Direct Vs Reverse Return Piping Systems - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
As flow through the system is reduced, a new system curve
is established. See the 0.375 L/s curve in Figure 69. When the
flow is reduced, the new pressure loss in source and supply and
return piping can be calculated using the formula:
flow 2 p 2
2
=
flow 1 p 1
Where:
flow = present flow
1
flow = next flow
2
p = present pressure drop
1
p = next pressure drop
2
For example: If flow = 2.5 L/s, flow
1
99 kPa, then p = 2.2 kPa.
2
p 2
2
( )
0.375
=
2.5 99
p = 99 x 0.0225
2
= 2.2 kPa
At low flow, the pressure drops through the coils, coil piping,
and heat source tend to disappear and nearly all of the now
elevated pump pressure appears across the partially closed
valves V1, V2, and V3. This can cause valve noise, poor control,
or failure of valves to seat. Control solutions are discussed in
following sections.
SUPPLY
HEAT
EXCHANGER
36 kPa
DROP
DESIGN DROPS,
ALL COILS &
PUMP
75 L/s
143 kPa
RETURN
= 0.375 L/s, and p
2
12 kPa 3 kPa
DROP DROP
HEAT/ HEAT/
24 kPa
COOL COOL
DROP COIL COIL
1 2
VALVES
V1
24 kPa
DROP
B1
36 kPa 30 kPa
DROP DROP
3 kPa
12 kPa
DROP
DROP
Fig. 70. Direct Return Piping System
240
PUMP 1750 RPM
210
CURVE
180
150
SYSTEM
CURVE
120
AT 0.375 L/s
90
60
30
0
0 0.625 1.25
=
1
Fig. 69. System and Pump Curves for a
Closed System at Various Loads.

DIRECT VS REVERSE RETURN PIPING SYSTEMS

Distribution system control solutions vary dependent upon
whether the designer chose a direct or reverse return piping system.
Systems are sometimes configured as a combination of both; a
high-rise building could, for example be reverse return on the
riser and direct return on the floor run-outs. Direct return systems
are usually lower cost and used in smaller installations. Reverse
return systems are used in both small and large installations.
3 kPa 6 kPa
DROP DROP
HEAT/ 12.5 L/S
COOL PER
COIL AHU COIL
3
V3
V2
B3
B2
24 kPa 12 kPa
DROP DROP
3 kPa 6 kPa
DROP DROP
ENGINEERING MANUAL OF AUTOMATIC CONTROL
346
SYSTEM CURVE
OPERATING POINT
AT DESIGN OF
2.5 L/s
COIL,
SUPPLY/RETURN
PUMP CURVE
PIPING 99 kPa
AT 817 RPM
VALVE 33 kPa
1.875 2.5 3.125 3.75 4.375 5
L/s
DESIGN
CONIDITION
3 kPa 3 kPa
DROP DROP
HEAT/ HEAT/ HEAT/
COOL
COOL COOL
COIL
COIL COIL
4 5 6
V4 V5
V6
B4 B5 B6
6 kPa 0 kPa
DROP DROP
3 kPa 3 kPa
DROP DROP
M15283
COIL,
SUPPLY/
RETURN
PIPING 2.2 kPa
VALVE162 kPa
50% LOAD
CONDITION
C4604