Dual Chiller Control - Carrier AquaForce B Series Controls, Start-Up, Operation, Service And Troubleshooting Instructions

Dual Chiller Control

The dual chiller function allows for master/slave control of two
units installed in parallel or series arrangement supplying
chilled fluid on a common loop. The chillers must be linked by
the Carrier Comfort Network
same bus.
When the units are installed for parallel operation and chilled
water control is done on the outlet side of the units, the dual
chiller accessory kit (P/N 00EFN900044000A) is required. The
kit includes additional leaving fluid temperature thermistors
that must be installed on the common chilled water leaving
piping as described in the Installation Instructions for the kit.
The leaving fluid temperature sensors will be connected to
each chiller as described in the installation instructions. When
the chilled water control is done on the inlet side of the parallel
units no additional temperature sensor is required. See the
Field Wiring section in the 30XV Installation Instructions for
dual chiller LWT sensor control wiring. When chillers are con-
figured to operate in series mode no additional chilled water
temperature sensor is required.
The master chiller will monitor all external commands such as
start/stop, demand limiting, or set point select, and needs to be
started in Master operating type. The commands are transmitted
automatically to the slave unit, which must operate in CCN (Net-
work) mode. The slave chiller has no action in the master/slave
operations; it will only verify that CCN communication with the
master chiller is correct. If the master chiller is turned off while the
master/slave function is active, then the slave chiller will be
stopped. Under certain circumstances, the slave unit may be start-
ed first to balance the run times of the two units. In the event of a
communication failure between the two units, each unit will return
to an autonomous operating mode until the fault is cleared. If the
master unit is stopped due to an alarm, the slave unit is authorized
to start and therefore the slave unit configurations should be veri-
fied with desired set points.
The CCN communication port for the Master and Slave
chillers must be joined using a shielded cable in order to avoid
communication issues.
The master/slave linkage will not be allowed to operate if any
one of the slave chiller CTRL_PNT, DEM_LIM, LAG_LIM,
or LCW_STPT variables has a force priority higher than a
control force. In that case, the master/slave operations will not
be allowed or will be disabled.
The control algorithm relies on several parameters that must be
field configured for operation. Both chillers must be on the
same CCN bus with different addresses. On both chillers, Mas-
ter/Slave Select (Main Menu

Master/Slave Config Master/Slave Select) must be enabled
(set to 1 [Master] or 2 [Slave]). The water piping arrangement
must be specified with the Chiller in Series variable (Main

Menu Configuration Menu
Chiller in Series). The Master chiller must be programmed
with the Slave Address (Main Menu
 
Master/Slave Config
al programming parameters may be configured to meet appli-
cation requirements.
The Lead Lag Select variable (Main Menu

Menu Master/Slave Config
which chiller is the lead machine. The options are: Always Lead,
Lag Once Failed Only, and Lead/Lag Runtime Select. Under Run-
time Select control, the lead chiller will change based on the time
increment selected in the Lead/Lag Balance Delta configuration

(Main Menu Configuration Menu

Lead/Lag Balance Delta). If the run hour difference between
the master and the slave remains less than the Lead/ Lag Balance
Delta, the chiller designated as the lead will remain the lead
chiller. The Lead/Lag changeover between the master and the
slave chiller due to hour balance will occur during chiller operat-
ing odd days, such as day 1, day 3, and day 5 of the month, at
® network and operate on the

Configuration Menu

Master/Slave Config

Configuration Menu
Slave Address). Additional option-

Configuration

Lead Lag Select) determines

Master/Slave Config
12:00 a.m. If a lead chiller is not designated, the master chiller will
always be designated the lead chiller.
The dual chiller control algorithm has the ability to delay the
start of the lag chiller in two ways. The Lead Pulldown Time
parameter (Main Menu

Slave Config Lead Pulldown Time) is a one-time delay ini-
tiated after starting the lead chiller, before checking whether to
start an additional chiller. This time delay gives the lead chiller
a chance to remove the heat that the chilled water loop picked
up while inactive during an unoccupied period. The second
time delay, Lead/Lag Start Timer (Main Menu

tion Menu Master/Slave Config
is a time delay imposed between the last stage of the lead
chiller and the start of the lag chiller. This prevents enabling
the lag chiller until the lead/lag delay timer has expired.
A quicker start of the lag chiller can be accomplished by con-
figuring the Lag Unit Pump Control parameter (Main Menu
Configuration Menu
Pump Control). If the difference between the common leaving
water temperature and the set point is greater than the config-
ured value, then the lag chiller will start.
A minimum on time for the lag chiller can be programmed
with the Lag Minimum Running Time configuration (Main

Menu Configuration Menu
Lag Minimum Running Time). This parameter causes the
control to run the lag chiller for the programmed minimum on
time. The Lag Unit Pump Control (Main Menu

tion Menu Master/Slave Config
trol) can be configured such that the pump can be on or off
while the chiller is off. This parameter is only active in Parallel
Chiller Operation.
The lead chiller is started first and the lag chiller will be main-
tained at zero percent capacity through master forcing the lag
demand limit value (LAG_LIM) to 0%. The lag water pump
will be maintained off. When the lead chiller cannot be loaded
anymore (because it is loaded at its full available capacity or at
the master demand limit value) then the lag start timer is start-
ed. When the lag start time has elapsed, if the error on the mas-
ter controlled set point is greater than the dead band (start_di)
and if the pulldown time is elapsed, then the lag chiller water
pump will be turned on (if required by configuration) and the
lag chiller will be allowed to start through the master chiller
forcing the lag chiller demand limit value (LAG_LIM) to its
own demand limit value. To ensure that the lag chiller will be
unloaded first in case of water load decrease, the lead chiller
set point error will be reset downwards by 4°F (2.2°C) provid-
ed that the lead capacity is not zero.

Each dual chiller application, Parallel and Series, is described
separately below.
DUAL CHILLER CONTROL FOR PARALLEL APPLICA-

TIONS
To configure the master chiller for parallel applications, see Ta-
ble 26. To configure the slave chiller for parallel applications,
see Table 27.
33

Configuration Menu


Lead/Lag Start Timer)
 
Master/Slave Config

Master/Slave Config


Lag Unit Pump Con-

Master/
Configura-

Lag Unit

Configura-