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

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
As flow through the system is reduced, a new system curve
is established. See the 6 gpm curve in Figure 69. When the
flow is reduced, the new head loss in source and supply and
return piping can be calculated using the formula:
gpm 2 h 2
  2
=
 
gpm 1 h 1
 
Where:
gpm 1 = present flow
gpm 2 = next flow
h 1 = present pressure drop
h 2 = next pressure drop
For example: If gpm 1 = 40, gpm 2 = 6, and h 1 = 33 ft, then h 2
= 0.74 ft.
h 2
2
( ) 6
=
40 33
h = 33 x 0.0225
2
= 0.74 ft
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 head 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
12 FT
DROP
DESIGN DROPS,
ALL COILS &
VALVES
PUMP
1200
GPM
48 FT HEAD
RETURN
ENGINEERING MANUAL OF AUTOMATIC CONTROL
4 FT 1 FT 1 FT
DROP DROP DROP
HEAT/ HEAT/
8 FT
COOL COOL
DROP
COIL COIL
1 2
V1 V2
8 FT
DROP
B1 B2
12 FT 10 FT
DROP DROP DROP
4 FT 1 FT 1 FT
DROP DROP
DROP
Fig. 70. Direct Return Piping System
80
PUMP 1750 RPM
70
CURVE
60
50
SYSTEM
CURVE
40
AT 6 GPM
30
20
10
0
0 10 20
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.
2 FT
DROP
HEAT/ 200 GPM
HEAT/
COOL PER
COOL
COIL AHU COIL
COIL
3
4
V3
B3
8 FT 4 FT
DROP
2 FT
DROP
348
SYSTEM CURVE
OPERATING POINT
AT DESIGN OF 40
GPM
COIL,
SUPPLY/RETURN
PUMP CURVE
PIPING 33FT
AT 817 RPM
VALVE 11FT
30 40 50 60 70 80
GPM
DESIGN
CONIDITION
1 FT 1 FT
DROP DROP
HEAT/
HEAT/
COOL
COOL
COIL
COIL
5 6
V4 V5
V6
B6
B4 B5
2 FT 0 FT
DROP DROP
1 FT 1 FT
DROP DROP
M15058
COIL,
SUPPLY/
RETURN
PIPING 0.74FT
VALVE 54FT
50% LOAD
CONDITION
C1050