Carrier 30GTR Specifications download pdf (Page 27)

Head Pressure Control

COMFORTLINK™ UNITS (With EXV) — The Main Base

Board (MBB) controls the condenser fans to maintain the

lowest condensing temperature possible, and thus the high-

est unit efficiency. The fans are controlled by the saturated

condensing temperature set from the factory. The fans can

also be controlled by a combination of the saturated con-

densing temperature, EXV position and compressor super-

heat. 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 = 2), 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 ex-

cessive 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 min-

utes 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 greater 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 dis-

charge pressure during pull-down. Fan sequences are shown

in Fig. 17.

Motormastert Option — For low-ambient operation, the lead

fan(s) in each circuit can be equipped with the Motor-

master III head pressure controller option or accessory. Wind

baffles and brackets must be ﬁeld-fabricated for all units us-

ing accessory Motormaster III controls to ensure proper cool-

ing cycle operation at low-ambient temperatures. The fans

controlled are those that are energized with the lead com-

pressor in each circuit. All sizes use one controller per cir-

cuit. Refer to Fig. 17 for condenser 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 per-

centage and compressor superheat control begins.At this point,

the EXV opens gradually to provide a controlled start-up to

prevent 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 seconds 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 seconds, the liquid line solenoid opens. At shutdown, the

liquid line solenoid closes when the lead compressor has

stopped.

0 200 400 600 800 1000

TIME (SECONDS)

2 STARTS

3 STARTS

DEADBAND EXAMPLE

LWT (F)

MODIFIED

DEADBAND

STANDARD

DEADBAND

LWT (C)

LEGEND

LWT — Leaving Water

Temperature

Fig. 16 — Deadband Multiplier

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 83 84

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Carrier 30GTR Specifications Page 27

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