Carrier Air Conditioner specifications Head Pressure Control, Pumpout

DEADBAND EXAMPLE

2 STARTS

L (C)WT

8

7

6

5

	47
	46
	45
(F)	44
WTL
	43
	42
	41
	0	200	400	600	800	1000
			TIME (SECONDS)			3 STARTS

LEGEND	STANDARD
LWT — Leaving Water Temperature	DEADBAND
	MODIFIED
	DEADBAND
	Fig. 17 — Deadband Multiplier

Low Cooler Suction Temperature — To avoid freezing the cooler, the control will compare the circuit Cooler Suction tem- perature (T5/T6) with a predetermined freeze point. If the cool- er fluid selected is Water, the freeze point is 34 F (1.1 C). If the cooler fluid selected is Brine, the freeze point is 8° F (4.4 ° C) below the cooling set point (or lower of two cooling set points in dual set point configurations). If the cooler suction tempera- ture is 24° to 29° F (13.3° to 16.1° C) below the cooler leaving water temperature and is also 2° F (1.1° C) less than the freeze point for 5 minutes, Mode 7 (Circuit A) or Mode 8 (Circuit B) is initiated and no additional capacity increase is allowed. The circuit will be allowed to run in this condition. If the cooler suc- tion temperature is more than 30° F (16.7° C) below the cooler leaving water temperature and is also 2° F (1.1° C) below the freeze point for 10 minutes, the circuit will be stopped without going through pumpdown.

Cooler Freeze Protection — The control will try to prevent shutting the chiller down on a Cooler Freeze Protection alarm by removing stages of capacity. The control uses the same freeze point logic as described in the Low Cooler Suction Tem- perature section above. If the cooler leaving fluid temperature is less than the freeze point plus 2.0° F (1.1° C), the control will immediately remove one stage of capacity. This can be repeat- ed once every 30 seconds.

MOP (Maximum Operating Pressure) Override — The con- trol monitors saturated condensing and suction temperature for each circuit. Based on a maximum operating set point (saturat- ed suction temperature), the control may lower the EXV posi- tion when system pressures approach the set parameters.

Head Pressure Control

COMFORTLINK™ UNITS (With EXV) — The Main Base Board (MBB) controls the condenser fans to maintain the low- est condensing temperature possible, and thus the highest unit efficiency. The fans are controlled by the saturated condensing temperature set from the factory. The fans can also be con- trolled by a combination of the saturated condensing tempera- ture, EXV position and compressor superheat. Fan control is a configurable decision and is determined by the Head Pressure Control Method (HPCM) setting in the Configuration Mode under the OPT1 sub-mode. For EXV control (HPCM = 1), when the position of the EXV is fully open, T3 and T4 are less than 78 F (25.6 C), and superheat is greater than 40 F (22.2 C), fan stages will be removed. When the valve is less than 40% open, or T3 and T4 are greater than 113 F (45 C), fan stages will be added. At each change of the fan stage, the system will wait one minute to allow the head pressure to stabilize unless either T3 or T4 is greater than 125 F (51.6 C), in which case all MBB-controlled fans will start immediately. This method allows the unit to run at very low condensing temperatures at part load.

During the first 10 minutes after circuit start-up, MBB- controlled fans are not turned on until T3 and T4 are greater than the head pressure set point plus 10 F (5.6 C). If T3 and T4 are greater than 95 F (35 C) just prior to circuit start-up, all MBB-controlled fan stages are turned on to prevent excessive discharge pressure during pull-down. Fan sequences are shown in Fig. 17.

UNITS WITH TXV — The logic to cycle MBB-controlled fans is based on saturated condensing temperature only, as sensed by thermistors T3 and T4 (see Fig. 8 and 10). When either T3 or T4 exceeds the head pressure set point, the MBB will turn on an additional stage of fans. For the first 10 minutes of each circuit operation, the head pressure set point is raised by 10° F (5.6° C). It will turn off a fan stage when T3 and T4 are both below the head pressure set point by 35° F (19.4° C). At each change of a fan stage the control will wait for one minute for head pressure to stabilize unless T3 and T4 is great- er than 125 F (51.6 C), in which case all MBB-controlled fans start immediately. If T3 and T4 are greater than 95 F (35.0 C) just prior to circuit start-up, all MBB-controlled fan stages are turned on to prevent excessive discharge pressure during pull- down. Fan sequences are shown in Fig. 18.

Motormaster® Option — For low-ambient operation, the lead fan(s) in each circuit can be equipped with the Motormaster III head pressure controller option or accessory. Wind baffles and brackets must be field-fabricated for all units using accessory Motormaster III controls to ensure proper cooling cycle opera- tion at low-ambient temperatures. The fans controlled are those that are energized with the lead compressor in each circuit. All sizes use one controller per circuit. Refer to Fig. 18 for con- denser fan staging information.

Pumpout

EXV UNITS — 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. If a circuit is starting within 15 minutes of the last shutdown, the pumpout cycle will be skipped.

The pumpout cycle starts immediately upon starting the lead compressor and keeps the EXV at minimum position for 10 seconds. The EXV is then opened an additional percentage and compressor superheat control begins. At this point, the EXV opens gradually to provide a controlled start-up to pre- vent liquid flood-back to the compressor.

At shutdown, the pumpout cycle continuously closes the EXV until all lag compressors are off and the EXV is at 0%. The lead compressor continues to run for an additional 10 sec- onds and is then shut off.

TXV UNITS — Pumpout is based on timed pumpout. On a command for start-up, the lead compressor starts. After 15 sec- onds, the liquid line solenoid opens. At shutdown, the liquid line solenoid closes when the lead compressor has stopped.

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