Danfoss VLT HVAC Basic Drive FC 101 Design Manual page 23

Introduction to VLT® HVAC B...
2.6.20 Primary Pumps
2 2 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
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.
2.6.21 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:
Illustration 2.15
22
®
VLT HVAC Basic Drive Design Guide
Flowmeter
F
Frequency
converter
®
MG18C402 - VLT is a registered Danfoss trademark
The first method uses a flow meter. Because the desired
flow rate is known and is constant, a flow meter installed
at the discharge of each chiller, can be used to control the
pump directly. Using the built-in PI 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 very similar to trimming the pump impeller, except it
does not require any labor and the pump 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 operates
at this speed any time the chiller is staged on. Because the
primary loop does not 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 remains appropriate. In the
event the flow rate needs to be increased later in the
systems life, the frequency convertercan simply increase
the pump speed instead of requiring a new pump
impeller.
Flowmeter
F
Frequency
converter