Variable Speed Pump Control; Decoupled Variable Speed Pump Control, Direct Return - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

SUPPLY
C
C
H
H
I
I
L
L
L
L
E
E
R
R
1
2
DP
1
RETURN
In these examples, the design differential pressure across
Load 1 is 84 kPa (Fig. 74), and pressure bypassing occurs at
103.5 kPa. The maximum setpoint for DP-2 (Fig 78) should
be about 87 or 90 kPa. This initial setpoint is then slowly
lowered based upon the percent-open values of load control
valves V1 through V6, to a minimum value of 30 to 36 kPa.
Specification:
Anytime either chiller pump starts, DP-2 shall be enabled
to control pressure reduction valve V8 at an initial setpoint of
60 kPa. Anytime any load control valve is greater than
95 percent open, the DP-2 setpoint shall be incremented at the
rate of 1.5 kPa every 2.0 minutes up to a maximum of 90 kPa.
Anytime all control valves are less than 80 percent open the
DP-2 setpoint shall be decremented at the same rate to a
minimum value of 36 kPa. All values shall be user adjustable.
Specification Discussion:
The value of 2.0 minutes in the specification assumes that
valves V1 through V6 are controlled from discharge air
temperature and should recover in less than 2 minutes from
V8-caused changes. If V1 through V6 were controlled from
space temperature directly, the time rate for V8 adjustments
may need to be extended to 12 to 15 minutes to allow
incremental space temperature control recovery from the flow
reductions brought about by V8. Valve V8 is normally open
and line size to minimize its pressure drop during full load
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
V8
HEAT/ HEAT/
COOL COOL
COIL COIL
1
DP
2
V1
V7
B1
Fig. 78. High AHU Valve Differential Pressure Control
353
HEAT/
COOL
COIL
3
2
V2 V3
B2 B3
operation. This valve is applicable for direct or reverse return
piping configurations where significant piping friction losses
migrate out to the control valves upon low flow conditions.
In reverse return systems, locate Valve V8 in the main line
piping just before the AHU 1 takeoff and the DP-2 sensor across
AHU 1 set for approximately 57 kPa, this should be adequate
for any variation in uneven loading. Allow 57 kPa for close-off
and good control. Resetting the DP-2 setpoint lower is
unnecessary in this constant-speed pumping reverse return
example.

VARIABLE SPEED PUMP CONTROL

Decoupled Variable Speed Pump
Control, Direct Return
Similar to Figure 74, Figure 79 decouples the loads from the
source pumps and heat exchangers and uses of a variable speed
pump to provide significant energy savings at reduced loads and
simpler control. Variable speed pump control matches pump speed
to system flow demands. If source devices perform well with
variable flow, the primary pumps may be replaced with variable
speed pumps and controlled similarly to the decoupled example.
NOTE: The variable speed pump pressure is only 108 kPa
since the primary pumps account for the 36 kPa chiller
pressure drop.
HEAT/
HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
6
4 5
V4 V5
B6
B4 B5
M15065
V6