Operation - Honeywell AUTOMATIC CONTROL 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 ten feet (from A to B). At
75 percent flow (375 gpm), the valve drop with both pumps
operating increases to over 41 ft (C to E). With one pump and
75 percent flow the valve drop is about 16 ft (C to D). When
flow is reduced to 50 Percent, the valve drop is about 55 ft for
one pump (F to G) or 63 ft 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 42 ft, energizes the second pump and on a pumping differential
rise to 77 ft, switches back to one pump. The 42 ft 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 77 ft) 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,
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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
80
70
1 PUMP
60
50
40
30
SYSTEM
20
CURVE
10
0
100 200
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 head are sharply reduced at
light loads.
346
CONTROL
VALVE (S)
LOAD
DESIGN
OPERATING
POINT
H
K
E
J
B
G
A
D
I
C
SYSTEM CURVE
MINUS
F
CONTROL VALVE
300 400 500
FLOW (GPM)
C2409A