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■ The Application
■■
Primary pumps in a primary/secondary pumping
system, such as shown in Figure 1, can be used to
maintain a constant flow through devices that
encounter operation or control difficulties when
exposed to variable flow. The primary/secondary
pumping technique decouples the primary
production loop from the secondary distribution
loop. This allows devices such as chillers to obtain
constant design flow and operate properly while
allowing the rest of the system to vary in flow.
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■ The Design
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In Chilled Water systems, the Primary loop or
production loop, consists of pumps sized to handle
the chillers designed flow rate at a discharge pressure just high enough to circulate the water through
the chiller and the rest of the primary piping loop.
This loop should be as small as possible, just large
enough to allow the secondary system to be
attached. This minimizes the resistance of the
primary loop and therefore the energy consumption
of the non-regulated constant flow pumps.
The primary pump flow is traditionally controlled by
throttling valves or balance valves on the discharge
of the pumps, figure 1. The pumps are often
oversized due to safety margin in the designs. By
adding losses to the pumping circuit with the
throttling valves, the proper design flow rate can be
established, figure 2 (moving from flow 1 to Design
flow).
Another method used is to trim the pumps impeller.
Once the system is operating, the balancing
contractor can determine the actual pressure drop
in the primary loop.
Once the actual pressure requirement of the pump
is established, the pumps impeller can be removed,
trimmed to the proper diameter, rebalanced, and
reinstalled in the pump.
Decreasing the diameter of the pumps impeller
reduces both the capacity and pressure of the
pump as desired, but also has an impact on the
pumps efficiency, figure 3.
VLT® 6000 HVAC
As the evaporator flow rate decreases in a chiller,
the chilled water begins to become over-chilled. As
this happens, the chiller attempts to decrease its
cooling capacity. If the flow rate drops far enough,
or too quickly, the chiller cannot shed its load
sufficiently and the chillers low evaporator
temperature safety trips the chiller requiring a
manual reset. This situation is common in large
installations especially when two or more chillers in
parallel are installed if primary/secondary pumping is
not utilized.
Primary system Secondary system
CHILLER
CHILLER
Fig. 1 - Traditional Primary/Secondary design
Pressure
Design - P2
P1
Pressure
100%
75%
Design
flow
Fig. 2 - Throttling valve
Efficiency
curves
Flow 1
86%
84%
MN.60.D1.02 - VLT is a registered Danfoss trademark
Design flow Flow 1
Fig. 3 - Impeller trimming
Impeller
curves
Flow
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■ The new standard:
■■
Depending on the size of the system and the size of
the primary loop, the energy consumption of the
primary loop can become substantial.
A VLT frequency converter can be added to the
primary system, to replace the throttling valve and/or
trimming of the impellers, leading to reduced
operating expenses (fig. 4). Two control methods are
common:
The first method uses a flow meter. Because the
desired flow rate is known and is constant, a flow
meter can be installed at the discharge of each
chiller can be used to control the pump directly.
Using the built-in PID controller, the VLT frequency
converter will always maintain the appropriate flow
rate, even compensating for the changing resistance
in the primary piping loop as chillers and their pumps
are staged on and off.
Flowmeter
F
Flowmeter
F
VLT® 6000 HVAC
The other method is local speed determination. The
operator simple decreases the output frequency
until the design flow rate is achieved.
Using a VLT frequency converter to decrease the
pumps speed is very similar to trimming the pumps
impeller, except it doesnt require any labor and the
pumps efficiency remains higher, figure 5. The
balancing contractor simply decreases the speed of
the pump until the proper flow rate is achieved and
leaves the speed fixed. The pump will operate at
this speed any time the chiller is staged on.
Because the primary loop doesnt have control
valves or other devices that can cause the system
curve to change and the variance due to staging
pumps and chillers on and off is usually small, this
fixed speed will remain appropriate. In the event the
flow rate needs to be increased later in the systems
life, the VLT frequen-cy converter can simply
increase the pumps speed instead of requiring a
new pump impeller.
P
VLT
CHILLER
CHILLER
Fig. 4
The VLT frequency converter solution
Flow
Fig. 5
Pump efficiencies with variable speed
(Figure courtesy of ITT Bell & Gossett)
Figure 5 shows that the pumps efficiency remains
constant as the speed is reduced to obtain the
design flow. This differs from the results of the
trimming the impeller where the efficiency decreases (fig. 3).
MN.60.D1.02 - VLT is a registered Danfoss trademark
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