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TRG-TRC016-EN 51
period two
Chilled-Water System Design
notes
Let’s summarize: When designed and operated correctly, the distribution loop
of the primary-secondary system has the following characteristics:
n Variable water flow. Only the amount of water that is actually used at the
load terminals is pumped throughout the distribution loop. Under most
operating conditions, this flow rate is less than the design flow rate,
resulting in reduced pumping energy.
n Load diversity. Not all of the load terminals peak at the same time.
Therefore, the quantity of water that flows at any given time is less than
the constant water flow required in a system using three-way valves.
This allows for reduced distribution pump and pipe sizes.
n Higher return-water temperature at all loads. Properly-operating two-
way valves do not allow unused chilled water to bypass the load terminals.
Water is only allowed to enter the return pipe after it has accomplished
vague useful cooling. If the system is operating properly, the temperature
of the water returning from the load terminals will be at least as high as it
is at design load conditions, and may actually rise at part-load conditions.
This warm return water is especially advantageous in systems using heat
recovery, free cooling, or preferential loading of chillers. These options will
be discussed in Period Three.
Distribution Loop Characteristics
▲ Reduced pump
energy use
▲ Distribution loop
sized for system
diversity
▲ Higher return-water
temperatures
Figure 57