Carrier 30GN040-420 operating instructions Head Pressure Control, Pumpout

Head Pressure Control

EXV UNITS (All 30GN units except 040 and 045 with op- tional brine) Ð The microprocessor controls the condenser fans in order to maintain the lowest condensing temperature possible, thus the highest unit efficiency. Instead of using the conventional head pressure control methods, the fans are con- trolled by the position of the EXV and suction superheat.

As the condensing temperature drops, the EXV opens to maintain the proper suction superheat. Once the EXV is fully open, if the condensing temperature continues to drop, the suction superheat begins to rise. Once the suction superheat is greater than 40 F (22.2 C), a fan stage is removed after 2 minutes.

As the condensing temperature rises, the EXV closes to maintain the proper suction superheat. Once the EXV has closed to 39.5% open (600 steps open), a fan stage is added after 2 minutes.

During start-up, all the condenser fans are started when the condensing temperature reaches 95 F (35 C) to prevent excessive discharge pressure during pulldown. See Table 5 for condenser fan sequence of operation.

For low-ambient operation, the lead fan in each circuit can be equipped with the optional or accessory Motormaster

IIIhead pressure controller. This control has its own sensor which is mounted on a return bend in the liquid portion of the condenser. It will vary the fan speed to maintain a satu- rated condensing temperature of 100 F. The controls auto- matically default to condensing temperature control during this ®rst stage of condenser-fan operation. When subsequent fan stages start, the controls revert to EXV fan control.

TXV UNITS (30GN040,045 with optional brine only) Ð Head pressure control is based on set point control. The micro- processor stages the condenser fans to maintain the set point temperature speci®ed by the controller.

Pumpout

EXV UNITS Ð When the lead compressor in each circuit is started or stopped, that circuit goes through a pumpout cycle to purge the cooler and refrigerant suction lines of refrigerant.

The pumpout cycle starts immediately upon starting the lead compressor and continues until the saturated suction tem- perature is 10° F (5.6° C) below the saturated suction tem- perature at start-up, is 10° F (5.6° C) below the cooler leav- ing ¯uid temperature, or reaches a saturated suction temperature of ±15 F (±26 C). No pumpout is necessary if the saturated suction temperature is below ±15 F (±26 C). At this point, the EXV starts to open and continues to open gradually to provide a controlled start-up to prevent liquid ¯ood-back to the compressor.

At shutdown, the pumpout cycle continues until the satu- rated suction temperature for that circuit is 10° F (5.5° C) below the saturated suction temperature when pumpout is initiated, or saturated suction temperature reaches ±15 F (±26 C). At that point, the compressor shuts down and the EXV continues to move until fully closed.

TXV UNITS Ð Pumpout is based on timed pumpout. On a command for start-up, the lead compressor starts. After 10 seconds, the liquid line solenoid opens. At shutdown, the liquid line solenoid closes, and the lead compressor contin- ues to run for 10 seconds before stopping.

Keypad and Display Module (Also Called HSIO

or LID) Ð The only function of this module is to allow the operator to communicate with the processor. It is used to enter con®gurations and set points and to read data, perform tests, and set schedules. This device consists of a keypad with 6 function keys, 5 operative keys, 12 numeric keys (0 to 9, ·, and -), and an alphanumeric, 8-character LCD. See Fig. 4. See Table 6 for key usage.

IMPORTANT: When entering multiple character in- puts beginning with a zero, a decimal point must be entered in place of the ®rst zero. When entering an in- put of zero, only the decimal point need be entered.

ACCESSING FUNCTIONS AND SUBFUNCTIONS Ð See Tables 6 - 8. Table 7 shows the 6 functions (identi®ed by name) and the subfunctions (identi®ed by number).

AUTOMATIC DEFAULT DISPLAY Ð When keypad has not been used for 10 minutes, display automatically switches to the rotating automatic default display. This display has 7 parts, listed below, which appear in continuous rotating sequence.

DISPLAY	EXPANSION
TUE 15:45	TODAY IS TUE, TIME IS 15:45 (3:45 PM)
LOCAL ON	UNIT IN LOCAL MODE
CLOCK ON	UNIT IS ON VIA CLOCK SCHEDULE
8 MODE	TEMPERATURE RESET IN EFFECT
COOL 1	NUMBER OF STAGES IS 1
2 ALARMS	THERE ARE 2 ALARMS
3 MINS	3 MINUTES REMAINING IN THE OFF-TO-ON
	TIME DELAY

Fig. 4 Ð Keypad and Display Module

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