Q: We are updating our building cooling system. It was mentioned that the update would include primary-secondary pumps, what does this mean?
A: Primary-secondary (P-S) pumping is incorporated into hydronic system designs for a variety of applications. The most frequent application of P-S pumping is used in chilled water systems where there is a need to have fairly constant flow through the chiller evaporator and variable flow through the distribution system. P-S pumping also allows a designer to break up very large, complicated systems into smaller, more manageable subsystems. P-S pumping can also provide different supply water temperatures to each secondary loop without mixing valves.
Image 1. Piping schematic of two pumping loops sharing a common pipe (Images courtesy of Hydraulic Institute)The fundamental operating principle of P-S pumping lies in the interconnection of the piping loops. Image 1 is a piping schematic where the two pumping loops share a small section of piping, referred to as the common pipe. The common pipe is located between two standard tees that are spaced approximately three pipe diameters in length apart. If the pressure drop in the common pipe is minimized, the flow rate in each loop will be independent of the other.
For more information on pumps used in building services, refer to the Pump Application Guideline for Commercial Building Services, which will be published in spring 2019, at www.pumps.org.