Carrier 69NT40-511-199, 69NT40-521 manual Snap Freeze Option, Defrost, Arctic

4.4.6Defrost

Refer to section 3.1.4 (Code 27) for description of the defrost interval selector and automatic defrost initiation.

The defrost cycle (see Figure 4-5) consists of two distinct sub-cycles. The first sub-cycle is the de-ice cycle, the second is a probe check cycle.

Defrost may take place any time the DTS allows, and no shutdown alarms are active. With these conditions satisfied, defrost is initiated when one of the following conditions becomes true:

a.The manual defrost switch (MDS) is closed by the user. Refer to Figure 2-7 or Figure 2-8 for location. The MDS is ignored during Pre-Trip.

b.The defrost interval timer reaches or exceeds the defrost interval selected and set by the user.

c.During Pre-Trip (auto, not manual) defrost can occur during the advanced Pre-Trip tests P-8 and P-10. Defrost is forced during advanced Pre-Trip test P-9.

d.When the probe diagnostic logic determines that a probe check is necessary based on the temperature values currently reported by the supply and return probes.

e.When bulb mode is active, and the defrost termination sensor (DTS) is between 0_C and 10_C (32_F and 50_F).

When the defrost mode is initiated, the controller relay contacts (TH) close to supply power to the heat contactor (HR) and in turn, energize the defrost heaters. The defrost light is illuminated.

Relay TC and TD open to de-energize the compressor contactor and cool light. Also relay TN opens to de-energize the condenser fan contactor (CF).

Relay TE and TV open to stop the evaporator fan motors.

The in-range light remains illuminated during defrost.

When the coil tube sheet temperature reaches 25.6_C (78_F), [4_C and 25.6_C (39.2_F and 78_F) if configured for and operating in bulb mode], the defrost termination sensor (DTS) causes the controller to end the defrost cycle and the unit returns to its normal function. Under certain circumstances, defrost may also be forced to terminate through special communication commands.

Upon completion of the de-ice phase of defrost, the controller will perform a probe check cycle. The purpose of the probe check cycle is to perform a periodic check of the controller sensors to detect malfunctions or drift in the sensed temperature that is too small to be detected by the normal sensor out of range tests. The system will run for eight minutes in this condition. At the end of the eight minutes, the primary supply, primary return and DataCORDER sensor temperatures will be compared. The controller probe alarms will be set or cleared based on the conditions seen.

The 54.5_C (130_F) heat termination thermostat (HTT) will open the circuit if the defrost mode does not terminate at 25.6_C (78_F). If termination does not occur within 2.0 hours, the controller will terminate defrost. An alarm will be given of a possible DTS failure.

When the return air falls to 7_C (45_F), the controller checks to ensure the defrost termination sensor (DTS) has dropped to 10_C or below. If it has not, a DTS failure alarm is given and the defrost mode is operated by the return temperature sensor (RTS).

Snap Freeze Option:

NOTE

Controller configuration variable 33 must be set to SnAP to activate this option, refer to Table 3-1.

If the probe check portion of defrost is required (depending on the configuration of probe check), snap freeze will run after the probe check cycle. Otherwise, snap freeze will run immediately following the de-ice portion of defrost.

The snap freeze cycle consists of running the compressor without the evaporator fans running for a period of four minutes with both suction solenoid (SSV) and suction modulation (SMV) valves fully open.

If current limiting activates (see section 3.1.4, Cd32) during snap freeze, the state of the SSV and SMV valves may change. When the snap freeze cycle is completed, defrost is formally terminated.

4.4.7Arctic

With arctic mode enabled, if the ambient is colder than -10.0_C there is a 30 minute time delay at startup for any of the components in the system, except for the controller and the compressor crankcase heater (CCH), which should be active at this point. In arctic mode, the CCH is energized for 30 minutes to warm the oil in the compressor, and boil off any liquid refrigerant that may be present in the crankcase.

T-268-07 4-12