Head Pressure Control (Variable Speed Fans); Head Pressure Control (Fixed Speed Fans) - Carrier AquaForce 30XV140 Controls, Start-Up, Operation, Service And Troubleshooting Instructions

Override #14: Low LWT (Leaving Water Temperature) —
This override stops the compressors if LWT < freeze +
freeze_ov (freeze = Main Menu  Configuration Menu 
Service Parameters  Brine Freeze Setpoint; freeze_ov =
Main Menu  Configuration Menu  Service Parameters
 Freeze Override 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 immediate-
ly without alarm at 35 F (1.7 C).
Override #15: Compressor Disabled — This override is
shown when either of the compressors are disabled through
Main Menu  Configuration Menu  Compressor Enable
menu.
Override #16: High Discharge Pressure — This override at-
tempts to avoid a high pressure failure. If the saturated con-
densing 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  Service Parameters  Evaporator
Fluid Type), this override gets activated at suction pressure
(SP) < 27.7 psig (191 kPa) minus 5 psig (34.5 kPa). 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  Service Parameters  Evaporator Fluid Type),
this override gets activated at suction pressure (SP) < Brine
Freeze Setpoint (converted to psig) (Main Menu  Config-
uration Menu  Service Parameters  Brine Freeze Set-
point) 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 Delay — This override is activated
when the unit is in off state (manually stopped or because of
alarm shutdown) and is requested to start through Local On,
Remote On, CCN On or after alarm shutdown reset.
Override #56: Isolation Valve Opening Delay — This over-
ride mode is activated when the actuated ball valves (if
equipped) on the discharge lines are opening (approxi-
mately 2-minute delay).
Override #59: Low Oil Level — This override is only effec-
tive when the circuit is not running. The override will prevent
the circuit from starting up with a low oil level.
Override #62: High Compressor Motor Temperature
This override prevents the compressor motor temperature
from rising above the high temperature limit, but still allows
the chiller to run close to the high temperature limit by un-
loading the compressor. If the motor temperature is greater
than 195.8 F (91 C), the compressor will not load. This
override will control the loading to the compressor to main-
tain 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 stop-
ping the compressor depending on the conditions. The in-
crease in capacity happens when the DGT > 201 F (93.9 C)
to lower DGT. The control seeks to control the DGT at
190 F 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 evapo-
rator leaving water temperature is close to the freeze point
or lower than the control point. The compressor restarts if
the DGT goes below 186.4 F (85.8 C) and more than 5 min-
utes have passed. This override has priority over almost ev-
ery other one (including the Demand Limit override).
Override #67: DGT off Protection (Discharge Gas Tempera-
ture) — This override is activated before the unit starts and
prevents the unit from starting if the DGT is still greater than
DGT activation point or if 5 minutes has not passed since the
last high DGT shutdown.
Override #77: Oil Pressure at Start — This override is acti-
vated 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 possi-
ble unit capacity. The unit shuts down and/or is on hold until
the demand limit is changed to higher than the minimum
unit capacity.
Table 29 — Capacity Control Overrides
NO.
0 Normal Operation
2 Low Suction Pressure
6 EWT < control point
7 Ramp Loading
9 Demand Limit Reached
10 Flow switch is open
11 Customer Interlock is closed
12 Flow Available Delay
14 Low LWT
15 Compressor Disabled
—
16 High Discharge Pressure
23 Low SP
34 Low SST
53 ON OFF Delay
56 Evaporator Heater Isolation Valve Opening Delay
59 Low Oil Level
62 High Compressor Motor Temperature
66 High Discharge Gas Temperature
67 DGT Off Protection
77 Oil Pressure at Start
91 Demand Limit
Head Pressure
The head pressure is controlled through the Touch
Fans) —
—
Pilot 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 function 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  Fan_CFG  Optimization Enable On, Off
[xt_enable]). By default the xt_enable variable is "On" which
means the optimization control is active. It can be set to "Off"
to deactivate the optimization. The frequency adjustment
during optimization control can be adjusted through the vari-
able os_d_amp (Service  Fan_CFG  Opt Disturbance
Ampl. (Hz) [os_d_amp]). By default os_d_amp is 5 Hz and
can be lowered to 3 Hz if the unit load demand is very steady.
Lowering the frequency below 3 Hz is not recommended and
could cause head pressure control instability, causing load and
power draw oscillations. All the other optimization parameters
in table (Service  Fan_CFG) are not recommended for
change, however are explained briefly. The Stability Sample
42
DESCRIPTION
Control (Variable Speed