Honeywell AUTOMATIC CONTROL Engineering Manual page 316

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
ANYTIME ANY AHU VALVE IS OPEN GREATER THAN
MINUTES AND THE TIME IS BEFORE
ENABLES THE CHILLER SYSTEM CONTROLS.
AT CHILLER START-UP, THE CHILLED WATER TEMPERATURE SETPOINT IS
ANYTIME ALL AHU CHILLED WATER VALVES ARE LESS THAN
SETPOINT INCREMENTS UP AT THE RATE OF
MINUTES UP TO A MAXIMUM OF
ANYTIME ANY AHU CHILLED WATER VALVE IS FULL OPEN, THE CHILLED WATER
TEMPERATURE SETPOINT DECREMENTS DOWN AT THE SAME RATE TO A MINIMUM
45
OF DEGREES.
Features
1. Automatic start-stop and setpoint optimization of chiller.
2. User friendly monitoring and adjustment.
3. Optimized unoccupied operation.
4. Chiller cannot start late shortly before the unoccupied
period begins.
Conditions For Successful Operation
1. Control network, software, and programming advises
chiller controller of AHU chilled water demands.
2. Interlock and control wiring coordinated with the chiller
manufacturer.
3. Appropriate cooling tower and control.
4. For single-chiller systems without primary-secondary
pumping, three-way air handling unit valves may be used
for 80 to 85 percent of the chilled water flow (Small
valves, up to 15 to 20 percent total flow, may be two-
way, which are simpler to pipe.) If all two-way valves
are provided on single pump systems, chilled water flow
or pressure controls (See DUAL CENTRIFUGAL
CHILLERS) are provided to maintain the required flow
(varies with chiller manufacturers) through the chiller.
Do not use three-way valves when diversity is used in
the chiller system design.
NOTE: Little pumping energy can be saved on a
single-pump single-chiller system by using
two-way AHU control valves since the chiller
usually requires high flow anyway.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLED WATER SYSTEM
CHILLED WATER SETPOINT
45 46
MINIMUM CURRENT
1545
52
DEGREES.
Fig. 10. Single Chiller Control Dynamic Sequence Display.
52
MAXIMUM
% FOR MORE THAN
20
THE CHILLED WATER PUMP STARTS AND
85
0.3
DEGREES EVERY
5. During the unoccupied period the 80 percent load limiting
parameter (see SPECIFICATION following) is based on
the assumption that AHUs are VAV and are operating
under a reduced maximum cfm setpoint during all
unoccupied cooling modes (see the Air Handling System
Control Applications) to save fan energy and place the
chiller operation in the maximum efficiency range.
6. Chilled water temperature reset from AHU chilled water
valve position requires:
a. No valve always full open.
b. Maximum of 30 to 40 valves. With too many valves
the probability that one valve will always be open is
very great.
c. Zone setpoint discipline is maintained. Lowering
setpoints to resolve complaints may result in one or
more valves being always open.
7. Chilled water temperature reset timing increments are
compatible with valve control response. If the temperature
reset is too fast, the valve cannot adjust to the new
temperature, resulting in instability.
Specification
The chiller system operation shall be enabled anytime the
time of day is less than 1545 and any AHU chilled water valve
is open greater than twenty percent for more than three minutes.
Anytime the chiller system is enabled, the chilled water pump
shall run.
306
3
46 .
% OPEN, THE
5.5
M10495