period two
Chilled-Water System Design
notes
Distribution Loop Characteristics
▲Reduced pump energy use
▲Distribution loop sized for system diversity
▲Higher return-water temperatures
Figure 57
Let’s summarize: When designed and operated correctly, the distribution loop of the primary-secondary system has the following characteristics:
nVariable 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.
nLoad 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.
nHigher 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.
