Specifications
2.7.21 Primary Pumps
Primary pumps in a primary/secondary pumping system
can 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. Decoupling allows
devices such as chillers to obtain constant design flow and
operate properly while allowing the rest of the system to
vary in flow.
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 low
evaporator temperature safety trips the chiller, requiring a
manual reset. This situation is common in large instal-
lations, especially when two or more chillers in parallel are
installed if primary/secondary pumping is not used.
2.7.22 The VLT Solution
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 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. Two
control methods are common:
The first method uses a flow meter. Because the desired
flow rate is known and constant, a flow meter installed at
the discharge of each chiller can control the pump directly.
Using the PID controller, the frequency converter always
maintains 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.
The other method is local speed determination. The
operator simply decreases the output frequency until the
design flow rate is achieved.
Using a frequency converter to decrease the pump speed
is similar to trimming the pump impeller, but more
efficient. The balancing contractor simply decreases the
speed of the pump until the proper flow rate is achieved
and leaves the speed fixed. The pump operates at this
speed any time the chiller is staged on. Because the
primary loop lacks control valves or other devices that can
change the system curve, and the variance due to staging
pumps and chillers on and off is small, this fixed speed
remains appropriate. If the flow rate must be increased
later in the life of the system, the frequency converter can
simply increase the pump speed instead of requiring a
new pump impeller.
Frequency
converter
Frequency
converter
CHILLER
CHILLER
Flowmeter
Flowmeter
FF
130BB456.10
Illustration 2.15 Frequency Converters used with Primary Pumps in a Primary/Secondary Pump System
Introduction VLT® HVAC Drive FC 102 Design Guide
28 MG16C102 - Rev. 2013-08-20
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