VLT
®
6000 HVAC
MG.60.B1.02 - VLT is a registered Danfoss trademark
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Primary pumps
Primary pumps in a primary/secondary pumping
system 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.
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 chiller’s 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.
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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. 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.
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 doesn’t require any labor and the
pumps efficiency remains higher. 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 doesn’t 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 frequency
converter can simply increase the pumps speed
instead of requiring a new pump impeller.
CHILLER
F
CHILLER
F
Flowmeter
Flowmeter