Honeywell AUTOMATIC CONTROL Engineering Manual page 317

Anytime chilled water flow is proven via a chilled water pump
current sensing relay, the chiller controls shall be enabled to
operate under factory controls, subject to a chiller software ON-
OFF-AUTO function (chilled water flow must still be proven
in the "ON" mode). Provide control and interlock wiring per
the chiller manufacturers recommendation.
Upon a call for chilled water, the chiller controls shall start the
condenser water pump and energize the cooling tower fan controls.
When condenser water flow is proven via a condenser water
pump current sensing relay, the chiller shall start, operate, and
load under chiller factory controls to maintain the chilled water
temperature setpoint, 46F at start-up.
Anytime all chilled water valves are less than 85 percent open,
the chilled water temperature setpoint shall be incremented at
a rate of 0.3F every 10 minutes up to a maximum of 52F.
Anytime any chilled water valve is full open, the chilled water
temperature setpoint shall be decremented at a rate of 0.3F
degrees every 10 minutes down to a minimum of 45F.
The maximum allowable percentage of chiller full load
electrical current shall be commandable from the BMCS, and
shall be 80 percent during all unoccupied periods of operation.
MULTIPLE CHILLER SYSTEM
CONTROL APPLICATIONS
Multiple chiller systems offer standby capacity and improved
economy at partial loads. Multiple chiller systems may be piped
for either parallel or series chilled water flow.
In the parallel piped arrangement (Fig. 11), return chilled
water is divided among the chillers then recombined after
chilling. Two methods of operation at light loads are depicted.
One uses a pump and a check valve for each chiller. The other
uses a common pump with an isolation valve for each chiller.
Multiple pumps with check valves allow one chiller and the
associated pump to be shut down during light load conditions
to save energy and require that the system be able to operate
with the reduced flow. The check valves prevent reverse flow
through the shut down chiller. Use of a common pump and
isolation valves require that the operating chiller be able to
withstand full system flow. The isolation valves allow the
operating chiller to supply only the chilled water temperature
required to meet system demands. Without the isolation valves,
half of the water flows through the chiller which is shut down
and is not cooled. When the uncooled water is mixed with the
cooled water, the temperature will be the average of the water
temperatures. As a result, the on-line chiller must supply water
cool enough so that the average will satisfy the primary sensor
and thus the system. To meet this requirement the on-line chiller
may need to supply water close to the freezing point.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
is the primary capacity control. The temperature low limit
control prevents the outlet temperature of each chiller from
going too low. A return water temperature sensor can be used
in conjunction with a supply water temperature sensor to turn
off one chiller in light load conditions.
ISOLATION
VALVES
USED WITH
COMMON
PUMP
drop is higher if the chillers are not designed for higher flow.
At partial loads, compressor power consumption is lower than
for the parallel arrangement.
are piped in series counterflow to balance loading. When piped
series-counterflow, Chiller 1 receives warmer condenser and
chilled water while Chiller 2 receives colder entering condenser
and chilled water. This makes refrigerant head approximately
the same for each chiller. The controls may be set to shutdown
either chiller at partial loads.
307
The temperature sensor in the common chilled water supply
Fig. 11. Parallel Piped Chillers.
In the series arrangement (Fig. 12) chilled water pressure
When the condensers of series units are water cooled, they
CHILLER 1
CONDENSER
COMPRESSOR
EVAPORATOR
LOW
LIMIT
Fig. 12. Series Piped Chillers.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER 2
CONDENSER
PRIMARY
SENSOR
COMPRESSOR
EVAPORATOR
LOW
LIMIT
CHILLER 1
CONDENSER
COMPRESSOR
LOAD
EVAPORATOR
LOW
LIMIT
COMMON
PUMP
C2691
CHILLER 2
CONDENSER
COMPRESSOR
EVAPORATOR
PRIMARY
SENSOR
LOAD
C2692