4.19 - Two Units Master/Slave Control
4.19.1 - General
This function allows to perform master/slave plant control between two chillers linked by the CCN network. Master
and slave chiller need to operate on the same bus. Master/slave operations shall operate in cooling or heating mode.
To operate properly, the master and the slave chillers must have an additional CHWS sensor located on the
common chilled leaving piping when the water control is done on the outlet side. This CHWS sensor shall be
connected on each chiller basic board. When the water system control is done on the inlet side no additional CHWS
sensor is required.
Parallel chiller control with dedicated pumps is recommended. Chiller must start and stop its own water pump
located on its own piping. If pumps are not dedicated for each chiller pimping, chiller isolation valves are required:
each chiller must open and close its own isolation valve through the control (valve shall be connected to the pump
outputs).
Water system control with no individual chiller pump control is feasible. It is not recommended when the system load
is low or if the control point is close from the freeze water tripping point (because of system instability that can be
caused by the freeze protection override). System flow is constant even if the master or the slave chiller is stopped.
In that case, the system pump shall be controlled by the master chiller.
Series chillers with dedicated primary pumping and constant flow shall be feasible but not recommended. Pump
start/stop must be controlled externally. Each series chiller must control its own isolation and bypass valves: stopped
chiller must be isolated from water flow (chiller pump command shall be used for valves control. Each chiller must
open and close its own chilled isolation valve).
The master chiller shall monitor all external commands as start/stop, demand limiting or setpoint select. It needs to
be started in
Master
operating type. The slave chiller must operate in CCN mode. If the master chiller is turned off
while the master/slave function is active then, the slave chiller shall be stopped.
The master/slave linkage shall not be allowed to operate if any one of the slave chiller CTRL_PNT, DEM_LIM,
LAG_LIM, CTRL_PNT HC_SEL or LCW_STPT variables has a force priority higher than a control force. In that case,
the master slave operations shall not be allowed or shall be disabled.
The control of the slave chiller shall be done through commands emitted by the master chiller. The slave chiller has
no action in the master/slave operations: it shall only verify that CCN communication with its master is correct.
The master function shall provide the ability to select a lead chiller among the master and the slave. Selection shall
be based on the delta between the master and the slave run hours and shall try to optimize the runtime hours. If this
function is unused then, the lead chiller shall always the master. Lead/lag changeover between the master and the
slave due to hour balance shall occur during chiller operations odd days at 12:00 a.m. or at startup.
The lead chiller shall always be started first and the lag chiller shall be maintained at zero percent capacity through
master forcing the lag demand limit value (LAG_LIM) to 0%. The lag water pump shall 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 started. When the lag start timer is elapsed, if the error on the master controlled
setpoint is greater than 3^F and if the pulldown time is elapsed then, the lag chiller water pump shall be turned on (if
required by configuration) and the lag chiller shall be allowed to start through master 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 then, the lead chiller setpoint error shall be reset of upwards 4^F provided that the lag capacity
is not zero.
CARRIER SA
USER'S GUIDE FOR
30GX&HX PRO_DIALOG 4 V.2
ECG-UG-02-002
Rev: Original
Page: 53/98
©
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ARRIER
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