period two
Chilled-Water System Design
notes
Primary-Secondary System Rules
▲The bypass pipe should be free of restrictions
◆Sized for minimal pressure drop
◆Avoid random mixing of supply- and
◆No check valve
Figure 48
The bypass pipe is common to both production and distribution loops. The purpose of the bypass pipe is to hydraulically decouple the production (primary) and distribution (secondary) pumps. Because water can flow freely between the supply and return pipes for both loops, a change in flow in one loop does not affect the flow in the other loop.
The actual extent of hydraulic decoupling depends on the pressure drop due to the bypass pipe. Total decoupling is accomplished only if the bypass pipe is free from restrictions and large enough to produce no pressure loss at all flow rates. Because zero pressure loss is not practical, some insignificant pump coupling will exist. Bypass pipes are typically sized so that the water velocity in the pipe will be 10 to 15 ft/s [3 to 4.5 m/s], based on the water flowing through the bypass pipe at the design flow rate of the largest chiller in the system. Additionally,
to further minimize pressure drop, the bypass pipe is usually relatively short in length. To prevent random mixing of the supply and return water streams, however, the minimum length of the bypass pipe is typically 5 to 10 pipe diameters.
When designing the bypass pipe, an important issue to keep in mind is that the bypass pipe must be kept free of unnecessary restrictions. For example, a check valve must not be installed in the pipe. Restrictions cause hydraulic coupling that can result in unacceptable chiller flow variations or unstable, and potentially harmful, system pressure variations due to the resulting series- pumping effects. If a manual isolation valve is required for service, it should be large enough to ensure that it does not add significant pressure drop to the bypass pipe.
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