Carrier GTR040-420 Specifiche Scaricare il pdf (Pagina 28)

Low Entering Fluid Temperature Unloading

— When the

entering fluid temperature is below the control point, the con-

trol will attempt to remove 25% of the current stages being

used. If exactly 25% cannot be removed, the control removes

an amount greater than 25% but no more than necessary. The

lowest stage will not be removed.

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 Med. Brine, the freeze point is COOL

set point CSP.3 minus 8° F (4.4° C).

If the cooler suction temperature 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 suction 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.

Brine Freeze Set Point

— If the cooler medium is glycol or

other suitable corrosion-inhibited antifreeze solution, then

CSP.3 should be set to the freeze point of the solution. This will

allow proper unit operation and prevent unnecessary Low

Cooler Suction Temperature and Cooler Freeze Protection

Alarms.

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.

High Condensing Temperature Unloading

— Every 10 sec-

onds the control checks for the conditions described below.

Unloaders and compressors will be cycled as needed to reduce

the saturated condensing temperature to below the configured

maximum condensing temperature. The factory default maxi-

mum condensing temperature is 154 F. If the condensing tem-

perature of a circuit is below the maximum condensing temper-

ature and within 12° of the limit, then the circuit capacity is not

allowed to increase. If the condensing temperature of a circuit

is below maximum condensing temperature and within 2° of

the limit and rising for 60 seconds, then an unloader will turn

on or a compressor will shut off to reduce unit capacity. If the

saturated condensing temperature is above the maximum con-

densing temperature, then unloaders will immediately turn on

or compressors will turn off to reduce unit capacity.

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 point which is 113 F (45 C) as a factory default.

The fans can also be controlled by a combination of the saturat-

ed condensing temperature, EXV position and compressor su-

perheat. Fan control is a configurable decision and is deter-

mined by the Head Pressure Control Method (HPCM) setting

in the Configuration Mode under the OPT1 sub-mode.

The default control mode is setpoint control (HPCM = 2)

where the fans modulate based on highest saturated condensing

temperature (T3 or T4) of the two refrigerant circuits. 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). If Motormaster® control is present, the set

point is raised 15° F (8.3° C) or an additional 5° F (2.8° C) if

within the first 10 minutes of operation in each circuit. The

Motormaster control offsets apply to the first additional stage

of fans only. The MBB 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 35 seconds 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 discharge pres-

sure during pulldown. Fan sequences are shown in Fig. 18.

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 re-

moved. When the valve is less than 40% open, or T3 and T4

are greater than 113 F (45 C), fan stages will be added. For the

first 10 minutes of each circuit operation, the head pressure set

point is raised by 10° F (5.6° C). If Motormaster control is

present, the set point is raised 15° F (8.3° C) or an additional

5° F (2.8° C) if within the first 10 minutes of operation in each

circuit. The Motormaster control offsets apply to the first addi-

tional stage of fans only. At each change of the fan stage, the

system will wait 35 seconds to allow the head pressure to stabi-

lize 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 tem-

peratures at part load.

Motormaster Option

— For low-ambient operation below

0° F (–18.0 C), the lead fan(s) in each circuit can be equipped

with the Motormaster controls as a factory or field-installed

0 200 400 600 800 1000

TIME (SECONDS)

2 STARTS

3 STARTS

DEADBAND EXAMPLE

WT (F)

MODIFIED

DEADBAND

STANDARD

DEADBAND

WT (C)

Fig. 17 — Deadband Multiplier

LEGEND

LWT —

Leaving Water Temperature

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 103 104

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Carrier GTR040-420 Specifiche Pagina 28

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