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12 Chiller System Design and Control SYS-APM001-EN
Primary System Components
valves may be either three-way or two-way. As previously discussed, three-
way valves require constant water flow, while two-way valves allow the water
flow in the system to vary. As flow varies, the pump may simply ride its curve
or use a method of flow control such as a variable-speed drive. Refer to the
chapter “System Configurations” on page 42 for a detailed discussion of
distribution-system options.
Figure 11. Simplified distribution system
The distribution system may contain other components, such as an
expansion tank, control valves, balancing valves, check valves, and an air
separator, to name a few. The density, and therefore the volume, of the water
in a “closed” chilled-water distribution system varies as it undergoes
changes in temperature. The expansion tank allows for this expansion and
contraction of water volume.
Pumping arrangements
Variations on three basic pumping arrangements are common. They are
referred to as constant flow, primary-secondary (decoupled) flow, and
variable-primary flow (VPF). The implications and nuances of each of these is
discussed in greater detail in “System Configurations” on page 42.
Constant flow system
When a chiller is on, a constant speed pump dedicated to it is on, and there
need not be any other pumps operated in the system (Figure 12). This is a
simple system and makes the most sense when there will only be one chiller
operated at a time in the system. Challenges with this system arise at part
load when chillers are in the parallel arrangement (refer to “Parallel Chillers”
on page 42). To solve some of these problems, the chillers can be placed in
Expansion
Tank
Pump
Chiller
Distribution
Piping
Loads
Figure 12. Constant flow system
Load
Chillers
CV
Pump
Three-Way
Control
Valve