period two
Chilled-Water System Design
notes
Tertiary Pumping
distribution pump
tertiary pump
bleedline
Figure 56
In very large systems, a primary-secondary-tertiary pumping configuration is sometimes used. The primary pumps circulate water through the chillers. The secondary distribution pumps circulate the water around the distribution loop. The individual load terminals are decoupled from the distribution loop and each load terminal has a dedicated tertiary pump.
A “load terminal” in this case may be an individual cooling coil or an entire building. In systems that use tertiary pumping, the load terminal must be controlled so that only the quantity of water required is drawn from the distribution loop. Water must not be allowed to flow into the return piping until it has experienced the proper temperature rise. The two-way valve modulates to maintain the design return-water temperature. A constant-volume tertiary pump circulates water through the load terminal.
Some tertiary pumping systems use a small bleed line to ensure that water will be immediately available when the tertiary pump is started, and to provide an accurate control signal when the two-way valve is closed. It also keeps the distribution pump from “dead-heading,” or trying to pump when all of the two-way valves are closed. If a bleed line is used, it should be of a much smaller diameter than the rest of the piping.
