Operation - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
PIPING
HEAT
EXCHANGE
PUMP
CURVE
LOW
FLOW
LOW FLOW
CURVE
FLOW
Fig. 65. System Operation with One Pump,
Design and Low Flow Condition.

Operation

Multiple pumps may be connected either in parallel or in
series into the system. In the dual parallel pump configuration
of Figure 66 a single pump can usually handle 75 to 80 percent
of the total flow. The system curves show that at design
conditions the control valve drop is 30 kPa (from A to B). At
75 percent flow (21 L/s), the valve drop with both pumps
operating increases to over 125 kPa (C to E). With one pump
and 75 percent flow the valve drop is about 50 kPa (C to D).
When flow is reduced to 50 Percent, the valve drop is about
165 kPa for one pump (F to G) or 190 kPa for two pumps (F to
H). Dual parallel pumps save energy and provide redundancy
for 75 to 80 percent of the flow. They do not provide much
relief for high valve pressure drops at low flow.
The pump curves and the system curves indicate possible
pump start/stop setpoints. One scenario on a pumping
differential fall to 130 kPa, energizes the second pump and on
a pumping differential rise to 235 kPa, switches back to one
pump. The 130 kPa pumping differential corresponds to a point
just before the 1-pump curve intersects the system curve (I),
the point at which a single pump no longer can support the
system. When the second pump is started, the operating point
moves to the 2-pump curve and when the control valves have
settled out will be at about Point J. It will vary along the
2-pump curve down to B or up to K. When the operating point
reaches K (about 235 kPa) the system switches back to a single
pump and the operating point is now on the 1-pump curve
until the differential pump pressure drops to I, at which time
the cycle repeats. See PLOTTING A SYSTEM CURVE for
statement on use of ideal system curve for determining
setpoints when coil loading may not be proportional.
Again a reminder to exercise caution when using the ideal
system curves for switching pumps on and off. The ideal curves
are valid only at full and no load conditions, the rest of the time
the actual curve is somewhere above the ideal. Since setpoint
determination is not possible without the actual system curve,
CONTROL
VALVE(S)
LOAD
DESIGN
100% FLOW
DESIGN
PIPING
DROP
DESIGN
VALVE
DROP
C2411
the lag pump stop setpoint should have a significant margin of
safety incorporated. The lag pump start setpoint should be
controlled by a differential pressure controller and have the
software requirement that one control valve be full open for
four minutes before starting.
Time delays must be built in to the control sequence to prevent
rapid switching between one pump and two pump operation. With
each change in pump operation, all control valves must adjust to
new steady-state conditions. The adjustment process often causes
overshoot or undershoot until temperature stability returns and
no switching should take place during this time. Depending upon
the type of temperature control loops, switch-lockout period can
vary from 5 minutes for relatively fast discharge air control to
over 30 minutes for relatively slow space control.
PUMP 1
PUMP 2
HEAT
EXCHANGER
2 PUMPS
240
210
1 PUMP
180
150
120
90
SYSTEM
CURVE
60
30
0
6 12
Fig. 66. System Operation with Two Pumps in Parallel.
Series pumps (Fig. 67), though rarely used in HVAC systems,
are useful where both flow and pressure are sharply reduced at
light loads.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
344
CONTROL
VALVE (S)
LOAD
DESIGN
OPERATING
POINT
H
K
E
J
B
G
A
D
I
C
SYSTEM CURVE
MINUS
F
CONTROL VALVE
18 24 30
FLOW (L/s)
M15282