Head Pressure Control (Variable Speed Fans) - Carrier AquaForce B Series Controls, Start-Up, Operation, Service And Troubleshooting Instructions

Override #9: Demand Limit
This override mode is active when a command to limit the ca-
pacity is received and the current capacity requirement meets
or exceeds the demand limit value. If the current unit capacity
is greater than the active capacity limit value, the unit unloads
per unloading scheme. The current capacity will stop increas-
ing when it reaches the capacity limit value minus 3%.
Override #10: Flow Switch is Open
This override prohibits compressor operation until the evapora-
tor flow switch is closed.
Override #11: Customer Interlock is Closed
This override prohibits compressor operation until the custom-
er interlock is opened.
Override #12: Flow Available Delay
This override prohibits chiller operation until flow has started.
Override #14: Low LWT (Leaving Water Temperature)
This override stops the compressors if LWT < freeze + freeze_ov

(freeze = Main Menu Configuration Menu

rameters Brine Freeze Setpoint; freeze_ov = Main Menu

Configuration Menu Service Parameters
Offset). The goal is to stop the unit without having an alarm if the
LWT goes too low so that the unit can start automatically without
the need to reset alarm. For example, freeze is 34°F (1.1°C); the
user can decide to add a threshold to force the compressors to stop
immediately without alarm at 35°F (1.7°C).
Override #15: Compressor Disabled
This override is shown when either of the compressors are dis-
abled through Main Menu
pressor Enable menu.
Override #16: High Discharge Pressure
This override attempts to avoid a high pressure failure. If the
saturated condensing temperature for the circuit is above the
high pressure threshold the compressor is unloaded while the
fan is run at maximum frequency.
Override #23: Low SP (Suction Pressure)
When the unit is configured with evaporator fluid as water

(Main Menu Configuration Menu

Evaporator Fluid Type = Water), this override is activated at
SP <22.7 psig (156.5 kPa) (6°F SST). In this mode the circuit
will not be allowed to load further until the SST goes above 30°F
(–1.1°C). When the unit is configured with evaporator fluid as

brine (Main Menu Configuration Menu

ters Evaporator Fluid Type = Med Brine), this override is
activated when SST < Brine Freeze Setpoint (Main Menu

Configuration Menu Service Parameters
Setpoint) minus 5 psig (34.5 kPa).
Override #34: Low SST (Saturated Suction Temperature)
The compressor is not allowed to start if the SST is lower than
–13°F (–25°C).
Override #53: ON OFF ON Delay
This override is activated when the unit is in off state (recycle,
manually stopped, or because of alarm shutdown). The control
will remain in this state for the next 3 minutes. This is to re-
duce short cycling.
Override #56: Isolation Valve Opening Delay
This override mode is activated when the actuated ball valves
(if equipped) on the discharge lines are opening (approximate-
ly 2-minute delay).
Override #59: Low Oil Level
This override is only effective when the circuit is not running.
The override will prevent the circuit from starting up with a
low oil level.

Service Pa-

Freeze Override
 
Configuration Menu

Service Parameters

Service Parame-

Brine Freeze
Override #62: High Compressor Motor Temperature
This override prevents the compressor motor temperature from
rising above the high compressor motor temperature limit, but
still allows the chiller to run close to the high temperature limit
by unloading the compressor. If the motor temperature is great-
er than 195.8°F (91°C), the compressor will not load. This
override will control the loading to the compressor to maintain
a maximum motor temperature of 194°F (90°C). The circuit
will come out of this mode if the motor temperature falls below
190.4°F (88°C) or if motor temperature is below 195.8°F
(91°C) and water temperature is established.
Override #66: High Discharge Gas Temperature (DGT)
This override avoids high DGT tripout by either increasing
compressor capacity or decreasing the capacity and stopping
the compressor depending on the conditions. The increase in
capacity happens when the DGT > 201°F (93.9°C) to lower
DGT. The control seeks to control the DGT at 190°F (87.8°C)
and if the DGT goes below 186.4°F (85.8°C) the unit goes to
normal control. The decrease in capacity followed by unit stop
happens if Override 66 is activated and evaporator leaving wa-

ter temperature is close to the Brine Freeze Setpoint (Main

Menu Configuration Menu
than the Control Point (Main Menu
The compressor restarts if the DGT goes below 186.4°F
(85.8°C) and more than 5 minutes have passed. This override
has priority over almost every other one (including the De-
mand Limit override).
Override #67: DGT off Protection (Discharge Gas Temperature)
This override is activated before the unit starts and prevents the
Com-
unit from starting if the DGT is still greater than DGT activa-
tion point or if 5 minutes has not passed since the last high
DGT shutdown.
Override #77: Oil Pressure at Start
This override is activated when the unit has just started passed the
start timer and OP<(SP+19.5) AND OP<(0.7*(DP-SP)+SP).
In this override the unit freezes the compressor loading and
waits until the oil pressure exceeds the above conditions and
then exits to normal mode of operation.
Override #91: Demand Limit
This override is activated when the demand limit is set lower
than the minimum possible unit capacity. The unit shuts down
and/or is on hold until the demand limit is changed to higher
than the minimum unit capacity.

Head Pressure Control (Variable Speed Fans)


The head pressure is controlled through the Carrier Controller
display by adjusting fan speed through variable speed drive(s).
The command sent to the drive is at a frequency to maintain the
lowest condensing temperature possible, and thus, the highest
unit efficiency. The frequency command sent is based on a func-
tion of compressor capacity, OAT, and leaving fluid temperature.
If the capacity is stable and no overrides have occurred recently,
an algorithm attempts to optimize fan frequency based on total
power feedback. The optimization control can be turned on and
off through the Network Service Tool (Service
Optimization Enable On, Off [xt_enable]).
Fan control continuously monitors all inputs and outputs and the
transitions between the modes are defined based on continuous
measurements of 2 inputs (Discharge Pressure and Discharge Gas
Temperature).
Fan modes of operation include the following:
• STANDARD: Normal mode of operation before using the
optimum-seeking algorithm.
• WAITOPT, OPTIMZE: Trying to optimize the fan fre-
quency during the optimum-seeking algorithm.
• FREEZE: Fan frequencies are frozen after completion of
the optimum-seeking algorithm cycle. The fan control
48

Service Parameters) or lower

General Parameters).

Fan_CFG
