Wa t e r
I nlet
Wa t e r
Outlet
BASIN
F lo w or pr essur e sensor
C ondenser
W a t er pump
T hr ottling
v alv e
Supply
CH
IL
LE
R
e3
0b
b4
52
.1
0
Drive
Illustration 14: Condenser Pumps
3.1.2.5 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 oper-
ate 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 at-
tempts 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.
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 drive 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:
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 drive 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 drive 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 sys-
tem’s life, the drive can simply increase the pump speed instead of requiring a new pump impeller.
AJ363928382091en-000101 / 130R0983
24 | Danfoss A/S © 2021.04
Product Overview
VLT® Flow Drive FC 111
Design Guide