Variable Speed Pump Control; Decoupled Variable Speed Pump Control, Direct Return - Honeywell AUTOMATIC CONTROL 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 28 ft (Fig. 74), and pressure bypassing occurs at 34 ft. The
maximum setpoint for DP-2 (Fig 78) should be about 29 or 30
ft. 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 10 to 12 ft.
Specification:
Anytime either chiller pump starts, DP-2 shall be enabled to
control pressure reduction valve V8 at an initial setpoint of 20
ft. Anytime any load control valve is greater than 95 percent
open, the DP-2 setpoint shall be incremented at the rate of 0.5
ft every 2.0 minutes up to a maximum of 30 ft. 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
12 ft. 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
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
HEAT/
COOL
COIL
3
2
V2 V3
B3
B2
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 take-off and the DP-2 sensor across
AHU 1 set for approximately 19 ft, this should be adequate for
any variation in uneven loading. Allow 19 ft 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 head is only 36 ft since the
primary pumps account for the 12 ft chiller head.
355 ENGINEERING MANUAL OF AUTOMATIC CONTROL
HEAT/
HEAT/ HEAT/
COOL
COOL COOL
COIL COIL COIL
6
4 5
V4 V5
B6
B4 B5
M15065
V6