Primary Pumps; The Vlt Solution - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Product Overview

3.1.20 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 overchilled. 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 2
or more chillers in parallel are installed if primary/
secondary pumping is not utilized.

3.1.21 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. 2 control
methods are common:
MG18C702
Design Guide
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.
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. If the
flow rate needs to be increased later in the system's life,
the frequency converter can simply increase the pump
speed instead of requiring a new pump impeller.
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