C
H
IL
L
E
R
C
H
IL
L
E
R
F lo wmet er
F lo wmet er
F
F
e
3
0
b
b
4
5
6
.1
0
Drive
Drive
Illustration 15: Primary Pumps
3.1.2.6 Secondary Pumps
Secondary pumps in a primary/secondary chilled water pumping system distribute the chilled water to the loads from the primary
production loop. The primary/secondary pumping system is used to hydronically de-couple 1 piping loop from another. In this case,
the primary pump is used to maintain a constant flow through the chillers while allowing the secondary pumps to vary in flow,
increase control and save energy.
If the primary/secondary concept is not used in the design of a variable volume system when the flow rate drops far enough or too
quickly, the chiller cannot shed its load properly. The chiller’s low evaporator temperature safety then trips the chiller requiring a
manual reset. This situation is common in large installations especially when 2 or more chillers in parallel are installed.
The VLT Solution
While the primary-secondary system with 2-way valves improves energy savings and eases system control problems, the true ener-
gy savings and control potential is realized by adding drives.
With the proper sensor location, the addition of drives allows the pumps to vary their speed to follow the system curve instead of
the pump curve. This results in the elimination of wasted energy and eliminates most of the overpressurization that 2-way valves
can be subjected to.
As the monitored loads are reached, the 2-way valves close down. This increases the differential pressure measured across the load
and the 2-way valve. As this differential pressure starts to rise, the pump is slowed to maintain the control head also called setpoint
value. This setpoint value is calculated by summing the pressure drop of the load and the 2-way valve together under design condi-
tions.
N O T I C E
When running multiple pumps in parallel, they must run at the same speed to maximize energy savings, either with individual
dedicated drives or 1 drive running multiple pumps in parallel.
AJ363928382091en-000101 / 130R0983 | 25
Danfoss A/S © 2021.04
Product Overview
VLT® Flow Drive FC 111
Design Guide