Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual page 150

MICROPROCESSOR-BASED/DDC FUNDAMENTALS
Sun, weather, and occupancy (building utilization) dictate
load reset demands. The sun and weather effects are relatively
slow and occur as the sun and seasons change. Occupancy
changes are abrupt and occur over brief periods of time.
If a chiller plant with 6.5 C design chilled water temperature
is controlled to increase chilled water temperature any time all
control valves are less than 80 percent open. Two air handling
units with different control sequences are compared.
1. Control valves are on a VAV AHU chilled water coil and
are part of a 13 C discharge air temperature control loop.
The load reset sequence events are:
a. The most demanding AHU valve closes to below
80 percent open.
b. The load reset program raises the chilled water
temperature setpoint.
c. The chiller unloads to maintain the raised
setpoint.
d. As the chilled water temperature increases, the
discharge air temperature increases.
e. The discharge air temperature controls open the
AHU valves to maintain the discharge air
temperature setpoint.
f. The most demanding AHU valve opens to greater
than 80 percent but less than 95 percent.
g. The other AHU valves open increasing the chiller
load.
h. The two temperature loops stabilize in time. The
chiller loop is usually set for a fast response and
the discharge air loop is set for a slow response.
2. Control valves are on single zone AHU chilled water coil,
controlled from space temperature at 24.5 C.
a. The most demanding AHU valve closes to below
80 percent open.
b. The load reset program raises the chilled water
temperature setpoint.
c. The chiller unloads to maintain the raised
setpoint.
d. As the chilled water temperature increases, the
discharge air temperature increases.
e. The space temperature control opens the valves
to maintain the space temperature setpoint. This
response takes several minutes in space
temperature control.
f. The most demanding AHU valve opens to greater
than 80 percent but less than 95 percent.
g. The other AHU valves open increasing the chiller
load.
h. The two temperature loops stabilize in time. The
chiller loop is usually set for a fast response and
the discharge air loop is set for a slow response.
The events of the first scenario occur within seconds because
both loops (leaving water temperature controlling the chiller
load and discharge air temperature controlling chilled water
flow) are close coupled and fast. Because the two loops oppose
each other (a chilled water temperature rise causes discharge
air temperature to rise which demands more chilled water flow),
a few minutes must be allowed for system stabilization. The
chilled water temperature control loop should be fast and keep
the chilled water near the new setpoint while the AHU
temperature loops slowly adjust to the new temperature.
Hysteresis is a critical load reset parameter. Water temperature
is raised if all valves are less than 80 percent open but, is not
lowered until one valve is greater than 95 percent open. This
15 percent dead band allows lengthy periods of stability between
load reset increases and decreases. Properly tuned load reset
programs do not reverse the commands more than once or twice
a day.
Scenario 1 initial parameters could be; command chilled
water temperature increments of 0.2 kelvins, a load reset
program execution interval of 4.0 minutes, a decrement
threshold of 80 percent, (most demanding valve percent open),
an increment threshold of 95 percent (most demanding valve
percent open), a start-up chilled water temperature setpoint of
7 C, a maximum chilled water temperature setpoint of 10.5 C,
and a minimum chilled water temperature setpoint of 6.5 C.
The load reset chilled water temperature program may include
an AUTO-MANUAL software selector and a manual chilled
water temperature setpoint for use in the manual mode.
Unlike scenario 1, the events within scenario 2 occur over
several minutes (not seconds) because when the chilled water
temperature setpoint is raised, it takes several minutes for the
water temerature rise and the resulting air temperature increase
to be fully sensed by the space temperature sensor.
Scenario 2 parameters could be the same as scenario 1 with
the exception of the execution interval which should be about
15 minutes.
All parameters should be clearly presented and easily
commandable. Figure 10 is an example of dynamic data display
for scenario 1.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
140