Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 359 / 518

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
349
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 16 ft in order to
get the 200 gpm through the valve (8 ft) and the coil (8 ft). To
get the 200 gpm from AHU 5 to AHU 6, the drop across the
piping at AHU 5 is 18 ft (16 ft required to get the flow through
AHU 6 plus 2 ft to overcome the supply and return piping drops
between AHU 5 and AHU 6). The AHU 5 balancing valve is
set to prevent the 18 ft drop across AHU 5 from forcing more
than 200 gpm to pass through the AHU 5 coil and control valve.
The balancing valve B5 then is set to take a 2 ft drop at 200
gpm. Similarly, set a 4 ft, 8 ft, 10 ft, and 12 ft 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 16 ft 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 16
ft, 12 ft 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 6 ft 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 head 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
1200
GPM
48 FT HEAD
HEAT
EXCHANGER
12 FT
DROP
200 GPM
PER
AHU COIL
1 FT
DROP
1 FT
DROP
2 FT
DROP
1 FT
DROP
1 FT
DROP
4 FT
DROP
SUPPLY
8 FT
DROP
8 FT
DROP
DESIGN DROPS,
ALL COILS &
VALVES
0 FT
DROP
1 FT
DROP
1 FT
DROP
2 FT
DROP
1 FT
DROP
1 FT
DROP
4 FT
DROP
6 FT
DROP
M15059