GE AF-600 FP Design And Installation Manual page 71

Application Set-up Examples
6.3.18 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,
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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 are
installed in parallel and primary/secondary pumping is not
utilized.
6.3.19 The AF-600 FP 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.
An adjustable frequency drive 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:
Figure 6.13
6-14
AF-600 FP Design and Installation Guide
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 be used to control the
pump directly. Using the built-in PID controller, the
adjustable frequency drive 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 simply decreases the output frequency until the
design flow rate is achieved.
Using a adjustable frequency drive to decrease the pump
speed is very similar to trimming the pump impeller,
except it doesn't 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 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 because 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 adjustable frequency
drive can simply increase the pump speed instead of
requiring a new pump impeller.
DET-768A