Dual Pumps, Dual Chillers, Pressure Bypass; 90 Percent Chiller Flow, Direct Return; Ahu Valve High Differential Pressure Control - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS

Dual Pumps, Dual Chillers, Pressure Bypass,

90 Percent Chiller Flow, Direct Return

Dual chiller pressure bypass systems are popular because if
a chiller, tower, or pump fails, there is part load redundancy
and better part load efficiency.
In Figure 77, the bypass sensor and valve is in the same
location as the single chiller system (Fig. 75). The valve is sized
for approximately 500 gpm.
The differential pressure setpoint is the same as for the single
pump/chiller system (34 ft) when both pumps are running. When
only one pump/chiller is operating, the piping between the
pump/chiller and differential pressure sensor carry a minimum
of 540 gpm (90 percent flow for one chiller) and the piping
friction drop (head) falls from 4 ft to 0.8 ft, thus:
2
( ) 540
4 = h 2 = 0.8 ft.
1200
With a single pump/chiller operating, the differential pressure
setpoint for 90 percent flow is 38.8 ft.
Setpoint = 50 ft (pump drop) – 9.6 (chiller drop) – 0.8 (piping
head) – 0.8 (piping drop) = 38.8 ft.
This is up from the 34 ft with both chillers operating.
SUPPLY
C
C
H
H
I
I
L
L
L
L
E
E
R
R
1
2
PUMPS
600 GPM
48' HEAD
EACH
(DESIGN)
RETURN
SYSTEM STRAINER
ENGINEERING MANUAL OF AUTOMATIC CONTROL
HEAT/
COOL
COIL
1
DP
V1
B1
Fig. 77. Dual Pumps, Dual Chillers—Pressure Bypass
With digital controls, the differential pressure setpoint offset
adjustment when only one chiller/pump is operating is handled
by a software routine (dual chiller/pump setpoint plus 4.8 ft.)
invoked anytime only one pump and one chiller are operating.
One method using pneumatic controls uses two pressure
controllers with separate setpoints. The primary controller is
set at 38.8 ft with the secondary controller set at 34 foot setpoint
and configured as a low limit device during periods of single
pump/chiller operation. During periods of dual pump/chiller
operation, the 34 foot setpoint controller is used alone.

AHU Valve High Differential Pressure Control

As noted in the discussion of Figure 75, the differential
pressure controlled bypass valve in constant speed pumping
systems is adequate to maintain a high flow through chillers,
but does little to prevent high differential pressures across the
AHU load control valves. In the previous example the load
control valve differential pressure varied from 8 ft at design
load to 38.8 ft at the single chiller, light-load mode of operation.
If it is expected that the light-load differential pressures will
exceed the load control valves close-off rating, locate a throttling
valve in the common load piping (valve V8 in Figure 78) to
reduce the load pressures while still allowing adequate pressure
to maintain chiller flows. Select either a double-seated or
balanced cage type valve for the high differential pressure.
HEAT/
HEAT/
COOL
COOL
COIL
COIL
3
2
V3
V2
B3
B2
354
HEAT/
HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
6
4 5
V4 V5
B6
B4 B5
M15064
V6