Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

Page: 357 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
347
The Figure 70 supply piping runs out to the coils decreasing in
size between AHU 1, 2, 3, 4, 5, and 6. The return lines between
each AHU are typically sized the same as the respective supply
lines. The drop across AHU 6 must be 48 kPa in order to get the
12.5 L/s through the valve (24 kPa) and the coil (24 kPa). To get
the 12.5 L/s from AHU 5 to AHU 6, the drop across the piping at
AHU 5 is 54 kPa (48 kPa required to get the flow through AHU 6
plus 6 kPa to overcome the supply and return piping drops between
AHU 5 and AHU 6). The AHU 5 balancing valve is set to prevent
the 54 kPa drop across AHU 5 from forcing more than 12.5 L/s to
pass through the AHU 5 coil and control valve. The balancing
valve B5 then is set to take a 6 kPa drop at 12.5 L/s. Similarly, set
a 12 kPa, 24 kPa, 30 kPa, and 36 kPa drop respectively across
balancing valves B4, B3, B2, and B1.
If this is a variable flow loop with a variable speed pump and
the pump is controlled to produce 48 kPa across AHU 6, the control
issue here is: When AHUs 2, 3, 4, 5, and 6 are off (no flow beyond
AHU 1), then, the drop across AHU 1 is only 48 kPa, 36 kPa of
which the AHU 1 balancing valve needs for design flow. Proper
solutions are presented later in this section.
Supply piping is the same for a reverse return system (Fig. 71)
as for the direct return system (Fig. 70). The return flow is reversed
such that the return piping increases in size between AHUs 1, 2,
3, 4, 5, and 6. A full size return line is run back to the source room
from AHU 6. In this example, the pump is the same as the direct
return, since the return line from AHU 6 also takes a 18 kPa drop.
If the AHUs and source were positioned in a circular or hex pattern
such that Coil 6 is closer to the pump, the run from AHU 6 back
to the source would be shorter, and the reverse return piping
pressure would be less than for the direct return, and the piping
cost would be similar. In reverse return systems, balancing is
usually only a trimming exercise.
COUPLED VS DECOUPLED PIPING SYSTEMS
Piping systems requiring constant flow through primary
equipment (chillers, boilers) and variable flow to AHUs may be
coupled or decoupled. See CHILLER SYSTEM CONTROL and
BOILER SYSTEM CONTROL for examples. Primary flow
control for coupled systems and secondary flow control for
decoupled systems are discussed later.
Fig. 71. Reverse Return Piping System
RETURN
HEAT/
COOL
COIL
1
V1
B1
HEAT/
COOL
COIL
2
V2
B2
HEAT/
COOL
COIL
3
V3
B3
HEAT/
COOL
COIL
4
V4
B4
HEAT/
COOL
COIL
5
V5
B5
HEAT/
COOL
COIL
6
V6
B6
PUMP
75 L/s
143 kPa
HEAT
EXCHANGER
36 kPa
DROP
12.5 L/s
PER
AHU COIL
3kPa
DROP
3 kPa
DROP
6 kPa
DROP
3 kPa
DROP
3 kPa
DROP
12 kPa
DROP
SUPPLY
24 kPa
DROP
24 kPa
DROP
DESIGN DROPS,
ALL COILS &
VALVES
0 kPa
3 kPa
DROP
3 kPa
DROP
6 kPa
DROP
3 kPa
DROP
3 kPa
DROP
12 kPa
DROP
18 kPa
DROP
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