Specifications

2.7.23 Secondary Pumps

Secondary pumps in a primary/secondary chilled water pumping system are used to distribute the chilled water to the loads

from the primary production loop. The primary/secondary pumping system is used to hydronically de-couple one 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 design concept is not used, and a variable volume system is designed, 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.

2.7.24 The VLT Solution

While the primary-secondary system with 2-way valves improves energy savings and eases system control problems, the

true energy savings and control potential is realised by adding frequency converters.

With the proper sensor location, the addition of frequency converters 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 over-pressurisation, 2-way valves can be

subjected too.

As the monitored loads are reached, the 2-way valves close down. This increases the differential pressure measured across

the load and 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 up the pressure drop of the load and 2-way valve under

design conditions.

Note that when running multiple pumps in parallel, they must run at the same speed to maximize energy savings, either

with individual dedicated drives or one running multiple pumps in parallel.

Frequency

converter

Frequency

converter

CHILLER

130BB454.10

Illustration 2.22 The VLT Solution

Introduction to VLT® HVAC D... Design Guide