ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
309
NOTE: When using two-way AHU valves with this
coupled chiller configuration, exercise care in
optimizing the chilled water temperature. With both
chillers running, raising the chilled water tempera-
ture results in greater flow and a smaller ∆T which
must be considered in the chiller shedding strategy.
For an example of a pressure bypass system, see
DUAL PUMPS, DUAL CHILLERS, PRESSURE
BYPASS, 90 PERCENT CHILLER FLOW,
DIRECT RETURN.
Specification
The chiller system operation shall be enabled anytime the
time of day is before 1545 and any AHU chilled water valve is
open greater than twenty percent for greater than three minutes.
Anytime the chiller system is enabled, the lead chilled water
pump shall run.
Anytime the lead chiller has run longer than 90 minutes, the
chilled water temperature has been greater than 0.5 kelvins above
the chilled water temperature setpoint for greater than 4 minutes,
and the time is less than 1545, the off chilled water pump shall start.
Anytime both chillers are running and the chiller plant water
differential temperature has been less than 2.4 kelvins for greater
than 4 minutes, the chilled water pump with the longest “on”
duration since the last start shall stop and remain off at least 30
minutes.
Anytime chilled water flow is proven via a chilled water pump
current sensing relay, the respective 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, 6.5°C at start-up.
Anytime all chilled water valves are less than 80 percent open,
the chilled water temperature setpoint shall be incremented at a
rate of 0.15 kelvins every 5 minutes up to a maximum of 10°C.
Anytime any chilled water valve is full open, the chilled water
temperature setpoint shall be decremented at a rate of
0.15 kelvins every 5 minutes down to a minimum of 5.5°C.
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.
Fig. 16. Multiple Equal Sized Chillers Control Graphic.
SIMILAR MULTIPLE CENTRIFUGAL CHILLERS CONTROL APPLICATIONS
Equal Sized Centrifugal Chillers Control
C
H
I
L
L
E
R
4
7.0
C
H
I
L
L
E
R
3
7.0
C
H
I
L
L
E
R
2
7.0
C
H
I
L
L
E
R
1
7.0
7.0
T
SP4
T
SP3
T
SP2
T
SP1
T
SP
7.0
13.0 13.0
T
D
T
RP
T
RS
T
S
= SUPPLY TEMPERATURE
= RETURN TEMPERATURE
= SUPPLY TEMPERATURE, SECONDARY
= SUPPLY TEMPERATURE, PRIMARY
= FLOW
= DECOUPLER TEMPERATURE
7.0
T
SS
7.4
T
SS
P
S1
P
S2
T
R
T
SS
T
SP
F
T
D
P
P4
P
P3
P
P2
P
P1
TEE B
TEE A
SECONDARY
FLOW LOOP
NOTE: F = SUM OF PRIMARY PUMP FLOWS
BASED UPON TEST-AND-BALANCE
DATA AND PUMP ON-OFF STATUS.
P
F
S
= SECONDARY FLOW
= PRIMARY FLOW,
= DIFFERENTIAL TEMPERATURE, PRIMARY
= DIFFERENTIAL TEMPERATURE, SECONDARY
F
P
T
P
T
S
WHEREF
S
F
P
=
T
P
∆
∆
T
S
∆
∆
PRIMARY FLOW LOOP
DECOUPLER
LINE
FORWARD
FLOW
BACKWARD
FLOW
M15257
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