Single Pump, Pressure Bypass, Direct Return; Valve Location And Sizing; Differential Pressure Sensor Location - Honeywell AUTOMATIC CONTROL Engineering Manual

The examples in this section on Flow And Pressure Control
Solutions use a distribution system that has six equal loads
(coils) as shown in Figure 73. These control solutions are:
1. Single constant speed pump, single chiller system, two-
way AHU control valves, and pressure bypass valve to
control chiller flow to a minimum of 90 percent full flow.
a. direct return.
b. reverse return.
2. Dual constant speed pumps, dual chiller systems, and
pressure bypass valve to control chiller flow to a minimum
of 90 percent full flow.
a. direct return.
b. reverse return.
3. High control valve differential pressure control.
4. Decoupled variable speed secondary pumping system
with two-way AHU control valves.
a. direct return.
b. reverse return.
SUPPLY
1' 1'
DROP DROP
EACH COIL AND ASSOCIATED PIPING TAKES AN 8' DROP
HEAT/ 200 GPM
HEAT/ HEAT/
COOL PER
COOL COOL
COIL COIL COIL AHU COIL
3
1 2
EACH CONTROL VALVE (NOT SHOWN) TAKES AN 8' DROP
B3
B2
B1
1'
1'
DROP
DROP
RETURN
BALANCING
VALVE
Fig. 73 Typical Example Loads.

Single Pump, Pressure Bypass, Direct Return

Figure 74 analyzes Figure 70 pumping system at full flow
and at half flow. The flow reduction at half flow is taken evenly
across each coil. At half flow with no pressure bypass the control
valve pressure drops increase from 8 ft to 44 ft as system friction
drops reduce to one-forth of the design values and the pump
head rises from 48 to 54 ft.
Figure 75 shows a pressure controlled bypass valve set to
maintain 90 percent minimum flow through the chiller to satisfy
the chillers minimum flow requirement. At 90 percent flow
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
2' 1' 1'
DROP DROP DROP
HEAT/ HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
4 5 6
B4 B5
2'
1' 1'
DROP DROP DROP
M15060
through the chiller (1080 gpm), the chiller and equipment room
piping drops are 81 percent of design (90 percent squared).
The pump curve (not shown) indicates a pump head of 50 ft at
1080 gpm.

VALVE LOCATION AND SIZING

Since the system piping between Loads 1 and 2 is designed
for only 1000 gpm and the low load bypass flow could exceed
that, the bypass valve is located remotely before Load 1. If
necessary to locate the bypass valve after Load 1, redesign the
piping to carry the 90 percent flow.
If the differential pressure sensor is located across the main
lines at Load 1 as shown in Figure 75 (see DIFFERENTIAL
PRESSURE SENSOR LOCATION), the best place for the
bypass valve is the same location. Because the differential
pressure is lower than in the chiller room, valve wear is less.
The valve is sized for approximately 1000 gpm with a 34
foot drop. A double seated valve is appropriate to reduce actuator
close off requirements and the inherent leakage will not be a
significant factor.

DIFFERENTIAL PRESSURE SENSOR LOCATION

As previously stated the chiller design flow is 1200 gpm at
12 ft of head and requires a minimum flow of 1080 gpm. At
1080 gpm the pump curve shows a head of 50 ft.
From the formula:
2
gpm 2 h 2
 
=
 
gpm 1 h 1
 
Calculate the drop across the chiller as 9.6 ft.
B6
h 2
2
( ) 1080
=
1200 12
12 (0.90) 2 = h 2
h 2 = 12 x 0.81 = 9.6 ft.
Similarly calculate the reduced drop in the supply and return to
Load 1 as 3.2 ft.
With the differential pressure controller located across Load 1,
the setting is:
50 ft – 9.6 ft – 3.2 ft – 3.2 ft = 34 ft
This location provides a lower head across the load control
valves at light loads than if located across the pump and chiller.
To ensure the best sensing, be sure that the system strainer is
located up stream from the differential pressure controller return
pickup, so that a dirty strainer is not sensed as an increasing
pressure drop (decreasing flow).
351 ENGINEERING MANUAL OF AUTOMATIC CONTROL