Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 156

MICROPROCESSOR-BASED/DDC FUNDAMENTALS
staged by a PI algorithm with software heat anticipation. See
Figure 15. During reheat, the control mode changes to constant
volume, variable discharge temperature.
MAX
CONSTANT VOLUME,
FLOW
VARIABLE DISCHARGE TEMPERATURE
REHEAT
FLOW
MIN
FLOW
COLD
Fig. 15. Control Sequence for VAV Cooling with
Sequenced Electric Reheat.
SYSTEM-LEVEL CONTROLLER
System-level controllers are variable-function devices applied
to a wide variety of mechanical systems. These controllers can
accommodate multi-loop custom control sequences and have
control integrated with energy management and building
management functions. The examples that follow cover direct
digital control functions for a system-level controller. Integrated
building management functions are covered in the Building
Management System Fundamentals section.
Where the examples indicate that user entered values are
furnished (e.g., setpoint), or that key parameters or DDC
operator outputs will have display capability, this represents
sound software design practice and applies whether or not the
controller is tied into a central building management system.
Data is entered or displayed in non-BMS applications by a
portable operator's terminal or by a keypad when display is
integral with the controller.
A five-step approach can be used to define DDC programs.
1. Develop a system flow schematic as a visual
representation of the process to be controlled. The
schematic may be provided as a part of the plans and
specifications for the job. If not, a schematic must be
created for the system.
2. Add actuators, valves, sensors, setpoints, and operational
data required for control and operation.
3. Write a detailed sequence of operation describing the
relationship between inputs, outputs, and operational data
points.
CONSTANT
DISCHARGE
TEMPERATURE,
VARIABLE
DEAD BAND VOLUME
HOT
HEATING COOLING
SETPOINT SETPOINT
SPACE LOAD
4. Develop a detailed flowchart of the control sequence
using either DDC operators or a programming logic flow
diagram. Programs written totally in a high-level language
use the logic flow diagram.
5. Write the program using either DDC operators (Table 1)
or high-level language statements.
An example of this approach follows for control of a hot
water converter:
Step 1—Develop flow schematic of the process to be controlled
(Fig. 16).
C2686
HOT WATER SUPPLY
HOT WATER RETURN
Fig. 16. Schematic of Steam to Hot Water Converter.
Step 2—Identify required sensors, actuators, and operational data
(Fig. 17). Refer to the Chiller, Boiler, and Distribution
System Control Applications section for a symbol legend.
OUTSIDE
OUTSIDE
AIR
AIR
16
-6
-20
HOT WATER RESET
PUMP-ON
SETPOINT
AUTO
11
HOT
WATER
RETURN
HOT WATER
PUMP
Fig. 17. Schematic Illustrating Sensors, Actuators, and
Operational Data for Steam to Hot Water Converter.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
146
STEAM TO
HOT WATER
CONVERTER
HOT WATER
SETPOINT
SETPOINT
67
50
77
SCHEDULE
SP
IN OUT
PID
67
HOT WATER
CONVERTER
60
ON
STEAM
M15035
PERCENT
58
OPEN
STEAM
VALVE
STEAM TO
M15139