Carrier Supra 950MT Operation & Service Manual

Also see for SUPRA 950Mt: Operation and service manual Operator's manual

62-11863 3-223.15 MODES OF OPERATION3.15.1 Heat/Cool ModeOnce the engine is considered to be running, the unit microprocessor maintains the set point temperature byswitching between heat and cool modes.3.15.2 Defrost CycleDefrost is an independent cycle that overrides cooling and heating functions, and de-ices the evaporator(s) asrequired. During Defrost mode, the microprocessor displays DF on the right hand temperature display. The lefthand display continues to display the set point.There are two ways of initiating a defrost.• Method one: Defrost may be initiated automatically at preset intervals by the defrost timer in the microprocessor.• Method two: Press the MANUAL DEFROST key.a. Initiating Automatic DefrostWhen defrost time entered via Functions has elapsed (see Table 3–1), a defrost is initiated. Whenever a defrostcycle is initiated, the defrost timer is reset to zero. The microprocessor holds the last entered defrost interval inmemory. The defrost timer runs only when a compartment defrost temperature sensor (DTS) is sensed at or below45°F (7°C). The lowest number compartment will go into null mode, fans only. The highest number compartmentwill initiate defrost first by turning off the compartment fans and using both hot gas and electric heat to completedefrost. After compartment defrost terminates, there will be a one minute operation in cool without fans by the com-partment that just completed defrost to allow excess moisture to freeze on the coil. A recovery time of 15 minuteswill be utilized by both compartments. The highest number compartment will then go into null mode, fans only. Thelowest compartment will initiate defrost by first turning off compartment fans and using both hot gas and electricheat to complete defrost. After compartment defrost terminates, there is a one minute operation in cool without fansby the compartment that just completed defrost to allow excess moisture to freeze on the coil.b. Manual DefrostPressing the manual defrost on the cab command will put the lowest number compartment into null mode, fansonly. The highest number compartment will initiate defrost first by turning off compartment fans and using both hotgas and electric heat to complete defrost. After compartment defrost terminates, there will be a one minute opera-tion in cool without fans by the compartment that just completed defrost to allow excess moisture to freeze on thecoil. The highest number compartment will then go into null mode, fans only. The lowest compartment will initiatedefrost by first turning off compartment fans and using both hot gas and electric heat to complete defrost. There isa one minute operation in cool without fans by the compartment that just completed defrost to allow excess mois-ture to freeze on the coil.c. Defrost TerminationAfter initiation, defrost mode terminates when the Defrost Temperature Sensor (DTS) is sensed at 50°F (10°C) indi-cating that the evaporator is de-iced and the defrost cycle is considered to be complete. Once a defrost has beeninitiated, the defrost interval timer resets to zero time. The compressor operates in four-cylinder operation duringdefrost and also uses electric heat. The length of defrost will depend on condition of evaporator coil(s). If a twocompartment system is used and one of the compartments’ Defrost Thermostat Sensor (DTS) is above 50°F(10°C), defrost will terminate and the system will continue to operate in temperature control mode.c. Fail Safe Defrost TerminationIf the defrost cycle does not complete within 45 minutes, the defrost cycle is terminated for that individual compart-ment by the microprocessor and the defrost cycle will continue within the next compartment. The Manual Defrostswitch overrides this mode and starts a new 45-minute cycle.When defrost override is active, the appropriate alarm is indicated. If the run relay is de-energized during defrost,defrost is terminated and/or if a shutdown alarm occurs, defrost will be terminated.3.15.3 Continuous or Start-Stop Operationa. Auto Start FailureIf the unit fails to start, shuts down on a safety, or fails to run for the minimum run time three consecutive times, theStart/Fail alarm is activated.