the fault occurs when the unit is running but the suction pressure is not low the relays that control the valves and superheat boards may be going bad. If the fault occurs right at startup the positive start relay may be going bad. Low Differential Pressure Failure The Low Differential Pressure Failure for a compressor is caused when the difference between the discharge and suction pressure is less than 60 Psig for a period exceeding 20 seconds. This failure will only occur when the compressor has been operating for a period of 90 seconds or longer as determined by compressor status digital input. The Differential Pressure Failure for a compressor can be reset by depressing the compressor reset button. This fault is dependent on the suction and discharge transducers as well as the compressor status input. To troubleshoot check the transducers against calibrated gauges to make sure they are reading correctly. Check the compressor status input as described in Mechanical Low Pressure above. Check to make sure that the fans are cycling at the appropriate pressure. Check to make sure that the expansion modules or controller is registering the values correctly. Low ambient starts, especially on units not equipped with B20 Flood Control, can lead to difficulty building proper differential pressure.

4 Crankcase Temperature Lockout If the Crankcase Temperature is lower than the Ambient Temperature + the CCT Setpoint (CC Cal Tmp on the FX16 display) the compressor will not be allowed to operate. Once the Crankcase Temperature rises above the CC Cal Tmp for a time period (CCT Delay) the compressor will be allowed to operate. Once the compressor is on the Crankcase Temperature lockout is no longer active. This lockout is dependent on ambient and crankcase temperature. To troubleshoot check the temperature sensors with a calibrated temperature recording device as close to the temperature sensor as possible to make sure they are reading correctly. Check to make sure that the crankcase heater is operational. Check to make sure that the expansion modules or controller is registering the temperatures correctly.

5 Sample compressor page; Compressor 1 (menu selection) o TXV (refrigeration valve(s) command status: Closed/Open) o Compressor (compressor operating status: Off/On) o Suction (compressor / circuit suction pressure: to 116 Psi) o Suction Limiting (compressor suction pressure limiting status: No/Hold/Yes) o Discharge (compressor / circuit discharge pressure: 0 to 435 Psi) o Discharge Limiting (compressor discharge pressure limiting status: No/Hold/Yes) o Current (compressor current: Amps) o Current Limiting (compressor current limiting status: No/Hold/Yes) o Manual On/Off (status of compressor manual on/off switch: Off/On) o Low Diff Press (status of low differential pressure failure: No/Yes) o Mech Low Press (status of mechanical low pressure failure: No/Yes) o Failure (status of circuit failure: No/Yes) o Crank CaseTemp (crankcase temperature: F) o CC Cal Tmp (ambient temperature + CCT Setpoint: F) o Crank Case Lock (status of crankcase low temperature lockout: Yes/No)