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Primary & Secondary Pumping Systems

Primary & Secondary Pumping Systems


As we all know, a system with only primary pumping at a constant volume will operate inefficiently for most of its lifetime.


The primary-secondary system solves one problem in the older system: high pumping energy consumption. But how does a primary-secondary system save pumping energy?


First, the system starts by separating the piping into two sides: production (primary) and distribution (secondary). The primary side contains the chillers and the primary pumps, while the secondary side contains the secondary pumps and the cooling units (FCU/AHU).


During times of partial load, which make more than 95 % of operational hours, the cooling coils won't be needing the same flow rate of chilled water because the heat load is less. This is controlled by a 2-way valve which regulates the flow of chilled water through the cooling coils based on the room's temperature.


To save costs and eliminate the need for bypass lines around cooling units, the secondary pumps contain a VSD (variable speed drive) that can regulate their operating point (flow, head) depending on the system's load.


But how does the pump's VSD know that the system's load has changed?

Using DPS.

Differential pressure sensors (DPS) are positioned at the end of each chilled water branch, measuring the difference in pressure between the supply and the return lines. The DPS sends a signal to the pumps' VSD to either increase or decrease the pumps' speed of rotation, and hence, the pumps' flow rate.


When the cooling load decreases, the 2-way valves restrict the flow to a certain degree, and the pressure difference between supply and return lines increases. The DPS can sense that change in pressure difference.


The opposite occurs when the load increases, demanding more flow. The 2-way valves open and the pressure difference drops, causing the DPS to send a signal to the pumps' VSD to slow down and decrease the pumps' flow rate.


Using variable-flow secondary pumping with constant-flow primary pumping allows for three things at once:


1. Maintain constant flow of chilled water through chillers ✔


2. Allow for a variable secondary pump's flow rate, and hence variable pump energy consumption, depending on the cooling load required


3. Select low-head primary pumps 



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