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TRG-TRC016-EN 65
period three
System Variations
notes
Notice also that the VPF system must continue to include a bypass. Although
a control valve prevents flow in the bypass for most system operating
conditions, the modulating valve and bypass are required to ensure that the
water flow through the system remains above the minimum flow limit of the
operating chillers. This bypass may be in the same location as in the primary-
secondary system, or it may be a three-way valve on a few of the cooling coils.
Although VPF systems have been successfully installed and operated, they
are more complex both to design and to operate when compared to a primary-
secondary system. The sequencing of chillers and pumps requires a thorough
understanding of system dynamics because flow rates will vary through every
operating chiller. The control system needs to avoid cycling (restarting the
chiller too soon) and maintain the rate-of-flow variation through the chiller
evaporators within the allowable limits. This becomes very complicated as
the number of chillers increases.
Another important consideration when investigating VPF systems is the fact
that they take more time and planning to design and commission properly than
other systems. The system design engineer must thoroughly define the control
sequence early in the design process, and clearly communicate it to the
controls provider. Also, the system operators must understand how the VPF
system works; therefore, training is mandatory. The success of a system design
is directly related to the ability of the operator to carry out the design intent.
Critical VPF System Requirements
▲ Chillers must handle variable evaporator flow
▲ System must include a bypass
▲ System-level controls must limit the rate-of-
flow change
▲ Adequate time to design and commission
controls
▲ Operator must understand the system
Figure 73