Carrier 69NT40-561-019, Operation And Service Manual

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4–13 T-359
During defrost the evaporator fans will cycle ON Low Speed once DTS reaches 0°C (32°F) and STS is below -5°C
(23°F). Once the evaporator fans cycle ON the controller will monitor STS at 1 second intervals and calculate the
difference between the current STS reading and the previous STS reading.
If STS becomes warmer that -5°C (23°F) or the calculated difference of STS remains below 0.1°C (0.18°F) for 10
consecutive readings or DTS reaches 20°C (68°F), the evaporator fans will turn off for 3 minutes. At the end of 3
minutes, the controller will begin checking the conditions for evaporator fans “PuLs” operation again.
4.4 PROTECTION MODES OF OPERATION
4.4.1 Evaporator Fan Operation
Opening of an evaporator fan internal protector will shut down the unit.
4.4.2 Failure Action
Function code Cd29 may be operator set to select action the controller will take upon system failure. The factory
default is full system shutdown. Refer to
Table 4–5 .
4.4.3 Generator Protection
Function codes Cd31(Stagger Start, Offset Time) and Cd32 (Current Limit) may be operator set to control start up
sequence of multiple units and operating current draw. The factory default allows on demand starting (no delay) of
units and normal current draw. Refer to
Table 4–5 .
4.4.4 Compressor High Temperature, Low Pressure Protection
The controller monitors compressor discharge pres sure, and temperature and suction pressure. If dis charge pres-
sure or temperature rises above the allowed limit or suction pressure falls below the allowed limit, the compressor
will be cycled off and on every 3 minutes. Condenser and evaporator fans continue to operate during the compres-
sor off cycle.
If high compressor dome temperature occurs, the controller will allow additional refrigerant to be released into the
system in order to provide cooling to the evaporator coil and compressor dome. The controller is alerted to high
compressor dome temperatures via the CPDS when the ambient temperature is greater than 43.3°C, the return air
temperature is less than -17.5°C and if the compressor discharge temperature is greater than 117.7°C.
Dome temperature control logic will disengage when return air temperature and ambient temperature return to
allowed limits or when compressor turns off.
If the suction pressure low limit is triggered, the DUV will energize to raise the suction pressure.
4.4.5 Perishable Mode - System Pressure Regulation
In perishable mode, system pressures may need to be regulated at ambient temperatures of 20°C (68°F) and
below. Once below this ambient temperature, the con denser fan may cycle on and off based on limits imposed for
discharge pressure. For extremely cold ambient temperatures, -18°C (0°F), heater cycling may occur within normal
system operation based on discharge pressure limits.
4.4.6 Condenser Fan Override
When configuration variable CnF17 (Discharge Temperature Sensor) is set to “In” and CnF48 (Condenser Fan
Switch Override) is set to “On”, the condenser fan switch override logic is activated. If condenser cooling water
pressure is sufficient to open the water pressure switch (de-energizing the condenser fan) when water flow or pres-
sure conditions are not maintaining dis charge temperature, the logic will energize the condenser fan as follows:
1. If the DUV is less than 80% open when the controller calls for it to be100% open, the condenser fan is ener-
gized. When the DUV is 100% open, the fan will de-energize.
2. If DPT reading is invalid or out of range (AL 65), the condenser fan is energized and will remain energized
until system power is cycled.
3. If the system is running on condenser fan override and the high pressure switch opens, the condenser fan is
energized and will remain energized until the system power is cycled.