Honeywell AUTOMATIC CONTROL User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
348
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:
Fig. 70. Direct Return Piping System
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
= 33 x 0.0225
=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.






gpm
2
gpm
1
2
=
h
2
h
1
()
6
40
2
=
h
2
33
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.
1 FT
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
12 FT
DROP
1 FT
DROP
1 FT
DROP
2 FT
DROP
1 FT
DROP DROP
4 FT
DROP
10 FT
DROP
8 FT
DROP
4 FT
DROP
2 FT
DROP
0 FT
DROP
M15058
PUMP CURVE
AT 817 RPM
PUMP 1750 RPM
CURVE
C1050
SYSTEM CURVE
SYSTEM
CURVE
AT 6 GPM
OPERATING POINT
AT DESIGN OF 40
GPM
COIL,
SUPPLY/RETURN
PIPING 33FT
COIL,
SUPPLY/
RETURN
PIPING 0.74FT
VALVE 54FT
VALVE 11FT
80706050403020100
GPM
DESIGN
CONIDITION
50% LOAD
CONDITION
0
10
20
30
40
50
60
70
80
TOTAL HEAD IN FEET

Honeywell AUTOMATIC CONTROL Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL
Category	Controller
Language	English